Yahoo - AFP, Mariƫtte Le Roux, 11 April 2018
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Melting sea ice in the Arctic produces fresh water, slowing the circulation of denser salt water and thereby slowing warming currents |
A system of
Atlantic Ocean currents that regulates global weather is at its feeblest in
1,600 years, weakened partly by climate change, said researchers Wednesday,
warning of trouble ahead.
Two studies
in the journal Nature concluded the system, dubbed the Atlantic Meridional
Overturning Circulation (AMOC), is in decline -- validating a long-held
scientific suspicion.
The
weakening, agreed the teams, was the result of melting sea ice, glaciers, and
ice-shelves gushing freshwater -- less dense than salty ocean water -- into the
North Atlantic.
"This
freshwater caused the AMOC to weaken because it prevented the waters getting
dense enough to sink," David Thornalley of the University College London,
who co-authored one of the studies, told AFP.
The AMOC is
often likened to a conveyer belt redistributing warmth through the ocean, so
regulating weather over land as well.
Its
currents take warm, salty Gulf Stream water northward, releasing heat into the
atmosphere and warming Western Europe.
As the
water cools in the cold, Nordic seas, it sinks to great depths and travels
southward all the way to Antarctica, eventually circulating back up to the Gulf
Stream, and so on.
"If
the system continues to weaken, it could disrupt weather patterns from the
United States and Europe to the African Sahel, and cause more rapid increase in
sea level on the US east coast," said the Woods Hole Oceanographic
Institution, which participated in the research.
Apart from
warming Europe, the system transports important nutrients, oxygen, and the
larvae of marine organisms such as corals and fish between ocean regions.
It also
controls the ability of the seas to absorb and store carbon dioxide -- the most
abundant of the Earth-warming greenhouse gases.
More
beasts from the east
For the
first study, Thornalley and a team consulted historical "records"
locked up in the ocean floor -- sand grains deposited by ocean currents.
The larger
the grains in a sediment layer, the stronger the current must have been.
The data
revealed the AMOC was relatively stable from about the year 400 to 1850, then
started weakening around the start of the industrial era.
The second
study looked at sea-surface temperature patterns to conclude the AMOC has
declined by about 15 percent in the last half century -- likely in response to
global warming caused by humankind's burning of coal, oil, and natural gas.
The two
teams found a similar degree of AMOC weakening, but over very two different
time periods.
This has
implications for identifying the trigger -- whether it was manmade or natural
changes in the climate.
"The
initial weakening that we show ocurred at the end of the Little Ice Age is
likely part of natural processes ending the Little Ice Age," Thornalley
said.
"However,
the fact that AMOC has remained weak and weakened further throughout the 20th
century, with a noticeable decline since about 1950, is very likely due to
human factors."
The studies
cannot conclude whether the AMOC will regress further.
If it does,
there will likely be further cooling in the North Atlantic, increased winter
storms for Europe, a possible southward shift in the tropical rainfall belt,
and faster sea-level rise off the US eastern seaboard, said the teams.
Global
warming, however, will likely more than counter any cooling of Europe due to a
weaker AMOC, said Thornalley.
According
to the European Union-funded ATLAS research project, another study participant,
commercial fisheries may be affected as the positions and depths of ocean
currents shift, and some regions are starved of oxygen-rich waters.
"A
weakening of the AMOC can also result in temperature increases or decreases of
several degrees centigrade, affecting some high-value fisheries as well as
abundances of plankton, fish, sea birds, and whales," it said in a
statement.
Further
weakening of the system "will leave more carbon dioxide in the atmosphere
where it causes warming," added Thornalley -- perpetuating the vicious
circle.
The
findings imply that models used to project global warming scenarios had likely
underestimated the contribution of a weakening AMOC, said the teams.
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