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.”
No comments:
Post a Comment