Kvale, Karin Frances , Turner, Katherine, Keller, David P. and Meissner, Katrin (2018) Asymmetric dynamical ocean responses in warming icehouse and cooling greenhouse climates. Environmental Research Letters, 13 (12). Art.Nr. 125011. DOI 10.1088/1748-9326/aaedc3.

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Abstract

Warm periods in Earth's history tend to cool more slowly than cool periods warm. Carbon cycle feedbacks play a major role in these dynamics, from the slower rate of recovery of ocean carbon export production, to the slower re- establishment of geosphere carbon reservoirs, relative to rates of loss. Here we explore one- differences in how the global ocean takes up and gives up heat and carbon in forced rapid warming and cooling climate scenarios. We force an intermediate- complexity earth system model using two atmospheric CO2 scenarios. A ramp-up (1% per year increase in atmospheric CO2 for 150 years) starts from an average global CO2 concentration of 285 ppm to represent warming of an icehouse climate. A ramp- down (1% per year decrease in atmospheric CO2 for 150 years) starts from an average global CO2 concentration of 1257 ppm to represent cooling of a greenhouse climate. Atmospheric CO2 is then held constant in each simulation and the model is integrated an additional 350 years. The ramp-down simulation shows a weaker response of surface air temperature to changes in radiative forcing relative to the ramp-up scenario. This weaker response is due to a relatively large and fast release of heat from the ocean to the atmosphere. This asymmetry in heat exchange in cooling and warming scenarios exists mainly because of differences in the response of the ocean circulation to forcing. In the ramp-up, increasing stratification and weakening of meridional overturning circulation slows ocean carbon and heat uptake. In the ramp-down, cooling accelerates meridional overturning and deepens vertical mixing, accelerating the release of carbon and heat stored at depth. Though idealized, our experiments offer insight into differences in ocean dynamics in icehouse and greenhouse climate transitions.

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