The future will be hot, at least, hotter than the present. But temperature isn’t the only thing that will change with global warming — so will the water cycle. Some places will become drier, some wetter. In fact, some have already become wetter.



One of the “places” that’s already wetter is the atmosphere. It contains about 4% more water vapor worldwide than it did just a few decades ago, which has a profound effect on climate:

It’s a direct result of global warming: warmer air tends to hold more water vapor. It’s also one of the main feedbacks in global warming, since water vapor is also a greenhouse gas, so increased water vapor due to global warming will amplify global warming.

The global atmosphere is one thing — but most of us live on land, and in a single location. The GHCN doesn’t just store historical temperature data, it also offers historical precipitation data. There are about 2500 observing locations covering the continental U.S., and they’re located here:

I took the monthly precipitation and computed anomaly (its departure from the mean for the same month). Then I combined the anomaly data since 1900, for all stations in each very large grid, 5 deg. latitude tall and 10 deg. longitude wide, covering most of the continental U.S. Then I smoothed the combined precipitation anomaly data, to get an idea how precipitation may have trended over the last century-and-a-decade-and-a-year. This is certainly an imperfect procedure, since precipitation may not show the same degree of long-distance spatial correlation that temperature does (and for other reasons too). Nonetheless, it’s at least a first glance at what the data may be telling us — an exploratory analysis, but hardly a definitive one.

And here’s the result, graphing the smoothed time evolution for each grid on top of the map of station locations (click the graph for a larger, clearer view):

For each grid box, the bottom is an anomaly of -10 mm/month average, the top is an anomaly of +20 mm/month.

It surprised me that so many of the grid boxes show a recent upward trend. This is particular true for New England, and much of the Midwest.

It’s also worth noting that precipitation isn’t the only factor is how wet or dry a region is. As temperatures rise evaporation will increase, so some areas may become desertified even if their precipitation doesn’t decline. But precipitation is certainly a major factor! Some parts of the U.S. already show signs of change, and I regard that as trouble. I like the way weather has been distributed throughout my lifetime — we’re adapted to it. I expect that, unfortunately, both the places that get wetter and those that get drier will be ill prepared for the change.

And of course, changes in precipitation in one region can have a profound impact on other regions. Just ask those who live near the Mississippi River.