Saltier tropical oceans and fresher ocean waters near the poles ...
- Saltier tropical oceans and fresher ocean waters near the poles further
signs of global climate change's impacts
Arlington, Va.-Tropical ocean waters have become dramatically saltier
over the past 40 years, while oceans closer to Earth's poles have become
fresher, scientists report in the December 18th issue of the journal
Nature. These large-scale, relatively rapid oceanic changes suggest that
recent climate changes, including global warming, may
be altering the fundamental planetary system that regulates evaporation
and precipitation and cycles fresh water around the globe.
The study was conducted by Ruth Curry of the Woods Hole Oceanographic
Institution (WHOI); Bob Dickson of the Centre for Environment, Fisheries,
and Aquaculture Science in Lowestoft, U.K.; and Igor Yashayaev of the
Bedford Institute of Oceanography in Dartmouth, Canada.
"This study is important because it provides direct evidence that the
global water cycle is intensifying," said Elise Ralph, associate director
of the National Science
Foundation's (NSF) physical oceanography program, which funded the
research. "This is consistent with global warming hypotheses that
suggest ocean evaporation will
increase as Earth's temperature does. These issues are particularly
important as pressure on freshwater resources has become critical in many
areas around the world."
An acceleration of Earth's global water cycle can potentially affect
global precipitation patterns that govern the distribution, severity and
frequency of droughts, floods and storms. It would also exacerbate global
warming by rapidly adding more water vapor-itself a potent, heat-trapping
greenhouse gas-to the atmosphere. And it could continue to freshen North
Atlantic Ocean waters to a point that could disrupt ocean circulation and
trigger further climate changes.
The oceans and atmosphere continually exchange fresh water. Evaporation
over warm, tropical and subtropical oceans transfers water vapor to the
atmosphere, which transports it toward both poles. At higher latitudes,
that water vapor precipitates as rain or snow and ultimately returns to
the oceans, which complete the cycle by circulating fresh water back
toward the equator. The process maintains a balanced distribution of
water around our planet.
The oceans contain 96 percent of the Earth's water, experience 86 percent
of planetary evaporation, and receive 78 percent of planetary
precipitation, and thus represent a
key element of the global water cycle for study, the scientists said.
Because evaporation concentrates salt in the surface ocean, increasing
evaporation rates cause detectable spikes in surface ocean salinity
levels. In contrast, salinity decreases generally reflect the addition
of fresh water to the ocean through precipitation and runoff from the
Curry, Dickson, and Yashayaev analyzed a wealth of salinity measurements
collected over recent decades along a key region in the Atlantic Ocean,
from the tip of Greenland to the tip of South America. Their analysis
showed the properties of Atlantic water masses have been changing-in some
cases radically-over the five decades for which
reliable and systematic records of ocean measurements are available, the
They observed that surface waters in tropical and subtropical Atlantic
Ocean regions became markedly saltier. Simultaneously, much of the water
column in the high
latitudes of the North and South Atlantic became fresher.
This trend appears to have accelerated since 1990-when 10 of the warmest
years since records began in 1861 have occurred. The scientists estimated
that net evaporation rates over the tropical Atlantic have increased by
five percent to ten percent over the past four decades.
These results indicate that fresh water has been lost from the low
latitudes and added at high latitudes, at a pace exceeding the ocean
circulation's ability to compensate, say
the scientists. Taken together with other recent studies revealing
parallel salinity changes in the Mediterranean, Pacific, and Indian
Oceans, a growing body of evidence
suggests that the global hydrologic cycle has revved up in recent
Among other possible climate impacts, an accelerated evaporation -
precipitation cycle would continue to freshen northern North Atlantic
waters. The North Atlantic is one
of the few places on Earth where surface waters become dense enough to
sink to the abyss. The plunge of this great mass of cold, salty water
helps drive a global ocean circulation system, often called the Ocean
Conveyor. This Conveyor helps draw warm Gulf Stream waters northward in
the Atlantic, pumping heat into the northern regions that significantly
moderates wintertime air temperatures, especially in Europe.
If the North Atlantic becomes too fresh, its waters would stop sinking
and the Conveyor could slow down. Analyses of ice cores, deep-sea
sediment cores, and other geologic
evidence have clearly demonstrated the Conveyor has abruptly slowed down
or halted many times in Earth's history. That has caused the North
Atlantic region to cool significantly and brought long-term drought
conditions to other areas of the Northern Hemisphere over time spans as
short as years to decades.
Melting glaciers and Arctic sea ice, another consequence of global
warming, are other sources of additional fresh water to the North
Atlantic. An accelerated water cycle also
appears to be increasing precipitation in higher latitudes, contributing
to the freshening of North Atlantic waters and increasing the possibility
of slowing the Conveyor.
A cooling of the North Atlantic region would slow the melting process,
curtail the influx of fresh water to the North Atlantic. The Conveyor
would again begin to circulate ocean waters. But global warming and an
accelerated water cycle would continue to bring fresh water to high
latitudes-possibly enough to maintain a cap on the Conveyor
even if the Arctic melting ceased. Monitoring Earth's hydrological cycle
is critical, the scientists said, because of its potential near-term
impacts on Earth's climate.
The research was also supported by the Framework V Programme of the
European Community, the National Oceanic and Atmospheric Administration's
on the Ocean's Role in Climate, and the Ocean and Climate Change
Institute at the Woods Hole Oceanographic Institution.
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