Editor's commentsThis is the final paper reporting the results of analyses of ice cores drilled at the Vostok station in Antarctica. The record extends back 420 thousand years. From the ice cores it is possible to determine CO2, CH4, and N2O concentrations and isotopic composition which can be used to estimate air temperature. The record extends over four glacial-interglacial cycles. The results show a strong correlation between air temperature and atmospheric green-house gases (Co2 and CH4). The authors suggest that in all four cases Milankovitch cycles possibly with changes in local solar radiation initiate the glacial terminations which are amplified by a rise in CO2 concentrations and then by reduced reflection (decreased albedo) and increased absorption of solar radiation. Considering the large gas-age/ice age uncertainty (1,000 years or more), the authors say it is premature to infer whether the increase in CO2 concentration occurs before or after the air temperature increase at the start of glacial terminations.

The late Quaternary period (the past one million years) is punctuated by a series of large glacial–interglacial changes with cycles that last about 100,000 years. Glacial-interglacial climate changes are documented by complementary climate records largely derived from deep sea sediments, continental deposits of flora, fauna and loess, and ice cores. These studies have documented the wide range of climate variability on Earth. They have shown that much of the variability occurs with periodicities corresponding to that of the precession, obliquity and eccentricity of the Earth’s orbit1. But understanding how the climate system responds to this initial orbital forcing is still an important issue in palaeoclimatology, in particular for the generally strong 100,000-year (100-kyr) cycle.

Ice cores give access to palaeoclimate series that includes local temperature and precipitation rate, moisture source conditions, wind strength and aerosol fluxes of marine, volcanic, terrestrial, cosmogenic and anthropogenic origin. They are also unique with their entrapped air inclusions in providing direct records of past changes in atmospheric trace-gas composition.

Figure Vostok time series and insolation. Series with respect to time (GT4 timescale for ice on the lower axis, with indication of corresponding depths on the top axis) of
  • a CO2
  • b isotopic temperature of the atmosphere
  • c CH4
  • d atmospheric delta-O18
  • e mid-June insolation at 65 degrees N (in Wm-2)

Sequence of events in last four glacial terminations

Properties change in the following sequence during each of the last four glacial terminations, as recorded in Vostok. First, the temperature and atmospheric concentrations of CO2 and CH4 rise steadily, whereas the dust input decreases. During the last half of the temperature rise, there is a rapid increase in CH4. This event coincides with the start of the delta-oxygen-18 (δ 18O) decrease. The authors believe that the rapid CH4 rise also signifies warming in Greenland, and that the deglacial delta-oxygen-18 decrease records rapid melting of the Northern Hemisphere ice sheets.

Figure Vostok time series during glacial terminations. Variations with respect to time (GT4) of
  • dust
  • temperature (proxy is delta-deuterium)
  • CO2
  • CH4
  • delta-O18-atm
These results suggest that the same sequence of climate forcing operated during each termination:
  • orbital forcing (with a possible contribution of local insolation changes) followed by two strong amplifiers,
  • greenhouse gases acting first, then
  • deglaciation and ice-albedo feedback.
Considering the large gas-age/ice age uncertainty (1,000 years or more), the authors say it is premature to infer the sign of the phase relationship between CO2 and temperature at the start of terminations.

The authors point out an intriguing feature of the deglacial CH4 curves is that the atmospheric concentration of CH4 rises slowly, then jumps to a maximum value during the last half of the deglacial temperature rise.

Present-day atmospheric burdens of CO2 and CH4 seem to have been unprecedented during the past 420,000 years.
NATURE, VOL 399, 3 JUNE 1999