  We document an intriguing property of zonal-mean surface warming in
3,000-year general circulation model (GCM) simulations under instantaneous 2,
4, 8, or 16xCO2 increases: across CO2 magnitudes and time periods from the
first decade to the thirtieth century, the hemispherically symmetric,
global-mean-normalized response is quasi-invariant. For a given CO2 value, the
hemispherically antisymmetric component evolves in time from initially weak
Southern Ocean warming and strong Arctic warming to comparably polar-amplified
warming in the two hemispheres. For a given time period, the mean-normalized,
full response (i.e. including both symmetric and antisymmetric components) is
remarkably consistent across CO2 values. Simulations in a moist energy balance
model (MEBM) with radiative feedbacks diagnosed from the 4xCO2 GCM simulation
at different timescales reproduce this quasi-invariant mean-normalized
symmetric warming component. This also holds in complementary MEBM simulations
with symmetrized inputs, indicating little role for covarying antisymmetric
components of the radiative feedback parameter and temperature response. In
800-year GCM simulations wherein CO2 is unchanged but cloud albedo is
artificially modified in different latitude bands, the mean-normalized,
symmetric temperature response is again nearly time-invariant, but its
meridional structure is influenced by the meridional structure of the imposed
albedo perturbations. The non-normalized temperature response at high
latitudes, however, depends more on the value of forcing imposed locally than
on its meridional structure.
