AP Photo/Rick Gunn, Nevada Appeal
A dying piñon pine tree on the Pinenut Range of East Carson City, Nevada.
The last few decades have seen larger areas of the world enduring longer—and drier—droughts. What's more, many of those droughts have been hotter than in the past.
Why is this happening? It's because warming is altering the water cycle—hotter soils lose their moisture faster, intensifying drought conditions. But heat also leads to more evaporation from the ocean. That evaporation transfers huge amounts of energy into the atmosphere and feeds more moisture into rain and snow. As a result, some areas will experience more intense rainfall and others will be left drier than before.
Based on changes in rain- and snowfall, temperature, and evaporation rate, areas of North and South America and northern Eurasia have become wetter over the past century. Other regions, including parts of Africa, are now significantly drier.
The 1930s drought that turned the southern American Great Plains into what we now call the Dust Bowl was an example of a climate pattern—driven by sea-surface temperatures in the Atlantic and Pacific—that had always been typical of the region. Now, warming may make such droughts more frequent and more intense.
But even in the first part of the twentieth century, humans made the situation worse. By the early 1900s, homesteaders had plowed the Plains deeply, replacing hardy, drought-adapted grasses with drought-sensitive strains of corn. In past droughts dead grasses had formed a tough protective mat over the soil. In the 1930s, the plowed soil simply dried up and blew away.
Life on the Edge
The Sahel is a semiarid region stretching from one side of Africa to the other, just south of the Sahara. The starkest drying trends on the planet have been measured there; during the drought of 1972 to 1984, nearly 100,000 people died, and 750,000 more would have perished without food aid. It is difficult to link individual extreme climatic events like the Sahelian drought directly to climate change, but such events illustrate the sort of problems climate change will create.
Many hardy piñon pines (Pinus edulis) of the American Southwest had survived droughts of the 1950s and earlier, yet 40 to 80 percent died in the drought of 2003. Why? Because this drought was hotter, so soils were even drier than in the past. Bark beetles then infested the stressed trees, which killed them.
Studying Tree Rings to Learn About Global Climate
By taking biopsy-like samples from centuries-old Siberian pines, scientists have reconstructed a 300-year record of temperature changes for the Arctic and the Northern Hemisphere.