Dual modes of the carbon cycle since the Last Glacial Maximum

The most conspicuous feature of the record of past climate contained in pol ar ice is the rapid warming which occurs after long intervals of gradual co oling. During the last four transitions from glacial to interglacial condit ions, over which such abrupt warmings occur, ice records indicate that the CO2 concentration of the atmosphere increased by roughly 80 to 100 parts pe r million by volume (refs 1-4). But the causes of the atmospheric CO2 conce ntration increases are unclear. Here we present the stable-carbon-isotope c omposition (delta(13)CO(2)) Of CO2 extracted from air trapped in ice at Tay lor Dome, Antarctica, rom the Last Glacial Maximum to the onset of Holocene times. The global carbon cycle is shown to have operated in two distinct p rimary modes on the timescale of thousands of years, one when climate was c hanging relatively slowly and another when warming was rapid, each with a c haracteristic average stable-carbon-isotope composition of the net CO2 exch anged by the atmosphere with the land and oceans. delta(13)CO(2) increased betiveen 16.5 and 9 thousand years ago by slightly more than would be estim ated to be caused by the physical effects of a 5 degrees C rise in global a verage sea surface temperature driving a CO2 efflux from the ocean, but our data do not allow specific causes to be constrained.