The question of when the current interglacial period will end and a new glacial period will begin is of paramount importance when attempting to predict the future of Earth's climate.

How does the present climate compare to the climate during other interglacials? Are current temperatures, sea levels, and carbon dioxide (CO2) concentrations unprecedented, or should they be considered within the expected range? When, if ever, is the current interglacial going to end? These are among the important questions that climate science seeks to answer. Ice cores, loess deposits, pollen analysis, and other data and techniques provide information that comes together to form a detailed picture that will help answer them.

Over the last three million years, there have been forty to fifty glacial/interglacial cycles. Scientists have sought to characterize conditions during these periods using a variety of approaches. Plant remains, particularly pollen, have been analyzed to estimate temperature and humidity. The carbonate shells of planktonic marine organisms, preserved as fossils in the sediments beneath the sea, have been analyzed to infer sea surface temperatures. It is difficult to separate regional effects from global ones, and often data from different time periods is only available in different locations, so acquiring a global picture is not easy.

In general, interglacial climate is seen as being quite similar to the current climate. In fact, the term is often restricted to periods during which temperatures were at least as high as they have been during the Holocene, the name given to the current interglacial.

Cores have been drilled out of the ice in Greenland and Antarctica. Within the ice are bubbles of air that have been preserved since the ice was formed (Luthi, 2008). The deepest core, from an area called Dome C in Antarctica, has samples of air from 800,000 years ago.

These core samples provide detailed information on eight complete glacial cycles. Ratios of oxygen istopes O18 to O16 are used to infer how much of the Earth's water was tied up in glacial ice, since glaciers sequester O16 and thereby increase the proportion ofO18 in the ocean. The concentration of deuterium (H2) in a sample, moreover, can be used to infer the temperature of the air when the snow formed.

Age is determined by combining depth, snow accumulation rates, ice flow rates, compaction rates, and so on, in a complex but reproducible way and calibrating the results by using radiometric dating techniques on volcanic dust incorporated in the ice. Dust is examined in detail, and often its place of origin can be determined. While no two cycles are identical, they all share a number of traits.

Each cycle has a saw-tooth shape. A glacial advance ends abruptly, with rapid melting of the ice and a rapid rise in air temperatures. For example, in MIS 9, the temperature rose 13œ Celsius in eight thousand years. This warming can stop abruptly, with cooling starting almost immediately, or it can taper off, warming at a slower rate for perhaps a dozen millennia, before cooling begins. Of the eight previous terminations in the Dome C core, four started cooling immediately and four tapered off. Once cooling begins, it is far more gradual than is warming, continuing, with some reversals, for about 100,000 years. Ice is sequestered as cooling occurs, and CO2 levels fall.

The air temperatures in Antarctica were higher than the average for the current millennium during each of the last four interglacials, but lower during those more than 500,000 years old. The Holocene (MIS 1) began 11,700 years ago. The three most recent interglacials before the Holocene (MIS 5, 7, and 9) had durations of about twelve thousand years or less, so simply by looking at the ice core data one might guess that the next glacial advance may be imminent (Ruddiman, 2008). However, a theory proposed by Milutin Milankovi6 suggests things may not be this simple.

Periodic variations in some of the orbital parameters of the Earth are known to control the timing of glacial cycles. One past interglacial during which those variations were similar to today's was MIS 11, which lasted for twenty-eight thousand years and had warming taper off after an early rapid rise. However, MIS 19, which also had similar orbital variations, saw cooling right after its initial warming and was of shorter duration. Perhaps the Milankov cycles are similar to lit matches thrown into the woods. When they are thrown will determine the timing of the fires produced, but the scale and intensity of the fires are dependent on how much fuel is available, how dry it is, the weather, relative humidity, and a host of other factors.

The Earth is currently in an interglacial state and has been in that state for nearly twelve thousand years. Geology and oceanography have shown that over the last three million years the Earth has switched between glacial and interglacial states some forty to fifty times. With no anthropogenic influence, one would conclude that continental glaciers will advance again, the only question being when. In the presence of anthropogenic influences, however, it is not clear that the Earth will return to another glacial state.

Although there is some discussion over whether the orbital conditions for triggering a glacial advance have already occurred or are yet to occur, it is still prudent to examine the conditions on the planet today to evaluate the risk posed by another glacial advance. Of particular interest is why, during times of elevated CO2 and elevated temperatures, former interglacials succumbed to the minor fluctuations of solar energy produced by those orbital conditions.

References

1. Luthi, Dieter, et al. "High-Resolution Carbon Dioxide Concentration Record 650,000-800,000 Years Before Present." Nature 453, no. 7193 (2008): 379-82.

2. Ruddiman, William F. Earth's Climate Past and Future. 2d ed. New York: W. H. Freeman, 2008.

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