Atmospheric CO₂ seasonality and the air-sea flux of CO₂

Abstract

Abstract. The amplitude, phase, and form of the seasonal cycle of atmospheric CO2 concentrations varies on many time and space-scales (Peters et al., 2007). Intra-annual CO2 variation is primarily driven by seasonal uptake and release of CO2 by the terrestrial biosphere (Machta et al., 1977; Buchwitz et al., 2007), with a small (Cadule et al., 2010), but potentially changing (Gorgues et al., 2010) contribution from the ocean. Variability in the magnitude, spatial distribution, and seasonal drivers of terrestrial Net Primary Productivity (NPP) will be induced by, amongst other factors, anthropogenic CO2 release (Keeling et al., 1996), land-use change (Zimov et al., 1999) and planetary orbital variability, and will lead to changes in CO2atm seasonality. Here I describe two separate mechanisms by which co-variability of the seasonal cycles in atmospheric CO2 concentration, ocean temperature, and sea-ice extent could potentially lead to rapid changes in the air-sea flux of CO2 at high latitudes. One mechanism responds to an increase in CO2atm seasonality by pumping CO2 into the ocean, and the other by releasing CO2 from the ocean (in a relative sense). The relative importance of the two mechanisms is determined by the seasonal extent of sea-ice, the net sign of their operation may therefore have interesting implications for glacial-interglacial and future climate change. To capture the described feedbacks within earth system models, CO2atm concentrations must be allowed to evolve freely, forced only by anthropogenic emissions, rather than prescribed CO2atm concentrations. The decision to prescribe CO2atm concentrations within model simulations for the fifth IPCC climate assessment (Taylor et al., 2009) may therefore result in an underestimation of changes in marine CO2 sources and sinks.