Centennial-scale changes in the global carbon cycle during the last deglaciation

摘要

Global climate and the concentration of atmospheric carbon dioxide (CO_2) are correlated over recent glacial cycles. The combination of processes responsible for a rise in atmospheric CO_2 at the last glacial termination (23,000 to 9,000 years ago), however, remains uncertain. Establishing the timing and rate of CO_2 changes in the past provides critical insight into the mechanisms that influence the carbon cycle and helps put present and future anthropogenic emissions in context. Here we present CO_2 and methane (CH_4) records of the last deglaciation from a new high-accumulation West Antarctic ice core with unprecedented temporal resolution and precise chronology. We show that although low-frequency CO_2 variations parallel changes in Antarctic temperature, abrupt CO_2 changes occur that have a clear relationship with abrupt climate changes in the Northern Hemisphere. A significant proportion of the direct radiative forcing associated with the rise in atmospheric CO_2 occurred in three sudden steps, each of 10 to 15 parts per million. Every step took place in less than two centuries and was followed by no notable change in atmospheric CO_2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO_2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO_2 associated with the glacial termination, given the strong covariance of CO_2 levels and Antarctic temperatures. Our data establish a contribution from an abrupt, centennial-scale mode of CO_2 variability that is not directly related to Antarctic temperature. We suggest that processes operating on centennial timescales, probably involving the Atlantic meridional overturning circulation, seem to be influencing global carbon-cycle dynamics and are at present not widely considered in Earth system models.