|Fig. 1: The Vostok Research Station. Source: Wikimedia Commons (Courtesy of the U.S. National Oceanic and Atmospheric Administration)|
An astounding amount of information about the history of our planet lies embedded deep within its ice. Dust and ash, sea salt, traces of radioactive isotopes, and tiny bubbles of air preserve clues about ancient climatological conditions.  Perhaps the most critical insight, attributed to Danish geophysicist Willi Dansgaard, is that the ratio of oxygen isotopes in the ice can be used to infer the temperature when it formed. By extracting ice cores from Greenland, Dansgaard was able to map out temperature fluctuations dating back thousands of years. His discoveries won him the highest honors in environmental science and gave rise to the essentially new field of paleoclimatology. Today, researchers are continuing his work at ice sources around the world. 
In light of contemporary concerns about rising global temperatures, it is more important than ever to understand how the climate has evolved historically. One of Dansgaard's most famous results, the discovery of the so-called Dansgaard-Oeschger event that bears his name, has attracted attention as a potential factor in climate change. A DO event consists of a period of rapid warming, typically over the span of a few decades, followed by a more gradual cooling. At least twenty such events are thought to have occurred over the course of the last glacial (about 120,000 to 10,000 years ago) at a rate of roughly one event every 1,470 years.  Although no comparably massive temperature changes have occurred over the past few millennia, some evidence suggests this effect may have continued well beyond the end of the last glacial, just at a much lower amplitude. The "little ice age" from the 16th to the 18th century might even be the most recent cold phase.  While this would imply that we are not yet due for a warm phase, the DO cycle could conceivably be interacting with anthropogenic factors today, complicating efforts to predict climate change. 
Accounting quantitatively for the periodicity of DO events has proved difficult, and it remains an active and controversial subject of research. Teams using different methods to date the ice have tended to reach dissimilar conclusions; furthermore, the strength of the effect seems to depend on how the data is analyzed: results have ranged from 95% confidence in the periodicity to claims that the apparent cycle can be entirely attributed to statistical noise. [1,4] Those scientists who believe the phenomenon is real have proposed a wide variety of explanations for it, both internal and external to the climate system itself. The most recent analysis suggests that the cycle is far too regular to have originated within the Earth system.  Rather, it seems that external, astrophysical phenomena might have been responsible for the abrupt temperature changes.
Perhaps the most intriguing piece of climate data to emerge from the ice is the apparent correlation between certain atmospheric gases, including carbon dioxide and methane, and global temperatures. [1,5] This result has not been perfectly straightforward to interpret, as carbon dioxide levels have consistently peaked after the temperature peaks, not before. While the carbon dioxide therefore must not have triggered the warming episodes, they may have contributed to their spread and severity. [5,6] However, without a precise understanding of the climate's sensitivity to these gas levels, it is difficult to predict in detail how our present day climate will respond to artificially altered levels of carbon dioxide. The plentiful data in the ice indicates the many ways in which our knowledge of natural climate change is expanding, but until these climatological processes are fully understood and disentangled from one another, we cannot claim to know precisely how they will interact with anthropogenic effects.
 S. Rahmstorf, "Timing of Abrupt Climate Change: A Precise Clock," Geophys. Res. Lett. 30, 1510 (2003).
 D. Martin, "Willi Dansgaard Dies at 88; Read Climates in Old Ice," New York Times, 28 Jan 11.
 M. Schulz, "On the 1470-Year Pacing of Dansgaard-Oeschger Warm Events," Paleoceanography 17, 1014 (2002).
 P. D. Ditlevsen, K. K. Andersen and A. Svensson, "The DO-Climate Events Are Probably Noise Induced: Statistical Investigation of the Claimed 1470 Years Cycle," Climate of the Past, 3, 1289 (2007).
 C. Lorius et al., "The Ice-Core Record: Climate Sensitivity and Future Greenhouse Warming," Nature 347, 139 (1990).
 N. Caillon et al., "Timing of Atmospheric CO2 and Antarctic Temperature Changes Across Termination III," Science 299, 1728 (2003).