A rainy day in Chicago (Bernt Rostad via Flickr/Creative Commons)

A rainy day in Chicago (Bernt Rostad via Flickr/Creative Commons)

A new study shows changes in precipitation are affected by human activities and can’t be explained by natural phenomena, such as El Niños and La Niñas.

Scientists from the Lawrence Livermore National Laboratory in California published the study in the Proceedings of the National Academy of Sciences.

The researchers suggest two mechanisms will probably cause changes in the distribution and intensity of precipitation worldwide because of human release of greenhouse gases that trap heat and deplete the ozone.

First, there are some thermodynamic changes, caused by the increasing global temperatures, which will likely result in already wet regions of the world getting wetter while the dry areas will become drier.

The researchers also think increased temperatures could change global atmospheric circulation patterns – movement of air at all levels of the atmosphere – which might move storm tracks and push current 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,” said Kate Marvel, the study’s lead author. “External influences such as the increase in greenhouse gases are responsible for the changes.”

To reach their conclusions, the researchers compared various climate model predictions with global observations from 1979-2012 that were provided by the Global Precipitation Climatology Project.

(Flóra Soós via Flickr/Creative Commons)

(Flóra Soós via Flickr/Creative Commons)

The scientists found natural climate phenomena alone couldn’t explain the ongoing changes in global precipitation patterns. They also noted that any fluctuations in climate brought on by natural causes could either intensify or shift precipitation towards the poles, but it’s very rare for both to take place together naturally.

“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, another co-author of the study. “The fact that we see both of these effects simultaneously in the observations is strong evidence that humans are affecting global precipitation.”

The researchers said their studies helped them identify a “fingerprint pattern” that can explain the simultaneous changes in precipitation locations and intensity brought on by external forces such as warming caused by human activities.

“We have shown that the changes observed in the satellite era are externally forced and likely to be from man,” Bonfils said.