Man-made pollutants may be causing the expansion of Earth\'s tropical belt northward toward the pole, U.S. researchers say. A study led by climatologist Robert J. Allen at the University of California, Riverside, suggests black carbon aerosols and tropospheric ozone, emitted predominantly in the Northern Hemisphere\'s low- to mid-latitudes, are most likely pushing the boundary of the tropics further poleward in that hemisphere. Such tropical belt expansion would affect large-scale atmospheric circulation, especially in the subtropics and mid-latitudes, researchers said. \"If the tropics are moving poleward, then the subtropics will become even drier,\" Allen said in a UCR release Wednesday. \"If a poleward displacement of the mid-latitude storm tracks also occurs, this will shift mid-latitude precipitation poleward, impacting regional agriculture, economy, and society.\" The tropics have widened by 0.7 degrees latitude per decade since 1979, caused by warming from greenhouse gases in both the Northern and Southern Hemispheres, researcher said. \"Both black carbon and tropospheric ozone warm the tropics by absorbing solar radiation,\" Allen said. \"Because they are short-lived pollutants, with lifetimes of one-two weeks, their concentrations remain highest near the sources: the Northern Hemisphere low- to mid-latitudes. \"The question to ask is how far must the tropics expand before we start to implement policies to reduce the emissions of greenhouse gases, tropospheric ozone and black carbon that are driving the tropical expansion?\" Allen said.