Title: Can climate sensitivity be constrained by current observations?
Presenter: Kate Marvel
 
What changes should we expect in response to human emissions of greenhouse gases?  How hot is the 
Earth really going to get?  Equilibrium climate sensitivity (ECS) is a purely theoretical but useful metric of 
climate change, defined as the planet's long-term response to the abrupt doubling of atmospheric carbon 
dioxide.  The ECS range arising from CMIP models has remained stubbornly large despite many advances 
in climate modeling and computing power.  It has been suggested in recent years that satellite and in situ 
observations of the recent past may help to narrow this range.  However, the usefulness of this exercise 
depends on whether the present-day climate is predictive of the far future climate.  In this talk, I'll 
discuss the reasons the present may be dissimilar to equilibrium climate in the CMIP5 models.  Models 
run in AMIP mode with observed SSTs yield lower inferred ECS than models run over the same time 
period in unconstrained "historical" mode, which in turn yield lower inferred ECS values than models run 
for 140 years after abrupt CO2 increase.  The inter-model spread in AMIP sensitivity estimates is 
explained by model differences in tropical marine low cloud cover.  However, we find no evidence that 
model skill in representing the low cloud changes in response to observed SSTs is related to long-term 
climate changes. Our results suggest a need for caution in any attempt to use current climate conditions 
to constrain behavior in the long term.