By Justin Gillis, The New York Times, April 26, 2012
New research
suggests that global warming is causing the cycle of
evaporation and rainfall over the oceans to intensify more than scientists had
expected, an ominous finding that may indicate a higher potential for extreme
weather in coming decades.
By measuring changes in salinity on the ocean’s surface, the
researchers inferred that the water cycle had accelerated by about 4 percent
over the last half century. That does not sound particularly large, but it is
twice the figure generated from computerized analyses of the climate.
If the estimate holds up, it implies that the water cycle could
quicken by as much as 20 percent later in this century as the planet warms,
potentially leading to more droughts and floods.
“This provides another piece of independent evidence that we need to
start taking the problem of global warming seriously,” said Paul J. Durack, a researcher at the Lawrence Livermore National Laboratory
in California and the lead author of a
paper being published Friday in the journal Science.
The researchers’ analysis found that over the half century that began
in 1950, salty areas of the ocean became saltier, while fresh areas became
fresher. That change was attributed to stronger patterns of evaporation and
precipitation over the ocean.
The new paper is not the first to find an intensification of the water
cycle, nor even the first to calculate that it might be fairly large. But the
paper appears to marshal more scientific evidence than any paper to date in
support of a high estimate.
“I am excited about this paper,” said Raymond
W. Schmitt, a senior scientist at the Woods Hole Oceanographic Institution in
Massachusetts, who offered a critique of the work before publication but was
otherwise not involved. “The amplification pattern that he sees is really quite
dramatic.”
The paper is the latest installment in a long-running effort by
scientists to solve one of the most vexing puzzles about global warming.
While basic physics suggests that warming must accelerate the cycle of
evaporation and rainfall, it has been difficult to get a handle on how much
acceleration has already occurred, and thus to project the changes that are
likely to result from continued planetary warming.
The fundamental problem is that measurements of evaporation and
precipitation over the ocean — which covers 71 percent of the earth’s surface,
holds 97 percent of its water and is where most evaporation and precipitation
occurs — are spotty at best. To overcome that, scientists are trying to use the
changing saltiness of the ocean’s surface as a kind of rain gauge.
That works because, as rain falls on a patch of the ocean, it freshens
the surface water. Conversely, in a region where evaporation exceeds rainfall,
the surface becomes saltier.
The variations in salinity are large enough that they can be detected
from space, and NASA recently sent up a new satellite, Aquarius,
for that purpose. But it will take years to obtain results, and scientists like
Dr. Durack are trying to get a jump on the problem by using older observations,
including salinity measurements taken by ships as well as recent measurements
from an army of robotic floats launched in an international program called Argo.
Dr. Schmitt cautioned that the work by Dr. Durack and his co-authors,
the Australian researchers Susan
E. Wijffels and Richard J. Matear, would need to be
scrutinized and reproduced by other scientists.
Another expert not involved in the work, Kevin
E. Trenberth of the National Center for Atmospheric Research in
Boulder, Colo., said that Dr. Durack had produced intriguing evidence that
global warming was already creating changes in the water cycle at a regional
scale. But Dr. Trenberth added that he doubted that the global intensification
could be as large as Dr. Durack’s group had found. “I think he might have gone
a bit too far,” he said.
Assuming that the paper withstands scrutiny, it suggests that a global
warming of about 1 degree Fahrenheit over the past half century has been enough
to intensify the water cycle by about 4 percent. That led Dr. Durack to project
a possible intensification of about 20 percent as the planet warms by several
degrees in the coming century.
That would be approximately twice the amplification shown by the
computer programs used to project the climate, according to Dr. Durack’s
calculations. Those programs are often criticized by climate-change skeptics
who contend that they overestimate future changes, but Dr. Durack’s paper is
the latest of several indications that the estimates may actually be
conservative.
The new paper confirms a long-expected pattern for the ocean that also
seems to apply over land: areas with a lot of rainfall in today’s climate are
expected to become wetter, whereas dry areas are expected to become drier.
In the climate of the future, scientists fear, a large acceleration of
the water cycle could feed greater weather extremes. Perhaps the greatest risk
from global warming, they say, is that important agricultural areas could dry
out, hurting the food supply, as other regions get
more torrential rains and floods.
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