More evidence turned up this week indicating that climate change impacts are already underway—this time in rainfall patterns. It's pretty hard to clearly link climate change to individual droughts, like the summer of 2012 in the United States, or specific storms, like Hurricane Haiyan that devastated the Philippines last week. These events are driven by a complex set of factors, including natural variations. But new research that tracked a broad look at precipitation patterns found that they have already shifted beyond the bounds of natural variations.

Typically, precipitation change is the afterthought in climate predictions—temperatures increases are carefully projected by models, but precipitation patterns are more complex and subject to more natural variation. We know things will change, but we're not sure exactly how—that seems to be the general answer to important questions about storm frequency and droughts.

To study these complex patterns on a global scale, scientists first needed to smooth the noise of natural variation from the data while holding on to the key patterns. This required some rather involved statistics.

A team of scientists from Lawrence Livermore National Laboratory used winter precipitation data from 1979-2012 and compared it to the precipitation shifts predicted by climate change models. They focused on two variables. The first is a phenomenon they call the thermodynamic changes—basically an intensification of the existing hydrogeological cycle, where increasing temperatures cause dry regions to get drier and wet regions to get wetter. The second factor they looked at was what they call dynamic changes, which involve the shifting of major circulation patterns towards the north and south poles. Basically, as the globe gets warmer, the tropics expand in both directions from the equator.

The variability in both of these metrics can be quite high, but these natural variations have a key feature: they don't occur in sync. In other words, if tropical weather patterns expand a bit northward one year, it shouldn't increase the odds of an intensified water cycle. By analyzing the two together, the researchers showed that external forces (mostly greenhouse gases and ozone depletion) are pushing both variables beyond their normal range at the same time.

This the first study to show that globally, precipitation has begun to shift in the ways that climate models predict, which seems like bad news, given that the predictions mean more severe droughts and intensified storms. But there is some good news. When it comes to the impact of ozone levels on precipitation, human actions, such as banning chemicals or reducing emissions, can have a meaningful impact on the pace of the planet's change.

PNAS, November 2013 DOI: 10.1073/pnas.1314382110