Science Source

How closely do changes in surface and column water vapor follow Clausius–Clapeyron scaling in climate change simulations?

Analyzes the factors governing the rate of change in the amount of atmospheric water vapor in simulations of climate change
Estimates the global-mean amount of water vapor to increase at a differential rate of 7.3% K − 1 with respect to global-mean surface air temperature in the multi-model mean
Finds larger rates of change result if the fractional change is evaluated over a finite change in temperature (e.g., 8.2% K − 1 for a 3 K warming), and rates of change of zonal-mean column water vapor range from 6 to 12% K − 1 depending on latitude
Finds there are deviations from Clausius–Clapeyron scaling of zonal-mean column water vapor in the tropics and mid-latitudes, but they largely cancel in the global mean
Finds surface specific humidity increases at a rate of 5.7% K − 1, considerably lower than the rate for global-mean water vapor
Finds surface specific humidity closely follows Clausius–Clapeyron scaling over ocean, but there are widespread decreases in surface relative humidity over land (by more than 1% K − 1 in many regions), and it is argued that decreases of this magnitude could result from the land/ocean contrast in surface warming