Dear EarthTalk: How are droughts and wildfires caused by global warming? I thought warming mostly brought on wet and flooded conditions.
—David Mossman, Albuquerque, NM
By throwing the planet’s climate regulation systems out of whack, global warming is likely to cause more extreme weather events of every kind, including additional precipitation and flooding in some cases and more drying and drought in others—sometimes within the same region.
According to the Union of Concerned Scientists (UCS), a leading non-profit, increased temperatures on the Earth’s surface due to global warming accelerates evapotranspiration, an otherwise natural process that takes moisture from land, plants and water bodies and moves it skyward into the atmosphere.
“In drier regions, evapotranspiration may produce periods of drought—defined as below-normal levels of rivers, lakes and groundwater, and lack of enough soil moisture in agricultural areas,” reports UCS. “Precipitation has declined in the tropics and subtropics since 1970. Southern Africa, the Sahel region of Africa, southern Asia, the Mediterranean, and the U.S. Southwest, for example, are getting drier.” Even areas that are typically wet, says the group, can experience long, dry spells between extreme rainy periods.
This drying trend is expected to continue through mid-century as the amount of land affected by drought grows significantly. Water resources in affected areas are predicted to decline by as much as 30 percent. “These changes occur partly because of an expanding atmospheric circulation pattern known as the Hadley Cell—in which warm air in the tropics rises, loses moisture to tropical thunderstorms, and descends in the subtropics as dry air,” adds UCS. “As jet streams continue to shift to higher latitudes, and storm patterns shift along with them, semi-arid and desert areas are expected to expand.”
The National Wildlife Federation (NWF) adds that the sea level rise expected to accompany global warming (as the polar ice caps melt) could further complicate matters for water-constrained areas by contaminating critical inland underground freshwater reserves with salt (so-called “saltwater intrusion”).
Another effect of unmitigated global warming will be a increased frequency of large wildfires and an expansion of burned over areas in already fire-prone regions like much of the Western U.S. NWF reports that researchers there are already noticing longer fire seasons, drier conditions persisting later into the year, and an increased frequency of lightning as thunderstorms are becoming more frequent and severe. The group adds that forest fires are expected to burn over twice as much of today’s affected areas across 11 western states by later this century if conservative predictions about warming come true.
So what can be done? NWF stresses that every one of us can play a role by cutting back on our fossil fuel use (less driving and flying, less home heating and cooling, more efficient appliances, etc.). Another way to help is to take into account our own water use and making a concerted effort to cut back and conserve this most vital of all natural resources. NWF also wants land managers and policymakers to consider global warming when choosing water management strategies to meet multiple demands and to work to protect natural forest and wetland systems that absorb flood waters and provide efficient water storage.