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

Abstract

Global climate and the concentration of atmospheric carbon dioxide (CO2) are correlated over recent glacial cycles. The combination of processes responsible for a rise in atmospheric CO2 at the last glacial termination (23,000 to 9,000 years ago), however, remains uncertain. Establishing the timing and rate of CO2 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 CO2 and methane (CH4) 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 CO2 variations parallel changes in Antarctic temperature, abrupt CO2 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 CO2 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 CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO2 associated with the glacial termination, given the strong covariance of CO2 levels and Antarctic temperatures. Our data establish a contribution from an abrupt, centennial-scale mode of CO2 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.

Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

“…This resolution allows us to delineate isotopic fingerprints of rapid shifts in CO 2 that were previously impossible to resolve. Our study complements recent precise observations of CO 2 concentration variations during the last deglaciation, which revealed abrupt centennial-scale changes (13) (Fig. 2).…”

“…During the initial 35-ppm CO 2 rise from 17.6 to 15.5 ka, we find a 0.3‰ decrease in δ 13 C-CO 2 that is interrupted by a sharp minimum coincident with rapid increases in CO 2 and CH 4 around 16.3 ka (13,14) (Fig. 2).…”

“…2). The 16.3-ka feature in the CO 2 and CH 4 concentration records, which corresponds to a 0.1‰ negative excursion in δ 13 C-CO 2 , has been plausibly tied to the timing of Heinrich event 1 (13,14) and signals a mode switch in the deglacial Significance Antarctic ice cores provide a precise, well-dated history of increasing atmospheric CO 2 during the last glacial to interglacial transition. However, the mechanisms that drive the increase remain unclear.…”

“…(14) and discrete CO 2 (blue) (13) concentration data plotted with Taylor Glacier CO 2 and δ 13 C-CO 2 data (this study) (red markers, black line is a smoothing spline), the five-point running Keeling intercept with shading indicating the R 2 for each time interval. Blue bars indicate intervals of rapid CO 2 rise identified in the WAIS Divide ice core (13).…”

Abrupt North Atlantic circulation changes in response to gradual CO2 forcing in a glacial climate state

“…The freshwater perturbation can be considered to represent North Atlantic (NA) meltwater input associated with the surface mass balance of the surrounding ice sheets and/or freshwater injection associated with ice-surging events during Heinrich Stadials. The atmospheric CO 2 concentration varies gradually between 185 ppm and 245 ppm at a rate of 0.05 ppm yr −1 , representative of the observed rate of CO 2 changes during the last deglaciation 26 . This set-up provides a surrogate for Heinrich stadialinterstadial transitions during glacial periods (especially during the last deglaciation) to test the robustness of the simulated changes in experiment CO2_Hys.…”