Hydrology
Rainfall affected by climate change

Global climate change will likely cause significant changes in the world’s rainfall patterns, according to researchers working on the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report summary, released Feb. 2 (see Geotimes online, Web Extra, Feb. 2, 2007). The report provides the first estimates of just how and where the landscape will change due to rainfall.

“The places that are wet are going to get wetter, and the places that are dry are going to get drier,” says Susan Solomon, a senior scientist at NOAA and a co-chair on the new report. Northern areas, such as Canada and Norway, will become more Mediterranean — hot, dry summers and cool, wet winters — while regions now considered Mediterranean will see less rainfall overall, researchers concluded.

These precipitation changes stem from the combination of increased liquid water available as the two major ice sheets in Antarctica and Greenland melt, followed by evaporation that puts more moisture into the atmosphere. With more water in the atmosphere, rainfall increases worldwide.

Only higher latitudes, however, will benefit from that increased rainfall: Though more moisture is retained in the atmosphere everywhere, evaporation in the lower latitudes will counter it, says Andrew Weaver, a climatologist at the University of Victoria, also an author on the report. Thus, he says, the subtropics become drier, and the higher latitudes become wetter.

Indeed, models have converged on a “rich get richer” distribution of precipitation change, so the wet areas in middle and higher latitudes get wetter, whereas subtropics get drier, says Philip Mote, a climatologist at the University of Washington. And we are already seeing drying in much of the subtropics, from Mexico to the Mediterranean, Southern Africa and Australia, Solomon adds. The IPCC report describes longer, more intense droughts over wider areas since the 1970s, particularly in the tropics and subtropics, caused by rising temperatures and decreased rainfall. “I’m particularly disturbed,” Mote says, “about the projections for Mexico and North Africa, where models are generally agreeing on big reductions.”

The other issue with changes in precipitation patterns is that the greatest changes will come in winter and spring, when rainfall would traditionally have been heaviest in many parts of the world, Weaver says. If precipitation does increase dramatically in certain seasons rather than spread out through the year, as models suggest, he says, much of that water will either cause flooding, or simply drain straight out to sea instead of being able to collect in reservoirs for future human use (see Geotimes, March 2006).

Although scientists have long thought that warming temperatures would cause the global water cycle to intensify, meaning a possible exacerbation of extremes such as droughts and floods, too many questions remained unanswered in models to actually point to long-term effects. But with advances in modeling since the first IPCC assessment in 1990, model projections are now closely matching observed patterns, Solomon says.

“Rainfall prediction is now where temperature prediction was in 1990,” Solomon says. At the time of the 1990 report, she says, the observed warming was just beginning to stray beyond past observations of typical, normal temperature variability. Now, a similar pattern with rainfall is beginning to emerge, she says, as rainfall begins to fall outside the norms of variability.

Along with precipitation changes, the report predicts an increase in storms such as typhoons and hurricanes. “Storms take heat from the subtropics to higher latitudes,” Weaver says. “When you warm up the subtropics, you evaporate more water, and you’re taking that water northward, so the rains are bigger events.”

Sally Adee
Geotimes contributing writer

Links:
"Climate report points finger at fossil fuels," Geotimes online, Web Extra, Feb. 2, 2007
"Global Water Supply Takes Some Heat," Geotimes, March 2006

Back to top