"It has long been known that climate models predict that the amount of water vapour goes up at roughly the same rate as the water vapour holding capacity, corresponding to constant relative humidity," Paul O'Gorman of MIT told environmentalresearchweb. "But the relative humidity is not exactly constant in simulations, and we wanted to ascertain exactly how much these small changes in relative humidity affected the rates of changes in the total amount of water vapour, and how much the rates of change varied regionally. We also wanted to compare the changes in the amount of water vapour near the surface and in the whole atmosphere."

By using climate models from the World Climate Research Programme's Coupled Model Intercomparison Project (CMIP), O'Gorman and colleague Caroline Muller calculated the rates of change in the amount of atmospheric water vapour between the last 20 years of the 20th century and the last 20 years of the 21st century under the A1B emissions scenario.

"We showed that the systematic changes in relative humidity had almost no effect on the rate of change of global water vapour in climate-model simulations of global warming, even though they mattered locally," said O'Gorman. "We also showed that near-surface water vapour increased at a smaller fractional rate than total water vapour, and that there were widespread decreases in near-surface relative humidity over land. The changes near the surface over land may be related to the amplified warming over land compared with over ocean – roughly 50% greater."

While it's useful to know the rates of change in the amount of surface- and column-water vapour for a number of reasons, write the researchers in ERL, related quantities, such as precipitation rates and cloud liquid water amounts, may have a different thermodynamic dependence. One exception is the case of tropical precipitation extremes, where precipitation rates will roughly scale with surface-specific humidity.

"The decreases in near-surface relative humidity over land under global warming may be important for the hydrological cycle and warming rate over land," said O'Gorman. "We will further study their implications for the climate system."

The researchers reported their work in ERL, in a Focus Issue on "Anticipated changes in the global atmospheric water cycle".