Monitoring of the Southern Ocean using arrays of anchored and drifting instruments reveals freshening of deep waters around Antarctica. Some of the extra melting of ice around the edge of Antarctica is flowing into the sea and getting carried down to the deep ocean by ocean currents. Image: Steve Rintoul/CSIRO |
A
clear change in salinity has been detected in the world’s oceans, signaling
shifts and acceleration in the global rainfall and evaporation cycle tied
directly to climate change.
In
a paper published in Science, Australian scientists from the
Commonwealth Scientific and Industrial Research Organisation (CSIRO) and
Lawrence Livermore National Laboratory (LLNL) reported changing patterns of
salinity in the global ocean during the past 50 years, marking a clear symptom
of climate change.
Lead
author Paul Durack said that by looking at observed ocean salinity changes and
the relationship between salinity, rainfall, and evaporation in climate models,
they determined the water cycle has become 4% stronger from 1950 to 2000. This
is twice the response projected by current generation global climate models.
“These
changes suggest that arid regions have become drier and high rainfall regions
have become wetter in response to observed global warming,” said Durack, a
post-doctoral fellow at Lawrence Livermore National Laboratory.
Scientists
monitor salinity changes in the world’s oceans to determine where rainfall has
increased or decreased. “It provides us with a gauge—a method of
monitoring how large-scale patterns of rainfall and evaporation (the climate
variables we care most about) are changing,” Durack said.
With
a projected temperature rise of 3 C by the end of the century, the researchers
estimate a 24% acceleration of the water cycle is possible.
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Scientists
have struggled to determine coherent estimates of water cycle changes from
land-based data because surface observations of rainfall and evaporation are
sparse. According to the team, global oceans provide a much clearer picture.
“The
ocean matters to climate—it stores 97% of the world’s water; receives 80% of
all surface rainfall, and it has absorbed 90 percent of the Earth’s energy
increase associated with past atmospheric warming,” said co-author,
Richard Matear of CSIRO’s Wealth from Oceans Flagship.
“Warming
of the Earth’s surface and lower atmosphere is expected to strengthen the water
cycle largely driven by the ability of warmer air to hold and redistribute more
moisture.”
He
said the intensification is an enhancement in the patterns of exchange between
evaporation and rainfall, and with oceans accounting for 71% of the global
surface area, the change is clearly represented in ocean surface salinity
patterns.
In
the study, the scientists combined 50-year observed global surface salinity
changes with changes from global climate models and found “robust evidence
of an intensified global water cycle at a rate of about 8 percent per degree of
surface warming,” Durack said.
Durack
said the patterns are not uniform, with regional variations agreeing with the
‘rich get richer’ mechanism, where wet regions get wetter and dry regions
drier.
He
said a change in freshwater availability in response to climate change poses a
more significant risk to human societies and ecosystems than warming alone.
“Changes
to the global water cycle and the corresponding redistribution of rainfall will
affect food availability, stability, access and utilization,” Durack said.
Susan
Wijffels, co-chair of the global Argo project and a co-author on the study,
said maintenance of the present fleet of around 3,500 profilers is critical to
observing continuing changes to salinity in the upper oceans.
