The Intergovernmental Panel on Climate Change (IPCC) says that up to one--third of forests have already been affected by climate change. On average, forest productivity has increased slightly. Thriving and expanding forests absorb more CO2, which is a negative feedback for global warming. Of course, enough tropical forests are being cut down that this positive effect is more than made up for.

Regional warming has had a devastating effect on forests in the Southwestern United States. Temperature increases coupled with a multiyear drought have been blamed for the largest loss of trees in a single location ever recorded. More than 45 million Piñon pines (Pinus cembroides), a stubby, nut-bearing tree, have died in New Mexico, where the plant is the state tree. The direct cause is the Piñon bark beetle (Ips confuses), but scientists say that higher temperatures are really to blame. This is because the spread of beetles is slowed by frost, but now that there are fewer frosts, the beetles are able to move into areas that were once inhospitable. Bark beetles are in Arizona's ponderosa pine forests, in Utah's spruces, and in Colorado's Douglas firs. In Alaska and British Columbia, 14 million acres (56,660 square km) of spruce have been killed by bark beetles.

"It's the type of thing we can expect more of with global warming," Professor David Breshears of the University of Arizona told National Geographic News in December 2005. "There is reason to believe other systems could get whacked the way the Southwest did."

Other diseases are ravaging western forests. Warmer temperatures allow the mountain pine beetle (Dendroctonus ponderosae) to complete its life cycle in one year rather than its previous two. Consequently, the beetles have increased in population, which has resulted in an increase in pine blister rust (Cronartium ribicola) in Rocky Mountain forests. The rust is a fungus transmitted by the beetles. Trees that are damaged or killed by bark beetles or pine blister rust are far more susceptible to the spread of forest fires.

Due to high ocean temperatures in the Caribbean and Atlantic basins and to ongoing deforestation of the Amazon rain forest, in 2005 the Amazon basin began the longest and worst drought since record keeping began. In some areas, water levels have dropped so low that the communities that depend on streams for transportation are completely isolated. Crops rot because they cannot be transported to market, and children cannot get to school. People living on the world's largest river are unable to find fresh water to drink. Fish die in the shallow water due to lack of oxygen, killing freshwater dolphins and other predators, and forcing people to depend on government food packages. Because streams also remove human waste, when the streams dry up, the resulting sewage backup raises fears of cholera and other waterborne illnesses. In the remaining stagnant pools, mos--quitoes breed in increasing numbers, which has the potential to raise malaria levels in local populations.

Longer--term drought causes trees to die and become fuel for fires, which the Amazon has recently experienced. In the Western United States, warmer temperatures and earlier springs have caused an increase in the number, duration, and destructiveness of wildfires since the mid-1980s. Studies show that the cause of the fires was

Trees killed by bark beetle in the San Bernardino Mountains of California. The trees were weakened by drought, ozone, and years of fire exclusion and ecological change. (CDF-California Department of Forestry)

increased spring and summer temperatures and earlier snowmelt. Similar changes in wildfire increases have been seen in other parts of the Americas.

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