Unit 12: Earth's Changing Climate // Section 5: Global Cooling: The Pleistocene Epoch
During the Pleistocene epoch, which began about 2 million years ago, Earth's average temperature has always been cold enough to maintain ice at high latitudes. But Pleistocene climate has not been constant: ice coverage has fluctuated dramatically, with continental ice sheets advancing and retreating over large parts of North America and Europe. These peak glacial periods are often referred to as "Ice Ages" or "Glacial Maxima." During the Pleistocene, Earth has experienced more than 30 swings between prolonged glacial periods and brief warmer interglacial phases like the one we live in today.
Glacial advances and retreats shaped Earth's topography, soils, flora, and fauna (Fig. 7). During glaciation events, huge volumes of water were trapped in continental ice sheets, lowering sea levels as much as 130 meters and exposing land between islands and across continents. These swings often changed ocean circulation patterns. During the most extreme cold phases, ice covered up to 30 percent of the Earth's surface.
Figure 7. Pleistocene glacial deposits in Illinois
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Source: Courtesy Illinois State Geological Survey.
As glaciers advanced and retreated at high latitudes, ecosystems at lower latitudes evolved to adapt to prevailing climate conditions. In North America, just south of the advancing glaciers, a unique type of grass steppe supported distinctive cold-adapted fauna dominated by large mammals such as the mammoth, woolly rhinoceros, and dire wolf.
Why did Pleistocene temperatures swing back and forth so dramatically? Scientists point to a combination of factors. One main cause is variations in Earth's orbit around the sun. These variations, which involve the tilt of the Earth's pole of rotation and the ellipticity of the Earth's orbit, have regular timescales of 23,000, 41,000, and 100,000 years and cause small changes in the distribution of solar radiation received on the Earth (footnote 5). The possibility that these subtle variations could drive changes in climate was first proposed by Scottish scientist James Croll in the 1860s. In the 1930s, Serbian astronomer Milutin Milankovitch developed this idea further. Milankovitch theorized that variations in summer temperature at high latitudes were what drove ice ages—specifically, that cool summers kept snow from melting and allowed glaciers to grow.
However, changes in summer temperature due to orbital variations are too small to cause large climate changes by themselves. Positive feedbacks are required to amplify the small changes in solar radiation. The two principal feedbacks are changes in Earth's albedo (the amount of light reflected from the Earth's surface) from snow and ice buildup, and in the amount of CO2 in the atmosphere.
Ice core samples from the Vostok station and the European Project for Ice Coring in Antarctica (EPICA) document that CO2 levels have varied over glacial cycles. From bubbles trapped in the ice, scientists can measure past concentrations of atmospheric CO2. The ice's chemical composition can also be used to measure past surface temperatures. Taken together, these records show that temperature fluctuations through glacial cycles over the past 650,000 years have been accompanied by shifts in atmospheric CO2. GHG concentrations are high during warm interglacial periods and are low during glacial maxima. The ice cores also show that atmospheric CO2 concentrations never exceeded 300 parts per million—and therefore that today's concentration is far higher than what has existed for the last 650,000 years (Fig. 8).
Figure 8. Vostok ice-core CO2 record
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Source: © Jean-Marc Barnola et al., Oak Ridge National Laboratory.
One important lesson from ice cores is that climate change is not always slow or steady. Records from Greenland show that throughout the last glacial period, from about 60,000 to 20,000 years ago, abrupt warming and cooling swings called Dansgaard-Oeschger, or D-O, events took place in the North Atlantic. In each cycle temperatures on ice sheets gradually cooled, then abruptly warmed by as much as 20°C, sometimes within less than a decade. Temperatures would then decline gradually over a few hundred to a few thousand years before abruptly cooling back to full glacial conditions. Similar climate fluctuations have been identified in paleoclimate records from as far away as China.
These sharp flips in the climate system have yet to be explained. Possible causes include changes in solar output or in sea ice levels around Greenland. But they are powerful evidence that when the climate system reaches certain thresholds, it can jump very quickly from one state to another. At the end of the Younger Dryas—a near-glacial phase that started about 12,800 years ago and lasted for about 1,200 years—annual mean temperatures increased by as much as 10°C in ten years (footnote 6).