ScienceWeek

PALEOCLIMATE: ON GLOBAL COOLING IN THE PLIOCENE EPOCH

The following points are made by A. C. Ravelo et al (Nature 2004 429:263):

1) Global climate change of the past 4 million years (Myr) includes the end of the early Pliocene warm period (5-3 Myr ago) and significant intensification of Northern Hemisphere glaciation (NHG) 2.75 Myr ago. The amplitude of 10^(4)-10^(6)-year climate oscillations increased as climate cooled. The past 4 Myr, unlike the more recent past, can be studied to assess climate theories that involve climate components with relatively long timescales of response (for example, deep ocean, cryosphere), theories that predict different behavior in warm versus cold conditions, and theories that are best tested by examining changes to average conditions that are large relative to the seasonal signal. In addition, well-understood changes in solar heating (Milankovitch cycles) occur on long timescales, providing an excellent natural experiment to examine climate responses to perturbations in the Earth's radiative balance.

2) Relative to today, the Pliocene warm period was characterized by 3 deg C higher global surface temperatures, 10-20 m higher sea level, enhanced thermohaline circulation(1,2), slightly reduced Antarctic ice sheets, emerging but small Northern Hemisphere ice coverage(3), and 30% higher atmospheric CO2 concentrations(1,4). A small decrease in carbon dioxide concentration could explain the cooling at the end of the warm period if coupled with positive feedbacks, as suggested for the onset of significant Antarctic glaciation(5). However, whether these feedbacks primarily involved low- or high-latitude processes has been controversial.

3) Although high-latitude feedbacks (for example, related to ocean heat transport or ice albedo) may have accelerated cooling once NHG began, the impact of glaciation on global-scale cooling still needs to be explored. Alternatively, long-term reorganization of tropical conditions could have strongly influenced global climate, as occurs interannually with the El Nino-Southern Oscillation phenomenon. Even small changes in tropical temperature patterns can profoundly affect extratropical conditions on geological timescales. Thus, low-latitude tectonic events (restriction of Panamanian or Indonesian seaways) may have changed the distribution of heat between basins, causing reorganization of climate patterns, the end of the warm period, and ultimately intensification of NHG. Yet the global impact of these tectonic events has not been adequately examined. Finally, bidirectional high-low latitude interactions may explain important features of the transition.

4) In summary: The Earth's climate has undergone a global transition over the past 4 million years, from warm conditions with global surface temperatures approximately 3 deg C warmer than today, smaller ice sheets, and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation 2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. The authors compare climate records from high latitudes, subtropical regions, and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. The authors also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. The authors suggest their results indicate that mean low-latitude climate conditions can significantly influence global climate feedbacks.

References (abridged):

1. Raymo, M. E., Grant, B., Horowitz, M. & Rau, G. H. Mid-Pliocene warmth: Stronger greenhouse and stronger conveyer. Mar. Micropaleo. 27, 313-326 (1996)

2. Ravelo, A. C. & Andreasen, D. H. Enhanced circulation during a warm period. Geophys. Res. Lett. 27, 1001-1004 (2000)

3. Haywood, A. M., Valdes, P. J. & Sellwood, B. W. Global scale palaeoclimate reconstruction of the middle Pliocene climate using the UKMO GCM: initial results. Glob. Planet. Change 25, 239-256 (2000)

4. Van der Burgh, J., Visscher, H., Dilcher, D. L. & Kürschner, W. M. Paleoatmospheric signatures in Neogene fossil leaves. Science 260, 1788-1790 (1993)

5. DeConto, R. M. & Pollard, D. Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2. Nature 421, 245-249 (2003)

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

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PALEOCLIMATE: SEAWATER TEMPERATURE AND GLOBAL COOLING

The following points are made by Roger Francois (Nature 2004 428:31):

1) Since the beginning of the Tertiary, 65 million years ago, the Earth has experienced a long-term cooling trend(1). The reasons invoked to explain the trend are complex, involving changes in major ocean currents and continental topography, the amount of solar energy reflected by the Earth, concentrations of atmospheric CO2, and continental glaciations. In contrast, Sigman et al(2) highlight a simple yet largely overlooked factor, which may have contributed significantly to cooling during the past 3 million years or so. It is based on the temperature-dependent density of sea water, and the interplay with the other main contributor to density, salinity.

2) One of the principal determinants of the Earth's mean temperature is the level of atmospheric CO2, which regulates its radiative balance. One possible contributor to global cooling during the Tertiary is thus a reduction of atmospheric CO2 concentration with time(3). Such a reduction would have resulted from a change in the balance between CO2 emitted by volcanoes and ocean ridges, and CO2 uptake by chemical weathering of the continental crust, which between them control the amount of carbon present in the ocean, on continents, and in the atmosphere on a timescale of millions of years. A variety of oceanic processes determine the partitioning of CO2 between atmosphere and ocean, and produce higher-frequency variability in atmospheric CO2 on timescales of 10^(4) to 10^(5) years. Particularly well documented are cyclical variations, which occurred during the past 500,000 years in concert with variations in Earth's orbital parameters and which resulted in lower atmospheric CO2 during glacial periods(4).

3) One of the leitmotifs that are used to try to explain this lower glacial CO2 is stratification in the upper ocean at high latitudes. Some 20 years ago, this mechanism was identified as a particularly effective means of sequestering carbon in the deep sea at the expense of the atmosphere(5). In the modern ocean, there is extensive vertical mixing at high latitudes, particularly in the cold waters surrounding Antarctica. This mixing returns to the atmosphere some of the CO2 that accumulates in the deep sea from the decay of sinking organic matter produced in the sunlit surface waters. Density stratification of the upper ocean in these regions would prevent such return and keep CO2 sequestered in the deep sea. This finding prompted an extensive search in the sedimentary record for evidence that such stratification happened during the recurring ice ages of the past 500,000 years. Some compelling but not universally accepted evidence for glacial stratification in the Antarctic emerged from this work, but with no clear consensus about the mechanism responsible.

4) Sigman et al(2) have also found evidence for enhanced ocean stratification at high latitudes starting 2.7 million years ago, a period coinciding with the initiation of glaciation in the Northern Hemisphere. That evidence comes from the past record of the activity of diatoms, one of the main members of the phytoplankton, whose cell walls are made from opal (a mineral composed of amorphous silica). Sigman et al(2) report a dramatic decrease in opal accumulation rates in the sediment deposited in the North Pacific and the Antarctic Ocean at that time, which they interpret as a reduction in opal production by diatoms and in the phytoplankton productivity of both regions. They also measured the nitrogen isotopic composition of bulk sediment, and the results suggest that these fewer diatoms used either a higher (in the North Pacific) or a similar (in the Antarctic) fraction of the nitrate nutrients supplied to the sunlit layer by vertical mixing.

References (abridged):

1. Zachos, J. et al. Science 292, 686-693 (2001)

2. Sigman, D. M., Jaccard, S. L. & Haug, G. H. Nature 428, 59-63 (2004)

3. Pearson, P. N. & Palmer, M. R. Nature 406, 695-699 (2000)

4. Petit, J. R. et al. Nature 429, 429-436 (1999)

5. Sarmiento, J. L. & Toggweiler, J. R. Nature 308, 621-624 (1984)

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

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PALEOCLIMATE: SOLAR ENERGY AND CLIMATE CYCLES

The following points are made by Katharina Billups (Nature 2004 427:686):

1) During the 1920s, Milutin Milankovitch (1879-1958), a Serbian mathematician, calculated the effects of alterations in Earth's motion around the Sun on the amount of solar energy reaching different latitudes(1). Since then, some of the long-period, cyclic changes seen in archives of past environmental conditions on Earth have been explained by these changes in "insolation". For example, a 41,000-year cycle in a climate record is commonly ascribed to changes in the planet's tilt, which affects insolation at high latitudes, and a 19,000-23,000-year cycle is ascribed to changes in the planet's wobble, which dominates insolation at low and middle latitudes(2,3).

2) Eighty years after Milankovitch's innovative work, Liu and Herbert(4) have unearthed surprising evidence that the views based on Milankovich's conclusions are incomplete. Liu and Herbert provide a record of low-latitude sea surface temperature (SST) that is in phase with the 41,000-year rhythm reminiscent of high-latitude insolation. Although their record represents only the past 1.8 million years, a small portion of Earth's history, their findings force us to think further about a climate system that we already knew to be complex.

3) How does the geological record document past climate change? Perhaps the most ubiquitous recorders of past climate change are the fossil shells of foraminifera, calcareous marine organisms. To a large degree, the oxygen-isotope ratio of the calcium carbonate shell of foraminifera reflects the oxygen-isotope ratio in sea water, which in turn is a function of the hydrologic cycle and thus the amount of fresh water stored as ice at the poles. Because ice-sheet growth and decay are affected by cycles in insolation, the (water O-18)/(water O-16) ratio of sea water varies at the same frequency as does the O-18/O-16 ratio of the carbonate shells of the foraminifera in the world ocean. Over time, foraminifers accumulating on the bottom of the ocean build an archive of information about glacial to interglacial climate change.

4) These types of record provide the backbone of palaeoclimate studies. For example, over the past 5 million years perhaps the most profound change in Earth's climate history was the onset of glaciation in the Northern Hemisphere, recorded by the increase in foraminiferal (O-18)/(O-16) ratios between 3 million and 2 million years ago. However, the isotopic composition of sea water, and hence global ice extent, is not the only variable determining foraminiferal (O-18)/(O-16) ratios; water temperature affects the preference of one isotope over the other incorporated in calcium carbonate during calcification. So although foraminiferal (O-18)/(O-16) ratios by themselves do provide information about large-scale climate change as described above, they cannot contribute unequivocally to our understanding of the relationship between individual climate variables such as ice volume and water temperature.(5)

References (abridged):

1. Milankovitch, M. Serb. Akad. Beogr. Spec. Publ. 132 (1941)

2. Hays, J. D. et al. Science 194, 1121 1132 (1976)

3. Berger, A. L. J. Atmos. Sci. 35, 2362 2367 (1978)

4. Liu, Z. & Herbert, T. D. Nature 427, 720 723 (2004)

5. Wefer, G., Berger, W. H., Bijma, J. & Fischer, G. in Use of Proxies in Paleoceanography (eds Fischer, G. & Wefer, G.) 1 68 (Springer, Berlin, 1999)

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