A lowering effect of reconstructed Holocene changes in sea surface temperatures on the atmospheric CO2 concentration

Abstract

One of the mechanisms proposed to explain the roughly 20 ppmv increase in atmospheric CO2 concentration from the early to late Holocene is a warming of the ocean surface that reduces CO2 solubility (Indermühle et al., 1999). Here we show that this hypothesis is not supported by reconstructed changes in sea surface temperatures (SSTs) because of an inhomogeneous distribution of SST changes across the globe during the course of Holocene. While alkenone‐based SST reconstructions compiled in the GHOST database (Kim et al., 2004; Kim and Schneider, 2004) suggest a net warming of the surface ocean on a global scale by 0.2 ± 0.2°C, both data and model results support a significant cooling trend for the North Atlantic during the last 8000 years. In response to the reconstructed cooling of the North Atlantic by 1.1 ± 0.2°C, a zonally averaged model of oceanic biogeochemistry simulates a drawdown of atmospheric CO2 by 7 ± 0.8 ppmv, while a reconstructed warming of the Pacific Ocean by 0.6 ± 0.4°C counterbalances this effect by about 1 ppmv. On a global scale, this model simulates a lowering of atmospheric CO2 from the Holocene to pre‐industrial times by 6 ± 2 ppmv due to changes in SSTs, while more complex, three‐dimensional biogeochemistry model indicates a moderate decrease by 1 ppmv after 300 years of the model integration. Our study suggests that changes in SSTs may have altered atmospheric CO2 in a direction opposite to the observed trend and that other mechanisms, presumably related to the changes in carbonate chemistry, could be responsible for the CO2 increase during the Holocene.