We study how the vertical distribution of relative humidity (RH) affects
climate sensitivity, even if it remains unchanged with warming. Using a
radiative-convective equilibrium model, we show that the climate
sensitivity depends on the shape of a fixed vertical distribution of
humidity, tending to be higher for atmospheres with higher humidity. We
interpret these effects in terms of the effective emission height of
water vapor. Differences in the vertical distribution of RH are shown to
explain a large part of the 0 to 30% differences in clear-sky
sensitivity seen in climate and storm-resolving models. The results
imply that convective aggregation reduces climate sensitivity, even when
the degree of aggregation does not change with warming. Combining our
findings with relative humidity trends in reanalysis data shows a
tendency toward Earth becoming more sensitive to forcing over time.
These trends and their height variation merit further study.