  Climate models simulate a strong land-ocean contrast in the response of
near-surface relative humidity to global warming: relative humidity tends to
increase slightly over oceans but decrease substantially over land. Surface
energy balance arguments have been used to understand the response over ocean
but are difficult to apply over more complex land surfaces. Here, a conceptual
box model is introduced, involving moisture transport between the land and
ocean boundary layers and evapotranspiration, to investigate the decreases in
land relative humidity as the climate warms. The box model is applied to
idealized and full-complexity (CMIP5) general circulation model simulations,
and it is found to capture many of the features of the simulated changes in
land relative humidity. The box model suggests there is a strong link between
fractional changes in specific humidity over land and ocean, and the greater
warming over land than ocean then implies a decrease in land relative humidity.
Evapotranspiration is of secondary importance for the increase in specific
humidity over land, but it matters more for the decrease in relative humidity.
Further analysis shows there is a strong feedback between changes in
surface-air temperature and relative humidity, and this can amplify the
influence on relative humidity of factors such as stomatal conductance and soil
moisture.
