In this Real News Network interview, Levke Caesar, discusses how Atlantic ocean currents have weakened due to global warming and are closer to catastrophic collapse than any time in the last 1,600 years. This decline could cause rapid sea level rise on the East Coast of North America. Caeser is lead author of one of two peer-reviewed studies published in Nature that reported these findings.
DIMITRI LASCARIS: 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.
LEVKE CAESAR: Yeah, thanks for having me.
DIMITRI LASCARIS: So first, Levke, please explain the Atlantic meridional overturning circulation, also known as AMOC, and why it is important.
LEVKE CAESAR: 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 AMOC 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.
DIMITRI LASCARIS: What are the key findings from your study, and why should we be concerned about those findings?
LEVKE CAESAR: 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, but we 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.
DIMITRI LASCARIS: 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 States?
LEVKE CAESAR: 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.
DIMITRI LASCARIS: 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?
LEVKE CAESAR: 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.
DIMITRI LASCARIS: 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?
LEVKE CAESAR: Yes.
DIMITRI LASCARIS: 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?
LEVKE CAESAR: 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.
DIMITRI LASCARIS: 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.
LEVKE CAESAR: Yeah, thanks to you.
DIMITRI LASCARIS: And this is Dimitri Lascaris reporting from Montreal, Canada for the Real News.