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Climate Change Has Intensified the Global Water Cycle

Climate scientists have been saying for years that one of the many downsides of a warming planet is that both droughts and torrential rains are both likely to get worse. That’s what climate models predict, and that’s what observers have noted, most recently in the IPCC’s report on extreme weather, released last month. It makes physical sense, too. A warmer atmosphere can absorb more water vapor, and what goes up must come down — and thanks to prevailing winds, it won’t come down in the same place.

The idea of changes to the so-called hydrologic cycle, in short, hangs together pretty well. According to a new paper just published in Science, however, the picture is flawed in one important and disturbing way. Based on measurements gathered around the world from 1950-2000, a team of researchers from Australia and the U.S. has concluded that the hydrologic cycle is indeed changing. Wet areas are getting wetter and dry areas are getting drier. But it’s happening about twice as fast as anyone thought, and that could mean big trouble for places like Australia, which has already been experiencing crushing drought in recent years.

More than 3,000 robotic profiling floats provide crucial information on upper layers of the world's ocean currents. Credit: Alicia Navidad/CSIRO.

The reason for this disconnect between expectation and reality is that the easiest place to collect rainfall data is on land, where scientists and rain gauges are located. About 71 percent of the world is covered in ocean, however. “Most of the action, however, takes place over the sea,” lead author Paul Durack, a postdoctoral fellow at Lawrence Livermore National Laboratory, said in a telephone interview.  In order to get a more comprehensive look at how water is exchanged between the surface and the atmosphere, that’s where Durack and his colleagues went to look.

Nobody has rainfall data from the ocean, so Durack and his collaborators looked instead at salinity — that is, saltiness — in ocean waters. The reasoning is straightforward enough. When water evaporates from the surface of the ocean, it leaves the salt behind. That makes increased saltiness a good proxy for drought. When fresh water rains back down on the ocean, it dilutes the seawater, so decreased saltiness is the equivalent of a land-based flood.

Fortunately, as the scientists make clear, research ships have been taking salinity measurements for decades in most of the planet’s ocean basins, so it’s possible to see where and how fast salinity has been changing. And it turns out that the saltiness has been increasing, especially in the waters surrounding Australia, southern Africa and western South America — all places where drought has increased as well.

The climate models weren’t really wrong, Durack hastened to add. “They’re accurately capturing the spatial patterns in hydrologic changes, and they’ve got the basic physics right. They’re just providing very conservative estimates of how big the changes are, and now we’re starting to understand that.”

Surface salinity changes for 1950 to 2000. Red indicates regions becoming saltier, and blue regions becoming fresher. Credit: Paul Durack.

This understanding is likely to grow dramatically: since 2000 or so, scientists have deployed some 3,500 autonomous Argo floats, which measure ocean temperature and salinity automatically and continuously — a much more reliable set of records than you can get from ships. And a little more than a year ago, NASA launched the Aquarius satellite, which measures salinity from space, with even more complete coverage. “I’m an early-career scientist,” Durack said, “and this is a great time to be getting into this field.”

Durack is also more aware than many scientists of the havoc changes in the planet’s water cycle can be. “I’m from Perth, in Western Australia,” he said, “where there’s been a fair amount of rain decline since the 1970’s.” The state government, noting projections from climate models that the drying would likely continue, has opted to build desalinization plants. “It’s expensive,” Durack said. “But if it’s not going to fall out of the sky, it has to come from somewhere.”