Warmer air allows for more water vapor. So scientists have long predicted that global warming will result in a more intense water cycle—the process by which water evaporates from the oceans, travels through the atmosphere and then falls as rain. Now new measurements of the ocean's salinity prove that prediction—and suggest that global warming strengthens the water cycle even more than anticipated.
"What we found is that regions that are salty in the main are becoming saltier" and areas that boast more rainfall are getting fresher, explains oceanographer Paul Durack of Lawrence Livermore National Laboratory, who led the research to be published in Science on April 27. "It's another independent estimate of how the climate is changing as we pump out CO2."
Durack and his colleagues from Australia looked at ocean salinity measurements from roughly 3,500 robot buoys—collectively known as Argo—that have been deployed since 2000. The saltiness of seawater reveals whether, on balance, more rain falls in that region than water evaporates or vice versa. But acquiring a truly global picture of ocean salinity was hampered in the past by weather. "You're quite a lunatic if you go south [in a research vessel] when some of those storms are brewing in the Southern Ocean," Durack notes.
The Argo floats don't have that problem, riding out storms underwater for the five or so years of their useful lives. Using a decade's worth of data from Argo, Durack and his colleagues created an understanding of ocean salinity in all seasons. Extending that knowledge to the millions of historical salinity readings from 1950 to 2000 reveals that, as predicted, evaporation strengthened in drier regions—making the oceans saltier there—and rainfall strengthened in wetter regions—making the surface seawater fresher. In fact, global warming of roughly 0.5 degree Celsius over that span strengthened the water cycle by roughly 4 percent, or around the amount predicted by physicists' equations. "Through a lot of investigation, it provides us with what we would have guessed," Durack says.
But that's not what scientists' computer models predicted. Though various computer models match the actual distribution of saltier and fresher regions quite well, the computer models underestimated the actual rate of water cycle intensification by half. That suggests that impacts on the water cycle of future warming of several degrees Celsius could be substantial, strengthening rainfall and evaporation by as much as 24 percent.
The stronger water cycle will mean stronger rains and intensified droughts over the oceans. And what holds at sea also extends over land, because the oceans cover 71 percent of the globe, hold 97 percent of the world's water, and have already sucked up 90 percent of the extra heat trapped by humanity's greenhouse gas pollution. The increase in warm water vapor as the globe heats will fuel more violent storms, whereas droughts in places like Australia, stuck in the middle of oceanic regions dominated by evaporation, will only grow more severe. Or as Durack says: "wet regions will get wetter and dry regions drier." Given that freshwater availability affects everything from food to energy, this may prove a more difficult challenge for human civilization and natural ecosystems to adapt to than that posed by global warming itself.