Abrupt changes in the global carbon cycle during the last glacial period

Abstract

During the last glacial period, atmospheric carbon dioxide (CO2) closely followed Antarctic temperature on millennial timescales. This strong correlation between Antarctic climate and atmospheric CO2 has led to suggestions that reorganizations of Southern Ocean circulation and/or biogeochemistry were the dominant cause of these variations. However, recent work also revealed centennial-scale changes in CO2 that appear unrelated to Antarctic climate and may represent additional modes of carbon cycle variability. Here we present a high-resolution CO2 record from the last glacial period from an ice core drilled in West Antarctica. This reconstruction precisely defines the timing of millennial and centennial CO2 variations with respect to Antarctic temperature and abrupt changes in Northern Hemisphere climate during Heinrich stadials and Dansgaard–Oeschger events. On the millennial scale, CO2 tracks Antarctic climate variability, but peak CO2 levels lag peak Antarctic temperature by more than 500 years. Centennial-scale CO2 increases of up to 10 ppm occurred within some Heinrich stadials, and increases of ~5 ppm occurred at the abrupt warming of most Dansgaard–Oeschger events. Regression analysis suggests that the CO2 variations can be explained by a combination of one mechanism operating on the timescale of Antarctic climate variability and a second responding on the timescale of Dansgaard–Oeschger events. Consistent with our statistical analysis, carbon cycle box-model simulations illustrate a plausible scenario where Southern Hemisphere processes contribute the majority of the CO2 variability during the last glacial period, but Northern Hemisphere processes are the crucial drivers of centennial-scale variability. Southern Hemisphere processes largely set Antarctic climate during the last glacial, though events in the Northern Hemisphere strongly impacted short, centennial-scale changes, according to an analysis of high-resolution carbon dioxide and temperature records from an Antarctic ice core.