The Atlantic circulation (AMOC) as part of the global overturning circulation of the oceans
in an animation from NASA, showing what happens globally to create the large, slow current called the thermohaline circulation.
Welcome to this week’s installment of “Don’t Mess with Geophysics.”
Last week, we
learned about
the possible destabilization of the Totten Glacier of East Antarctica,
which could unleash over 11 feet of sea level rise in coming centuries.
And now this week brings news of another potential mega-scale perturbation.
According to a
new study just out in Nature Climate Change
by
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research
and a group of co-authors, we’re now seeing a slowdown of the great
ocean circulation that, among other planetary roles, helps to partly
drive the
Gulf Stream off
the U.S. east coast.
The consequences could be dire – including
significant extra sea level rise for coastal cities like New York and
Boston.
The Gulf Stream system is one of Earth's most important heat transport systems.
Now, though, scientists have found evidence for a slowdown of this system, which could have major implications for climate.
(Photo : Gulf of Mexico, NASA/GSFC/Aqua MODIS)
A vast, powerful, and warm current, the Gulf Stream transports more water than “
all the world’s rivers combined,”
according to the National Oceanic and Atmospheric Administration.
But
it’s just one part of a larger regional ocean conveyor system
– scientists technically call it the “Atlantic meridional overturning
circulation” — which, in turn, is just one part of the larger global “
thermohaline” circulation (“thermohaline” conjoins terms meaning “temperature” and “salty”).
For
the whole system, a key driver occurs in the North Atlantic ocean.
Here, the warm Gulf Stream flows northward into cooler waters and splits
into what is called the North Atlantic Current.
This stream flows still
further toward northern latitudes — until it reaches points where
colder, salty water sinks due to its greater density, and then travels
back southward at depth.
This “overturning circulation” plays a
major role in the climate because it brings warm water northward,
thereby helping to warm Europe’s climate, and also sends cold water back
towards the tropics.
Here’s a helpful visualization, from Rahmstorf and
the Potsdam Institute, of how it works:
Graph of the Atlantic Meridional Overturning
Circulation by Stefan Rahmstorf from the Potsdam Institute for Climate
Impact Research.
Red colors are surface currents, blue colors are below
the surface. “NADW” stands for North Atlantic Deep Water.
And here’s a
wonderful video from NASA that visualizes the thermohaline circulation for the entire globe. Rahmstorf also has a
blog post up at RealClimate.org explaining his research.
The day after fiction
The
system above has a key vulnerability.
What keeps everything churning in
the North Atlantic is the fact that cold salt water is more dense than
warm water — so it sinks.
However, if too much ice melts in the region —
from, say Greenland — a freshening of the cold salt water could occur.
If the water is less salty it will also be less dense, reducing its
tendency to sink below the surface.
This could slow or even eventually shut down the circulation.
In the
scientifically panned 2004 blockbuster film “The Day After Tomorrow,” it
is
precisely such a shutdown that triggers a New Ice Age, and utter global disaster and chaos.
That’s not going to happen, say scientists.
Not remotely.
Nonetheless,
the new research finds that global warming does indeed seem to be
slowing down the circulation. And while hardly catastrophic, that can’t
be good news.
Among the very real effects, notes the Potsdam Institute’s
Rahmstorf, could be a possible increase in U.S. sea level if the whole
circulation were to break down — which would be seriously bad news for
cities like New York and Boston.
As more and more fresh water is released into the ocean by melting glaciers, the Atlantic's most powerful current, the Gulf Stream, is likely to slow down even more
The study uses a reconstruction
of sea surface temperatures in the North Atlantic to find that starting
in around 1970 or 1975, the overturning circulation started to weaken —
an event likely triggered by an unusual amount of sea ice traveling out
of the Arctic ocean, melting, and causing freshening.
The circulation
then started to recover in the 1990s, but “it seems this was only a
temporary recovery, and now it’s actually further weakened,” says
Rahmstorf.
Despite all the warming that’s taken place since 1970, one little blip of the North Atlantic (shown here in green) has begun to actually cool.
The hypothesized reason for further declines presented
by the paper is that the massive Greenland ice sheet may now be losing
enough freshwater due to melting to weaken the circulation.
And indeed,
it appears that a particular ocean region of the North Atlantic south of
Greenland and between Canada and Britain is becoming colder — an
indicator of less northward heat transport.
Rahmstorf points to a
recent release
by the National Climatic Data Center of the National Oceanic and
Atmospheric Administration, finding that the winter of December 2014
through February 2015 was the warmest on record for the globe as a
whole.
However, there were several anomalies — not just a cold winter
for the eastern U.S., but also
record cold temperatures in the middle of the North Atlantic:
According to the National Climatic Data Center,
the world just saw its warmest winter ever…except for in one spot in
the north Atlantic ocean (the deepest blue color above), which set a
record for cold. Which is not good. (NCDC)
“These new NOAA data got me quite worried because they indicate that
this partial recovery that we describe in the paper was only temporary,
and the circulation is on the way down again,” says Rahmstorf.
So
far, the study finds, we’re looking at a circulation that’s about 15 to
20 percent weaker.
That may not sound like much, but the paper suggests
a weakening this strong has not happened at any time since the year
900. Moreover, this is already more weakening than scientifically
expected — and could be the beginning of a further slowdown that could
have great consequences.
The U.N.’s Intergovernmental Panel on Climate Change, in 2013,
said
it was “very likely” that the Atlantic overturning circulation would
weaken over the course of this century, but gave a gigantic range of
from 1 to 54 percent, with best estimates at 11 and 34 percent.
We’re
already in that window, suggests the new study, and it’s only 2015.
So
what would happen if the circulation weakens even more substantially or
even shuts down?
Why the U.S. suffers from a Gulf Stream system slowdown
One thing that will
not happen
from a shutdown of the circulation is a sudden, dramatic freezing of
Europe.
It will certainly cool, relative to a world in which the
circulation remains robust — but that will be offset by rising average
temperatures due to global warming, says Rahmstorf.
The “Day After
Tomorrow” scenario will not come to pass.
However, there are many
other effects, ranging from dramatic impacts on fisheries to, perhaps
most troubling of all, the potential for extra sea level rise in the
North Atlantic region.
That may sound surprising, but here’s how it works.
We’re starting out from a situation in which sea level is “
anomalously low”
off the U.S. east coast due to the motion of the Gulf Stream.
This is
for at least two reasons.
First, explains Rahmstorf’s co-author Michael
Mann of Penn State University, there’s the matter of temperature
contrast: Waters to the right or east of the Gulf Stream, in the
direction of Europe, are warmer than those on its left or west.
Warm
water expands and takes up more area than denser cold water, so sea
level is also higher to the right side of the current, and lower off our
coast.
“So if you weaken the ‘Gulf Stream’ and weaken that
temperature contrast…sea level off the U.S. east coast will actually
rise!” explains Mann by e-mail.
But there’s another factor, too, involving what is called the “
geostrophic balance
of forces” in the ocean.
This gets wonky, but the bottom line result is
that “sea surface slope perpendicular to any current flow, like the
Gulf Stream, has a higher sea level on its right hand side, and the
lower sea level on the left hand slide,” says Rahmstorf.
(This would
only be true in the northern hemisphere; in the southern it would be the
opposite.)
Sea surface temperature anomaly on 20 March 2015.
Note that this is relative to a baseline 1979-2000, which is already a cold period in the subpolar Atlantic.
We’re on the left hand side of the Gulf Stream.
So weaken the flow,
and you also raise the sea level.
(For further explanation, see
here and
here.)
Indeed, researchers
recently found a
sudden, 4-inch sea level rise of the U.S. East Coast in 2009 and 2010,
which they attributed to a slowdown of the Atlantic overturning
circulation.
Rahmstorf says that “for a big breakdown of the
circulation, [sea level rise] could amount to one meter, in addition to
the global sea level rise that we’re expecting from global warming.”
Shutting
down the circulation would also almost certainly have effects on global
weather — changing around major planetary heat transport processes
tends to do that — though scientists don’t know yet what those would
look like.
So in sum: It appears that we’ve just seen yet another
surprise from the climate system — and yet another process, like the
melting of Antarctica, that seems to be happening faster than previously
expected.
And indeed, much like with that melting, the upshot if the
trend continues is an especially bad sea level rise for the United
States — the country
more responsible than any other on Earth for the global warming that we’re currently experiencing.
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