Exploring mechanisms for the decrease of atmospheric CO₂ during the last glacial maximum using a four-box ocean model

Abstract

A record of carbon dioxide for the last 400,000 years revealed that atmospheric CO2 decreased from a pre-industrial concentration of 280 ppmV to approximately 200 ppmV during the last glacial maximum (Petit et al., 1999). Several hypotheses as to why this happened have been suggested yet no one explanation has been able to fully account for this decrease. Deep water is the main sink for carbon in the ocean through the biological pump, where the organic matter and CaCO3 shells of dead surface biota sink. The stored carbon in the deep ocean is 'aired' in the southern polar ocean, where large amounts of deep water are upwelled to the surface. Stephens and Keeling (2000) have proposed that if the southern polar ocean was covered with ice during the last glacial maximum, this would have prevented carbon stored in the deep ocean to be released into the atmosphere, thereby reducing the concentration of atmospheric CO2. Stephens and Keeling (2000) created a six-box ocean to test this hypothesis and were able to produce a 67 ppmV decrease of atmospheric CO2 from the pre-industrial concentration when only the gas exchange between the southern polar ocean and the atmosphere was limited. Based on the Toggweiler and Sarmiento (1985) three-box ocean model, a four-box ocean model that splits the Toggiweiler and Sarmiento polar ocean box in to a northern and southern component was created. The four-box ocean model examined the sensitivity of atmospheric CO2 to limitations in the airsea gas exchange for the southern polar ocean. The four-box ocean was able to produce seventy percent of Stephens and Keeling's decrease in atmospheric CO2 when the air-sea gas exchange was limited in the southern polar ocean. In addition, the four-box ocean model calculated carbon-14 concentrations in the ocean, which provide a useful constraint on model results that was not presented in the Stephens and Keeling model. The atmospheric carbon dioxide in the four box model was found to be more sensitive to increasing biological productivity in the southern polar ocean than to the growth of the Antarctic ice sheet.