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ScienceWeek
SCIENCEWEEK
ScienceWeek
June 27, 2003
Vol. 7 Number 26A
An Online Digest of Research in the Sciences
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It has not been easy for humans to face time. Some, in recoiling
from the fearsome prospect of time's abyss, have toppled backward
into the abyss of ignorance.
-- Claude Albritton
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Section 1
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Part A - Symposium: Paleoclimate
1. Introduction
2. Reconstruction of Past Climate
3. The Last 1000 Years
4. Climate in the Holocene
5. Climate in the Quaternary
6. Other Aspects of Paleoclimate
Notices and Subscription Information
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Section 2
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1. INTRODUCTION
NOTES & TERMINOLOGY
The following geological time-scales are relevant:
Cenozoic: Approximately 65 million years ago to the present.
Quaternary: Approximately 2.6 million years ago to the present.
The Quaternary comprises 2 epochs: the Holocene (the last 10,000
years) and the Pleistocene (from the beginning of the Quaternary
to the Holocene).
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. El Nino alternates with an opposite cold phase
sometimes called "La Nina".
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.
The term "lithosphere" refers to the outer layer of the Earth,
comprising the crust and upper mantle, and extending to a depth
of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer. "Plate tectonics" is the current consensus theory
that the Earth's lithosphere is broken into fairly rigid plates,
seven or eight major plates and many smaller plates, and that
convection within the underlying less rigid "asthenosphere"
causes the plates (and the associated continents and crust) to
move relative to each other.
ON PALEOCLIMATOLOGY
Ancient climates are the subject of paleoclimatology, its
importance made clear by the glacials and interglacials of the
Quaternary. To understand the Quaternary, geologists used to
concentrate on the landforms and deposits left by glaciers or
icecaps, but climatologists, oceanographers and geochemists have
lately joined them, aware (partly because of the odd weather of
the last few decades) that climate changes on a human time scale
have major economic, political and social consequences.
The Earth's climate is an engine whose parts are the land, the
ocean and the atmosphere. The engine runs on heat from the Sun.
Some of this heat is reflected back into space, but much reaches
the surface and warms the soil, the air and the water. Over the
oceans the heat causes evaporation; clouds form, travel and
condense, and rain falls somewhere.
Near the equator, the sun's rays strike the earth at a high
angle, heating the surface more than do the rays that come in at
higher latitudes. The hot equatorial air, being light, rises and
flows north and south, distributing its warmth across the planet.
Rising air expands, cools and so is less able to hold moisture,
hence the equatorial rain belt. Aloft, the warm air gradually
cools and between 25 degrees and 35 degrees N and S latitude some
of it sinks and returns to the equator as a surface wind. The
sinking air is compressed and creates a subtropical high-pressure
zone. Because the compression gives the air a greater capacity to
hold moisture, the subtropics are quite dry.
The upper air continues toward the pole, cooling and sinking
gradually as it goes. From the polar region it returns as a cold
surface wind. The seasons complicate this simple picture because
in winter the temperate and polar zones are less or not at all
heated, with the result that a sharp temperature gradient forms
between 45 degrees and 60 degrees N and S, the "circumpolar
vortex".
Adapted from: Tjeerd H. Van Andel: New Views on an Old Planet: A
History of Global Change. 2nd Edition. Cambridge University Press
1994, p53.
ON GLOBAL CLIMATE CHANGE
Global climate usually changes little over the course of a human
lifetime, but a large and rapidly growing body of research has
begun to reveal just how variable it is on longer time scales.
Three hundred and fifty years ago, the world was in the depths of
a pro-longed cold spell called the "Little Ice Age", which
lingered for nearly half a millennium. Fifty thousand years ago,
in the middle of the last glacial period, large continental ice
sheets covered much of North America, Northern Europe, and
Northern Asia. Fifty million years ago, global temperatures were
so high that there were no large ice sheets at all. The speed at
which climate can change has also recently become clear:
Transitions between fundamentally different climates can occur
within only decades. In order to understand these variations, we
need to reconstruct them over a wide range of temporal and
geographical scales. The importance of this task is underlined by
the growing awareness of how profoundly human activity is
affecting climate. As with so many other complex systems, the key
to predicting the future lies in understanding the past... There
are few direct records of paleoclimate, so we therefore must rely
on "proxies" -- measurable parameters that provide quantitative
information about variables such as temperature, rainfall, or ice
volume to reconstruct it.
Adapted from: J. Smith and J. Uppenbrink: Science 2001 292:657
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2. RECONSTRUCTION OF PAST CLIMATE
TOWARD INTEGRATED RECONSTRUCTION OF PAST CLIMATES
Science 2003 300:589
The following points are made by K.E. Trenberth and B.L. Otto-
Bliesner:
1) Climate involves interactions among the atmosphere, the
oceans, the land surface and its vegetation and hydrology, and
the cryosphere. It naturally varies on time scales ranging from
interannual (El Nino) to millennia or longer. The instrumental
record of a hundred years or so is clearly inadequate to help us
understand these processes.
2) Paleoclimate reconstructions fill this void. Made up of
estimates of climate variables at times long before the
instrumental record, they are based on proxy indicators known to
be sensitive to climate. Examples include cores from long-lived
trees, ice sheets in Greenland and Antarctica, glaciers at high
elevations in the tropics, sediments, and corals. With
considerable ingenuity, these proxies have been used to derive
information about past climates, natural variability, and global
climate change.
3) The reconstruction of a time series of temperature or
precipitation at a single location is no mean achievement. To
synthesize results from previous reconstructions is even more
difficult and has only recently been credibly achieved after
considerable work, especially in statistical analysis (1).
However, it is becoming clear that a synthesis of data with more
physical credibility requires collaboration between paleoclimate
and climate dynamics experts (including modelers).
4) Two difficulties faced by climate reconstructions concern
chronological dating and what a proxy is really measuring. For
example, oxygen isotopes (measured by the O-18 isotopic ratio) in
corals are affected by both temperature and salinity of the
seawater in which the coral grows, resulting in an inherent
ambiguity. Other methods are based on trace element ratios
normalized to calcium (such as Sr/Ca and Mg/Ca) in the skeletons
of corals and shells. It is assumed that the trace metals are
incorporated into the skeleton at concentrations that depend on
growth temperature. However, as living organisms are complex,
their response varies and empirical calibrations are almost
always necessary.(2-5)
References (abridged):
1. M. E. Mann et al., Geophys. Res. Lett. 26, 759 (1999)
2. R. G. Fairbanks, M. Sverdlove, R. Free, P. H. Wiebe, A.W. H. B
, Nature 298, 841 (1982)
3. M. Werner, U. Mikolajewicz, M. Heimann, G. Hoffmann, Geophys.
Res. Lett. 27, 723 (2000)
4. K. E. Trenberth et al., J. Geophys. Res. 103, 14291 (1998)
5. M. Blackmon et al., Bull. Am. Meteorol. Soc. 82, 2357 (2001)
Related Material:
ROLE OF GROUND WATER IN GEOMORPHOLOGY, GEOLOGY, AND PALEOCLIMATE
OF THE SOUTHERN HIGH PLAINS, USA
Ground Water 2002 40:438
The following points are made by W.W. Wood:
1) Study of ground water in the Southern High Plains is central
to an understanding of the geomorphology, deposition of economic
minerals, and climate change record in the area. Ground water has
controlled the course of the Canadian and Pecos rivers that
isolated the Southern High Plains from the Great Plains and has
contributed significantly to the continuing retreat of the
westward escarpment.
2) Evaporative and dissolution processes are responsible for
current plateau topography and the development of the signature
20,000 small playa basins and 40 to 50 large saline lake basins
in the area. In conjunction with eolian processes, ground water
transport controls the mineralogy of commercially valuable
mineral deposits and sets up the distribution of fine
efflorescent salts that adversely affect water quality. As the
water table rises and retreats, lunette and tufa formation
provides valuable paleoclimate data for the Southern High Plains.
3) In all these cases, an understanding of ground water processes
contributes valuable information to a broad range of geological
topics, well beyond traditional interest in water supply and
environmental issues.
Notes:
The term "eolian (aeolian) processes" refers to processes
involving wind.
In this context, the term "lunette" refers in general to a broad
mound of windblown fine silt and clay.
The term "tufa" refers to sedimentary rock formed as a thin layer
around saline springs by the deposition of calcium carbonate (or
more rarely, silica).
Related Material:
LEAF SENSORS FOR PALEOCLIMATE CARBON DIOXIDE
Nature 2001 411:287
The following points are made by Gregory J. Retallak:
1) To understand better the link between atmospheric carbon
dioxide concentrations and climate over geological time, records
of past carbon dioxide are usually reconstructed from geochemical
proxies. Although these records have provided a broad picture of
carbon dioxide variation throughout the Phanerozoic eon (the past
570 million years), inconsistencies and gaps remain that still
need to be resolved.
2) The author presents a continuous 300-million-year record of
*stomatal abundance from fossil leaves of four genera of plants
that are closely related to the present Ginkgo tree. Using the
known relationship between leaf stomatal abundance and growing
season carbon dioxide concentrations, the author reconstructs
past atmospheric carbon dioxide concentrations.
3) For the past 300 million years, only two intervals of low
carbon dioxide are suggested by the data, with both intervals
coinciding with known ice ages in the *Neogene and early *Permian
eras. But for most of the Mesozoic era (65 to 245 million years
ago), carbon dioxide levels were high, with transient excursions
to even higher carbon dioxide concentrations. The author suggests
these results are consistent with some reconstructions of past
carbon dioxide and paleotemperature records, but the data
indicate that carbon dioxide reconstructions based on carbon
isotope proxies may be compromised by episodic outbursts of
isotopically high methane. The author suggests the results
support the role of water vapor, methane, and carbon dioxide in
greenhouse climate warming over the past 300 million years.
Notes:
"Stoma" are pores in the skin (epidermis) of a leaf or stem of a
vascular plant. The stoma are of variable aperture, the aperture
controlled by surrounding cells and providing regulated gas
exchange between the tissues of the plant and the atmosphere.
Neogene: The time-frame 23.3 to 1.64 million years ago.
Permian: The time-frame 290 to 245 million years ago.
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3. THE LAST 1000 YEARS
CORALS, CHEMISTRY, AND CLIMATE
Science 2002 296:277
The following points are made by D.P. Schrag and B.K. Linsley:
1) To most people, coral reefs are symbols of the extreme
biodiversity of the undersea world. But to many geochemists,
corals hold the key to understanding the recent temperature
history of the tropical oceans. Certain coral species grow like
trees, putting down layers of their skeletons every year, and can
live for 300 years or more. By examining the chemistry of the
coral skeleton, geochemists have been able to reconstruct the
temperature history of the tropical oceans spanning the last
several centuries, providing a better understanding of climate
oscillations such as El NiÏo. The Sr/Ca ratio of corals has
received particular attention, with claims of precision better
than 0.5õC (1).
2) Such records are not without controversy. A coral skeleton is
made of an inorganic mineral, aragonite (a form of calcium
carbonate), but it is precipitated through a biological process
that is not well understood. Furthermore, many corals used for
paleotemperature reconstructions have algal symbionts that may
affect the chemistry of the skeleton. This uncertainty has led
some to question the reliability of these temperature
reconstructions (2).
3) An old and continuing struggle in the field of
paleoceanography is that between those who want to use
geochemical proxies (imperfect as they are) to answer questions
about the history of the oceans and climate, and those who want
to first understand completely the causes of the geochemical
signals in biologically precipitated minerals. In the extreme,
both views are ridiculous. If we had never tried to use the
geochemistry of shells to study the history of the oceans, we
would know little about how climate has varied through Earth's
history. But we cannot be completely confident in our
reconstructions of past climates if we do not understand all the
factors that may contribute to the chemical signals we
measure.(3-5)
References (abridged):
1. J. W. Beck et al., Science 257, 644 (1992)
2. S. de Villiers, B. K. Nelson, A. R. Chivas, Science 269, 1247
(1995)
3. A. L. Cohen et al., Science 296, 331 (2002)
4. A. L. Cohen, G. D. Layne, S. R. Hart, P. S. Lobel,
Paleoceanography 16, 20 (2001)
5. H. M. Stoll, D. P. Schrag, Geochem. Geophys. Geosyst. 1,
1999GC000015 (2000)
Related Material:
ON PAST CLIMATIC CHANGES AND CLIMATE PREDICTION
Science 2002 297:206
The following points are made by Hugo Beltrami:
1) For climate predictions from general circulation models to be
interpreted with confidence, a robust record of past climatic
changes is required. Without such a record, natural variability
of the climate system cannot be separated from the possible
changes induced by human activity. Resolving this issue is
essential for addressing future climate change.
2) Two different approaches are widely used to reconstruct
Northern Hemisphere climatic change during the last 500 to 1000
years. Both show a warming in the 20th century, but for earlier
centuries they observe different patterns of climate change. Do
these disagreements reflect only differences in the spatial
distribution of sites, or are they due to intrinsic limitations
of the methods?
3) The first method uses large data sets of various temperature
proxies, such as tree rings and oxygen isotopes in ice cores, to
construct a model of past temperature change (1). The second
relies on geothermal data from boreholes worldwide to model
ground temperature changes and the energy balance at Earth's
continental surface (2-4).
4) Comparison of these multiproxy and geothermal paleoclimatic
models is difficult because of differences in the spatial
distribution of data. But preliminary comparison (5) yields some
important differences. In particular, they disagree over the
existence of a cold period between 1500 and 1800 A.D. Such a cold
spell is documented in all geothermal models but does not appear
as a strong signal in the multiproxy reconstructions (1).
References (abridged):
1. M. E. Mann et al., Geophys. Res. Lett. 26, 759 (1999)
2. S. Huang et al., Nature 403, 756 (2000)
3. R. N. Harris, D. S. Chapman, Geophys. Res. Lett. 28, 747
(2001)
4. H. Beltrami et al., Geophys. Res. Lett. 29,
10.1029/2001GL014310 (2002)
5. K. R. Briffa, T. J. Osborn, Science 295, 2227 (2002)
Related Material:
THE EVOLUTION OF CLIMATE OVER THE LAST MILLENNIUM
Science 2001 292:662
The following points are made by P.D. Jones et al:
1) The instrumental record is generally considered not to be long
enough to give a complete picture of climatic variability. Recent
records are also likely already influenced by human actions (1).
It is crucial, therefore, to extend the record of climatic
variability beyond the era of instrumental measurements if we are
to understand how large natural climatic variations can be, how
rapidly climate may change, which internal mechanisms drive
climatic changes on regional and global scales, and what external
or internal forcing factors control them (2).
2) At present, paleoclimatology is very far from achieving
season-specific histories for different climate variables at the
regional scale; which is the detailed picture we need in our
search for an unambiguous "fingerprint" of the climate response
to increasing greenhouse gas emissions (1,2). Interest has,
therefore, focused initially on large-scale climate features,
particularly mean hemispheric temperatures and the behavior of
major climate phenomena like the El Nino-Southern Oscillation
(ENSO) and the North Atlantic Oscillation (NAO) (3).
3) Relatively widespread (4) instrumental data exist for the land
and marine regions of the Northern Hemisphere (NH) back to the
mid-1850s (5). These data show that since 1861, annual average
temperatures have warmed by 0.6›C, but with a marked seasonal
contrast: winters have warmed by nearly 0.8›C and summers by only
0.4›C. The warming has occurred in two pronounced phases, from
about 1920 to 1945 and from 1975 to the present (5). Several
attempts have been made recently to extend this record of
temperature variations across the NH to cover the last 1000
years. All are based on the rationale that large-scale
temperature variability can be represented sufficiently well by
integrating data from a limited number of geographically
scattered indicators or "proxies" of variability of past climate.
4) In summary: Knowledge of past climate variability is crucial
for understanding and modeling current and future climate trends.
The authors review present knowledge of changes in temperatures
and two major circulation features -- El Nino-Southern
Oscillation (ENSO) and the North Atlantic Oscillation (NAO) --
over much of the last 1000 years, mainly on the basis of high-
resolution paleoclimate records. Average temperatures during the
last three decades were likely the warmest of the last
millennium, about 0.2›C warmer than during warm periods in the
11th and 12th centuries. The 20th century experienced the
strongest warming trend of the millennium (about 0.6›C per
century). Some recent changes in ENSO may have been unique since
1800, whereas the recent trend to more positive NAO values may
have occurred several times since 1500. The authors suggest that
uncertainties will only be reduced through more extensive spatial
sampling of diverse proxy climatic records.
References (abridged):
1. T. P. Barnett, et al., Bull. Am. Meteorol. Soc. 80, 2631
(1999)
2. J. T. Houghton et al., Eds., Climate Change 1995: The Science
of Climate Change (Cambridge Univ. Press, Cambridge, 1996)
3. M. E. Mann, Weather 56, 91 (2001)
4. The data are adequate to obtain estimates of decadal-mean NH
temperature with a standard error of 0.1›C or less.
5. P. D. Jones, M. New, D. E. Parker, S. Martin, I. G. Rigor,
Rev. Geophys. 37, 173 (1999)
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4. CLIMATE IN THE HOLOCENE
VARIABILITY OF EL NINO/SOUTHERN OSCILLATION ACTIVITY AT
MILLENNIAL TIMESCALES DURING THE HOLOCENE EPOCH
Nature 2002 420:162
The following points are made by C.M. Moy et al:
1) The variability of El Nino/Southern Oscillation (ENSO) during
the Holocene epoch, in particular on millennial timescales, is
poorly understood. Paleoclimate studies have documented ENSO
variability for selected intervals in the Holocene, but most
records are either too short or insufficiently resolved to
investigate variability on millennial scales(1-3).
2) A sediment core about 9 m long, retrieved from the lake Laguna
Pallcacocha in the southern Ecuadorian Andes, has been used to
investigate Holocene ENSO variability (4). It was proposed that
the hundreds of light-colored, inorganic clastic laminae in the
sediment core were probably deposited during ENSO-driven episodes
of alluvial deposition in the Laguna Pallcacocha drainage basin.
This hypothesis was based on the observation that light-colored
laminae deposited in the past 200 yr generally correlated with
known El Nino events in instrumental and historical records. The
greyscale record, which was used to quantify the distribution of
light-colored laminae, exhibited significant variance in the ENSO
band (2 8 yr). However, gaps between adjacent core sections
precluded the identification of significant millennial-scale
variance.
3) The authors present a record of sedimentation in Laguna
Pallcacocha, southern Ecuador, which is strongly influenced by
ENSO variability, and covers the past 12,000 years continuously.
The authors find that changes on a timescale of 2 8 years, which
we attribute to warm ENSO events, become more frequent over the
Holocene until about 1200 years ago, and then decline towards the
present. Periods of relatively high and low ENSO activity,
alternating at a timescale of about 2000 years, are superimposed
on this long-term trend. The authors attribute the long-term
trend to orbitally induced changes in insolation, and suggest
internal ENSO dynamics as a possible cause of the millennial
variability. However, the millennial oscillation will need to be
confirmed in other ENSO proxy records.(5)
References (abridged):
1. Thompson, L. G., Mosley-Thompson, E. & Thompson, P. A. El
Nino: Historical and Paleoclimate Aspects of the Southern
Oscillation (eds Diaz, H. F. & Markgraf, V.) 295-322 (Cambridge
Univ. Press, Cambridge, 1992)
2. Cook, E. R., D'Arrigo, R. D., Cole, J. E., Stahle, D. W. &
Villalba, R. El Nino and the Southern Oscillation (eds Diaz, H.
F. & Markgraf, V.) 297-323 (Cambridge Univ. Press, Cambridge,
2000)
3. Tudhope, A. W. et al. Variability in the El Nino:Southern
oscillation through a glacial-interglacial cycle. Science 291,
1511-1517 (2001)
4. Rodbell, D. T. et al. An 15,000-year record of El Nino-driven
alluviation in southwestern Ecuador. Science 283, 516-520 (1999)
5. Vuille, M., Bradley, R. S. & Keimig, F. Climate variability in
the Andes of Ecuador and its relation to tropical Pacific and
Atlantic sea surface temperature anomalies. J. Clim. 13, 2520-
2535 (2000)
Related Material:
CULTURAL RESPONSES TO CLIMATE CHANGE DURING THE LATE HOLOCENE
Science 2001 292:667
The following points are made by Peter B. deMenocal:
1) In the spring of 1785, the geologist James Hutton (1726-1797)
presented a lecture to the Royal Society of Edinburgh that
changed scientific inquiry into natural processes. The essence of
his view was simple enough: The present is the key to
understanding the past. Hutton recognized that slow geologic
processes such as erosion or uplift could produce sedimentary
strata or mountain ranges. In 1795, he wrote that "we find no
vestige of a beginning -- no prospect of an end... Not only are
no powers to be employed that are not natural to the globe, no
actions to be admitted of except those of which we know the
principle and no extraordinary events to be alleged in order to
explain a common experience ..." (1). This view was not accepted
by most natural scientists at the time because it required full
acceptance of the expanse of geologic time and rejection of the
prevalent views of a young Earth. Future generations of
scientists, however, most notably Charles Darwin half a century
later, were encouraged by this new way of thinking to interpret
their observations on the basis of what they knew of modern
processes.
2) To understand how and why climates change, we have to invoke a
corollary to Hutton's view: The past must be used to understand
the present. Modern instrumental records are sufficiently long to
document climate phenomena that vary at interannual time scales,
such as El Nino, but they are too short to resolve multidecadal-
to century-scale climate variability that we know to exist from
detailed tree-ring, coral, and lake sediment records spanning the
past 500 to 1000 years (2,3). Similarly, the socioeconomic
impacts of recent El Nino/La Nina events are well documented (4),
but little is known about the societal impacts of longer period
climatic excursions. Without knowing the full range of climatic
variability at time scales of a few decades to a few millennia,
it is difficult to place our understanding of modern climate
variability, and its socioeconomic impacts, within the context of
how Earth climate actually behaves, both naturally and as a
result of anthropogenic increases of greenhouse gasses (3).
3) Excellent examples of the value of past climate records can be
gleaned from the history of drought in the US. Water
availability, rather than temperature, is the key climatic
determinant for life in semiarid expanses across the planet.
Drought often conjures up images of the Dust Bowl drought of the
1930s, which lasted approximately 6 years (1933-38) and resulted
in one of the most devastating and well-documented agricultural,
economic, and social disasters in the history of the US. The
drought was triggered by a large and widespread reduction in
rainfall across the American West, particularly across the
northern Great Plains (5). It displaced millions of people, cost
over $1 billion (in 1930s U.S. dollars) in federal support, and
contributed to a nascent economic collapse. Subsequent analysis
of the Dust Bowl drought has revealed that its tremendous
socioeconomic impact was, in part, due to wanton agricultural
practices and overcapitalization just before the drought, when
rainfall had been more abundant (5). A subsequent decadal-scale
drought in the 1950s was also severe but less widespread, mainly
impacting the American Southwest, where improved land use
practices and disaster relief programs mitigated its effects.
4) In summary: Modern complex societies exhibit marked resilience
to interannual-to- decadal droughts, but cultural responses to
multidecadal-to-multicentury droughts can only be addressed by
integrating detailed archaeological and paleoclimatic records.
Four case studies drawn from New and Old World civilizations
document societal responses to prolonged drought, including
population dislocations, urban abandonment, and state collapse.
The authors suggests that further study of past cultural
adaptations to persistent climate change may provide valuable
perspective on possible responses of modern societies to future
climate change.
References (abridged):
1. J. Hutton, Theory of the Earth, with proofs and illustrations
(Creech, Edinburgh, 1795)
2. J. T. Overpeck, Science 271, 1820 (1996)
3. ___ and R. Webb, Proc. Natl. Acad. Sci. U.S.A. 97, 1335 (2000)
4. M. Cane, G. Eshel, R. W. Buckland, Nature 370, 204 (1994)
5. R. A. Warrick, in Drought in the Great Plains: A Case Study of
Research on Climate and Society in the USA, J. Ausubel, A. K.
Biswas, Eds., NASA Proceedings Series: Climate Constraints and
Human Activities (Pergamon, New York, 1980)
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5. CLIMATE IN THE QUATERNARY
A FRESH LOOK AT GLACIAL FLOODS
Science 2002 296:1251
The following points are made by Steven M. Colman:
1) We tend to think of continental-scale ice sheets as rather
ponderous affairs, inexorably advancing southward over the
landscape and then slowly retreating to the north at the end of
each ice age. Over the last 20 years, however, evidence has
accumulated that this is a misconception. We now know that the
Laurentide Ice Sheet -- the largest ice-age glacier -- was
characterized by thin, marginal ice streams flowing rapidly on
low-friction beds and was unstable through much of its history
(1-3). The ice sheet periodically and abruptly discharged massive
amounts of ice into the North Atlantic (4), and abrupt coolings
and warmings occurred throughout the last ice age (5).
2) Recent research investigating the roles of glacial meltwater
and continental drainage in this glacial and climatic instability
suggest that the thermohaline circulation of the oceans is
sensitive to changes in the amount and location of freshwater
discharge.
3) Each time the ice sheet advanced beyond the Canadian Shield,
it confined large volumes of water in proglacial lakes at the ice
margin. This occurred because the ice sheet flowed over low-
relief terrain containing large basins (such as those occupied by
the modern Great Lakes), because ice flowed up-slope or against
preexisting drainage, and because the glacier depressed Earth's
crust under its own weight. As various lobes of the ice sheet
slithered forward or back, two major types of drainage event
occurred. First, the overall drainage of much of the continental
interior was periodically rerouted to different parts of the
ocean -- the Gulf of Mexico (via the Mississippi River), the
Arctic Ocean (via the Mackenzie River), the North Atlantic (via
the St. Lawrence and Hudson Rivers), and the Labrador Sea (via
the Hudson Strait). Second, large proglacial lakes, such as Lake
Agassiz, catastrophically drained in conjunction with some
rerouting events.
3) Each time the ice sheet advanced beyond the Canadian Shield,
it confined large volumes of water in proglacial lakes at the ice
margin. This occurred because the ice sheet flowed over low-
relief terrain containing large basins (such as those occupied by
the modern Great Lakes), because ice flowed up-slope or against
preexisting drainage, and because the glacier depressed Earth's
crust under its own weight. As various lobes of the ice sheet
slithered forward or back, two major types of drainage event
occurred. First, the overall drainage of much of the continental
interior was periodically rerouted to different parts of the
ocean -- the Gulf of Mexico (via the Mississippi River), the
Arctic Ocean (via the Mackenzie River), the North Atlantic (via
the St. Lawrence and Hudson Rivers), and the Labrador Sea (via
the Hudson Strait). Second, large proglacial lakes, such as Lake
Agassiz, catastrophically drained in conjunction with some
rerouting events. The amounts of fresh water involved in both of
these drainage changes were enormous.
References (abridged):
1. L. Clayton, J. T. Teller, J. W. Attig, Boreas 14, 235 (1985)
2. P. U. Clark, Quat. Res. 41, 19 (1994)
3. C. J. Patterson, Geology 26, 643 (1998)
4. G. Bond et al., Nature 360, 245 (1992)
5. W. Dansgaard et al., Nature 364, 218 (1993)
Notes:
The term "thermohaline" refers to the joint activity of salinity
and temperature in the oceans, and thermohaline circulation
refers to the convective process produced by thermohaline
gradients.
Related Material:
A HIGH-RESOLUTION PALEOCLIMATE RECORD SPANNING THE PAST 25,000
YEARS IN SOUTHERN EAST AFRICA
Science 2002 296:113
The following points are made by T,C. Johnson et al:
1) Tropical Africa was cooler and drier during the last glacial
maximum than it is today (1,2). However, we have little
information about higher frequency climate variability in the
African tropics during the last glacial period or about the
transition from ice age to interglacial conditions. Was there an
abrupt shift to warm and wetter conditions? Was there monotonic
evolution, or change by fits and starts? How does climate change
in the African tropics relate to the signals registered in the
ice sheets of Greenland and Antarctica? Some answers have been
forthcoming from studies of lake sediments throughout much of
Africa (1). But knowledge of the timing and nature of climate
variability in much of tropical Africa still eludes us, as does
an understanding of its role in the global climate system.
2) The authors present a high-resolution record of climate
dynamics from two piston cores spanning the past 25,000 years in
northern Lake Malawi. The authors recovered six piston cores and
seven multicores from the north basin of Lake Malawi in 1998 as
part of an expedition of the International Decade for the East
African Lakes (3). The percent biogenic silica in the cores
reflects the abundance of diatoms in the lake sediments. Other
sources of biogenic silica (e.g., phytoliths and sponge spicules)
are rare in comparison to diatoms. Diatoms dominate the
phytoplankton in Lake Malawi throughout most of the year,
especially during the dry windy season in austral winter, when
primary production in the lake is at a maximum (4,5). The percent
biogenic silica was converted to a mass accumulation rate based
on sediment porosity and density and the linear sedimentation
rates.
3) The authors suggest these cores provide a climate signal for
this part of tropical Africa spanning the past 25,000 years. The
biogenic silica mass accumulation rate was low during the
relatively dry late Pleistocene, when the river flux of silica to
the lake was suppressed. Millennial-scale fluctuations, due to
upwelling intensity in the late Pleistocene climate of the Lake
Malawi basin appear to have been closely linked to the Northern
Hemisphere climate until 11 thousand years ago. Relatively cold
conditions in the Northern Hemisphere coincided with more
frequent north winds over the Malawi basin, perhaps resulting
from a more southward migration of the Intertropical Convergence
Zone.
References (abridged):
1. F. Gasse, Quat. Sci. Rev. 19, 189 (2000)
2. D. A. Livingstone, in Biological Relationships Between Africa
and South America, P. Goldblatt, Ed. (Yale University Press, New
Haven, 1993), pp. 455-472
3. All six piston cores were analyzed for magnetic susceptibility
using a Geotek Multi-Sensor Core Logger (Northamptonshire, UK).
Water content and total organic carbon were determined at 10-cm
intervals down each core, by weight loss after drying and
coulometry, respectively. Stratigraphic correlations were
established among the cores and were based on sedimentary
structures, magnetic susceptibility, and percent abundance of
BSI: The cores contain an upper varved interval that averages
about a meter in thickness, overlying an interval that is about
2.5-m thick, consisting of alternating packets of varved and
nonvarved sediments. The varves in both intervals are about 0.5-
to 0.7-mm thick, and the packets are typically about 2- to 8-cm
thick. These overlie another continuously varved interval that
spans about 1.2 to 2 m, which, in turn, overlies a second
interval of alternating packets of varves and nonvarved sediment
of about a meter thickness. The bottom unit in the cores is a
homogenous silty clay, up to 3-m thick.
4. H. A. Bootsma, R. E. Hecky, Water Quality Report: Draft
Document [South African Development Community/Global
Environmental Facility (SADC/GEF), Lake Malawi Nyasa Biodiversity
Conservation Project, 1998]
5. G. Patterson, O. Kachinjika, in The Fishery Potential and
Productivity of the Pelagic Zone of Lake Malawi/Niassa, A. Menz,
Ed. (Natural Resources Institute, Overseas Development
Administration, Chatham, UK 1995), pp. 1-68
Related Material:
ICE SHEET COLLAPSE AND SEA LEVEL CHANGE
Science 2002 295:2376
The following points are made by Roberto Sabadini:
1) Sea level records contain fundamental information on Earth's
climatic changes and the relative motion between the sea surface
and the topography of the sea floor. On a global scale, they
provide a measure of how much water was locked up in ice sheets
and glaciers at different times in Earth's history. But this is
not all. Clark et al (1) show that geographic patterns of sea
level change can provide clues to the origin of meltwater pulses
during deglaciation.
2) Increasingly sophisticated models that simulate the phenomena
responsible for past sea level changes enable us to understand
the complex interactions among the various parts of the Earth
system -- hydrosphere, cryosphere, lithosphere, and atmosphere --
and their impact on climate and the biosphere. They are
complemented by existing and planned satellite and space missions
that aim to refine our capability to detect present-day sea level
changes (2-5). Clark et al (1) open new perspectives in our
ability to make use of such sea level data to understand the
interaction among the various parts of the Earth system and their
feedback on Earth's climate. Clark et al make use of a notion
they discovered in earlier work on the recent mass balance of
polar ice sheets: a meltwater pulse from ice sheet complexes
belonging to the cryosphere induces a distinct geographic sea
level signature -- a fingerprint -- that is not uniform or
eustatic over the oceans. Using a sophisticated sea level model
that accounts for the physical properties of Earth's lithosphere
and mantle and the latest techniques for analyzing fast, large
glacial discharges, Clark et al. demonstrate that this
fingerprint allows the source-point of a previously enigmatic
meltwater pulse from Earth's cryosphere at the end of the last
ice age to be identified. The pulse occurred 14,200 years before
the present and lasted for about 500 years; the associated sea
level rise exceeded 40 mm/year at Barbados and the Sunda Shelf, a
value that, compared with the modern sea level rise of 1 to 2
mm/year, testifies to the exceptionality of the event.
References (abridged):
1. P. U. Clark, J. X. Mitrovica, G. A. Milne, M. E. Tamisiea,
Science 295, 2438 (2002)
2. ESA SP-1233 (1), Reports for Mission Selection, The Four
Candidate Earth Explorer Core Missions (European Space Agency,
ESTEC, Noordwijk, Netherlands, 1999)
3. J. Wahr, M. Molenaar, J. Geophys. Res. 103, 30205 (1998)
4. M. K. Cheng, C. K. Shum, B. D Tapley, J. Geophys. Res. 102,
22377 (1997)
5. R. Devoti et al., Geophys. Res. Lett. 28, 855 (2001)
Related Material:
RANGE SHIFTS AND ADAPTIVE RESPONSES TO QUATERNARY CLIMATE CHANGE
Science 2001 292:673
The following points are made by M.B. Davis and R.G. Shaw:
1) Modern plant taxa have persisted through a long period of
variable climate, including glacial-interglacial cycles with
large changes in temperature, precipitation, and CO2
concentration, over the past 2.5 million years. Rates of climate
change varied widely: Regional temperature changes were as rapid
as several degrees Celsius within a few decades or as slow as 1
degree C per millennium.
2) The changes in species distribution evidenced by fossils
provide a detailed record of plant responses to these changes.
Hundreds of pollen diagrams, compiled in databases, provide
regional and continental records of tree abundances as they
changed through space and time (1-3). New pollen records
supplemented by macrofossils (4) and DNA recovered from fossil
pollen (5) provide increasing temporal and taxonomic detail. In
arid regions, where pollen-bearing sediments are less abundant,
plant fragments preserved in middens made by packrats (Neotoma)
and other rodents provide a spatially precise record of past
species distributions.
3) Changes in geographic distribution are so frequently
documented in the fossil record that range shifts are seen as the
expected plant response to future climate change. Beyond changes
in distribution, however, plants underwent genetic changes,
adapting to changes in climate during the Quaternary. Yet
adaptation at the population level is seldom considered in the
literature describing Quaternary environments nor, with some
notable exceptions, in discussions of vegetation response to
anticipated global change.
4) The authors cite evidence of genetic adaptation to climate and
argue that the interplay of adaptation and migration has been
central to biotic response to climate change. The authors suggest
that rapid climate change challenges this process, pushing
populations to limits of adaptation, thus influencing regional
ecosystem properties as well as the persistence of taxa.
References (abridged):
1. B. Huntley, T. Webb III, Eds., Vegetation History (Kluwer
Academic, Dordrecht, Netherlands, 1988)
2. T. Webb, III, P. J. Bartlein, S. P. Harrison, K. H. Anderson,
in Global Climates Since the Last Glacial Maximum, H. E. Wright
Jr. et al., Eds. (Univ. of Minnesota Press, Minneapolis, MN,
1993), vol. 1, pp. 415-467
3. S. T. Jackson, et al., Quat. Sci. Rev. 16, 1 (1997)
4. S. T. Jackson, et al., Quat. Sci. Rev. 19, 489 (2000)
5. Y. Suyama, et al., Genes Genet. System. 71, 145 (1996)
Related Material:
SEA LEVEL CHANGE THROUGH THE LAST GLACIAL CYCLE
Science 2001 292:679
The following points are made by K. Lambeck and J. Chappell:
1) Sea levels have fluctuated throughout geological time,
periodically encroaching or retreating across coastal plains.
Changes in the relative positions of sea and land surfaces are
indicative of vertical movements of the land, changes in ocean
volume, or, in most cases, of both. Global changes occur on time
scales of millions of years, with amplitudes on the order of
several hundred meters (1,2) and are associated mainly with plate
tectonics-induced changes in ocean basin geometry.
2) During the Quaternary, the dominant contribution to sea level
change has been the periodic exchange of mass between ice sheets
and oceans: ice ages being times of sea level lowstands and
interglacials being times of relative highstands. Superimposed on
the global signals are more regional and local changes caused by
uplift and subsidence of the coastal zone or by changes in
regional and local climate. At decadal, annual, and shorter
intervals, the climate-, meteorology-, and tide-driven changes
become important. Observations of sea level change also indicate
considerable spatial variation. The observed signals vary
substantially from site to site, even when the localities lie
relatively near to each other such as the Scandinavian ngerman
and And ya sites: At the first site, sea level has fallen nearly
200 m in the past 9000 years, whereas at the second, the level
9000 years ago was near the present level. In contrast, at
Barbados, sea level was about 30 m below the present level at
that time. In southern England, levels have risen slowly over the
past 7000 years, but along the Australian margin they have fallen
by a few meters during the same interval. The relative sea level
change therefore exhibits complex temporal and spatial patterns
that contain information about a range of Earth and climate
processes.
3) In summary: Sea level change during the Quaternary is
primarily a consequence of the cyclic growth and decay of ice
sheets, resulting in a complex spatial and temporal pattern.
Observations of this variability provide constraints on the
timing, rates, and magnitudes of the changes in ice mass during a
glacial cycle, as well as more limited information on the
distribution of ice between the major ice sheets at any time.
Observations of glacially induced sea level changes also provide
information on the response of the mantle to surface loading on
time scales of 10^(3) to 10^(5) years. Regional analyses indicate
that the earth-response function is depth dependent as well as
spatially variable. Comprehensive models of sea level change
enable the migration of coastlines to be predicted during glacial
cycles, including the anthropologically important period from
about 60,000 to 20,000 years ago.(3-5)
References (abridged):
1. A. Hallam, Annu. Rev. Earth Planet. Sci. 12, 205 (1984)
2. P. R. Vail, J. Hardenbol, R. G. Todd, Am. Assoc. Petrol. Geol.
Mem. 36, 129 (1984)
3. M. Nakada and K. Lambeck, Nature 33, 36 (1988)
4. A. M. Tushingham and W. R. Peltier, J. Geophys. Res. 96, 4497
(1991)
5. W. R. Peltier, Rev. Geophys. 36, 603 (1998)
ScienceWeek http://www.scienceweek.com
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6. OTHER ASPECTS OF PALEOCLIMATE
ON COMPARATIVE PHYLOGEOGRAPHY AND PALEOCLIMATOLOGY
Proc. Nat. Acad. Sci. 2002 99:6112
The following points are made by A. Hugall et al:
1) Phylogeography seeks to reveal biogeographical history of
species and the habitats they occupy via (i) qualitative spatial
association of divisions between monophyletic clusters of alleles
with biogeographic barriers, and (ii) quantitative estimates of
historical population size (1-4). Much of this work has focused
on mitochondrial DNA; however, stochastic variance limits our
confidence in reconstructions of history from a single gene. One
approach solving this limitation is to sample more genes (5). A
more common approach is comparative phylogeography, in which
sequence variation is surveyed at a single gene for multiple
species across the same landscape. A limitation here is that
histories of local extinction and recolonization may vary among
species despite a common history of habitat fluctuation.
2) To improve inference of historical biogeography, we need to
incorporate spatially explicit evidence from paleoecology into
interpretation of species' phylogeography. Some recent studies
have promoted the use of fossil evidence along with
phylogeography to estimate historical distributions, or have
examined sequence variation in the fossils themselves. However,
appropriate fossils are sparse or nonexistent for most taxa.
3) The authors present an alternative approach in which spatial
models of predicted species distributions under serial
paleoclimates are compared with a molecular phylogeography, in
this case for a snail endemic to the rainforests of North
Queensland, Australia. The authors also compare the
phylogeography of the snail to those from several endemic
vertebrates and use consilience across all of these approaches to
enhance biogeographical inference for this rainforest fauna.
4) The snail mtDNA phylogeography is consistent with predictions
from paleoclimate modeling in relation to the location and size
of climatic refugia through the late Pleistocene-Holocene and
broad patterns of extinction and recolonization. There is general
agreement between quantitative estimates of population expansion
from sequence data (using likelihood and coalescent methods) vs.
distributional modeling. The snail phylogeography represents a
composite of both common and idiosyncratic patterns seen among
vertebrates, reflecting the geographically finer scale of
persistence and subdivision in the snail. The authors suggest
that in general this multifaceted approach, combining spatially
explicit paleoclimatological models and comparative
phylogeography, provides a powerful approach to locating
historical refugia and understanding species' responses to them.
References (abridged):
1. Avise, J. C. , Arnold, J. , Ball, R. M. , Bermingham, E. ,
Lamb, T. , Neigel, J. E. , Reeb, C. A. & Saunders, N. C. (1987)
Annu. Rev. Ecol. Syst. 18, 489-522
2. Slatkin, M. (1987) Science 236, 787-792
3. Templeton, A. R. , Routman, E. & Phillips, C. A. (1995)
Genetics 140, 767-782
4. Hewitt, G.M. (2001) Mol. Ecol. 10, 537-549
5. Edwards, S. V. & DeBeerli, P. (2000) Evolution 54, 1839-1854
Related Material:
DYNAMICS OF RECENT CLIMATE CHANGE IN THE ARCTIC
Science 2002 297:1497
The following points are made by R.E. Moritz et al:
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).
2) 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.
3) 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.
4) 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
Related Material:
THE ROLE OF THE SUN IN CLIMATE VARIATIONS
Science 2002 296:673
The following points are made by D. Rind:
1) How much the climate system is influenced by solar variability
has long been a subject of controversy, due largely to the
strictly empirical nature of the evidence. Observations of past
or current climate have been correlated with presumed variations
of solar irradiance or solar activity proxy records, and a de
facto cause and effect relation has been established. For those
convinced of the Sun's dominance, this is generally sufficient.
For critics, the correlations often do not extend sufficiently
long to establish statistical significance; nothing suffices
short of complete understanding of how the energy associated with
solar variability produces the responses at each step of the
process. Rarely is the latter achieved for any forcing of the
climate system, even when physical relations are apparent
(witness the search for the smoking gun of anthropogenic
greenhouse warming).
2) Empirical correlations do not necessarily imply causation,
especially when the climate data quality and dating is imperfect
and solar forcing is poorly known. However, the sheer number of
empirical Sun-climate relations defies ready dismissal. One
difficulty is that different sides typically adopt absolutist
views of the problem: either the Sun is responsible in a dominant
way or it is of no consequence whatsoever. The reality is that
Earth's atmosphere, land surface, and oceans are not passive
recipients of any forcing, be it solar variability, volcanic
eruptions, or altered greenhouse gas concentrations. Rather, the
entire interconnected system participates in the final climate
outcome via multiple, nonlinear feedbacks that can amplify or
diminish climate forcing as well as change the nature and
consistency of the response. To appreciate the solar effect, we
need to disentangle the contributions made by system feedbacks,
natural variability and other forcings.
3) The concept is well established that the Sun was 25 to 30%
less luminous 4.5 Ga (billion years ago), which should have
produced a completely ice-covered Earth for some 2 Gy (billion
years) (1). Yet free flowing water and the beginnings of life
were apparent 3.5 to perhaps more than 4 Ga (the "faint Sun
paradox"). Large amounts of greenhouse gases are presumed to have
been present in the atmosphere to offset the solar deficit,
although it is not understood precisely which gases. If it were
reducing gases, such as CH4 or NH3, organic mixing ratios would
be three orders of magnitude more easily generated by lightning
discharges, than if it were CO2 (2). But the Sun's ultraviolet
(UV) radiation would destroy the reducing gases in short order
(3). Regardless of the ultimate answer, it is apparent that what
would have been expected from solar forcing alone was not what
the climate system registered, due presumably to even greater
forcings or feedbacks such as altered greenhouse gas
concentrations.(4,5)
4) In summary: Is the Sun the controller of climate changes, only
the instigator of changes that are mostly forced by the system
feedbacks, or simply a convenient scapegoat for climate
variations lacking any other obvious cause? The author addresses
this question for suggested solar forcing mechanisms operating on
time scales from billions of years to decades. Each mechanism
fails to generate the expected climate response in important
respects, although some relations are found. The magnitude of the
system feedbacks or variability appears as large or larger than
that of the solar forcing, making the Sun's true role ambiguous.
As the Sun provides an explicit external forcing, a better
understanding of its cause and effect in climate change could
help us evaluate the importance of other climate forcings (such
as past and future greenhouse gas changes).
References (abridged):
1. C. Sagan and G. Mullen, Science 177, 52 (1972)
2. C. F. Chyba and C. Sagan, Nature 355, 125 (1992)
3. W. R. Kuhn and S. K. Atreya, Icarus 37, 207 (1979)
4. M. Chandler, E. Sohl, Eos 20 Spring Meeting Suppl. abstr.
U22A-06 (2001)
5. W. T. Hyde, T. J. Crowley, S. K. Baum, and W. R. Peltier,
Nature 405, 425 (2000).
Related Material:
TRENDS, RHYTHMS, AND ABERRATIONS IN GLOBAL CLIMATE 65 MA TO
PRESENT
Science 2001 292:686
The following points are made by J. Zachos et al:
1) Through study of sedimentary archives, it has become
increasingly apparent that during much of the last 65 million
years and beyond, Earth's climate system has experienced
continuous change, drifting from extremes of expansive warmth
with ice-free poles, to extremes of cold with massive continental
ice-sheets and polar ice caps. Such change is not unexpected,
because the primary forces that drive long-term climate, Earth's
orbital geometry and plate tectonics, are also in perpetual
motion.
2) Much of the higher frequency change in climate (10^(4) to
10^(5) years) is generated by periodic and quasi-periodic
oscillations in Earth's orbital parameters of eccentricity,
obliquity, and precession that affect the distribution and amount
of incident solar energy (1). Whereas eccentricity affects
climate by modulating the amplitude of precession and thus
influencing the total annual/seasonal solar energy budget,
obliquity changes the latitudinal distribution of insolation.
Because the orbital parameters vary with distinct tempos that
remain stable for tens of millions of years (2), they provide a
steady and, hence, predictable pacing of climate.
3) The orbitally related rhythms, in turn, oscillate about a
climatic mean that is constantly drifting in response to gradual
changes in Earth's major boundary conditions. These include
continental geography and topography, oceanic gateway locations
and bathymetry, and the concentrations of atmospheric greenhouse
gases (3). These boundary conditions are controlled largely by
plate tectonics, and thus tend to change gradually, and for the
most part, unidirectionally, on million-year (My) time scales.
Some of the more consequential changes in boundary conditions
over the last 65 My include: North Atlantic rift volcanism,
opening and widening of the two Antarctic gateways, Tasmanian and
Drake Passages (4); collision of India with Asia and subsequent
uplift of the Himalayas and Tibetan Plateau (5); uplift of Panama
and closure of the Central American Seaway; and a sharp decline
in pCO2. Each of these tectonically driven events triggered a
major shift in the dynamics of the global climate system.
Moreover, in altering the primary boundary conditions and/or mean
climate state, some or all of these events have altered system
sensitivity to orbital forcing, thereby increasing the potential
complexity and diversity of the climate spectrum. This would
include the potential for unusually rapid or extreme changes in
climate.
4) In summary: Since 65 million years ago (Ma), Earth's climate
has undergone a significant and complex evolution, the finer
details of which are now coming to light through investigations
of deep-sea sediment cores. This evolution includes gradual
trends of warming and cooling driven by tectonic processes on
time scales of 10^(5) to 10^(7) years, rhythmic or periodic
cycles driven by orbital processes with 10^(4)- to 10^(6)-year
cyclicity, and rare rapid aberrant shifts and extreme climate
transients with durations of 10^(3) to 10^(5) years.
References (abridged):
1. J. D. Hays, J. Imbrie, N. J. Shackleton, Science 194, 1121
(1976)
2. J. Laskar, F. Joutel, F. Boudin, Astron. Astrophys. 270, 522
(1993)
3. T. J. Crowley, K. G. Burke, Eds., Tectonic Boundary Conditions
for Climate Reconstructions, vol. 39 (Oxford Univ. Press, New
York, 1998)
4. L. A. Lawver, L. M. Gahagan, in (3), pp. 212-226
5. P. Copeland, in (15), pp. 20-40
Related Material:
NONGLACIAL RAPID CLIMATE EVENTS: PAST AND FUTURE.
Proc. Nat. Acad. Sci. 2000 97(4):1335
The following points are made by J. Overpeck and R. Webb:
1) Paleoclimatologists have been aware for decades that the
climate system is capable of behavior quite unlike that of the
present day. At the same time, however, paleoclimatology has
tended to follow the lead of the climate dynamics community, and
to pursue research primarily under the paradigm that the Earth's
climate tends to change gradually in response to slowly changing
climate forcing. For example, the first major revolution of
modern paleoclimatology was the identification of a slowly
oscillating Earth-Sun relationship as the pacemaker of the Ice
Ages (i.e., "Milankovitch Theory"). On the other end of the
climate variance spectrum, early successes with seasonal to
interannual climate prediction benefited from a well described
stable mode of the El Nino/Southern Oscillation (ENSO)
variability in the late 1980s.
2) However, a new paradigm of climate variability is emerging.
Rapid step-like shifts in climate variability that occur over
decades or less, as well as climatic extremes (e.g., drought)
that persist for decades, occurred repeatedly in recent Earth
history. For example, a substantial amount of work now focuses on
abrupt ice-age climate shifts, and significant research has also
focused on the abrupt climate shift that took place over the
Pacific basin in the mid-1970s (1). This mid-1970s shift
represents a transition into a mode of variability characterized
by frequent and strong El NiÏo conditions relative to earlier
decades.
3) The authors review a representative subset of the growing body
of paleoclimatic evidence regarding rapid climatic change since
the last deglaciation and highlight the conclusion that climate
variability in the past has changed substantially in response to
altered climate forcing. The authors focus on rapid climate
change of "warm climates" like those of today and the future,
rather than on the well described rapid changes of glacial or
deglacial climates in periods of time when the Earth's climate
system may have been heavily influenced by large Northern
Hemisphere ice sheets. The authors also focus on climate
phenomena (e.g., El Nino, monsoons, and drought) of primary
interest to society. The authors suggest the growing body of
paleoclimatic evidence indicates that the climate system may have
many potential surprises in store for scientists and society
alike.(1-5)
References (abridged):
1. Trenberth, K. E. (1990) Bull. Am. Meterol. Soc. 71, 988-993
2. Overpeck, J. T. (1996) Science 271, 1820-1821
3. Duplessy, J.-C. & Overpeck, J. T. (1994) The PAGES/CLIVAR
Intersection: Providing The paleoclimatic Perspective Needed To
Understand Climate Variability and Predictability (IGBP PAGES
Core Project Office, Bern, Switzerland)
4. Cole, J. E, Fairbanks, R. G. & Shen, G. T. (1993) Science 262,
1790-1793
5. Dunbar, R. B., Wellington, G. M., Colgan, M. W. & Glynn, P. W.
(1994) Paleoceanography 9, 291-316
Related Material:
IMPACT OF A PERMO-CARBONIFEROUS HIGH O2 EVENT ON THE TERRESTRIAL
CARBON CYCLE.
Proc. Nat. Acad. Sci. 2000 97:12428
The following points are made by D.J. Beerling and R.A. Berner:
1) Independent models predicting the Phanerozoic (past 600
million years) history of atmospheric O(2) partial pressure
(pO(2)) indicate a marked rise to approximately 35% in the Permo-
Carboniferous, around 300 million years before present, with the
strong potential for altering the biogeochemical cycling of
carbon by terrestrial ecosystems. This potential, however, would
have been modified by the prevailing atmospheric pCO(2) value.
2) The authors use a process-based terrestrial carbon cycle model
forced with a late Carboniferous paleoclimate simulation to
evaluate the effects of a rise from 21 to 35% pO(2) on
terrestrial biosphere productivity and assess how this response
is modified by current uncertainties in the prevailing pCO(2)
value.
3) The authors report that the results indicate that a rise in
pO(2) from 21 to 35% during the Carboniferous reduced global
terrestrial primary productivity by 20% and led to a 216-Gt (1 Gt
= 10^(12) kg) C reduction in the vegetation and soil carbon
storage, in an atmosphere with pCO(2) = 0.03%. However, in an
atmosphere with pCO(2) = 0.06%, the CO(2) fertilization effect is
larger than the cost of photorespiration, and ecosystem
productivity increases leading to the net sequestration of 117 Gt
C into the vegetation and soil carbon reservoirs. In both cases,
the effects result from the strong interaction between pO(2),
pCO(2), and climate in the tropics.
4) From this analysis, the authors deduce that a Permo-
Carboniferous rise in pO(2) was unlikely to have exerted
catastrophic effects on ecosystem productivity (with pCO(2) =
0.03%), and if pCO(2) levels at this time were >0.04%, the water-
use efficiency of land plants may even have improved.
Related Material:
PALEOCLIMATE AND AMERINDIANS: EVIDENCE FROM STABLE ISOTOPES AND
ATMOSPHERIC CIRCULATION
Proc. Nat. Acad. Sci. 2001 98:2485
The following points are made by M.B. Lovvorn et al:
1) Two Amerindian demographic shifts are attributed to climate
change in the northwest plains of North America: at approximately
11,000 calendar years before the present (yr BP), Amerindian
culture apparently split into foothills-mountains vs. plains
biomes; and from 8,000-5,000 yr BP, scarce archaeological sites
on the open plains suggest emigration during xeric "Altithermal"
conditions.
2) The authors reconstructed paleoclimates from stable isotopes
in prehistoric bison bone and relations between weather and
fractions of C(4) plants in forage. Further, the authors
developed a climate-change model that synthesized stable isotope,
existing qualitative evidence (e.g., palynological, erosional),
and global climate mechanisms affecting this midlatitude region.
3) The authors report the isotope data indicate significant
warming from approximately 12,400 to 11,900 yr BP, supporting
climate-driven cultural separation. However, isotope evidence of
apparently wet, warm conditions at 7,300 yr BP refutes emigration
to avoid xeric conditions. Scarcity of archaeological sites is
best explained by rapid climate fluctuations after catastrophic
draining of the Laurentide Lakes, which disrupted North Atlantic
Deep Water production and subsequently altered monsoonal inputs
to the open plains.
Related Material:
POSSIBLE ROLE OF OCEANIC HEAT TRANSPORT IN EARLY EOCENE CLIMATE
Paleoceanography 1995 10:347
The following points are made by L.C. Sloan et al:
1) Increased oceanic heat transport has often been cited as a
means of maintaining warm high-latitude surface temperatures in
many intervals of the geologic past, including the early Eocene
(approximately 50 million years ago). Although the excess amount
of oceanic heat transport required by warm high latitude sea
surface temperatures can be calculated empirically, determining
how additional oceanic heat transport would take place has yet to
be accomplished.
2) That the mechanisms of enhanced poleward oceanic heat
transport remain undefined in paleoclimate reconstructions is an
important point that is often overlooked. Using early Eocene
climate as an example, the authors consider various ways to
produce enhanced poleward heat transport and latitudinal energy
redistribution of the sign and magnitude required by interpreted
early Eocene conditions.
3) The authors report their interpolation of early Eocene
paleotemperature data indicate that an approximately 30% increase
in poleward heat transport would be required to maintain Eocene
high-latitude temperatures. This increased heat transport appears
difficult to accomplish by any means of ocean circulation if we
use present ocean circulation characteristics to evaluate early
Eocene rates. Either oceanic processes were very different from
those of the present to produce the early Eocene climate
conditions or oceanic heat transport was not the primary cause of
that climate. The authors believe that atmospheric processes,
with contributions from other factors, such as clouds, were the
most likely primary cause of early Eocene climate.
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