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Effects of Past Global Change on Life
The ice sheets in turn act as a source of forcing for other parts of the climate system, through several mechanisms. Like rock plateaus, the rise of these large domes of ice can rearrange the basic circulation of the atmosphere, with major effects that are sent far downstream to the south and east. For example, both marine geologic data and GCM experiments show that cold winds from the ice sheets can chill and freeze the surface of the North Atlantic Ocean at high latitudes (Ruddiman and McIntrye, 1984; Manabe and Broccoli, 1985). GCM experiments also show that chilling and freezing of the North Atlantic can in turn cool climates substantially over maritime regions of Eurasia and southward into the Mediterranean and even northern Africa (Rind et al., 1986). Loess records from China indicate that even in far southeastern Asia, aridity cycles follow the basic tempo of change in the size of ice sheets farther north (Heller and Liu, 1982; Kukla, 1987).
In addition, formation of deep water in the North Atlantic was suppressed when ice sheets were large (Boyle and Keigwin, 1985), probably because of changes in salinity created by the altered wind field (and possibly because of meltwater fluxes). Changes in rate of formation of NADW may also influence subsurface and surface circulation in the Southern Ocean (Weyl, 1968). Redistribution of nutrients and alkalinity in this region may, in turn, affect atmospheric CO2 levels and thus global climate (Broecker and Peng, 1989). To some extent, however, CO2 has also varied somewhat independently of Northern Hemisphere ice sheets over the past 150,000 yr (Barnola et al., 1987).
SUMMARY AND CONCLUSIONS
The abrupt climatic changes that marked the onset of the Recent ice age during the Pliocene Epoch followed a long interval of widespread cooling and accentuation of contrasts between wet and dry regional climates. Nonetheless, prior to about 3.1 Ma, climates in many areas of the North Atlantic region remained warmer and less seasonal than today. The Atlantic and Pacific Oceans exchanged temperate species of mollusks by way of the Arctic, the Mediterranean Sea was marginally tropical, and shallow seas off Virginia became briefly subtropical.
Oxygen isotope ratios for marine microfossils offer the best chronological record of the present ice age. They reveal that the deep-sea became permanently cooler at about 3.2 to 3.1 Ma, which implies that there was increased cooling at high latitudes. At this time, several subtropical species of plants disappeared from northwestern Europe, and a brief pulse of cooling extended at least as far south as the Mediterranean, where changes in the planktonic foraminiferal fauna record an estimated reduction in mean annual temperature of 2 to 4°C.
Climates warmed again in at least some areas during the 3.0 to 2.5 Ma interval, but at 2.5 to 2.4 Ma the modern ice age began. Mountain glaciers had been present earlier, but the expansion of large ice sheets at this time accounted for the strong isotopic signal recorded in deep-sea cores. Subsequent isotopic cycles in these cores and in loess reveal an orbitally forced periodicity of ~41,000 yr for about 1.5 m.y. and then of ~100,000 yr between 0.9 Ma and the present.
Terrestrial biotas underwent major changes in the vicinity of the North Atlantic during Late Pliocene time, with aridification playing at least as large a role as cooling. In most areas, the greatest changes took place at about 2.5 Ma, or a bit later. Throughout Africa, forests contracted and many species of mammals that had adapted to them died out; speciation soon produced many new species adapted to the expanding savannas. In Europe forests changed their character, in part through the disappearance of subtropical taxa, but they remained widespread so that mammals were little affected. Details of floral change during the Pliocene remain poorly known for many areas of North America; grasslands were widespread before the end of Miocene time, but about 55 genera of mammals disappeared near the end of the Pliocene.
Beginning slightly before 2.5 Ma, heavy extinction of shallow marine life occurred in the Western Atlantic and Caribbean. Some species survived by migrating offshore to relatively stable thermal regimes, but nearly all strictly tropical species of Florida died out. Heavy extinction ended in late Pleistocene time, after the onset of severe climatic oscillations at -0.9 Ma. Moderately heavy extinction also occurred in the Mediterranean and the North Sea. In general, shallow water benthic faunas of the North Atlantic region are impoverished today as a result of Plio-Pleistocene extinction.
Tectonic forcing has probably been responsible for persistent, longer-term climatic trends in the North Atlantic region during late Neogene time. Formation of the Isthmus of Panama slightly before 3 Ma should have increased the formation of deep water in the North Atlantic and decreased the formation of sea ice by reducing the influx of low-salinity water to the Atlantic. In OGCMs, closure of the Straits of Panama elevates temperatures in the southeastern United States, as actually happened, but fails to produce the observed general cooling at high latitudes. General circulation models suggest that late Neogene uplift of the Tibetan Plateau and other regions should have intensified meanders in westerlies and jet stream flow, producing aridification of the Mediterranean region and reduction of winter temperatures in the southeastern United States. Both predictions match actual events, although the second change is reflected in the fossil record only after mid-Pliocene cooling at high latitudes. It appears, how