Effects of sea ice on atmospheric pCO2: A revised view and implications for glacial and future climates

Sea ice is a key component in the global carbon cycle and climate system. In the traditional view, the sole effect of expanded sea ice coverage is to reduce the atmospheric pCO(2) by inhibiting air-sea gas exchange. However, this view neglects the effect that sea ice capping has on the biological production. By limiting light for photosynthesis, larger sea ice coverage would reduce the strength of the biological pump and therefore increase atmospheric pCO(2). Recently, Kurahashi-Nakamura et al. (2007) suggested that the opposing impact of biology on atmospheric pCO(2) will more than offset the gas exchange effect, such that atmospheric pCO(2) will actually increase with larger sea ice coverage. In an effort to resolve this controversy, we use an intermediate-complexity, global model of biogeochemistry and climate to determine the sensitivity of atmospheric CO2 concentration to changes in the sea ice coverage, driven by prescribed changes in sea ice albedo. When sea ice in our model is increased by 34% globally relative to the control run, gas solubility, ice capping effect and stratification increase, while biological production decreases; overall atmospheric pCO(2) is reduced by 9.4 ppmv. Our results broadly support the notion that the biological response of sea ice capping is as important as its physical response. Furthermore, we show that the overall change in atmospheric pCO(2) is indeed inversely related to sea ice coverage, but it is not because sea ice caps off gas exchange but because gas solubility is increased by lower temperatures that accompany sea ice expansion in our model simulations.