ScienceWeek

SCIENCEWEEK

ScienceWeek
May 2, 2003
Vol. 7 Number 18

An Online Digest of Research in the Sciences

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Environmental stability is fleeting.
Environmental change is perduring.
-- R.J. Huggett

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Section 1

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Symposium: Climate Change: Factors and Effects

1. Introduction
2. Climate Modeling
3. Climate and the Hydrological Cycle
4. Climate and Greenhouse Gases
5. Climate and Aerosols
6. Climate Change and Biological Populations
7. Climate Change and Human Disease

Notices and Subscription Information

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Section 2

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1. INTRODUCTION

ON CLIMATE CHANGE

"We derive our confidence that the climate will not change much
from its constancy and generally favorable state in the first
half of [the 20th century]. Before 1920 drought and famine could
be counted on to devastate India on the average once every 8.5
years. That risk dropped to half between 1920 and 1960, and the
population, previously controlled largely by famine, rose
accordingly. Early in this century Californians expected a truly
dry winter about once in seven years, but for the next 50 years
droughts arrived less than half as often. The 1960s changed all
that and the world's climate has returned to an earlier, less
benign state. At the same time rapid population growth and an
increasing need for advance planning in an ever more complex
world keep reducing our capacity for a swift and flexible
response.

"All this is perfectly normal. During the past two millennia our
climate has changed often, rapidly, and often drastically. Across
Europe, the Little Ice Age, lasting from about 1450 to 1850 AD,
was much colder than it is now. Severe winters were common, the
summers cool and damp, and glaciers crept down Alpine valleys. In
northern England, Scotland and Scandinavia, growing grain above
200 m (600 ft) became impossible, and lost harvests caused
widespread famines which stirred war and rebellion. In Iceland,
where human subsistence is always marginal, a cold spell in the
14th century and the Little Ice Age converted the country
permanently from a wheat-growing to a sheep-farming economy,
although the temperature fell only 1.5 to 2.0 degrees Celsius.

"The cold 14th century also contributed to the demise of the
first European colony west of the Atlantic. Vikings from Iceland
had settled Greenland in the warm centuries between 800 and 1200
AD, but the change to a colder climate sharply reduced their
yield from farming. Travel across an ocean sometimes ice-bound
even in summer cut the supply from Norway of essentials such as
timber and iron, and new blood for the small population failed to
arrive. Economic and political pressures in northern Europe that
shifted the focus away from the far west of the Viking world
administered the coup de grace.

"Not everywhere was the Little Ice Age a bad time. Early in the
i9th century, the prairies of North America were nourished by
rain far more than now, and the tales about the cornucopia out
west that seduced so many settlers in the 1840s were founded on
reality. Certainly, some of the tale-tellers were unconscionable
real estate crooks, but it was not their fault that the end of
the Little Ice Age arrived together with the new immigrants, who
found land and climate to be far less suitable than promised. The
marked decline in rainfall that followed would have decimated the
buffalo herds, had not the white man taken his toll a little
earlier and somewhat more thoroughly."

Tieerd H. Van Andel: New Views on an Old Planet: A History of
Global Change. Cambridge University Press 1994, p.47.

ON GLOBAL WARMING

"For mechanical energy, the liberator of human energy, industrial
civilization continues to depend upon fossil fuels. It does so
even with solar and other alternative energy technologies --
modes of solar-energy conversion, ocean-temperature conversion,
deep dry-earth heat and not excluding nuclear fission -- at the
ready. The four-fold increase in combustion of fossil fuels
during the past 50 years injects carbon dioxide into the
atmosphere in a volume exceeding 25 percent of the natural
turnover of the gas. Global warming, in consequence, threatens a
worldwide shift in climate prospectively catastrophic to
agriculture. The projected rise in sea level endangers the
habitations of a third of humankind. If the present doubling of
the world population is to be the last, a human existence must be
extended to all. That will require another four-fold increase in
energy. Such increase cannot come from fossil fuels; it must come
from alternative technologies.

"Human activity has raised the cycles of other gases in the
atmosphere -- nitrogen, for example, to twice its natural
turnover, and nitrogen and methane well in excess of that. Now it
is injecting into the atmosphere compounds new to nature -- the
notorious fluorocarbons, for example. About one untoward
consequence of these unnatural emissions there is no debate.
Observation has established and now monitors the ominous thinning
of the high-atmosphere ozone layer over the Southern Hemisphere.
Increase in high-energy solar radiation at the ground has begun
to disrupt photosynthesis, starting with the phytoplankton in the
Antarctic Ocean. This is a considerably more serious matter than
the rising incidence of skin cancer in that hemisphere and more
immediately menacing than global warming.

"The Montreal Convention of 1989 and its stiffening after-
amendments may yet curb release of the gases that most
insidiously disrupt the ozone layer. This international
convention was adopted within a decade of the publication of the
scientific paper establishing the peril. Here is ground for hope
that the human species will not lose the future open to the means
at its command."

Gerard Piel: The Age of Science. Basic Books 2001, p.380.

ON ANTHROPOGENIC CARBON AND FUTURE GLOBAL CLIMATE

Since 1958, atmospheric CO2 levels have been measured at a
monitoring station on Mauna Loa, Hawaii, far out in the Pacific
Ocean away from the principal industrial regions of the Western
Hemisphere. This 30-year record has a distinctive pattern that
unmistakably reflects human impact on the global flux of carbon.
The annual ups and downs in the concentration of atmospheric
carbon, which amount to [approximately] 12.7 billion metric tons
(bmt), result from the natural biological cycling of carbon. The
dominance of plant photosynthesis, which removes CO2 from the
air, is evident during the spring and summer; the dominance of
animal respiration, which releases CO2, into the air, is apparent
during the fall and winter. Superimposed on this natural seasonal
cycle is an upward drift in the total concentration of airborne
CO2, which is interpreted as the industrial byproduct of the
intensified use of fossil fuels by humans. This rising trend has
subsequently been confirmed bv measurements at stations in Alaska
and Antarctica, establishing the dependability of these data
without doubt.

"The actual increase in airborne levels of CO2 due to human
activity is startling in its magnitude. At present, the
atmosphere contains about 740 bmt of CO2, a value well above the
1958 level of <670 bmt measured at Mauna Loa, Hawaii, and far
above the estimated concentration of <600 bmt for the mid-
nineteenth century inferred from the study of gas bubbles in ice
cores. Isolated bubbles in glaciers preserve a chemical sample,
including the CO2 composition of the atmosphere at the time that
the ice formed, stretching back hundreds of thousands of years
into the geologic past. These estimates, which are considered to
be reliable, indicate that the amount of CO2 currently in the
atmosphere is [approximately] 25 to 30 percent above the amount
that existed a century ago and, more importantly, that CO2 levels
have been and continue to be increasing at an alarming rate. This
upward trend is attributed to the burning of fossil fuels, which
has released CO2 into the air at a rate that has increased by
[approximately] 3.5 percent for each year of this century. Energy
experts expect that fossil-fuel CO2 emissions will continue to
increase in the future, but at a lower rate of 1 to 2 percent per
year over the next 100 years. The increasing CO2 content of the
atmosphere is also partly the result of deforestation -- a
practice particularly rampant in areas covered by rain forests,
such as the Amazonian region of South America. This harvesting of
trees removes millions of acres of vegetation that would extract
atmospheric CO2 if left undisturbed; instead, these trees become
a source of atmospheric CO; when they are burned or allowed to
decompose.

"The implications of this measured and projected increase in
future emissions of atmospheric CO2 are far-reaching for the
earth's heat balance and, hence, for the global climate. Because
CO2 is an excellent absorber of infrared energy and the Earth
radiates infrared wave-lengths, the increasing levels of
atmospheric CO2 will trap terrestrial heat and lead to a warmer
Earth in the not too distant future."

Paul R. Pinet: Oceanography. West Publishing 1992, p.441.

ON THE HYDROSPHERE

"The hydrosphere is all the Earth's waters. It includes liquid
water, water vapor, ice and snow. Water in the oceans, in rivers,
in lakes and ponds, in ice sheets, glaciers, and snow fields, in
the saturated and unsaturated zones below ground, and in the air
above ground is all part of the hydrosphere. Some people set the
ambits of the hydrosphere to exclude the waters of the
atmosphere.

"The hydrosphere presently holds about 1,384,120,000 km^(3) of
water in various states. By far the greatest portion of this
volume is stored in the oceans. A mere 2.6 per cent [36,020,000
km^(3)] of the hydrosphere is fresh water. Of this, 77.23 per
cent is frozen in ice caps, icebergs, and glaciers. Groundwater
down to 4 km accounts for 22.21 per cent, leaving a tiny fraction
stored in the soil, lakes, rivers, the biosphere, and the
atmosphere.

"Water is the chief component of the hydrosphere. Concentrations
of constituents dissolved in water vary in different parts of the
hydrosphere. Sodium and chlorine are the main constituents of sea
water. The acidity of waters in the hydrosphere is highly
variable. The most acid natural waters on Earth occur in volcanic
crater lakes. These may have a pH less than 0. Water free of
carbon dioxide in contact with ultramafic rocks may have a pH of
12. Soils in desert basins and rich in sodium carbonate and
sodium borate may be equally alkaline. In most environments, 9 is
the upper limit of pH."

Richard J. Huggett: Environmental Change: The Evolving Ecosphere.
Routledge 1997, p.137.

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2. CLIMATE MODELING

CONSTRAINTS ON RADIATIVE FORCING AND FUTURE CLIMATE CHANGE FROM
OBSERVATIONS AND CLIMATE MODEL ENSEMBLES

R. Knutti et al (University of Bern, CH) discuss future climate
change, the authors making the following points:

1) The assessment of uncertainties in global warming projections
is often based on expert judgment, because a number of key
variables in climate change are poorly quantified. In particular,
the sensitivity of climate to changing greenhouse-gas
concentrations in the atmosphere and the radiative forcing
effects by aerosols are not well constrained, leading to large
uncertainties in global warming simulations(1).

2) The expected future warming of the climate system and its
potential consequences increase the need for climate projections
with clearly defined uncertainties and likelihood estimates(2).
The IPCC provides these probabilities for most of their findings
in the recently published Third Assessment Report(1). However,
some of the most important uncertainties -- such as the projected
surface warming -- are still based on expert judgment, and are
only given as ranges derived from different models. The evidence
that part of the observed warming of both atmosphere and
ocean(3,4) is caused by anthropogenic emissions of greenhouse
gases and aerosols(1,5) may help assess climate models, and has
been used to scale model projections for the next few decades.

3) The authors present a Monte Carlo approach to produce
probabilistic climate projections, using a climate model of
reduced complexity. The uncertainties in the input parameters and
in the model itself are taken into account, and past observations
of oceanic and atmospheric warming are used to constrain the
range of realistic model responses. The authors obtain a
probability density function for the present-day total radiative
forcing, giving 1.4 to 2.4 W m^(-2) for the 5–95 per cent
confidence range, narrowing the global-mean indirect aerosol
effect to the range of 0 to –1.2 W m^(-2). Ensemble simulations
for two illustrative emission scenarios suggest a 40 per cent
probability that global-mean surface temperature increase will
exceed the range predicted by the Intergovernmental Panel on
Climate Change (IPCC), but only a 5 per cent probability that
warming will fall below that range.

References (abridged):

1. Houghton, J. T. et al. (eds) Climate Change 2001: The
Scientific Basis (Cambridge Univ. Press, Cambridge, 2001).

2. Schneider, S. H. What is "dangerous" in climate change? Nature
411, 17-19 (2001).

3. Jones, P. D., New, M., Parker, D. E., Martin, S. & Rigor, I.
G. Surface air temperature and its changes over the past 150
years. Rev. Geophys. 37, 173-199 (1999).

4. Levitus, S., Antonov, J. I., Boyer, T. P. & Stephens, C.
Warming of the world ocean. Science 287, 2225-2229 (2000).

5. Santer, B. D. et al. A search for human influences on the
thermal structure of the atmosphere. Nature 382, 39-46 (1996)

Nature 2002 416:719

Related Background:

CARBON DIOXIDE AND CLIMATE CHANGE

The term "greenhouse effect" refers to the blockade of longwave
(infrared) radiation from the Earth into space by trace
constituents of the atmosphere, primarily water vapor, carbon
dioxide, ozone, methane, nitrous oxide, and various
chlorofluorocarbons -- gases referred to collectively as the
"greenhouse gases". The sensitivity of the biosphere to the
greenhouse effect has existed throughout the history of life on
Earth: the infrared radiation-blocking gases return radiated heat
to the ground, accounting for approximately 70 percent of the net
input of energy to the Earth's surface.

In this context, the term "radiative forcing" refers in general
to climate changes produced by changes in incoming or outgoing
radiative heat to and from the Earth's surface.

The term "tectonic" refers in general to deformations of the
Earth's crust and consequent structural changes, and "tectonic
timescales" are the long timescales (e.g., tens of millions of
years) over which such deformations occur.

T. J. Crowley and R.A. Berner (2 installations, US) present a
commentary and analysis of current research relating carbon
dioxide and climate change, the authors making the following
points:

1) The authors point out that it has long been known that on
timescales of tens of millions of years, intervals of continental
glaciation were interspersed with intervals of little or no ice.
The magnitude of warmth during these warm intervals is
impressive: for example, at times during the Cretaceous period
(approximately 65 to 145.6 million years ago) duck-billed
dinosaurs roamed the northern slope of Alaska, and deep and
bottom waters of the ocean, now near freezing, could reach a
balmy 15 degrees celsius.

2) The authors point out that in the 1980s, a convergence of
results from paleoclimate data and geochemical and climate models
suggested that such long-term variations in climate were strongly
influenced by natural variations in the carbon dioxide content of
the atmosphere. Recently, some geochemical results have raised
concerns about the validity of this conclusion, since carbon
dioxide concentrations over the past 65 million years appear to
have reached low levels well before the most recent phase (the
past 3 million years) of Northern Hemisphere glaciation. A study
spanning the Phanerozoic time-frame (the past 570 million years)
also suggests some decoupling between times of predicted high
carbon dioxide and some climate indices.

3) In order to reevaluate the validity of the assumed carbon
dioxide-climate link, the authors compared estimates of
Phanerozoic carbon dioxide variations and net radiative forcing
with the continental glaciation record and low-latitude
temperature estimates. The authors report that the first order
agreement between the carbon dioxide record and continental
glaciation continues to support the conclusion that carbon
dioxide has played an important role in long-term climate change.

4) The authors conclude that to weigh the merits of the carbon
dioxide paradigm, it may be necessary to expand the scope of
climate modeling. For factors responsible for the presence or
absence of continental ice, the carbon dioxide model works very
well. In contrast, there are substantial gaps in our
understanding of how climate models distribute heat on the planet
in response to carbon dioxide changes on tectonic time scales.
"Given the need for better confidence in some of the paleoclimate
data, and unanticipated complications arising from altered
tectonic boundary conditions, it may be hazardous to infer that
existing discrepancies between models and data cloud
interpretations of future anthropogenic greenhouse gas
projections."

Science 2001 292:870

Related Background:

EARTH SCIENCES: AN ALTERNATIVE SCENARIO FOR GLOBAL WARMING

Earth's global surface temperature has increased by approximately
0.5 degrees centigrade since 1975, a relative "burst" of warming
that has apparently taken the global temperature to its highest
level in the past 1000 years, and there is a growing consensus
that the warming is at least in part a consequence of increasing
anthropogenic *greenhouse gases. These gases cause a global
"climate forcing", i.e., an imposed perturbation of the energy
balance of the Earth with space. There are many competing natural
and anthropogenic climate forcings, but increased greenhouse
gases are estimated to be the largest forcing and to result in a
net positive forcing, especially during the past few decades.
Evidence supporting this interpretation has been provided by
observed heat storage in the ocean, which is positive and which
is of the magnitude of the energy imbalance estimated from
climate forcings for recent decades.

J. Hansen et al (5 authors at 3 installations, US) present a
discussion of global warming, the authors making the following
points:

1) The authors point out that a common view is that the current
global warming rate will continue or accelerate. The authors,
however, argue that rapid warming in recent decades has been
driven mainly by non-carbon dioxide greenhouse gases such as
chlorofluorocarbons, methane, and N(sub2)O, and not by the
products of fossil fuel burning, carbon dioxide, and *aerosols,
the positive and negative climate forcings of which are partially
offsetting.

2) The authors point out that the growth rate of non-carbon
dioxide greenhouse gases has declined in the past decade. If
sources of methane and O(sub3) precursors were reduced in the
future, the change in climate forcing by non-carbon dioxide
greenhouse gases in the next 50 years could be near zero.
Combined with a reduction of *black carbon emissions and
plausible success in slowing carbon dioxide emissions, this
reduction of non-carbon dioxide greenhouse gases could lead to a
decline in the rate of global warming, reducing the danger of
dramatic climate change.

3) The authors suggest that such a focus on air pollution has
practical benefits that unite the interests of developed and
developing countries, although assessment of ongoing and future
climate change requires composition-specific long-term global
monitoring of aerosol properties.

[Editor's note: After its publication several weeks ago, this
paper became controversial and received considerable publicity.
The senior author, James Hansen, is noted for helping to alert
the world to global warming in 1988, and this recent paper has
been interpreted as a reversal of his ideas concerning the
dangers of fossil fuel, carbon dioxide, and aerosol emissions,
and publicized by those opposed to the Kyoto Protocol on climate
change. For an account of reaction to this paper, see: Nature
2000 407:7.]

Proc. Nat. Acad. Sci. 2000 97:9875

Notes:

*greenhouse gases: The physical basis of the so-called
"*greenhouse effect" is essentially simple: carbon dioxide gas is
transparent to visible light but relatively opaque to infrared
radiation. The same is true of glass. Relatively high-energy
visible light radiation from the sun passes inward through the
atmosphere, warms the surface of the Earth, which then radiates
lower energy in the form of infrared radiation (heat) back to the
atmosphere. But if the atmosphere has a concentration of infrared
impenetrable gases such as carbon dioxide, the infrared radiation
cannot pass out, and the surface of the Earth underlying the
atmosphere cannot cool, and the surface of the Earth thus will
continue to grow hotter.

*aerosols: The term "aerosol" refers to a dispersion in which a
finely divided solid is suspended in air and the particles are of
colloidal dimensions. The term "colloidal dimensions" refers to
the range approximately 1 nanometer to 100 nanometers in
diameter.

*black carbon: (carbon black) Amorphous (i.e., non-crystalline)
carbon.

Related Background:

ON GLOBAL CLIMATE CHANGE

Environmental change involves jumps, fluctuations, and trends,
the environment changing through the operation of the internal
machinery of the *ecosphere (biosphere), and through the external
agencies of cosmic and geological forces. Evidence of past
environmental change, almost always incomplete, derives from
geochemical, physical, biological, historical, and instrumental
sources. In recent years, high-speed computers have allowed
researchers to manipulate complicated and reasonably realistic
models of environmental change, with modeling particularly useful
for studying changes in *sedimentary basins, biogeochemical
cycles, and climate. General circulation models, run with
appropriate boundary conditions, predict climates of the past,
and these predicted climates can be compared with paleoclimatic
indicators.

R.B. Alley et al (3 authors 3 installations, US) present a review
of current research on global climate change, the authors making
the following points:

1) Prediction of climate change requires observational
constraints on the current climate state, knowledge of the way
the coupled air-ocean-ice-earth-life system behaves, and
information on changing forcings such as solar variability.
Studies of past climate are also required to focus model-building
efforts on climate components that are likely to change, and to
allow testing of the ability of models to predict time-evolution
of the system.

2) The last few million years have been generally cold and icy
compared with the previous hundred million years but have
alternated between warmer and colder conditions. These
alternations have been linked to changes over tens of thousands
of years in the seasonal and latitudinal distribution of sunlight
on Earth caused by features of Earth's orbit. Globally
synchronous climate change despite some hemispheric asynchrony of
the forcing is explained at least in part by lowering carbon
dioxide during colder times in response to changes in ocean
chemistry. We live in one of the warmer times of these orbital
cycles; the coolest times brought glaciation to nearly one-third
of the modern land area.

3) Studies of past climate changes indicate that the Earth system
has experienced greater and more rapid changes over larger areas
that was generally believed possible, with jumping between
fundamentally different modes of operation in as little as a few
years. Most of the last 100,000 years or longer has been
characterized by large and abrupt regional-to-global climate
changes, and agriculture and industry have developed during
anomalously stable climatic conditions. New high-resolution
analysis of sediment cores indicates these past changes have been
caused by "*band jumps" between modes of operation of the climate
system. Recurrence of such band jumps is possible and might be
affected by human activities.

Proc. Nat. Acad. Sci. 1999 96:9987

Notes:

*ecosphere (biosphere): In general, the term "biosphere" refers
to the portion of the planet capable of supporting life. It
ranges from elevations of approximately 10,000 meters above sea
level to the deep ocean, and a few hundred meters below the
surface of the soil. The biosphere consists of the hydrosphere,
the lower atmosphere (troposphere), and the surface of the
*lithosphere, all three regions inhabited by metabolically active
organisms.

*lithosphere: In current geology, the lithosphere is the
approximately 100 kilometer rigid upper layer of the crust and
upper mantle of the Earth.

*sedimentary basins: The term "sedimentary basin" refers to a
subsiding area of the Earth's crust, which permits the net
accumulation of sediment, i.e., material derived from pre-
existing rock, from biogenic sources, or precipitated by chemical
processes.

*band jumps: In this context, the term "band jump" refers to an
abrupt change from one range of variation to another.

Related Background:

ON THE ACCURACY OF CLIMATE SIMULATIONS

T.M. Smith et al (National Climatic Data Center, US) discuss
climate simulations, the authors making the following points:

1) It is a fundamental tenet of the scientific method that
theories must be consistent with observations. To test our
understanding of the climate system, we must evaluate how
accurately climate models reproduce not only today's climate (1),
but also the climate of the past. Over the past decade, the
observed climate record has become more complete, allowing the
climatic effects of natural agents and human-related changes in
atmospheric composition (collectively referred to as "climate
forcing") to be estimated. We can now test how well climate
models simulate century-scale variations in the observed climate
record. There have been numerous intercomparisons of various
climate model simulations of 20th-century climate, based on the
best available estimates of the climate forcing (2).

2) A standard assumption in these intercomparisons is that the
model simulations should reproduce as closely as possible
observed climate variability. This assumption must, however, be
viewed with caution. Observational errors, sampling errors, and
time-dependent biases degrade the climate record. Considerable
effort has been spent at minimizing these biases (3,4), yet
problems remain. Consider for example the worldwide record of sea
surface temperatures, which dates back to the 19th century. At
present, several different estimates of time-dependent bias
adjustments and the effects of incomplete and changing spatial
sampling can be used to correct the observational record before
1942 (2-5). Different assumptions and adjustment techniques lead
to additional uncertainty in the climate record.

3) Climate models are not perfect either. Errors evolve in
climate simulations as a result of incomplete physical
understanding and limited knowledge of past (or future) climate
forcing. These errors must be considered along with the
uncertainty related to climate chaos, which occurs because of
nonlinear interactions in the global climate system. Climate
chaos errors can be addressed through repeated runs of a climate
model with the same forcing, but different starting conditions.
These ensemble simulations can then be used to estimate the
magnitude of the uncertainty introduced by a chaotic climate
system (2).

References (abridged):

1. B. A. Wielicki et al., Science 295, 841 (2002)

2. J. T. Houghton et al., Climate Change 2001: The Scientific
Basis. Contribution of Working Group I to the Third Assessment
Report of the Intergovernmental Panel on Climate Change
(Cambridge Univ. Press, Cambridge/New York, 2001)

3. C. K. Folland, D. E. Parker, Q. J. R. Meteorol. Soc. 121, 319
(1995)

4. T. M. Smith, R. W. Reynolds, J. Clim. 15, 73 (2002)

5. T. M. Smith, C. F. Ropelewski, J. Clim. 7, 949 (1994)

Science 2002 296:483

Related Background:

ON CLIMATE MODELING

Leonard A. Smith (London School of Economics, US) discusses
climate modeling, the author making the following points:

1) The traditional approach to climate modeling is to build the
most complicated model that will fit inside the largest computer
available, run it once, and see what happens. This approach
yields a single "best guess" forecast. Yet even in high school
physics, we learn that an answer without "error bars" is no
answer at all. Although it is a nontrivial task to assign
relevant uncertainty estimates to imperfect models of chaotic
systems undergoing transient changes in forcing, doing so is
conceivable. One alternative to devoting all our resources to one
best guess is to use the same computer resource to perform an
ensemble of model runs. This alternative would, of course,
require the use of simpler models, and a balance between running
different initial conditions (to cope with chaos), different
model parameterizations and parameter values (to identify tuning
issues), and different model structures (to mitigate model
error). A single best guess from a complicated model run without
good uncertainty estimates is impotent, whereas a beautiful set
of ensemble statistics on too simple a model is irrelevant. How
do we go about assigning resources between these two extremes?
And how can we tell which physical phenomena of economic and
social interest our current models might be able to forecast?

2) At best, our models hold only in certain circumstances. This
is true even for our "Laws of Physics". In climate forecasting,
to make any progress, we assume the "rosy scenario" holds: a)
nothing horrible happens that takes the model beyond its range of
validity (e.g., no asteroid collides with the Earth); and b) no
small but crucial feedback mechanism is missing from our model
(i.e., our model has a range of validity). As we are forced to
assume the rosy scenario, we can never make objective probability
statements on the basis of our climate simulations. What we can
do is establish their internal consistency: we can determine for
which phenomena and on which time scales our models might reflect
reality.

Proc. Nat. Acad. Sci. 2002 99:2487

Related Background:

ON GLOBAL CLIMATE MODELS

M. Stute et al (Barnard College, US) discuss global climate
models, the authors making the following points:

1) Global climate is a result of the complex interactions between
the atmosphere, cryosphere (ice), hydrosphere (oceans),
lithosphere (land), and biosphere (life), fueled by the
nonuniform spatial distribution of incoming solar radiation. We
know from climate reconstructions using recorders such as ice
cores, ocean and lake sediment cores, tree rings, corals, cave
deposits, and ground water that the Earth's climate has seen
major changes over its history.

2) An analysis of the temperature variations patched together
from all these data reveals that climate change occurs in cycles
with characteristic periods, for example, 200 million, 100,000,
or 4 to 7 years. For some of these cycles, particular mechanisms
have been identified, for example, climate forcing by changes in
the Earth's orbital parameters or internal oscillations of the
coupled ocean-atmosphere system. However, major uncertainties
remain in our understanding of the interplay of the components of
the climate system.

3) Paleoclimate reconstructions, in particular from ice cores,
also have demonstrated that climate can change over extremely
short periods of time such as a few years. Over the last century,
humans have altered the Earth's surface and the composition of
its atmosphere to the extent that these factors measurably affect
current climate conditions, and there is concern that perhaps
during one human generation we will gradually change climate
conditions or even trigger a rapid and much more dramatic shift:
we might be "poking an angry beast".

4) Major progress in our understanding of climate processes in
the past, present, and future has been made by the development of
numerical models that simulate climate at an increasing level of
detail. Recent breakthroughs in spatial coverage and temporal
resolutions of systems recording today's climate, and high-
resolution reconstruction of past climate conditions from diverse
sources using new past-climate indicators (proxies), make it
possible to validate climate models and thus improve their
reliability for future predictions.

Proc. Nat. Acad. Sci.2001 98:10529

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3. CLIMATE AND THE HYDROLOGICAL CYCLE

The term "hydrological cycle" (water cycle) refers to the
continuous circulation of water between the oceans, atmosphere,
and land. Water evaporates from the oceans (and lakes and rivers)
as water vapor in the atmosphere, where it may condense into
clouds. Subsequently, clouds release precipitation (rain, snow,
or hail), which falls on the land and either evaporates back into
vapor, or is absorbed by living systems, or runs off into streams
or rivers.

THE OCEAN'S ROLE IN ATLANTIC CLIMATE VARIABILITY

Martin Visbeck (Columbia University, US) discusses the ocean and
climate variability, the author making the following points:

1) Large climatic variations during the ice ages have been linked
to changes in the circulation of the Atlantic Ocean (1). During
the last 10,000 years, we have enjoyed a more stable climate with
comparatively mild century-scale fluctuations (2). Today, a
substantial part of the global year-to-year climatic variability
is caused by the El Nino-Southern Oscillation in the Pacific
Ocean (3).

2) The Pacific is three times as wide along the equator as the
Atlantic and can effortlessly influence climate around the globe.
The influence of the Atlantic is less wide-ranging but can
nevertheless be substantial, especially if its circulation
changes. One part of the international research program CLIVAR
(Climate Variability and Predictability) is beginning to shed
light on the mechanisms and predictability of Atlantic climate
variability (4).

3) Air-sea interactions in the tropical Atlantic cause
substantial year-to-year variability in the amount and timing of
rainfall along the east coast of South America from Brazil to the
Caribbean in boreal spring (March to May) and in western sub-
Saharan Africa in boreal fall (August to September). These
regions are near the Intertropical Convergence Zone (ITCZ), where
very warm surface temperatures cause rapid, high-reaching cloud
formation associated with strong precipitation. Changes in the
location of the warmest surface temperature cause north-south
displacements of the ITCZ and substantial regional rainfall
variability (5).

4) In the equatorial Atlantic, changes in the north-to-south
temperature distribution cause most of the observed ITCZ
variability. This is quite different from the Pacific, where El
Nino is associated with changes in the west-to-east surface
temperature. Atlantic surface-temperature anomalies just north of
the Equator can be triggered by El Niño, as well as by changes in
the strength of the trade winds associated with the North
Atlantic Oscillation (NAO). Changes in large-scale ocean
circulation may also alter tropical temperature gradients and
thus modulate the location and strength of the Atlantic ITCZ.
With rapid progress in the understanding of tropical Atlantic
variability (TAV), prospects are good for improved seasonal-to-
interannual rainfall predictions in the tropical Atlantic.

References (abridged):

1. W. S. Broecker, Science 278, 1582 (1997). [1582]

2. G. Bond et al., Science 278, 1257 (1997). [1257]

3. A. Kaplan et al., J. Geophys. Res. 103, 18,567 (1998). [ADS]

4. J. Marshall et al., Int. J. Climatol. 21, 1863 (2001).

5. R. T. Sutton, S. P. Jewson, D. P. Rowell, J. Clim. 13, 3261
(2000).

Science 2002 297:2223

Related Background:

RAPID WASTAGE OF ALASKA GLACIERS AND THEIR CONTRIBUTION TO RISING
SEA LEVEL

A.A. Arendt et al (University of Alaska, US) discuss glaciers and
sea level, the authors making the following points:

1) Mountain glaciers (1) constitute only about 3% of the
glacierized area on Earth, but they are important because they
may be melting rapidly under present climatic conditions and may
therefore make large contributions to rising sea level. Previous
studies (2-5), based on observations and model simulations of
glacier mass balance, estimated the contribution of all mountain
glaciers to rising sea level during the last century to be 0.2 to
0.4 mm/year. The range of uncertainty is large, and it stems from
insufficient measurements of glacier mass balance: Conventional
mass balance programs are too costly and difficult to sample
adequately the >160,000 glaciers on Earth. At present, there are
only about 40 glaciers worldwide with continuous balance
measurements spanning more than 20 years. High-latitude glaciers,
which are particularly important because predicted climate
warming may be greatest there, receive even less attention
because of their remote locations. Glaciers that are monitored
routinely are often chosen more for their ease of access and
manageable size than for how well they represent a given region
or how large a contribution they might make to changing sea
level. As a result, global mass balance data are biased toward
small glaciers (<20 km2) rather than those that contain the most
ice (>100 km2). Also, large cumulative errors can result from
using only a few point measurements to estimate glacier-wide mass
balances on an individual glacier.

2) Glaciers in Alaska and neighboring Canada (labeled "Alaska"
glaciers herein) cover 90,000 km2, or about 13% of the mountain
glacier area on Earth, and include some of the largest ice masses
outside of Greenland and Antarctica. Additionally, many of these
glaciers have high rates of mass turnover. However, they are
underrepresented by conventional mass balance studies, which
include only three or four long-term programs on relatively small
glaciers. Dyurgerov and Meier (5), by necessity, extrapolated the
data from these few small glaciers to estimate the contribution
of all Alaska glaciers to sea-level change, and they specifically
pointed to the need for further data in this region, especially
on the larger glaciers. The authors report they used airborne
laser altimetry to address this problem. They have measured
volume and area changes on 67 glaciers, representing about 20% of
the glacierized area in Alaska and neighboring Canada, and they
use these data to develop new estimates of the total contribution
of Alaska glaciers to rising sea level.

3) In summary: The authors have used airborne laser altimetry to
estimate volume changes of 67 glaciers in Alaska from the mid-
1950s to the mid-1990s. The average rate of thickness change of
these glaciers was -0.52 m/year. Extrapolation to all glaciers in
Alaska yields an estimated total annual volume change of -52 ± 15
km3/year (water equivalent), equivalent to a rise in sea level
(SLE) of 0.14 ± 0.04 mm/year. Repeat measurements of 28 glaciers
from the mid-1990s to 2000-2001 suggest an increased average rate
of thinning, -1.8 m/year. This leads to an extrapolated annual
volume loss from Alaska glaciers equal to -96 ± 35 km3/year, or
0.27 ± 0.10 mm/year SLE, during the past decade. These recent
losses are nearly double the estimated annual loss from the
entire Greenland Ice Sheet during the same time period and are
much higher than previously published loss estimates for Alaska
glaciers. They form the largest glaciological contribution to
rising sea level yet measured.

References (abridged):

1. Mountain glaciers are those not in Greenland and Antarctica

2. M. Meier, Science 226, 1418 (1984)

3. M. Meier, in Ice in the Climate System, W. R. Peltier, Ed.
(Springer-Verlag, Berlin, 1993), pp. 141-160

4. Z. Zuo and J. Oerlemans, Clim. Dyn. 13, 835 (1997)

5. M. Dyurgerov and M. Meier, Arctic Alpine Res. 29, 392 (1997)

Science 2002 297:382

Related Background:

GLOBAL COOLING AFTER THE ERUPTION OF MOUNT PINATUBO: A TEST OF
CLIMATE FEEDBACK BY WATER VAPOR

B.J. Soden et al (Princeton University, US) discuss global
cooling, the authors making the following points:

1) Water vapor plays a key role in regulating Earth's climate. It
is the dominant greenhouse gas (1) and provides the largest known
feedback mechanism for amplifying climate change (2). Because the
equilibrium vapor pressure of water increases rapidly with
temperature, it is generally believed that the concentration of
water vapor will rise as the atmosphere warms. If so, the added
radiative absorption from water vapor will act to further amplify
the initial warming. Current climate models suggest that this
provides an important positive feedback, roughly doubling the
sensitivity of the surface temperature to an increase in
anthropogenic greenhouse gases (3-5). If the actual feedback by
water vapor is substantially weaker than predicted by current
models, both the magnitude of warming and range of uncertainty
resulting from a doubling of CO2 would be substantially
diminished (5).

2) Despite the importance of water vapor feedback in determining
the sensitivity of Earth's climate, the fidelity of its
representation in climate models has remained a topic of debate
for more than a decade. The difficulty in verifying models partly
stems from the lack of observed climate variations that can
provide quantitative tests of the feedbacks in question.
Assessments of water vapor feedback are often based on regional,
seasonal, or interannual variations of Earth's climate, which
differ markedly in both cause and character from the more
uniform, radiatively forced perturbations that result from
increasing CO2. Thus, their conclusions are often qualitative,
and their relevance to feedbacks that arise from global warming
are often questioned.

3) It has long been recognized that volcanic eruptions provide a
valuable opportunity to observe the climate system's response,
albeit a transient one, to the presence of an external radiative
forcing. Strong volcanic eruptions inject large amounts of
sulfuric gas into the lower stratosphere where it combines with
water and oxygen to form small, yet optically important, aerosol
particles. Winds rapidly disperse the particles throughout the
lower stratosphere, resulting in a near-global perturbation to
the radiative energy balance. Because they are more effective at
scattering sunlight than absorbing longwave terrestrial
radiation, the net radiative effect of volcanic aerosols is to
cool the planet.

4) In summary: The sensitivity of Earth's climate to an external
radiative forcing depends critically on the response of water
vapor. The authors use the global cooling and drying of the
atmosphere that was observed after the eruption of Mount Pinatubo
to test model predictions of the climate feedback from water
vapor. The authors first highlight the success of the model in
reproducing the observed drying after the volcanic eruption.
Then, by comparing model simulations with and without water vapor
feedback, the authors demonstrate the importance of the
atmospheric drying in amplifying the temperature change and show
that without the strong positive feedback from water vapor the
model is unable to reproduce the observed cooling. The authors
suggest these results provide quantitative evidence of the
reliability of water vapor feedback in current climate models,
which is crucial to their use for global warming projections.

References (abridged):

1. J. Kiehl and K. Trenberth, Bull Am. Meteorol. Soc. 78, 197
(1997)

2. U. Cubasch, R. Cess, in Climate Change: The IPCC Scientific
Assessment, J. T. Houghton et al., Eds. (Cambridge Univ. Press,
Cambridge, 1990)

3. R. Wetherald and S. Manabe, J. Atmos. Sci. 45, 1397 (1988)

4. R. Cess, et al., J. Geophys. Res. 95, 16601 (1989)

5. I. Held, B. Soden, Annu. Rev. Energy Environ. 25, 441, (2000)

Science 2002 296:727

Related Background:

INTERPRETATION OF RECENT SOUTHERN HEMISPHERE CLIMATE CHANGE

D.W Thompson and S. Solomon (Colorado State University, US)
discuss climate change, the authors making the following points:

1) The atmosphere of the Southern Hemisphere (SH) high latitudes
has undergone pronounced changes over the past few decades. Total
column ozone losses have exceeded 50% during October throughout
the 1990s (1-3), and the Antarctic ozone "hole" reached record
physical size during the spring of 2000 (4). The lower polar
stratosphere has cooled by ~10 K during October-November since
1985 (5), and the seasonal breakdown of the polar vortex has been
remarkably delayed: from early November during the 1970s to late
December during the 1990s, in both the troposphere and the lower
stratosphere (1). At the surface, the Antarctic Peninsula has
warmed by several K over the past several decades, while the
interior of the Antarctic continent has exhibited weak cooling.
Ice shelves have retreated over the peninsula and sea-ice extent
has decreased over the Bellingshausen Sea, while sea-ice
concentration has increased and the length of the sea-ice season
has increased over much of eastern Antarctica and the Ross Sea.
The authors offer evidence that illuminates the connections
between these seemingly disparate trends.

2) In summary: Climate variability in the high-latitude Southern
Hemisphere is dominated by the SH annular mode, a large-scale
pattern of variability characterized by fluctuations in the
strength of the circumpolar vortex. The authors present evidence
that recent trends in the SH tropospheric circulation can be
interpreted as a bias toward the high-index polarity of this
pattern, with stronger westerly flow encircling the polar cap.
The authors argue that the largest and most significant
tropospheric trends can be traced to recent trends in the lower
stratospheric polar vortex, which are due largely to
photochemical ozone losses. During the summer-fall season, the
trend toward stronger circumpolar flow has contributed
substantially to the observed warming over the Antarctic
Peninsula and Patagonia and to the cooling over eastern
Antarctica and the Antarctic plateau.

References (abridged):

1. A. E. Jones and J. D. Shanklin, Nature 376, 409 (1995)

2. D. J. Hofmann, S. J. Oltmans, J. M. Harris, B. J. Johnson, J.
A. Lathrop, J. Geophys. Res. 102, 8931 (1997)

3. World Meteorological Organization, "Scientific assessment of
ozone depletion: 1998," Global Ozone Research and Monitoring
Project Rep. 44 (1999)

4. J. K. Angell et al., "Southern hemisphere winter summary 2000"
[National Oceanic and Atmospheric Administration (NOAA)/Climate
Prediction Center (CPC), Washington, DC (2000)]. Updates are
available on the Web at www.cpc.ncep.noaa.gov

5. K. E. Trenberth and J. G. Olson, J. Clim. 2, 1196 (1989)

Science 2002 296:895

Related Background:

THE CAUSE OF DECREASED PAN EVAPORATION OVER THE PAST 50 YEARS

M.L. Roderick and G.D. Farquhar (Australian National University,
AU) discuss the global water cycle, the authors making the
following points:

1) It is now well established that the surface of Earth has, on
average, warmed ~0.15°C decade1 over the past 50 years (1). One
expected consequence of this warming is that the air near the
surface should be drier, which should result in an increase in
the rate of evaporation from terrestrial open water bodies.
However, despite the observed increases in average temperature,
observations from the Northern Hemisphere show that the rate of
evaporation from open pans of water has been steadily decreasing
over the past 50 years (2). This trend is general (3,4) but not
universal (5). The contrast between expectation and observation
is called the "pan evaporation paradox".

2) It is important to understand why pan evaporation has
decreased despite the increases in average temperature in order
to make more robust predictions about future changes in the
hydrological cycle. Two proposals for the decline in pan
evaporation have been advanced: the first invokes changes in the
humidity regime over the pans, whereas the second invokes
reductions in solar irradiance resulting from more clouds and/or
aerosols (5) and is generally consistent with the independent
suggestion that increased pollution would weaken the hydrological
cycle. The first proposal is that pan evaporation has decreased
because evaporation from the environment surrounding the pan has
increased. The explanation is that in water-limited environments,
when the evaporation from the adjacent environment is high, the
air over the pan tends to be cooler and more humid, thereby
reducing evaporation from the pan. A subsequent analysis of
rainfall and streamflow data from water-limited environments in
both the former Soviet Union and the United States does
apparently show an increase in evaporation from the environment.
However, this explanation for decreasing pan evaporation is
unsatisfactory for two reasons. First, it only predicts changes
in pan evaporation in water-limited environments. The problem is
that some areas are not water-limited, and in wet environments
the evaporation from pans and the surrounding environment have
both declined. Further, if the proposed mechanism was the
important one, then the vapor pressure deficit should have
decreased. However, data from the United States show that its
average has remained virtually constant over the past 50 years.
This implies that the second proposal, based on the decrease in
solar irradiance, should be further investigated.

3) In summary: Changes in the global water cycle can cause major
environmental and socioeconomic impacts. As the average global
temperature increases, it is generally expected that the air will
become drier and that evaporation from terrestrial water bodies
will increase. Paradoxically, terrestrial observations over the
past 50 years show the reverse. The authors demonstrate that the
decrease in evaporation is consistent with what one would expect
from the observed large and widespread decreases in sunlight
resulting from increasing cloud coverage and aerosol
concentration.

References (abridged):

1. C. K. Folland et al., in Climate Change 2001: The Scientific
Basis, J. T. Houghton et al., Eds. (Cambridge Univ. Press,
Cambridge, 2001), pp. 99-181

2. T. C. Peterson, V. S. Golubev, P. Y. Groisman, Nature 377, 687
(1995)

3. N. Chattopadhyay and M. Hulme, Agric. For. Meteorol. 87, 55
(1997)

4. A. Thomas, Int. J. Clim. 20, 381 (2000)

5. S. Cohen, A. Ianetz, G. Stanhill, Agric. For. Meteorol. 111,
83 (2002)

Science 2002 298:1410

Related Background:

MAGNITUDE AND TIMING OF TEMPERATURE CHANGE IN THE INDO-PACIFIC
WARM POOL DURING DEGLACIATION

K. Visser et al (University of South Carolina, US) discuss ocean
temperature change, the authors making the following points:

1) Ocean–atmosphere interactions in the tropical Pacific region
have a strong influence on global heat and water vapor transport
and thus constitute an important component of the climate
system(1,2). Changes in sea surface temperatures and convection
in the tropical Indo-Pacific region are thought to be responsible
for the interannual to decadal climate variability observed in
extra-tropical regions(1,3), but the role of the tropics in
climate changes on millennial and orbital timescales is less
clear.

2) The authors analyze oxygen isotopes and Mg/Ca ratios of
foraminiferal shells from the Makassar strait in the heart of the
Indo-Pacific warm pool to obtain synchronous estimates of sea
surface temperatures and ice volume. The authors find that sea
surface temperatures increased by 3.5–4.0 degrees C during the
last two glacial—interglacial transitions, synchronous with the
global increase in atmospheric CO2 and Antarctic warming, but the
temperature increase occurred 2,000–3,000 years before the
Northern Hemisphere ice sheets melted. The authors suggest their
observations indicate that the tropical Pacific region plays an
important role in driving glacial—interglacial cycles, possibly
through a system similar to how El Niño/Southern Oscillation
regulates the poleward flux of heat and water vapor.

References (abridged):

1. Cane, M. A role for the tropical Pacific. Science 282, 59-61
(1998)

2. Pierrehumbert, R. Climate change and the tropical Pacific: The
sleeping dragon wakes. Proc. Natl Acad. Sci. 97, 1355-1358 (2000)

3. Hoerling, M., Hurrell, J. & Xu, T. Tropical origins for recent
North Atlantic climate change. Science 292, 90-92 (2001)

4. Broecker, W. Paleocean circulation during the last
deglaciation: a bipolar seesaw? Paleoceanography 13, 119-121
(1998)

5. CLIMAP Project Members. Seasonal reconstructions of the
Earth's surface at the last glacial maximum. Geol. Soc. Am. Map
Chart Ser. MC-36, 1-18 (1981)

Nature 2003 421:152

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4. CLIMATE AND GREENHOUSE GASES

CLIMATE SENSITIVITY UNCERTAINTY AND THE NEED FOR ENERGY WITHOUT
CO2 EMISSION

K. Caldeira et al (Lawrence Livermore National Laboratory, US)
discuss CO2 Emission, the author making the following points:

1) "Climate sensitivity" (T2X) is the global mean climatological
temperature change resulting from a doubling of atmospheric CO2
content. Climate sensitivity is thought, based primarily on
models, to lie in the range of 1.5° to 4.5 degrees C (1,2). Cloud
feedbacks remain the greatest source of uncertainty in model
predictions of global mean warming (3). Aerosols, non-CO2
greenhouse gases, internal variability in the climate system, and
land use change also affect Earth's temperature (2). Uncertainty
in aerosol radiative forcing precludes a more accurate,
observationally based estimate of climate sensitivity to a CO2
doubling (4,5).

2) In summary: The UN Framework Convention on Climate Change
calls for "stabilization of greenhouse gas concentrations at a
level that would prevent dangerous anthropogenic interference
with the climate system." Even if we could determine a "safe"
level of interference in the climate system, the sensitivity of
global mean temperature to increasing atmospheric CO2 is known
perhaps only to a factor of three or less. The authors
demonstrate how a factor of three uncertainty in climate
sensitivity introduces even greater uncertainty in allowable
increases in atmospheric CO2 concentration and allowable CO2
emissions. Nevertheless, unless climate sensitivity is low and
acceptable amounts of climate change are high, climate
stabilization will require a massive transition to CO2 emission-
free energy technologies.

References (abridged):

1. J. T. Houghton et al., Eds., Climate Change 1995: The Science
of Climate Change (Cambridge Univ. Press, UK, 1996)

2. U. Cubasch et al., in Climate Change 2001: The Scientific
Basis. Contribution of Working Group I to the Third Assessment
Report of the Intergovernmental Panel on Climate Change, J. T.
Houghton et al., Eds. (Cambridge Univ. Press, UK, 2001), pp. 525-
582

3. R. D. Cess, et al., J. Geophys. Res. 101, 12791 (1996)

4. N. G Andronova and M. E. Schlesinger, J. Geophys. Res. 106,
22605 (2001)

5. C. E. Forest, P. H. Stone, A. P. Sokolov, M. R. Allen, M. D.
Webster, Science 295, 113 (2002)

Science 2003 299:2052

Related Background:

DYNAMICS OF RECENT CLIMATE CHANGE IN THE ARCTIC

R.E. Moritz et al (University of Washington Seattle, US) discuss
Arctic climate change, the authors making the following points:

1) It is now well established that important changes occurred in
Arctic climate during the 20th century, including a marked
increase of surface air temperature (SAT) during 1970-2000 (1-3).
The warming was correlated with important but less well-
documented changes in many other Arctic climate and environmental
variables, such as precipitation, sea-ice extent, snow cover,
permafrost temperature, and vegetation distribution (2,4,5).
Because these changes had considerable impacts on people and
ecosystems in the Arctic and may also have global impacts through
a variety of climate feedback mechanisms, it is important to know
whether they will continue in the future. To project future
Arctic climate change with confidence requires an understanding
of how radiative forcing [e.g., from anthropogenic greenhouse gas
(GHG) concentrations] and internal variability (from the internal
dynamics of the climate system) contributed to the recent trends.

2) As is increasingly recognized, the response of the climate
system to radiative forcing may be closely linked to free modes
of internal variability, both in an observational and a dynamical
sense. For example, the Arctic Oscillation (AO) has been
simulated successfully as a purely free internal mode in
atmospheric general circulation models (GCMs). Also, the spatial
pattern of the recent trend in Arctic SAT strongly resembles the
SAT signature of the AO, whereas the AO index exhibited a
substantial positive trend. These correspondences, along with
physical reasoning supported by some GCM experiments, support the
hypothesis that the recent trend in the AO is a consequence of
anthropogenic radiative forcing that somehow excites this free
mode of variability. Though satisfactory understanding of forced
and free variability of Arctic climate remains elusive,
substantial progress has been made in the past 5 years or so on
the basis of statistical and dynamical analysis of historical
observations, paleoclimate reconstructions, physical theories,
and numerical climate modeling.

3) In summary: The pattern of recent surface warming observed in
the Arctic exhibits both polar amplification and a strong
relation with trends in the Arctic Oscillation mode of
atmospheric circulation. Paleoclimate analyses indicate that
Arctic surface temperatures were higher during the 20th century
than during the preceding few centuries and that polar
amplification is a common feature of the past. Paleoclimate
evidence for Holocene variations in the Arctic Oscillation is
mixed. Current understanding of physical mechanisms controlling
atmospheric dynamics suggests that anthropogenic influences could
have forced the recent trend in the Arctic Oscillation, but
simulations with global climate models do not agree. In most
simulations, the trend in the Arctic Oscillation is much weaker
than observed. In addition, the simulated warming tends to be
largest in autumn over the Arctic Ocean, whereas observed warming
appears to be largest in winter and spring over the continents.

References (abridged):

1. P. D. Jones, M. New, D. Parker, S. Martin, I. Rigor, Rev.
Geophys. 37, 173 (1999)

2. M. C. Serreze, et al., Clim. Change 46, 159 (2000)

3. J. K. Eischeid, C. B. Baker, T. R. Karl, H. F. Diaz, J. Appl.
Meteorol. 34, 2787 (1995)

4. J. Morison, K. Aagaard, M. Steele, Arctic 53, 359 (2000)

5. SEARCH SSC, SEARCH: Study of Environmental Arctic Change,
Science Plan, J. Morison et al., Eds. (Univ. of Washington,
Seattle, 2001), pp. 1-89

Science 2002 297:1497

Related Background:

DETECTION OF HUMAN INFLUENCE ON SEA-LEVEL PRESSURE

N.P. Gillett et al (University of Victoria, CA) discuss sea-level
pressure, the authors making the following points:

1) Greenhouse gases and tropospheric sulfate aerosols -- the main
human influences on climate -- have been shown to have had a
detectable effect on surface air temperature(1-3), the
temperature of the free troposphere and stratosphere(2,4) and
ocean temperature(5). Nevertheless, the question remains as to
whether human influence is detectable in any variable other than
temperature.

2) The authors report they detect an influence of anthropogenic
greenhouse gases and sulfate aerosols in observations of winter
sea-level pressure (December to February), using combined
simulations from four climate models. The authors find increases
in sea-level pressure over the subtropical North Atlantic Ocean,
southern Europe and North Africa, and decreases in the polar
regions and the North Pacific Ocean, in response to human
influence. The authors suggest their analysis also indicates that
the climate models substantially underestimate the magnitude of
the sea-level pressure response. This discrepancy suggests that
the upward trend in the North Atlantic Oscillation index
(corresponding to strengthened westerlies in the North Atlantic
region), as simulated in a number of global warming scenarios,
may be too small, leading to an underestimation of the impacts of
anthropogenic climate change on European climate.

References (abridged):

1. Tett, S. F. B., Stott, P. A., Allen, M. R., Ingram, W. J. &
Mitchell, J. F. B. Causes of twentieth-century temperature change
near the Earth's surface. Nature 399, 569-572 (1999)

2. Mitchell, J. F. B. et al. Climate Change 2001. The Scientific
Basis Ch. 12 (Cambridge Univ. Press, Cambridge, UK, 2001)

3. Allen, M. R. et al. Quantifying anthropogenic influence on
recent near-surface temperature change. Surv. Geophys. (in the
press)

4. Tett, S. F. B., Mitchell, J. F. B., Parker, D. E. & Allen, M.
R. Human influence on the atmospheric vertical temperature
structure: Detection and observations. Science 274, 1170-1173
(1996)

5. Barnett, T. P., Pierce, D. W. & Schnur, R. Detection of
anthropogenic climate change in the world's oceans. Science 292,
270-274 (2001)

Nature 2003 422:292

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5. CLIMATE AND AEROSOLS

CLIMATE EFFECTS OF BLACK CARBON AEROSOLS IN CHINA AND INDIA

S. Menon et al (NASA Goddard Institute for Space Studies, US)
discuss atmospheric aerosols, the authors making the following
points:

1) China has been experiencing an increased severity of dust
storms, commonly attributed to overfarming, overgrazing, and
destruction of forests (1). Plumes of dust from north China, with
adhered toxic contaminants, are cause for public health concern
in China, Japan, and Korea, and some of the aerosols even reach
the United States (2). Recent dust events have prompted Chinese
officials to consider spending several hundred billion yuan (~$12
billion) in the next decade to increase forests and green belts
to combat the dust storms (3).

2) Such measures may be beneficial in any case. However, the
authors suggest that the observed trend toward increased summer
floods in south China and drought in north China (4), thought to
be the largest change in precipitation trends since 950 A.D.(4),
may have an alternative explanation: human-made absorbing
aerosols in remote populous industrial regions that alter the
regional atmospheric circulation and contribute to regional
climate change. The authors suggest that if interpretation is
correct, reducing the amount of anthropogenic black carbon
aerosols, in addition to having human health benefits, may help
diminish the intensity of floods in the south and droughts and
dust storms in the north.

3) Similar considerations may apply to India and neighboring
regions such as Afghanistan, which have experienced recent
droughts. Atmospheric aerosols, which are fine particles
suspended in the air, comprise a mixture of mainly sulfates,
nitrates, carbonaceous (organic and black carbon) particles, sea
salt, and mineral dust. Black (elemental) carbon (BC) is of
special interest because it absorbs sunlight, heats the air, and
contributes to global warming (5), unlike most aerosols, which
reflect sunlight to space and have a global cooling effect. BC
emissions, a product of incomplete combustion from coal, diesel
engines, biofuels, and outdoor biomass burning, are particularly
large in China and India because of low-temperature household
burning of biofuels and coal (9).

4) In summary: In recent decades, there has been a tendency
toward increased summer floods in south China, increased drought
in north China, and moderate cooling in China and India while
most of the world has been warming. The authors used a global
climate model to investigate possible aerosol contributions to
these trends. The authors found precipitation and temperature
changes in the model that were comparable to those observed if
the aerosols included a large proportion of absorbing black
carbon ("soot"), similar to observed amounts. Absorbing aerosols
heat the air, alter regional atmospheric stability and vertical
motions, and affect the large-scale circulation and hydrologic
cycle with significant regional climate effects.

References (abridged):

1. H. W. French, New York Times, 14 April 2002, p. 3

2. R. B. Husar, et al., J. Geophys. Res. 106, 18317 (2001)

3. Reuters, http://in.news.yahoo.com/020514/64/1o02h.html 14 May
2002.

4. Q. Xu, Atmos. Environ. 35, 5029 (2001)

5. J. Hansen, M. Sato, R. Ruedy, A. Lacis, V. Oinas, Proc. Natl.
Acad. Sci. U.S.A. 97, 9875 (2000)

Science 2002 297:2250

Related Background:

AEROSOLS, CLIMATE, AND THE HYDROLOGICAL CYCLE

V. Ramanathan et al (University of California San Diego, US)
discuss global aerosols, the authors making the following points:

1) One of the most visible impacts of human activities is the
brownish haze that pervades many industrial regions as well as
the rural areas of the tropics and the subtropics that are
subjected to heavy biomass burning. Long-range atmospheric
transport transforms this haze into a regional-scale aerosol
layer. Well-known examples are the Arctic haze, the Indo-Asian
haze, the east Asian dust and haze traveling across the Pacific,
and the biomass burning and dust plumes from North Africa (Sahara
and Sahel regions) that spread over most of the subtropical
Atlantic. Unlike the long-lived greenhouse gases, which are
distributed uniformly over the globe, aerosol lifetimes are only
a week or less, resulting in substantial spatial and temporal
variations with peak concentrations near the source.

2) On account of the large spatial and temporal variability of
these aerosols, remote sensing from satellites delivers the most
reliable information about global aerosol distributions. The
measurable quantity from space is the "aerosol optical depth"
(AOD), which is derived from the solar radiation reflected to
space. The AOD is the vertical integral of the aerosol
concentration weighted with the effective cross-sectional area of
the particles intercepting (by scattering and absorption) the
solar radiation at the wavelength of interest. The globally and
annually averaged value of AOD (at 0.55 microns wavelength) is
approximately 0.12 (+- 0.04). Anthropogenic sources contribute
almost as much as natural sources to the global AOD.
Anthropogenic aerosols are typically in the submicrometer-to
micrometer-size range and are composed of numerous inorganic and
organic species falling under four broad categories: sulfates,
carbonaceous aerosols [black carbon and organic carbon, dust, and
sea salt. Global anthropogenic emissions of sulfates, organics,
and black carbon even exceed natural sources. Such a large
perturbation of the global aerosol loading is a major
environmental concern.

3) The authors suggest that in addition to human-made aerosols
enhancing scattering and absorption of solar radiation, they also
produce brighter clouds that are less efficient at releasing
precipitation. These in turn lead to large reductions in the
amount of solar irradiance reaching Earth's surface, a
corresponding increase in solar heating of the atmosphere,
changes in the atmospheric temperature structure, suppression of
rainfall, and less efficient removal of pollutants. These aerosol
effects can lead to a weaker hydrological cycle, which connects
directly to availability and quality of fresh water, a major
environmental issue of the 21st century.

Science 2001 294:2119

References (abridged):

1. J. E. Penner et al., in Climate Change 2001: The Scientific
Basis [Working Group I to the Third Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC), Cambridge Univ.
Press, Cambridge, 2001], pp. 289-348.

2. J. Haywood and O. Boucher, Rev. Geophys. 38, 513 (2000)

3. Y. J. Kaufman, et al., J. Geophys. Res. 102, 17051 (1997)

4. W. D. Collins, et al., J. Geophys. Res. 106, 7313 (2001)

Related background:

ANTHROPOGENIC ATMOSPHERIC AEROSOLS AND GLOBAL CLIMATE CHANGE

S.E. Schwartz and P.R. Buseck (2 installations, US) present a
commentary on recent research on anthropogenic atmospheric
aerosols, the authors making the following points:

1) Most considerations of global climate change caused by human
activities have focused on the warming influence of greenhouse
gases. However, aerosols are another important atmospheric
constituent that influences climate and that has been affected by
human activities. In general, aerosol particles increase
scattering and absorption of shortwave (solar) radiation,
increase cloud reflectance, enhance cloud lifetimes, and suppress
precipitation. These phenomena are all thought to exert a cooling
influence on climate. Recent data indicate that anthropogenic
aerosols reduce cloud droplet size and suppress precipitation
downward of major urban areas and industrial facilities, which is
consistent with earlier hypotheses.

2) The influences of aerosols on climate are more complex than
those of greenhouse gases. Bulk aerosol composition is highly
variable spatially and temporally because of different sources
and production mechanisms and short atmospheric residence times
(from less than a day to more than a month). Particles sizes
range from nanometers to microns, and within the same size class,
particles can exhibit widely different compositions and
morphologies, with different constituents present within the same
particle (e.g., 10 nanometer carbon spherules can be found
embedded within much larger sulfate particles). The
inhomogeneities in properties and geographical distribution of
aerosols make it difficult to characterize their influences on
climate and to represent these influences in models.

3) Recent analysis of the consequence of absorption of shortwave
radiation by aerosols indicates that the heating of the
atmosphere can evaporate clouds. Clouds exert both cooling and
warming influences on climate: cooling in the shortwave (because
of their reflectance), and warming in the longwave (because of
absorption and re-emission of thermal infrared radiation). The
shortwave component dominates, so a reduction in cloud coverage
would result in a net warming influence.

4) The authors conclude: "Recent studies demonstrate both the
importance of aerosol effects on climate and the complexity of
aerosol-cloud interactions. Unfortunately for those would like a
quick and accurate assessment of anthropogenic climate forcing
over the industrial period, the studies also demonstrate that
there is much to be learned before such an assessment can
confidently be given."

Science 2000 288:989

ScienceWeek http://www.scienceweek.com

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6. CLIMATE CHANGE AND BIOLOGICAL POPULATIONS

ECOLOGICAL EFFECTS OF CLIMATE FLUCTUATIONS

The term "El Nino" refers to an aperiodic intermittent (2 to 10
years) flow of unusually warm surface water along the western
coast of South America, the flow capable of causing abnormally
high rainfall in usually dry areas and severe local ecosystem
dislocations -- what is termed an El Nino "event". El Ninos are
regional phenomena, but they have global consequences. The name
"El Nino" ("The Child") arose because the phenomenon usually
occurs around Christmas. In 1986, M.A. Cane and S.A. Zebiak
proposed a model for making forecasts of El Nino several seasons
ahead by applying Newton's equations of motion and the laws of
thermodynamics to the dynamics of the ocean and atmosphere of the
tropical Pacific.

The term "Southern Oscillation" (SO) refers to a coherent
interannual fluctuation of atmospheric pressure over the tropical
Indo-Pacific region. The El Nino/Southern Oscillation phenomenon
(called ENSO), the interaction between El Nino and the Southern
Oscillation, is the strongest source of natural variability in
Earth's climate system. Although ENSO originates in the tropical
latitudes of the Pacific Ocean, its climate impact is felt
globally. Variations in major rainfall systems that are
attributed to ENSO range from droughts in Indonesia and Australia
to storms and flooding in Ecuador and the US. The crucial role of
the interaction between the ocean and the atmosphere in the
tropical Pacific was first postulated in 1969 by Jacob Bjerknes
(1897-1975), and the development of quantitative models has
progressed during the past 3 decades. The essence of the current
Bjerknes hypothesis, as it is called, is that ENSO arises as a
coupled cycle in which anomalies in sea surface temperature in
the Pacific cause the trade winds to strengthen or slacken and,
in turn, drive the changes in ocean circulation that produce
anomalous sea surface temperatures. Ocean-atmosphere feedback can
amplify perturbations in either the equatorial sea surface
temperature or what is called the Walker Circulation -- the
thermodynamic circulation of air parallel to the equator.
Although the oscillatory aspect of ENSO behavior is now
understood reasonably well, the irregularity of the observed
cycle is a subject of active research.

N.C. Stenseth et al (University of Oslo, NO) discuss climate
fluctuations, the authors making the following points:

1) Ecological processes are influenced by prevailing climatic
conditions (1). Early studies typically focused on local weather
parameters such as temperature, precipitation, and snow depth. By
so doing, an important dimension is overlooked: the holistic
nature of the climate system (2). Recently, therefore, increasing
attention has been given to large-scale patterns of climate
variability with marked ecological impacts on interannual and
longer time scales. Of particular interest are the North Atlantic
Oscillation (NAO) (3) and the El Nino-Southern Oscillation (ENSO)
(4,5). These patterns account for major variations in weather and
climate around the world and have been shown to affect
terrestrial vegetation, herbivores and carnivores, and marine
biology and fish stocks (4) through both direct and indirect
pathways.

2) Increasing awareness among and interactions between biologists
and climate scientists are rapidly advancing our insights into
the critical issue of the response of ecosystems to climate
variability and climate change. Moreover, mutual interest in
climate processes is serving as an impetus for new
interdisciplinary research. Climate as a synchronizing agent for
population fluctuations in space, the so-called "Moran effect",
has received much attention. Rainfall changes associated with
ENSO, for instance, produce a highly synchronic pattern of
massive germination of annuals, rodent outbreaks, and vertebrate
predators responses over arid and semiarid regions of South
America. Similarly, across the boreal forest of Canada, lynx and
snowshoe hare population cycles have a closely related dynamic
structure within climatologically based regions defined by the
spatial influences of the NAO. This indicates that climate
affects the hunting behavior (and success) of the lynx,
presumably through snow condition. Also, muskrat (Ondatra
zibethicus) and mink (Mustela vison) populations are more
synchronous over eastern Canada, where the influence of the NAO
is strongest.

3) In summary: Climate influences a variety of ecological
processes. These effects operate through local weather parameters
such as temperature, wind, rain, snow, and ocean currents, as
well as interactions among these. In the temperate zone, local
variations in weather are often coupled over large geographic
areas through the transient behavior of atmospheric planetary-
scale waves. These variations drive temporally and spatially
averaged exchanges of heat, momentum, and water vapor that
ultimately determine growth, recruitment, and migration patterns.
Recently, there have been several studies of the impact of large-
scale climatic forcing on ecological systems. The authors review
how two of the best-known climate phenomena -- the North Atlantic
Oscillation and the El Nino-Southern Oscillation -- affect
ecological patterns and processes in both marine and terrestrial
systems.

References (abridged):

1. B.-E. Sæther, Trends Ecol. Evol. 12, 143 (1997)

2. J. Namias and D. R. Cayan, Science 214, 869 (1981)

3. J. W. Hurrell, Science 269, 676 (1995)

4. S. G. Philander, El Niño, La Niña, and the Southern
Oscillation (Academic Press, New York, 1990)

5. K. E. Trenberth, in Encyclopedia of Ocean Sciences, J. Steele,
S. Thorpe, K. Turekian, Eds. (Academic Press, London, 2001), pp.
4 and 815.

Science 2002 297:1292

Related Background Brief:

ENVIRONMENTAL STOCHASTICITY AND POPULATION DYNAMICS OF LARGE
HERBIVORES: A SEARCH FOR MECHANISMS. Recently, the results from
several long-term individual-based population studies of
ungulates have been published. One major conclusion is that the
population dynamics of ungulates in predator-free environments is
strongly influenced by a combination of stochastic variation in
the environment, and population density. Both density dependence
and environmental stochasticity operate through changes in life
history traits, correlated with variation in body weight. This
generates delays in the response of the population to changes in
environment. In the absence of predation, a stable equilibrium is
therefore unlikely to exist between an ungulate population and
its food resources. This thorough understanding of the mechanisms
generating population fluctuations suggests that studies of
ungulates will provide an important source for examining effects
of long-term changes in the environment, for instance, resulting
from a climatic change. B.E. Saether: Trends in Ecology &
Evolution 1997 12:143.

Related Background Brief:

EL NINO EFFECTS ON THE DYNAMICS OF TERRESTRIAL ECOSYSTEMS. New
studies are showing that the El Nino Southern Oscillation (ENSO)
has major implications for the functioning of different
ecosystems, ranging from deserts to tropical rain forests. ENSO-
induced pulses of enhanced plant productivity can cascade upward
through the food web invoking unforeseen feedbacks, and can cause
open dryland ecosystems to shift to permanent woodlands. These
insights suggest that the predicted change in extreme climatic
events resulting from global warming could profoundly alter
biodiversity and ecosystem functioning in many regions of the
world. Our increasing ability to predict El Nino effects can be
used to enhance management strategies for the restoration of
degraded ecosystems. M. Holmgren et al: Trends Ecol Evol 2001 Feb
1;16(2):89.

Related Background Brief:

A LONG-TERM STUDY OF VERTEBRATE PREDATOR RESPONSES TO AN EL NINO
(ENSO) DISTURBANCE IN WESTERN SOUTH AMERICA. The authors report
they analyzed the putative effects of the El Nino Southern
Oscillation (ENSO) of 1991-92 in a semi-arid locality of northern
Chile. The authors obtained 30 months of pre ENSO data, followed
by 36 months of peak and post ENSO data (total = 5.5 yr). The
rainy winter of 1991 resulted in a three-fold increase in total
seed bank (perennial and ephemerals pooled) and in ephemeral (but
not perennial) herb cover. Seed and herbage eaters (rodents)
irrupted to population levels ca 20 times higher during the
breeding season of 1991 than the preceding wintering season.
Diurnal carnivorous predators (hawks, owls, and foxes) showed a
delayed response to the irruption, increasing from seven
individuals sighted during the wintering season of 1991 to 13
during the wintering season of 1992. A seemingly counterclockwise
trajectory of predator abundance versus prey levels suggested a
pattern of prey-driven dynamics, but confidence intervals were
likely broad. In this semiarid locality, it appears that ENSO
effects did not cascade down from higher to lower trophic levels,
but rather the opposite. In this bottom-up scenario, the authors
predict that as primary productivity varies with rainfall, so
should secondary (mammal prey densities), and tertiary
productivity (vertebrate predators). Long-term monitoring of this
terrestrial ecosystem is needed to test this prediction. F.M.
Jaksic et al: Oikos 1997 78:341.

Related Background:

POTENTIAL IMPACT OF CARBON DIOXIDE INJECTION ON DEEP-SEA LIFE

B.A. Seibel and P.J. Walsh (Monterey Bay Aquarium Research
Institute, US) discuss oceanic carbon dioxide injection, the
authors making the following points:

1) The potential for global warming has spurred the development
of various strategies to control the concentrations of greenhouse
gases, particularly carbon dioxide, in the atmosphere.
Technologies for carbon capture, storage, and sequestration to
reduce greenhouse gas concentrations are receiving increasing
attention. Because of its enormous volume, the ocean is an
attractive site for possible storage of carbon dioxide. First
proposed 25 years ago by C. Marchetti, disposal in the ocean is
now being actively explored.

2) Recent modeling studies indicate that carbon dioxide must be
released at great depths to avoid substantial outgassing. Direct
studies of the biological consequences of carbon dioxide
injection are in their infancy, but a large literature on the
physiology of deep-living animals indicates that they are highly
susceptible to the carbon dioxide and pH excursions likely to
accompany deep-sea carbon dioxide sequestration. Microbial
populations may be highly susceptible as well. The impacts of
ocean sequestration on deep-sea biota and the biogeochemical
cycles dependent on their metabolism are therefore of great
concern. Increased carbon dioxide results in decreases in
seawater pH. Primary responses of organisms to the consequent
internal acid-base imbalance include metabolic production and
consumption of acid-base equivalents, passive chemical buffering
of intra- and extracellular fluids, and active ion transport.

Science 2001 294:319

Related Background:

A GLOBALLY COHERENT FINGERPRINT OF CLIMATE CHANGE IMPACTS ACROSS
NATURAL SYSTEMS

C. Parmesan and G. Yohe (University of Texas Austin, US) discuss
climate change impacts, the authors making the following points:

1) Causal attribution of recent biological trends to climate
change is complicated because non-climatic influences dominate
local, short-term biological changes. Any underlying signal from
climate change is likely to be revealed by analyses that seek
systematic trends across diverse species and geographic regions;
however, debates within the Intergovernmental Panel on Climate
Change (IPCC) reveal several definitions of a "systematic trend".

2) The IPCC assessed the extent to which recent observed changes
in natural biological systems have been caused by climate change.
This was a difficult task despite documented statistical
correlations between changes in climate and biological changes(2-
5). With hindsight, the difficulties encountered by the IPCC can
be attributed to the differences in approach between biologists
and other disciplines, particularly economists. Studies in this
area are, of necessity, correlational rather than experimental,
and as a result, assignment of causation is inferential. This
inference often comes from experimental studies of the effects of
temperature and precipitation on the target species or on a
related species with similar habitats. Confidence in this
inferential process is subjective, and differs among disciplines,
thus resulting in the first divergence of opinion within the
IPCC.

3) The authors explore these differences, apply diverse analyses
to more than 1,700 species, and demonstrate that recent
biological trends match climate change predictions. Global meta-
analyses documented significant range shifts averaging 6.1 km per
decade towards the poles (or meters per decade upward), and
significant mean advancement of spring events by 2.3 days per
decade. The authors define a diagnostic fingerprint of temporal
and spatial "sign-switching" responses uniquely predicted by
twentieth century climate trends. Among appropriate long-
term/large-scale/multi-species data sets, this diagnostic
fingerprint was found for 279 species. This suite of analyses
generates "very high confidence" (as laid down by the IPCC) that
climate change is already affecting living systems.

References (abridged):

1. Intergovernmental Panel on Climate Change Third Assessment
Report Climate Change 2001: Impacts, Adaptation, and
Vulnerability (eds McCarthy, J. J., Canziani, O. F., Leary, N.
A., Dokken, D. J. & White, K. S.) (Cambridge Univ. Press,
Cambridge, 2001)

2. Easterling, D. R. et al. Climate extremes: observations,
modeling, and impacts. Science 289, 2068-2074 (2000)

3. Parmesan, C., Root, T. L. & Willig, M. Impacts of extreme
weather and climate on terrestrial biota. Bull. Am. Meteorol.
Soc. 81, 443-450 (2000)

4. Pounds, J. A. Climate and amphibian declines. Nature 410, 639-
640 (2001)

5. Otterson, G. et al. Ecological effects of the North Atlantic
Oscillation. Oecologia 128, 1-14 (2001)

Nature 2003 421:37

Related Background:

ECOLOGICAL RESPONSES TO RECENT CLIMATE CHANGE

G-R. Walther et al (University of Hannover, DE) discuss climate
change, the authors making the following points:

1) The Earth's climate has warmed by approximately 0.6  degrees C
over the past 100 years with two main periods of warming, between
1910 and 1945 and from 1976 onwards. The rate of warming during
the latter period has been approximately double that of the first
and, thus, greater than at any other time during the last 1,000
years(1). Organisms, populations and ecological communities do
not, however, respond to approximated global averages. Rather,
regional changes, which are highly spatially heterogeneous, are
more relevant in the context of ecological response to climatic
change.

2) In many regions there is an asymmetry in the warming that
undoubtedly will contribute to heterogeneity in ecological
dynamics across systems. Diurnal temperature ranges have
decreased because minimum temperatures are increasing at about
twice the rate of maximum temperatures. As a consequence, the
freeze-free periods in most mid- and high-latitude regions are
lengthening and satellite data reveal a 10% decrease in snow
cover and ice extent since the late 1960s. Changes in the
precipitation regime have also been neither spatially nor
temporally uniform. In the mid- and high latitudes of the
Northern Hemisphere a decadal increase of 0.5–1% mostly occurs in
autumn and winter whereas, in the sub-tropics, precipitation
generally decreases by about 0.3% per decade1.

3) There is now ample evidence that these recent climatic changes
have affected a broad range of organisms with diverse
geographical distributions(2-5). The authors assess these
observations using a process-oriented approach and present an
integrated synopsis across the major taxonomic groups, covering
most of the biomes on Earth. The authors focus on the
consequences of thirty years of warming at the end of the
twentieth century, and review the responses in (1) the phenology
and physiology of organisms, (2) the range and distribution of
species, (3) the composition of and interactions within
communities, and (4) the structure and dynamics of ecosystems,
highlighting common and contrasting features amongst the taxa and
systems considered.

4) In summary: There is now ample evidence of the ecological
impacts of recent climate change, from polar terrestrial to
tropical marine environments. The responses of both flora and
fauna span an array of ecosystems and organizational hierarchies,
from the species to the community levels. Despite continued
uncertainty as to community and ecosystem trajectories under
global change, The review of the authors exposes a coherent
pattern of ecological change across systems. Although we are only
at an early stage in the projected trends of global warming,
ecological responses to recent climate change are already clearly
visible.

References (abridged):

1. Climate Change 2001. Third Assessment Report of the
Intergovernmental Panel on Climate Change IPCC (WG I & II)
(Cambridge Univ. Press, Cambridge, 2001)

2. Hughes, L. Biological consequences of global warming: is the
signal already apparent? Trends Ecol. Evol. 15, 56-61 (2000)

3. Wuethrich, B. How climate change alters rhythms of the wild.
Science 287, 793-795 (2000)

4. McCarty, J. P. Ecological consequences of recent climate
change. Conserv. Biol. 15(2), 320-331 (2001)

5. Ottersen, G. et al. Ecological effects of the North Atlantic
Oscillation. Oecologia 128, 1-14 (2001)

Nature 2002 416:389

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7. CLIMATE CHANGE AND HUMAN DISEASE

ON GLOBAL CLIMATE CHANGE AND HEALTH

J.A. Patz and M. Khaliq (Johns Hopkins University, US) discuss
global climate change, the authors making the following points:

1) Global climate change is expected to have broad health
impacts.[1] If current warming trends continue, heat waves,
floods, and droughts and their attendant physical effects are
likely to become more frequent and severe. Warmer air
temperatures can influence the concentration of regional air
pollutants and aeroallergens. Less direct health impacts may
result from the disruption of ecosystems and of water and food
supplies, which in turn could affect infectious disease incidence
and nutritional status. Finally, sea-level rise could lead to
major population displacement and economic disruption.

2) Human activities related primarily to the burning of fossil
fuels and changes in land cover such as deforestation are
changing the concentration of atmospheric constituents or
properties of the earth's surface that help to absorb or scatter
radiant energy.[2] Since the preindustrial mid-1800s, increases
in concentrations of three major greenhouse gases, carbon
dioxide, methane, and nitrous oxide, have exceeded past changes
that occurred over the last 10 000 years; carbon dioxide alone
has increased by 30% since the late 1800s.[1] Warmer air, such as
that resulting from the greenhouse effect, can hold more moisture
and more quickly evaporate surface water, thereby increasing the
frequency of severe storms, floods, and droughts.[1]

3) According to the United Nations Intergovernmental Panel on
Climate Change (IPCC), "An increasing body of observations gives
a collective picture of a warming world and other changes in the
climate system.[3] During the 20th century, global average
surface temperature increased about 0.6 degrees C, global average
sea level rose 10 cm to 20 cm, and snow and ice cover
decreased.[2] The latest IPCC report predicts that if current
trends continue, sea level rise will rise 45 cm and global
temperatures will increase by 3 degrees C by the year 2100.[3]

4) Small changes in global mean temperatures can produce
relatively large changes in the frequency of extreme
temperatures.[2] Mortality rates increase at both hot and cold
extremes of temperature.[4] Increases in temperature have a
direct and substantial impact on excess mortality for elderly
individuals and individuals with pre-existing illnesses. Much of
the mortality attributable to heat waves is a result of
cardiovascular, cerebrovascular, and respiratory disease.[5] A
1995 heat wave in Chicago that caused 514 heat-related deaths (12
per 100 000 population) may be part of a recent trend of longer,
more frequent heat waves and record-setting temperatures. Long-
term global warming trends are further exacerbated by the "heat
island" effect, whereby high concentrations of heat-retaining
surfaces such as asphalt and tar roofs sustain higher
temperatures through the night. Heat waves also have the
secondary effect of worsening urban air pollution. Ozone, which
forms chemically from precursor pollutants, is the most
temperature-dependent air pollutant and may contribute to the
development of asthma in children.

References (abridged):

1. Patz JA, Engelberg D, Last J. The effects of changing weather
on public health. Ann Rev Public Health. 2000;21:271-307.

2. Intergovernmental Panel on Climate Change (IPCC). Climate
Change 2001: The Scientific Basis: Contribution of Working Group
I to the Third Assessment Report of the IPCC. Houghton J, Ding Y,
Griggs M, et al, eds. Cambridge, England: Cambridge University
Press; 2001.

3. McMichael A. Human health. In: IPCC Working Group II, ed.
Climate Change 2001: Impacts, Adaptation, and Vulnerability.
Cambridge, England: Cambridge University Press; 2001:453-485.

4. Curriero FC, Heiner KS, Samet JM, et al. Temperature and
mortality in 11 cities of the eastern United States. Am J
Epidemiol. 2002;155:80-87.

5. Kilbourne E. Heat waves. In: Noji E, ed. The Public Health
Consequences of Disasters. New York, NY: Oxford University Press;
1997:51-61.

J. Am. Med. Assoc. 2002 287:2283

Related Background:

CLIMATE CHANGE AND THE RESURGENCE OF MALARIA IN THE EAST AFRICAN
HIGHLANDS

S.I. Hay et al (University of Oxford, UK) discuss climate change
and malaria, the authors making the following points:

1) The public health and economic consequences of Plasmodium
falciparum malaria are once again regarded as priorities for
global development. There has been much speculation on whether
anthropogenic climate change is exacerbating the malaria problem,
especially in areas of high altitude where P. falciparum
transmission is limited by low temperature(1-4). The
International Panel on Climate Change has concluded that there is
likely to be a net extension in the distribution of malaria and
an increase in incidence within this range(5).

2) The resurgence of malaria caused by P. falciparum in the East
African highlands has been reported widely. From 1986 to 1998,
the tea estates of Kericho in western Kenya saw a rise in severe
malaria cases from 16 to 120 per 1,000 per year. In Kabale,
southwestern Uganda, the average monthly incidence has increased
from about 17 cases per 1,000 (1992–96 average) to 24 cases per
1,000 (1997–98 average). Gikonko in southern Rwanda has seen
annual incidence rise from 160 to 260 cases per 1,000 from 1976
to 1990. Muhanga in northern Burundi had an average of 18 malaria
deaths per 1,000 during the 1980s, which rose to between 25 and
35 deaths per 1,000 in 1991. These increases, considered
alongside evidence of a global increase in the average surface
temperature of 0.6 degrees C this century, have fuelled
speculation that temperature-related increases in transmission of
P. falciparum are already manifest. Although these claims have
met with robust counter argument, there has been no critical
examination of climate change at these sites.

3) The authors investigated long-term meteorological trends in
four high-altitude sites in East Africa, where increases in
malaria have been reported in the past two decades. The authors
demonstrate that temperature, rainfall, vapor pressure and the
number of months suitable for P. falciparum transmission have not
changed significantly during the past century or during the
period of reported malaria resurgence. A high degree of temporal
and spatial variation in the climate of East Africa suggests
further that claimed associations between local malaria
resurgences and regional changes in climate are overly
simplistic.

References (abridged):

1. Loevinsohn, M. E. Climatic warming and increased malaria
incidence in Rwanda. Lancet 343, 714-718 (1994). | PubMed | ISI |

2. McMichael, A. J., Haines, A., Sloof, R. & Kovats, S. Climate
Change and Human Health (World Health Organization, Geneva,
1996).

3. Epstein, P. R. et al. Biological and physical signs of climate
change: focus on mosquito-borne diseases. Bull. Am. Meteorol.
Soc. 79, 409-417 (1998). | Article | ISI |

4. Martens, P. How will climate change affect human health? Am.
Sci. 87, 534-541 (1999). | ISI |

5. McCarthy, J. J., Canziani, O. F., Leary, N. A., Dokken, D. J.
& White, K. S. Climate change 2001: Impacts, Adaptation, and
Vulnerability--Contribution of Working Group II to the Third
Assessment Report of the Intergovernmental Panel on Climate
Change (Cambridge Univ. Press, Cambridge, 2001).

Nature 2002 415:905

Related Background:

A HUMAN DISEASE INDICATOR FOR THE EFFECTS OF RECENT GLOBAL
CLIMATE CHANGE

Jonathan A. Patz (Johns Hopkins University, US) discusses global
climate change and human disease, the author making the following
points:

1) Connections between weather and disease are well established,
with many diseases occurring during certain seasons or erupting
from unseasonable flood or drought conditions. With new concerns
about global warming, accompanied by greater climate variability,
many recent studies have focused on disease fluctuations related
to short-term or interannual climate oscillations (e.g., from
weather extremes driven by El Nino). Yet, the nagging question
remains as to whether or not there has been any documented change
in human disease trends in response to long-term climate change,
since warming has already occurred over the last century (1,2).

2) This trend analysis has been elusive because of the scarcity
or inconsistent quality of health databases over long periods.
Additionally, strong confounding factors especially complicate
long-term trend analysis. Some of these include increasing trends
in travel, trade and migration, erratic disease control efforts,
emerging drug or pesticide resistance, human population growth,
urban sprawl, agricultural development, and variable reporting
biases. But Rodo et al. (3) have now succeeded in finding a
robust relationship between progressively stronger El Nino events
and cholera prevalence in Bangladesh, spanning a 70-year period;
their use of a uniquely high quality extensive cholera database
and innovative statistical methods were key. This study likely
represents the first piece of evidence that warming trends over
the last century are affecting human disease.

3) Rodo et al used innovative statistical methods to conduct a
time-series analysis of historical cholera data dating back to
1893 to examine the effect of nonstationary interannual
variability possibly associated with climate change. In the last
two decades, the El Niño Southern Oscillation (ENSO) differed
from previous decades (3). Since the 1980's, there has been a
marked intensification of the ENSO beyond that expected from the
known shift in the Pacific basin temperature regime that began in
the mid 1970s. The authors (Rodo et al) found that the
association of cholera incidence in the earlier half of the
century (1893-1940) is weak and uncorrelated with ENSO, whereas
late in the century (1980-2001), the relationship is strong and
consistent with ENSO. Past climate change, therefore, may have
already affected cholera trends in the region through intensified
ENSO events.(4,5)

References (abridged):

1.  Folland, C. K. & Karl, T. R. (2001) in Climate Change 2001:
The Scientific Basis, eds. Houghton, J., Ding, Y., Griggs, M.,
Noguer, M., van der Linden, P. & Dai, X. (Cambridge Univ. Press,
Cambridge, U.K.), p. 881.

2.  Easterling, D. R. , Horton, B. , Jones, P. D. , Peterson, T.
C. , Karl, T. R. , Parker, D. E. , Salinger, M. J. , Razuvayev,
V. , Plummer, N. , Jamason, P. & Folland, C. K. (1997) Science
277, 364-367[Abstract/Free Full Text].

3.  Rodó, X. , Pascual, M. , Fuchs, G. & Faruque, A. S. G. (2002)
Proc. Natl. Acad. Sci. USA 99, 12901-12906.

4.  Levitus, S. , Antonov, J. I. & Boyer, T. P. (2000) Science
287, 2225-2229[Abstract/Free Full Text].

5.  Kumar, K. K. , Rajagopalan, B. & Cane, M. A. (1999) Science
284, 2156-2159[Abstract/Free Full Text].

Proc. Nat. Acad. Sci. 2002 99:12506

Related Background:

ENSO AND CHOLERA: A NONSTATIONARY LINK RELATED TO CLIMATE CHANGE?

X. Rodo et al (University of Barcelona, ES) discuss climate
change and cholera, the authors making the following points:

1) A main issue at the center of the current debate on climate
change is the impact that any anthropogenic-induced changes will
have on human society (1,2). Of considerable importance among
these impacts are those affecting human health, particularly the
spread and intensification of water-born (1,3-5) and vector-born
diseases (1). A central difficulty that precludes quantitative
assessment of these impacts arises from the lack of studies
comparing past and present dynamics of infectious diseases over
sufficiently long time periods relevant to climate change. For
endemic diseases with a long history, such as cholera in
Bangladesh, the existence of historical records and on-going
surveillance programs make such a comparison possible.

2) Studies of climate and disease so far have been limited to two
main areas: the exploration of links between temporal patterns of
disease and the interannual variability of climate, mainly El
Nino/Southern Oscillation (ENSO), and the building of scenarios
for the geographical spread of disease with climate change. The
former addresses only interannual variability; it does not
address long-term change. Thus, for endemic diseases such as
cholera, one key question has been overlooked. Has the
interannual variability of climate become a stronger driver of
disease dynamics in recent decades? Currently, there is
considerable interest in the consequences of climate change for
the interannual variability of climate itself, particularly for
ENSO and associated phenomena. In this context, the question of a
nonstationary link between climate variability and disease
dynamics is central to address the effect of climate change.

3) In summary: The authors present quantitative evidence for an
increased role of interannual climate variability on the temporal
dynamics of an infectious disease. The evidence is based on time-
series analyses of the relationship between ENSO and cholera
prevalence in Bangladesh (formerly Bengal) during two different
time periods. A strong and consistent signature of ENSO is
apparent in the last two decades (1980-2001), while it is weaker
and eventually uncorrelated during the first parts of the last
century (1893-1920 and 1920-1940, respectively). Concomitant with
these changes, the Southern Oscillation Index (SOI) undergoes
shifts in its frequency spectrum. These changes include an
intensification of the approximately 4-yr cycle during the recent
interval as a response to the well documented Pacific basin
regime shift of 1976. This change in remote ENSO modulation alone
can only partially serve to substantiate the differences observed
in cholera. Regional or basin-wide changes possibly linked to
global warming must be invoked that seem to facilitate ENSO
transmission. For the recent cholera series and during specific
time intervals corresponding to local maxima in ENSO, this
climate phenomenon accounts for over 70% of disease variance. The
authors suggest this strong association is discontinuous in time
and can only be captured with a technique designed to isolate
transient couplings.

References (abridged):

1. Watson, R. T. , Zinyowera, M. C. & Moss, R. H. (1998) IPCC
Special Report on The Regional Impacts of Climate Change: An
Assessment of Vulnerability (Cambridge Univ. Press, Cambridge,
U.K.)

2. Huq, S. S. (2001) Science 294, 1617

3. Colwell, R. R. (1996) Science 274, 2025-2031

4. World Health Organization. (1996) Regional Health Report 1996
(W.H.O., Geneva).

5. Brandling-Bennett, A. D. & Pinheiro, F. (1996) Emerging
Infectious Diseases 2, 59-61

Proc. Nat. Acad. Sci. 2002 99:12901

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