Changes in global rain patterns linked to human activity

Variations in the location and intensity of global rainfall have been linked to global warming and human activities, scientists in California say. Researchers at the Lawrence Livermore National Laboratory say observed changes in global precipitation are directly affected by human activities and their effect on world temperatures and cannot be explained by natural variability alone. Emissions of heat-trapping and ozone-depleting gases created by humans affect the distribution of precipitation through two mechanisms, they said: Increasing temperatures are expected to make wet regions wetter and dry regions drier, while changes in atmospheric circulation patterns will push storm tracks and subtropical dry zones toward the poles. "Both these changes are occurring simultaneously in global precipitation and this behavior cannot be explained by natural variability alone," study lead author Kate Marvel said. "External influences such as the increase in greenhouse gases are responsible for the changes." Natural variability caused by natural processes such as El Ninos and La Ninas does not account for the changes in global precipitation patterns from 1979 through 2012 analyzed in the study, the scientists said. While such natural fluctuations in climate can lead to either regional changes in rainfall intensity or poleward shifts in precipitation, it is very rare for the two effects to occur together naturally, they said. "In combination, man-made increases in greenhouse gases and stratospheric ozone depletion are expected to lead to both an intensification and redistribution of global precipitation," co-author Celine Bonfils said. "The fact that we see both of these effects simultaneously in the observations is strong evidence that humans are affecting global precipitation."