Climate-model evaluation of the contribution of sea-surface temperature and carbon dioxide to the Middle Miocene Climate Optimum as a possible analogue of future climate change

Abstract

The Middle Miocene Climate Optimum (MMCO), which occurred at about 15 Ma, is the most recent global warming episode. Given the fact that no dramatic tectonic movement had taken place, this historical warming episode mirrors the present warming event induced mostly by human activities. Proxy data indicate that the MMCO had a global mean surface temperature of ∼3–4°C higher than the present, equivalent to the warming predicted for the next century by the mid-range scenarios of the IPCC Fourth Report (AR4). With this comparable magnitude of warming, it is therefore of scientific interest to examine whether or not the present warming is similar to the MMCO warming. Since the MMCO boundary conditions such as paleogeography and paleobathymetry were not greatly different from today, contentious scientific issues on possible forcing mechanisms can be assessed. The MMCO climate was simulated using the latest National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.1 and Land Model CLM3.0 coupled to a slab ocean model. Two simulations were conducted, the MG700 with a near-present equator–pole meridional sea-surface temperature (SST) gradient and a CO2 level of 700 ppmv and the HG350 with a higher equator–pole meridional SST gradient and a CO2 level of 350 ppmv. These two simulations give the closest proxy global mean surface temperature of 18.4°C with a difference of ±0.6°C. This study analyses these two simulations and provides an insight into possible mechanisms of the MMCO. Results show that the north and south poles respond differently to a 50% reduction in CO2 from the upper bound value of 700 ppmv (an approximate doubling of the present CO2) and a change in equator–pole meridional SST gradient. The Arctic response to a 50% reduction in CO2 is balanced by a 50% decrease in surface temperature but shows no response to the increase in equator–pole meridional SST gradient. The Antarctic situation is opposite to the Arctic: the western Antarctic's response to the increase in the equator–pole meridional SST gradient is nearly balanced by a doubling of the maximum surface temperature, but shows no response to a 50% reduction in CO2; the eastern Antarctic shows a weak response to either a 50% reduction in CO2 or to an increase in the equator–pole meridional SST gradient. This may explain why the Arctic warms much more than the Antarctic in the MMCO and the decoupling of CO2 with temperature as determined by the proxy SST.