Sack Lunch Seminar (SLS)
SLS --- Ric Williams (University of Liverpool)
|September 13th, 2017
"Mechanisms controlling how global-mean surface warming depends on cumulative carbon emissions"
Insight into how to avoid dangerous climate may be obtained from Earth system model projections, which reveal a near-linear dependence of global-mean surface warming on cumulative carbon emissions. This dependence of surface warming on carbon emissions is interpreted using theory and a framework based on a product of three terms: the dependence of surface warming on radiative forcing, the fractional radiative forcing from CO2 and the dependence of radiative forcing from CO2 on carbon emissions. Mechanistically each of these dependences varies, respectively, with ocean heat uptake, the CO2 and non-CO2 radiative forcing, and the ocean and terrestrial uptake of carbon. An ensemble of 9 Earth System models forced by up to 4 Representative Concentration Pathways are diagnosed up until year 2100. In all cases, the dependence of surface warming on carbon emissions evolves primarily due to competing effects of heat and carbon uptake over the upper ocean: there is a reduced effect of radiative forcing from CO2 due to ocean carbon uptake, which is partly compensated by enhanced surface warming due to a reduced effect of ocean heat uptake. Over the next decades, there are large inter-model differences in the sensitivity in surface warming to carbon emissions, which are mainly due to uncertainties in the climate sensitivity and ocean heat uptake. These uncertainties undermine our ability to predict how much carbon may be emitted before reaching a warming target.