Climate sensitivity and the direct effect of carbon dioxide in a limited-area cloud-resolving model
Romps, David — Lawrence Berkeley National Laboratory
Area of research
In this study, cloud-resolving simulations are shown to exhibit a dramatic increase in equilibrium climate sensitivity (ECS) at higher temperatures, due to both an increase in forcing and a decrease in the feedback parameter. In addition, it is found that the direct effect of carbon dioxide offsets a quarter of the warming-induced increase in precipitation, reduces the shallow cloud fraction by a small amount, and has no impact on convective available potential energy (CAPE).
Shallow clouds play an important role in the current climate through their enhancement of Earth’s albedo, so their direct response to elevated CO2 (often referred to as a “rapid adjustment”) and their response to warming are both of great interest. This study is the first to use a cloud-resolving model to map out the direct effect of CO2 on clouds and to show that equilibrium climate sensitivity (the warming you get from a doubling of CO2) increases dramatically with temperature (i.e., successive doublings cause greater warming).
Even in a small domain, it can be prohibitively expensive to run cloud-resolving greenhouse-gas warming experiments due to the long equilibration time. Here, a technique is introduced that reduces the computational cost of these experiments by an order of magnitude: instead of fixing the carbon-dioxide concentration and equilibrating the sea-surface temperature (SST), this technique fixes the SST and equilibrates the carbon-dioxide concentration. Using this approach in a cloud-resolving model of radiative-convective equilibrium (RCE), the equilibrated SST is obtained as a continuous function of carbon dioxide concentrations spanning 1 ppmv to nearly 10,000 ppmv, revealing a dramatic increase in equilibrium climate sensitivity (ECS) at higher temperatures. This increase in ECS is due to both an increase in forcing and a decrease in the feedback parameter. In addition, the technique is used to obtain the direct effects of carbon dioxide, i.e., the rapid adjustments, over a wide range of SSTs. Overall, the direct effect of carbon dioxide offsets a quarter of the increase in precipitation from warming, reduces the shallow cloud fraction by a small amount, and has no impact on convective available potential energy (CAPE).