A difference in the rate of warming in the Northern and Southern hemispheres could significantly alter tropical rainfall patterns, scientists say. Climatologists from the University of California, Berkeley, and the University of Washington say since the Northern Hemisphere is becoming warmer that the Southern Hemisphere, resulting shifts in precipitation patterns could increase or decrease seasonal rainfall in areas such as the Amazon, sub-Saharan Africa or East Asia, leaving some areas wetter and some drier than today. "A key finding is a tendency to shift tropical rainfall northward, which could mean increases in monsoon weather systems in Asia or shifts of the wet season from south to north in Africa and South America," UC Berkeley graduate student Andrew R. Friedman, who led the analysis, said. Growing differences in average temperatures in the hemispheres could drive the change, researchers said. "Tropical rainfall likes the warmer hemisphere," Berkeley geography Professor John Chiang said in a Berkeley release Wednesday. "As a result, tropical rainfall cares a lot about the temperature difference between the two hemispheres." Rainfall patterns fall into bands at specific latitudes, the researchers say, and a warmer Northern Hemisphere causes atmospheric overturning to weaken in the north and strengthen in the south, shifting rain bands northward. University of Washington researcher Dargan M.W. Frierson said the regions most affected by the shift are likely to be on the bands' north and south edges. "It really is these borderline regions that will be most affected, which, not coincidentally, are some of the most vulnerable places: areas like the Sahel where rainfall is variable from year to year and the people tend to be dependent on subsistence agriculture." The Sahel is a belt of semi-arid climate stretching across Africa between the Sahara desert in the north and savanna regions in the south. "We are making major climate changes to the planet and to expect that rainfall patterns would stay the same is very naive," Frierson said.