Global precipitation patterns are being moved in new directions by climate change, a new study has found.

The research, published yesterday in the journal Proceedings of the National Academy of Sciences, is the first study to find the signal of climate change in global precipitation shifts across land and ocean.

"It's worth saying that this is another grain of sand on that vast pile of evidence that climate change is real and is occurring," said study co-author Kate Marvel, a climate scientist at Lawrence Livermore National Laboratory.

Climate models predict that the addition of heat-trapping gases in the atmosphere will shift precipitation in two main ways. The first shift is in a strengthening of existing precipitation patterns. This is commonly called "wet get wetter, dry get drier."

Warmer air traps more water vapor, and scientists expect that additional water to fall in already wet parts of the Earth.

"But because precipitation has to be balanced by evaporation, we expect a [corresponding] increase in dry regions," Marvel said.

The second shift is a change in storm tracks, which should move away from the equator and toward the poles as atmospheric circulation changes.

Wrestling with natural variability
Scientists have looked for these changes in rainfall patterns, but they are often difficult to distinguish because there is so much natural variability in precipitation.

For example, El Niño generally makes wet regions wetter and dry regions drier, so when scientists see that happening, it has been difficult to say whether it is climate change or simply El Niño.

"It's really hard to tease out that signal," Marvel said.

What Marvel and her co-author, Celine Bonfils, were able to do in this new study was look at both the expected shifts together.

"What we did in the study is say, OK, people have looked for one or the other in the observations, to see if it's happening. What if we look for both happening at the same time?" Marvel said.

Doing this was useful for the research because, while El Niño makes wet areas wetter, it also contracts storm tracks toward the equator. That's the opposite of what would happen due to climate change.

So looking for both increased rainfall in wet areas and a shift in storm tracks away from the equator helped the researchers separate the signal of climate change from the noise of natural variability.

"If you find wet get wetter, dry get drier, occurring increasingly in tandem with poleward expansion, there's just almost no way that can happen naturally," Marvel said.

Finding a signal amid the 'noise'
Similar to many studies that try to tease out climate change's signal, the researchers compared observed data with climate models.

The models were run without the influence of greenhouse gases, so that the scientists could compare those results with the observations from 33 years of satellite precipitation data.

The models tell the researchers whether it is at all likely that natural variations, without the influence of greenhouse gases, could have produced the rainfall patterns and shifts in storm tracks the satellites have observed.

When the answer to that question is no, then the greenhouse gases are implicated as the culprit in changing how precipitation is falling worldwide.

Gabi Hegerl, a climate researcher at the University of Edinburgh who was familiar with the work, called it "very innovative."

"As precipitation is so important, this is a very useful finding and an exciting method," she said.

Francis Zwiers, a climate scientist and director of the Pacific Climate Impacts Consortium who had seen the research presented, agreed.

"I think this paper strengthens, substantially, the available body of literature that we have concerning the question of whether humans are having an effect on precipitation at the global scale," Zwiers wrote in an email.

Marvel noted the findings were not a surprise, but rather a confirmation of what scientists have long said would be the effects of climate change. "It would have been a much more surprising and exciting result if we had found that this wasn't happening," she said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500