Natural climate fluctuations and changes in vegetation due to widespread human activities like grazing, deforestation and agriculture can make regions hotter and drier or cooler and wetter, say scientists.

Nearly 100 studies in recent years support the claim that improving the vegetation in drylands regions may cause significant cooling in some of the world's hottest regions. In the eastern U.S., changes in land use may be overwhelming all other human effects, according to Gordon Bonan of the National Center for Atmospheric Research in Boulder, Colo.

Among these studies' findings:

• Temperatures along the Mexican-American border between the states of Arizona and Sonora can be more than 8 degrees Celsius cooler on the undergrazed U.S. side than the overgrazed Mexican side.

• The mean temperature on lands between the 10th and 50th northern latitudes warmed between 1901 and 1990 at a rate equal to 0.4 degrees C per century (see figure).

• The warming rate has been greater in areas designated by the United Nations as overgrazed, 0.6 degrees C per century; whereas areas the U.N. designates as "not overgrazed" have warmed at a rate of only 0.3 degrees C per century.

• Conversely, in the eastern U.S., changes in vegetation has resulted in a 1.0 degree C cooling.

Thus some scientists say international efforts to rehabilitate vegetation on drylands, such as the Sahelian or sub-Saharan region of Africa, could significantly cool areas thought to be hotter due to global warming.