Scientist Link Global Precipitation Changes to Human Activities

Scientists from Lawrence Livermore National Laboratory in California report that observed changes in global precipitation cannot be accounted for by natural variability alone and are a direct consequence of human activities.

The researchers, who published their work in the Proceeding of the National Academy of Sciences, report that the emissions from heat-trapping and ozone-depleting gasses affect the global distribution of precipitation though two climate mechanisms - thermodynamic change and changes in atmospheric circulation patterns - the former of which is known for making wetter regions wetter and drier regions drier, and the later is associated with pushing storm tracks and subtropical dry zones towards the poles.

"Both these changes are occurring simultaneously in global precipitation and this behavior cannot be explained by natural variability alone," said Kate Marvel the study's lead author. "External influences such as the increase in greenhouse gases are responsible for the changes."

By comparing their climate model prediction with more than three decades worth of global observations from the Global Precipitation Climatology Project, the team concluded that natural variability, such as the El Niño and La Niña weather patterns, do not account for changes in global precipitation patterns.

Furthermore, the researchers said, it is very rare for natural fluctuations in climate to lead to both the poleward shift and intensification of precipitation, which is what the global precipitation model suggests is happening.

"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," said Céline Bonfils, a study author also from Lawrence Livermore. "The fact that we see both of these effects simultaneously in the observations is strong evidence that humans are affecting global precipitation."

The researchers were able to identify a distinguishable "fingerprint" pattern that characterizes the concurrent weather events.

"Most previous work has focused on either thermodynamic or dynamic changes in isolation. By looking at both, we were able to identify a pattern of precipitation change that fits with what is expected from human-caused climate change," Marvel said.

Bonfils added that by focusing their large-scale analysis on the underlying mechanisms that drive global precipitation changes,"we have shown that the changes observed in the satellite era are externally forced and likely to be from man."