During the instrumental period the Earth's climate has varied within a relatively narrow range as compared to the distant past. Much can be learned about how the climate system operates by examining the characteristics of past extremes. Paleoclimatologists and paleoceanographers use a wide variety of chemical, biological, physical, and numerical tools to reconstruct ancient conditions, and they made many important contributions in 2002. In particular, better methods for reconstructing deep-ocean properties and new ideas about past tropical-extratropical teleconnections emerged.
Deep-Ocean Temperature and Salinity
Past deep-ocean properties are of great interest to researchers because the
meridional overturning circulation is an important player in major climatic
changes. While past sea-surface temperatures can be reconstructed in many ways,
estimates of deep-ocean paleotemperatures have long relied on the oxygen isotopic
composition (oxygen-18) of benthic foraminifera. Foraminiferal oxygen-18 depends
not only on calcification temperature but on the oxygen-18 of seawater, which
is, in turn, influenced by global ice volume and salinity. Paleotemperatures
based on oxygen-18 alone are therefore little more than "guesstimates."
Fortunately, methods introduced recently allow for the separation of temperature
from seawater oxygen-18.
One approach is to measure the concentration of magnesium in benthic foraminifera (Mg/Ca), which increases exponentially with calcification temperature. New measurements by Pam Martin et al. (Earth and Planetary Science Letters, v. 198, p. 193) suggest that the deep ocean was colder by about 2-4 degrees Celsius during glacial periods of the past 330,000 years. Coolings may have been caused by changes in deep ocean circulation (for example, the replacement of North Atlantic Deep Water by colder Antarctic Bottom Water) or by the formation of deep waters at colder surface temperatures. Katharina Billups and Dan Schrag (Paleoceanography, v. 17, n. 1) used paired benthic foraminiferal Mg/Ca and oxygen-18 measurements to infer the global ice volume history for the past 27 million years, which compares favorably with Bil Haq's classic sequence-stratigraphy-based sea-level curve. Active research is under way to refine the Mg/Ca temperature calibration and isolate possible ancillary effects.
Another way to separate the effect of seawater oxygen-18 is to measure it directly. Deep ocean water, with its characteristic oxygen-18 and salinity, diffuses into sediment pore waters at a rate of roughly 10 meters per 20,000 years. Over glacial-interglacial cycles, as the sequestering of fresh water into ice sheets causes mean ocean oxygen-18 and salinity to vary, a smoothed record is preserved in the pore waters. Seawater oxygen-18 and salinity during the Last Glacial Maximum, which peaked about 21,000 years ago, may therefore be reconstructed by measuring pore water profiles and modeling the diffusion history. Temperature then emerges from the combination of seawater oxygen-18 and benthic foraminiferal oxygen-18, and temperature plus salinity yields density.
Jess Adkins et al. (Science, v. 298, p. 1769) used these methods to show that the Last Glacial Maximum deep ocean was uniformly close to the freezing point of seawater. Whereas today the deep stratification is controlled largely by temperature, it was dominated by salinity during the Last Glacial Maximum. In particular, the deep Southern Ocean was found to be dramatically saltier and denser than the deep North Atlantic. Though these observations need to be confirmed at additional core sites, they suggest that glacial ocean dynamics were even more different from modern conditions than previously thought.
Ice Age El Niño-Southern Oscillation?
Paleoclimatologists are increasingly turning to the dynamics of the El Niño-Southern
Oscillation system to explain past climate patterns. Peter Molnar and Mark Cane
(Paleoceanography, v. 17, n. 2) suggested that the warm climate of the
early to mid-Pliocene, from about 5 to 2.7 million years ago, was characterized
by a virtually permanent El Niño-like state with enhanced atmospheric
heat transport to middle latitudes. The growth of Northern Hemisphere ice sheets
during the late Pliocene may have thus required a strengthening of the Walker
circulation and a shift toward more La Niña-like conditions. Within the
colder climate of the late Pleistocene, George Kukla et al. (Quaternary Research,
v. 58, p. 27) proposed that El Niño conditions may have supplied the
excess moisture needed for Northern Hemisphere ice sheet growth at the start
of the last glaciation about 115,000 years ago. A coupled ocean-atmosphere model
of the tropical Pacific suggests that changes in solar radiation caused by variations
in the Earth's orbit could have led to more frequent El Niño events and
less frequent La Niña events at this time.
El Niño-like conditions were also proposed for the end of the last glaciation, from about 20,000 to 15,000 years ago, on the basis of a reduced east-west gradient in tropical Pacific sea-surface temperatures reconstructed by Tom Koutavas et al. (Science, v. 297, p. 226). These data also suggest that the early-mid Holocene, about 8000 to 5000 years ago, was more La Niña-like than today, in agreement with previous modeling. Similarly, Christopher Moy et al. (Nature, v. 420, p. 162) inferred an increase in El Niño frequency since the early Holocene as evidenced by precipitation-triggered clastic sediment events in an Ecuadorian Andes lake. Both results could explain an apparent southward shift of the Intertropical Convergence Zone during the late Holocene as a response to weakening of the eastern tropical Pacific cold tongue. In contrast, oxygen isotopes in sea catfish otoliths presented by Fred Andrus et al. (Science, v. 295, p. 1508) imply that the Peruvian coast was actually warmer during the mid Holocene, which suggests less upwelling and therefore is inconsistent with La Niña-like conditions.
Finally, there is some indication that the El Niño-Southern Oscillation system varied with the millennial-scale Dansgaard-Oeschger climate cycles of the last ice age. Lowell Stott et al. (Science, v. 297, p. 222) used paired planktonic foraminiferal Mg/Ca and oxygen-18 to suggest that rainfall in the western tropical Pacific was reduced during cold stadials, which may reflect El Niño-like conditions. They further agued that stadial-interstadial El Niño-La Niña oscillations could explain various low- to mid-latitude records of Dansgaard-Oeschger variability. Additional work is required to verify these results and to determine whether the El Niño-Southern Oscillation system was driving or responding to Dansgaard-Oeschger cyclicity.
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