Climate feedbacks derived from theory and spatial contrasts in recent
climatology
Abstract
Climate feedbacks determine how much surface temperatures will
eventually warm to balance anthropogenic radiative forcing, but remain
difficult to constrain. The climate feedback due to some process X is
defined as the partial derivative of outgoing radiation at the top of
the atmosphere with respect to surface temperature following a change in
X, λX=-∂Rout/TS|X, with total climate feedback a summation from
all processes, λtotal=∑λX. Standard approaches evaluate climate
feedbacks from finite temporal changes in surface temperatures and
outgoing radiation, following observed or simulated perturbations to
climate state. However, this introduces significant linear combination
error (λtotal≠∑λX) when the applied perturbation is large enough to
achieve a good signal-to-noise ratio. This study presents a new
semi-empirical evaluation of non-cloud climate feedbacks, constrained
instead by spatial variation in outgoing radiation and climate state.
First, we observationally constrain functional relations for outgoing
radiation over ocean and land in terms of surface temperature, pressure,
relative humidity, the height of the tropopause, fractional clound
amount and latitude. Then, these functional relations are differentiated
with respect to surface temperature to calculate the climate feedbacks
for infinitesimal perturbation, eliminating linear combination error at
high signal-to-noise ratio. We find, when combined with a recent cloud
feedback estimate, a present-day total climate feedback of -0.99 (-0.75
to -1.22 at 66% range) Wm-2K-1. Our method is independent of temporal
variation approaches to evaluate climate feedback allowing Bayesian
combination to further reduce uncertainty.