The Pleistocene Ice Ages

After the end of the PETM, temperatures fell, ultimately bottoming out during the Pleistocene Ice Ages, which began 1.8 million years ago and ended 10,000 years ago. The Pleistocene was not a time of relentless cold: Glaciers advanced and retreated many times. At the height of the most recent glacial advance, between 18,000 and 22,000 years ago, glaciers covered much of Eurasia and North America, from New York City northward. Average global temperatures were about 10°F (5.5°C) colder, and sea level was about 395 feet (125 m) lower than today. The low sea level exposed the Bering land bridge, allowing humans and large animals to migrate from Asia into North America. The bountiful forests south of the ice sheets were home to giant ice age mammals such as cave bears, saber-toothed cats, and wooly mammoths.

During the warm periods, known as interglacials, temperatures were more than 2°F (1.1°C) higher, and sea level was about 16 feet (4.8 m) higher than today. CO2 was higher than during the glacial periods but never rose above 300 ppm. Interglacial periods lasted about 10,000 years (although one of them lasted as long as 27,000 years). Even the interglacial periods were broken up by relatively short cold spells. CO2 was stable at or below 280 ppm for at least 400,000 years.

— Temperature in degrees centigrade (compared with 350- 1960-1990 baseline)

— Atmospheric carbon dioxide (COz in ppm)

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400,000 300,000 200,000 100,000 Years before present

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400,000 300,000 200,000 100,000 Years before present

CO2 and temperature show the same pattern in the Vostok ice core from Antarctica over the past 400,000 years. Temperature and CO2 are high during interglacial periods and low during glacial periods. CO2 does not drive the initial rise in temperature during an interglacial, but it is a major contributor later. The rise in CO2 since 1958 has been picked up by the Mauna Loa monitoring station; temperature has not kept up with CO2.

The glacials and interglacials of the Pleistocene were caused by the Milankovitch and other natural cycles. Greenhouse gas levels also played a role. At the beginning of each glacial advance, CO2 and methane plunged and then resurged at the end. Ice cores from Greenland and Antarctica exhibit CO2 values that are 30% lower during glacial periods than during interglacial periods.

Climate changed quickly during the Pleistocene, with rapid transitions between glacial and interglacial periods. One especially dramatic temperature change took place early in the interglacial period that began 12,700 years ago. At about 10,500 years ago, as glaciers were retreating, the warming trend suddenly reversed. Temperatures in parts of the Northern Hemisphere fell as much as 20°F (11°C) in as little as 10 to 100 years. The summit of Greenland was 27°F (15°C) colder, and Great Britain was 9°F (5°C) colder than they are now. This climatic period, which lasted about 1,400 years, is called the Younger Dryas. At the end of the Younger Dryas, temperatures returned to normal in only about 10 years.

Such rapid and dramatic cooling was likely the result of a massive influx of freshwater from North America. When an enormous lake of glacial melt water that was held back by an ice dam was breached, freshwater flooded the North Atlantic. The freshwater was light and floated on the sea surface, shutting down thermohaline circulation. As a result, warm equatorial waters were stopped from flowing northward.

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