ScienceWeek

CLIMATOLOGY: ANCIENT CLIMATE AND FUTURE CLIMATE

The following points are made by D.P. Schrag and R.B. Alley (Science 2004 306:821):

1) Humans are changing the amount of carbon dioxide in the atmosphere by burning coal, oil, and gas. The current atmospheric CO2 concentration is higher than it has been for at least the past 430,000 years [1], and perhaps for tens of millions of years [2]. Over the next 100 years, without substantial changes in energy technology or economic development, the atmospheric CO2 concentration will rise to 800 to 1000 ppm [3]. This rise represents a spectacular uncontrolled experiment that humans are performing on Earth. The paleoclimate record may provide the best guess as to what may happen as a result.

2) One crude measure of how much the climate will warm in response to an increased atmospheric CO2 concentration is the climate sensitivity, often taken as the globally averaged warming expected from doubling the atmospheric CO2 concentration. This sensitivity is usually estimated as between 1.5 deg and 4.5 deg C on the basis of results from a suite of complex climate models and from efforts to explain temperature changes over the past century [4]. However, many uncertainties exist in that estimation, including large gaps in our understanding of water vapor and cloud feedbacks on climate.

3) The study of past climates provides information about the magnitude of, and causes for, many preinstrumental climate changes, allowing for comparison with climate models and an independent assessment of climate sensitivity. Periodic ice ages over the past 2 million years were paced by Earth's orbit around the Sun. However, the synchronous and substantial glaciation in both hemispheres requires some additional feedbacks beyond the orbital variations to amplify the climate response and make it uniform in both hemispheres. Changes in the atmospheric CO2 concentration are likely responsible for both [5]. The sea surface temperature in the Western Equatorial Pacific was about 3 deg C colder during the last ice age than it is today. Given that this warm and stable area of the world ocean was relatively unaffected by changes in high-latitude ice cover and in ocean circulation, the cooling must be explained predominantly by radiative effects associated with changes in atmospheric CO2 concentration. This observation yields a climate sensitivity that is on the high end of modern estimates, consistent with model simulations of the ice ages.

4) Likewise, warm episodes in Earth's history reveal a similar cautionary lesson. During the Eocene, 50 million years ago, palm trees grew in Wyoming and deep ocean temperatures were more than 10 deg C warmer than present. Because we do not know exactly how high the atmospheric CO2 concentration was at that time, we cannot use it as a direct measure of climate sensitivity. However, the extreme warmth at high latitudes -- especially during the winter in continental interiors -- cannot be simulated by climate models purely through elevating greenhouse gas concentrations. Special cloud feedbacks must be included that are not present in the models used to predict future climate change. This observation suggests that feedbacks may be missing from current models and that future climate change may be underestimated in these models, particularly at high latitudes.

References (abridged):

1. J. R. Petit et al., Nature 399, 429 (1999)

2. M. Pagani, M. A. Arthur, K. H. Freeman, Paleoceanography 14, 273 (1999)

3. Intergovernmental Panel on Climate Change, Climate Change 2001: The Science of Climate Change (Cambridge Univ. Press, Cambridge, 2001)

4. R. A. Kerr, Science 305, 932 (2004)

5. C. Lorius et al., Nature 347, 139 (1990)

Science http://www.sciencemag.org

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PALEOCLIMATE: ON THE BEGINNING OF THE LAST ICE AGE

The following points are made by Kurt M. Cuffey (Nature 2004 431:133):

1) The relatively warm and stable climate that humanity has enjoyed for the past 10,000 years will inevitably give way to a new ice age -- a tremendous environmental transformation that is destined to bury the sites of Boston, Edinburgh and Stockholm under glacial ice. In The Day After Tomorrow, the Hollywood movie most notable for its public abuse of thermodynamics, a new ice age starts in only one week. What does such a transition look like in reality?

2) A new ice core(1) that samples the entire 3-km thickness of the north-central Greenland ice sheet provides us with an unprecedentedly rich and precise view of the onset of the most recent ice age, some 120,000 years ago. And it is from the location of greatest interest -- the North Atlantic region, where rapid climate changes have been most dramatic in the past. This achievement is the result of the efforts of the multinational North Greenland Ice Core Project (NGRIP). Individual years of snow deposition are distinguishable for events as far back as 123,000 years in the past. A few years ago, such a resolution was thought to be unattainable.

3) The story of how the impossible became possible would itself make a fine movie, complete with dramatic scenes of enlightenment in the mass spectrometry lab as isotope analyses reveal past climatic changes. In the early 1990s, two deep ice cores recovered from central Greenland yielded a detailed environmental history extending 100,000 years back in time, through the entirety of the last glacial climate(2). These showed that climate was extremely unstable. Within the space of a few decades, the North Atlantic region could evidently warm by 10 C, while smaller changes of temperature and moisture occurred over wide areas of the planet(3).

4) Unfortunately, structural disturbance of the deepest ices(4) prevented these cores from revealing either the onset of the glacial period or events during the preceding warm interglacial period, known as the "Eemian", about 130,000 to 120,000 years ago. Initial reports to the contrary(5), highlighting apparent climate instabilities within the Eemian, were clearly mistaken, as the new report demonstrates(1). Yet the degree of climate stability during the Eemian is of intense interest: the Eemian was slightly warmer than the world is now, providing an analogue for a possible future climate warmed by atmospheric pollution.

References (abridged):

1. North Greenland Ice Core Project members Nature 431, 147-151 (2004)

2. Hammer, C., Mayewski, P. A., Peel, D. & Stuiver, M. (eds) J. Geophys. Res. 102 (C12), 26317-26886 (1997)

3. Severinghaus, J. P. & Brook, E. J. Science 386, 930-934 (1999)

4. Chappellaz, J., Brook, E., Blunier, T. & Malaize, B. J. Geophys. Res. 102, 26547-26557 (1997)

5. Greenland Ice-Core Project members Nature 364, 203-208 (1993)

Nature http://www.nature.com/nature

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OCEAN SCIENCE: GLOBAL WARMING AND THE NEXT ICE AGE

The following points are made by A.J. Weaver and C. Hillaire-Marcel (Science 2004 304:400):

1) A popular idea in the media is that human-induced global warming will cause another ice age. But where did this idea come from? Several recent magazine articles (1-3) report that abrupt climate change was prevalent in the recent geological history of Earth and that there was some early (albeit controversial) evidence from the last interglacial -- thought to be slightly warmer than preindustrial times (4) -- that abrupt climate change was the norm (5). Consequently, the articles postulate a sequence of events that goes something like this: If global warming were to boost the hydrological cycle, enhanced freshwater discharge into the North Atlantic would shut down the AMO (Atlantic Meridional Overturning), the North Atlantic component of global ocean overturning circulation. This would result in downstream cooling over Europe, leading to the slow growth of glaciers and the onset of the next ice age.

2) This view prevails in the popular press despite a relatively solid understanding of glacial inception and growth. What glacier formation and growth require is, of course, a change in seasonal incoming solar radiation (warmer winters and colder summers) associated with changes in Earth's axial tilt, its longitude of perihelion, and the precession of its elliptical orbit around the Sun. These small changes must then be amplified by feedback from reflected light associated with enhanced snow/ice cover, vegetation associated with the expansion of tundra, and greenhouse gases associated with the uptake (not release) of carbon dioxide and methane.

3) Several modeling studies provide outputs to support this progression. These studies show that with elevated levels of carbon dioxide, such as those that exist today, no permanent snow can exist over land in August (as temperatures are too warm), a necessary prerequisite for the growth of glaciers in the Northern Hemisphere. These same models show that if the AMO were to be artificially shut down, there would be regions of substantial cooling in and around the North Atlantic. Berger and Loutre (2002) specifically noted that "most CO2 scenarios led to an exceptionally long interglacial from 5000 years before the present to 50,000 years from now ... with the next glacial maximum in 100,000 years. Only for CO2 concentrations less than 220 ppmv was an early entrance into glaciation simulated." They further argued that the next glaciation would be unlikely to occur for another 50,000 years.

4) Although most paleoclimatologists would agree that the past is unlikely to provide true analogs of the future, past climate synopses are valuable for confronting the results of modeling experiments or for illustrating global warming. A reduction of the AMO due to a global warming-induced increase in freshwater supplies to the North Atlantic is often discussed in relation to a short event that occurred some 8200 years ago (8.2 ka). During this event, one of the largest glacial lakes of the Laurentide Ice Sheet, Lake Ojibway, drained into the North Atlantic through Hudson Strait, quickly releasing enormous quantities of fresh water. However, unequivocal evidence that this event resulted in a substantial reduction of the AMO has apparently not yet been obtained.

References (abridged):

1. S. Rahmstorf, New Scientist 153, 26 (8 February 1997)

2. W. H. Calvin, Atlantic Monthly 281, 47 (January 1998)

3. B. Lemley, Discover 23, 35 (September 2002)

4. IPCC, Climate Change 2001, The Scientific Basis. Contribution of Working Group I to the Third Scientific Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton et al., Eds. (Cambridge Univ. Press, Cambridge, 2001)

5. GRIP Project Members, Nature 364, 203 (1993)

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