Abstract
Uncertainty in climate feedbacks is the primary source of spread in
model projections of surface temperature response to anthropogenic
forcing. Cloud feedback persistently appears as the main source of
disagreement in future projections while the combined lapse-rate plus
water vapor (LR+WV) feedback is a smaller (~30%), but
non-trivial source of uncertainty in climate sensitivity. Here
observation-based emergent constraints are adopted to evaluate the
intermodel spread in these feedbacks. The observed interannual variation
provides a useful constraint on the long-term cloud feedback as
evidenced by the consistency between their global-mean values as well as
their similar regional contributions to the intermodel spread. However,
internal variability does not serve to constrain the long-term LR+WV
feedback spread, which we find is mostly associated with the relative
humidity response over the tropics. Model differences in hemispheric
warming asymmetries, induced primarily by ocean heat uptake differences,
also contribute to the spread in water vapor feedback.