Science (27 April 2012) Vol. 336, No. 6080, p.
405; doi: 10.1126/science.336.6080.405
- News & Analysis: Global Warming
The Greenhouse Is Making the Water-Poor Even Poorer
by Richard A. Kerr, Science News, April 27, 2012
A new study of the ocean's changing
salinity on page 455 confirms that this “rich get richer” mechanism of
water-cycle amplification
has been operating for the past half-century. “It's
a pretty striking result,” says oceanographer Raymond Schmitt of Woods
Hole Oceanographic Institution in Massachusetts.
The result also suggests that the water cycle is intensifying quickly
under
global warming—twice as fast as climate models have
been predicting.
Saltier or less so.
Ship-borne instruments (top) showed that places already saltier than average got saltier (reds) and those less salty got even more so (blues).
CREDITS (TOP TO BOTTOM): CSIRO; P. DURACK ET AL., SCIENCE 336 (27 APRIL)
Several studies of how fast water has been
evaporating and falling back as rain on a global scale had suggested an
acceleration
of the water cycle, but they had their limitations.
Using rain gauges on land was tricky given that heavy rains can be
sporadic
and gauge networks have been sparse. Satellites can
survey the globe, but they have been at it for only a couple of
decades.
So oceanographer Paul Durack of Lawrence
Livermore National Laboratory in California and colleagues from the
Commonwealth
Scientific and Industrial Research Organisation's
Marine and Atmospheric Research division in Australia looked to the
ocean.
After all, “that's where the water is,” as Schmitt
puts it.
The oceans cover 71% of the globe, hold 97% of its water, and
receive 80% of its precipitation. And gauging the
oceans' changing salinity provides a way of tracking water as it cycles
between atmosphere and ocean. If more rain falls
over an ocean than water evaporates from it, surface salinity drops
proportionately.
If evaporation outpaces rain, salinity rises.
Oceanographers were not surveying global
salinity intensively until 1999, when they started releasing
instrumented subsurface
floats under the Argo program. Argo floats now
number about 3,500. But because the ocean smoothes out rainfall's
patchiness,
even pre-Argo measurements reflect changes in the
global water cycle more accurately than the denser data available over
land.
Durack and colleagues used the Argo observations to
help correct for shortcomings in the salinity measurements made from
research
vessels between 1950 and 2000, such as
oceanographers' understandable reluctance to brave the stormy wintertime
high latitudes.
When Durack and colleagues analyzed the
1.7 million salinity measurements made worldwide in the second half of
the 20th century,
they found that the water-rich had indeed been
getting richer and vice versa (see figure). High-latitude and equatorial
parts
of the oceans, where greater precipitation keeps
surface waters less salty than average, became even less salty; the
central
regions of ocean basins, where evaporation
dominates and turns water saltier, became even saltier. Because 80% of
the water
cycle operates over oceans and much of the water
falling as rain over land comes from the ocean, the water cycle over
land
no doubt is behaving the same way.
After comparing the magnitude and
geographical pattern of salinity change in models and in the real world,
Durack and colleagues
concluded that the water cycle had sped up roughly
4%, while the surface warmed 0.5 °C. That 8% increase per degree of
warming
is about the rate of acceleration expected on the
basis of how much more water vapor warmer air can hold. The more
moisture
air can hold, the faster water can make the circuit
of the water cycle, like building a bigger pipe in a plumbing system.
But the warming seems to have intensified the real
world's water cycle twice as much as the typical global climate model
does.
At least some of the models, it seems, do not
properly simulate the water cycle's response to warming.
The new analysis “has provided decisive
evidence for a surprisingly rapid acceleration of the global water
cycle,” Schmitt
says. “This is the first time we're seeing this for
the hydrological cycle,” adds oceanographer Sydney Levitus of the U.S.
National Oceanic and Atmospheric Administration
(NOAA) in Silver Spring, Maryland. The pattern and rate of the change
“represents
yet another ‘fingerprint’ of global change.”
So wet places have been getting wetter
while dry places got drier. But worse is yet to come. If the world warms
2 °C to 3 °C
by the end of the century, as currently projected,
and the past is any guide to the future, the water cycle will accelerate
16% to 24%, Durack and colleagues point out.
“That's a pretty amazing projection,” says ocean modeler Stephen
Griffies of
NOAA's Geophysical Fluid Dynamics Laboratory in
Princeton, New Jersey, “and it just might be close to reality.”
Such a revved-up water cycle would have
“a lot of implications for how extreme events would change in a warming
climate,”
says meteorologist Brian Soden of the University of
Miami in Florida. Water cycling from the surface to the atmosphere
carries
heat energy that can ultimately fuel violent
storms, from tornadoes to tropical cyclones. The faster water cycles,
the more
abundant and more violent those storms might be.
And wet places getting wetter can lead to more severe and more frequent
flooding.
Dry places getting drier would mean longer and more
intense droughts. The Argo flotilla should have an interesting tale to
tell in the years to come.
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