This is Dimitri Lascaris, reporting for the Real News from Montreal, Canada.
Human-induced climate change is rapidly altering our physical world.
Climate change is intensifying extreme weather events like hurricanes, flooding, snowstorms,
and bone-chilling cold.
It is worsening droughts and deadly heat waves.
It is causing sea level rise through the melting of glaciers and ice sheets in Greenland and
Antarctica.
These effects have been much discussed in recent years, but a phenomenon that has received
less attention and is only beginning to be understood is the effect of climate change
on ocean currents.
Two new studies published in the peer-reviewed journal Nature posit that due to human-induced
global warming, the Atlantic meridional overturning circulation or AMOC, has weakened and is closer
to catastrophic collapse than it has been in the last 60-100 years.
Among other things, this weakening and possible collapse could cause rapid sea level rise
on the east coast of North America.
With us to discuss this, we are very pleased to be joined by the lead author of one of
these studies, entitled "Observed fingerprint of a weakening Atlantic Ocean overturning
circulation."
Joining us from Potsdam, Germany is Levke Caesar, lead author of the Nature study and
physicist at the Potsdam Institute.
Thank you for joining us today.
Yeah, thanks for having me.
So first, Levke, please explain the Atlantic meridional overturning circulation, also known
as AMOC, and why it is important.
Yes.
So this circulation, called Atlantic meridional overturning circulation, is one of the earth's
major ocean circulation system.
So it's actually not just a single current, but a system of currents that is characterized
by a northward flow of arm and salty waters in the upper layers of the Atlantic, and then
a southward flow of colder waters in the deep Atlantic.
Thereby it redistributes the heat on our planet, bringing heat from the tropics to the high
northern latitudes.
There the ocean releases heat to the atmosphere, which is why the A MOC has a large effect
on the climate of the North Atlantic region, including Europe and the U.S.
But despite its importance, the evolution of this overturning circulation, and especially
its response to anthropogenic greenhouse gas emission are poorly known for lack of direct
current measurements.
What are the key findings from your study, and why should we be concerned about those
findings?
What we did is we identified a kind of fingerprint of a slowdown of the Atlantic overturning
circulation that is seen in the trends of the sea surface temperatures in the North
Atlantic since the late 19th century.
Well this, what we call a fingerprint is a pattern that consists of a cooling trend of
the sea surface temperatures in the South Pole and North Atlantic, south and east of
Greenland, and an extreme warming trend in the Gulf Stream region.
And this pattern that we can also see in the observation can be best explained by a slowdown
of the overturning circulation.
Why?
Due to its role that I just explained as a heat carrier.
If the AMOC slows down, the northward heat transport into that region is reduced.
And this leads to this cooling in the South and the North Atlantic, and due to a more
complex interaction between the southward deep return flow of the overturning circulation
with the bottom topography of the ocean, the reduced overturning circulation is also associated
with a northward shift of the Gulf Stream that then leads to a warming east of the West
Coast.
And this fingerprint was identified for us in a high resolution climate model simulation,
butwe find the same fingerprint, that is, the same pattern in sea surface temperature
trends and the observed long term trend.
And we could show that this indicates a weakening of the overturning circulation by approximately
15 percent since the mid-20th century.
So now you ask why you should be concerned.
Well, this shows that the ocean and the ocean dynamics are responding to the increased greenhouse
gas emissions by us human.
And even though this has to be further studied, we think that the weakening of the Atlantic
overturning circulation may already have an impact on weather.
For example you know, one example is that these cold temperatures in the subpolar north
Atlantic correlate with high summer temperatures over Europe, and the heat wave in 2015 in
Europe has been linked to the cold temperatures in the subpolar North Atlantic in that year.
This sounds paradox at first, but this can be explained.
Since these low temperatures actually favor air pressure and distribution that channels
warm air into Europe, that can lead to a heat wave there.
And additionally we have also been studies that connect a weakening of the overturning
circulation with, as you already said, above average sea level rise at the U.S. coast affecting
cities like New York or Boston.
Why would this result, potentially, in sea level rise on the East Coast?
Is it because there's going to be an increase in temperatures on the East Coast, and water
expands in heat?
Is that the main reason, or there are other reasons?
And furthermore, are you able to estimate or at least give a range of the kind of sea
level rise that a collapse in the AMOC could precipitate on the East Coast of the United
State?
OK, so these are two questions.
The first is actually that due to the earth rotating all the time there is a force acting
on everything that's moving from the south to the north, like the overturning circulation,
so the Coriolis force.
And to kind of encounter this force there is actually a slope in the sea level that
is connected with this current that is going from lower levels at the coast and higher
in the mid-Atlantic.
And if this current slows down then this decline of the slope and sea level will be less, and
we have a rise of sea levels at the U.S. coast.
So this is basically physics, and this is known, this connection.
But talking about the amount of sea level rise we have to expect, this definitely still
has to be investigated, and it depends strongly on where on the earth you.
On our model simulation suggests that if we really have a complete collapse of the Atlantic
overturning circulation, then there might be a rise in dynamic sea level of up to, of
up to one meter in the North Atlantic region.
Why we have other regions, for example around Antarctica, where there will actually be lower
sea levels.
But we definitely need further investigations here.
Levka, climate change deniers would, of course, say that the warming of the earth and your
findings are part of the natural cycle of variability.
What did you and your co-authors find in your study that links the weakening of the AMOC
to human-caused global warming?
Well, this actually has not been really investigated by our study.
But there have already been several studies that can link greenhouse gas emissions to
a weakening of the overturning circulation, because it's a rather direct physical link.
So when we have , due to enhanced greenhouse gas emissions we have a warm globe, and this
leads, on the one hand, tied to more rainfall.
So enhanced precipitation over the North Atlantic region and the surrounding land areas.
This leads to an increased freshwater influx into the North Atlantic, which is also, again,
enhanced by the melting of the Greenland ice sheet and the melting of the sea ice over
the Arctic.
And these freshwater fluxes hinder the formation of deep water that's basically the driver
of the overturning circulation.
So let me expand this a little bit more.
As I said, the overturning circulation consists of a part of the North Atlantic where you
have water that is cooling, and then due to its northward flow, and then it's sinking
into the deep ocean, where we have the return flow.
And this engine of the overturning simulation is hindered due to enhanced global warming,
and this is due to the enhanced greenhouse gas emissions.
And that one meter, or potential one meter rise, I take it would be an addition to the
increase in sea level that would be, would result from the melting of glaciers and the
ice caps.
Is that correct?
Yes.
So would the shift away from the burning of fossil fuels reverse the trend that you've
observed, halt it, slow it down?
Or are we on that trajectory no matter what we do at this stage due to the amount of greenhouse
gases already emitted into the Earth's atmosphere?
Well, this is difficult to answer.
So the ocean normally reacts slower to changes in the atmosphere, which is on the one side
positive, because right now these ocean, the ocean is kind of acting as a buffer to increased
CO2 concentrations in the atmosphere.
And it will kind of slow down this increase as it takes up CO2.
But it also means that the ocean responds slower to the changes we've made.
So if we stop emitting greenhouse gas emissions right now we know, for example, that we still
have to expect for the sea level rise, because the ocean will keep up, continue taking up
carbon dioxide and heat.
Nevertheless, this shouldn't stop us from trying our best in reducing greenhouse gas
emissions.
Well, we've been speaking to Levka Caesar, lead author of a new study on the dramatic
weakening of the Atlantic meridional overturning circulation.
Thank you very much for joining us today, Levka.
Yeah, thanks to you.
And this is Dimitri Lascaris reporting from Montreal, Canada for the Real News.