The last 800,000 years of Earth’s climate is characterized by a sequence of glacial and interglacial cycles with an approximate period of 100,000 years. Understanding what causes these alternating cycles of warm and cold periods remains an important issue in palaeoclimatology. In January 1998, a collaborative ice-drilling project between Russia, the United States, and France at the Russian Antarctica Vostok research station revealed that over the past four cycles the atmospheric concentration of carbon dioxide correlates closely with Antarctic air-temperature. In particular during glacial terminations the correlation is too close to determine whether one precedes the other. The analysis also suggests a sequence of events that is characteristic of every termination; warming by orbital forcing, then increasing greenhouse gases, and followed by ice sheet melting and reduced albedo.
Glacial-interglacial climate changes are documented by climate records largely derived from deep sea sediments, continental deposits of flora, fauna and loess, and ice cores. A 1976 study by Hays, Imbrie and Shackleton revealed that some of the variation in Earth’s climate occurs with periodicities corresponding to small variations in the Earth’s orbit that change the amount of solar radiation reaching the Earth at high Northern latitudes. Called orbital forcing these changes have periods of about 41,000 and 26,000 years. However, that study and a more recent 1999 study by Raymo has shown that the observed dominant 100,000 glacial/deglacial cycle cannot be explained by orbital forcing alone. Ice sheet dynamics, greenhouse gas feedback, atmospheric dust, and other mechanisms have been suggested to account for the 100,000 year cycles.
Through proxies ice cores provide very granular data about atmospheric dust, local temperature (using delta-D as a proxy), and precipitation temperature (delta-18O as a proxy). In addition air bubbles trapped in the ice provide a direct record of past changes in atmospheric gas composition including carbon dioxide and methane. The most striking result of comparing these properties over the past 420,000 years from the Vostok ice cores is the very close correlation between changes in atmospheric temperature and CO2 concentration. The time resolution of the analytics methods is about 1,000 years. At this level of resolution the correlation between changes in temperature and CO2 concentration is too close to determine whether one precedes the other.
Comparison of atmospheric properties during last four glacial terminations
The other interesting result are parallels during the periods of rapid warming that correspond to shrinking ice sheets and sea level rise. At the end of the four deglaciations atmospheric temperature and concentrations of CO2 and CH4 rise steadily. At the same time the amount of dust deposited on the surface of Antarctica decreases. During the last half of the temperature rise, there is a rapid increase in CH4 concentration. This event coincides with the start of the delta-18O decrease. The authors suggest that the rapid CH4 rise signifies warming in Greenland, and that the delta-18O decrease reflects rapid melting of the Northern Hemisphere ice sheets. The authors suggest that the same sequence of climate forcing events operated during each termination. First solar warming caused by orbital forcing, which is amplified by growing greenhouse gas concentrations. This is followed by shrinking ice-sheets and ice-albedo feedback, decreasing reflection and greater absorption of solar radiation by the atmosphere as ice sheets shrink.
Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, J. R. Petit, J. Jouzel, D. Raynaud, N. I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V. M. Kotlyakov, M. Legrand, V. Y. Lipenkov, C. Lorius, L. PÉpin, C. Ritz, E. Saltzman & M. Stievenard, Nature 399, pages 429–436 (1999)