Another big wheel of the climate shift has turned. Scientist Levke Caesar: the Atlantic overturning current weakened due to climate change. Hundreds of millions of people on both sides of the Atlantic are affected. From Germany, Luis Samaniego forecasts European drought. Spain could be a desert within a lifetime. This is science at its best and most frightening. This is Radio Ecoshock 180502.

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The wild change in ocean currents portrayed in the disaster movie “The Day After Tomorrow” have now been confirmed by science. No, not the sudden freezing disaster shown by Hollywood. But if you are concerned with weather in the Eastern United States, Canada, the UK and Europe – or if you are worried global climate change itself – this is for you. Multiple layers of science demonstrate the great ocean current known popularly as the Gulf Stream, has weakened significantly since 1950.

Two papers on the overturning of Atlantic waters were published in the April edition of the prestigious journal Nature. One is led by the Potsdam Institute for Climate Impact Research or PIK. The well-known scientist Stefan Rahmsdorf is deeply involved, but here the lead author is Ph.D. candidate Levke Caesar. This is her first paper, and published in the top journal Nature no less!

The title of this important paper is: “Stronger evidence for a weaker Atlantic overturning” – as published in Nature on April 11th, 2018.

Before we get to the many impacts of a big change in the Atlantic, we need to understand what it is: the technical term “Atlantic meridional overturning circulation” or “AMOC”. See the Wikipedia explanation here, which says:

The Atlantic meridional overturning circulation (AMOC) is a system of currents in the Atlantic Ocean, characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, including the Gulf Stream, and a southward flow of colder, deep waters that are part of the thermohaline circulation. The AMOC is an important component of the Earth’s climate system.

The press release from PIK, the Potsdam Institute for Climate Impact Research, reports the Atlantic overturning current has reduced by about 15% since 1950.

See also this video with Levke from PIK “Stronger evidence for a weaker Atlantic overturning”


The press release says:

As the currents slow down, they bring less heat towards the north, causing widespread cooling of the northern Atlantic – the only ocean region that has cooled in the face of global warming. At the same time, the Gulf Stream shifts northwards and closer to shore and warms the waters along the northern half of the US Atlantic coast.

The reduction is counted by an ocean science measurement called a Sverdrup, named after the Norwegian scientist Harald Sverdrup. I looked into this. Wikipedia says “The entire global input of fresh water from rivers to the ocean is equal to about 1.2 Sverdrups.” That would mean the reduction in the Atlantic current described in this paper is about twice the total of all river water flowing into the sea globally. That is a huge volume of change!


ALEX: While researching this paper, I came across a 2014 study led by the University of Oxford scientist Yavor Kostov. It begins, quote:

We propose here that the Atlantic meridional overturning circulation (AMOC) plays an important role in setting the effective heat capacity of the World Ocean and thus impacts the pace of transient climate change.

If the Atlantic overturning slows down, that could hasten the arrival of climate impacts, because less excess heat is dropping down to the deep ocean.

FROM LEVKE: This is a complicated situation, Levke says, because we have two different forces operating on two different time scales. CO2 is more easily dissolved in cold water than in warm water, and the overturning takes carbon dioxide from the atmosphere and carries it to the deep ocean.

When the overturning circulation slows down, we will probably have a positive feedback on the CO2 in the atmosphere, that is, the ocean will take up less carbon dioxide from the atmosphere, and thus the CO2 concentrations in the atmosphere could rise more quickly. So we expect feedbacks on climate as the overturning circulation is changing, but we have to study them in more detail.


Several studies have shown, for example, that a slowdown of the Atlantic overturning exacerbates sea-level rise on the US coast for cities like New York and Boston.
– PIK Press Release

ALEX: How does the Gulf Stream affect the weather along the American eastern seaboard, and the maritime provinces of Canada, and in what seasons?

FROM LEVKE: Levke says this is really complex question, too complex to answer here and more study needed. But she says that what we know from other science is that the change in sea level can be affected by changes in the Gulf Stream. Changes to the Atlantic Meridional Overturning Circulation (AMOC) will lead to an increased sea level rise, she says.

We know the sea level will rise as the oceans warm generally, because hotter water expands. But in the specific case of sea level rise off the east coast of North America, there is a complex change in play, that takes some time to explain.

As you know the Earth is rotating, and therefore we have on all moving things on Earth the Coriolis force acting. This normally means that on the Northern Hemisphere that when some body is moving from the South to the North – and it doesn’t matter if it’s me or you or an ocean current – it is forced to move toward the right hand side. And so for a current to move from the South to the North without being deflected this much – the sea surface has to tilt, with lower sea levels at the U.S. Coast and higher sea levels in the mid-Atlantic.

And then you have a force acting from the mid-Atlantic from the high sea levels toward the low sea levels which can counteract this Coriolis force. So this is one point.

Then when the Coriolis force is stronger when a current is faster. Therefore this tilt has to be more steep when the current is faster. Now as the current weakens, the Coriolis force gets weaker and the tilt in sea level heights also becomes smaller. And this leads in the mid-Atlantic to lower sea levels, but at the U.S. east coast – higher sea levels.

This is a physical mechanism that is – actually it’s simple when you know about it… This has already been seen that we have a sea level rise at the U.S. coast due to slower ocean currents.”


In this interview, Levke says:

What we show in our study is that there is a change in the location of the Gulf Stream. This is actually also basic physics.

My summary would be that due to the topography of the ocean bottom, the returning lower component current of colder water is deflected away from the U.S. east coast, starting around the Gulf of Maine. When that recirculation is weakened, the Gulf Stream manages to follow the U.S. coast a little further North, heading out into the mid-Atlantic later. But that is not the only force changing where the Gulf Stream is located: winds are also a major factor. It’s complex. This paper investigates the overall pattern of this circulation, and not yet as a study of just the Gulf Stream.


ALEX: tell us about the second study published in the same issue of Nature, April 2018, led by David Thornalley of University College London. How does that relate to your study?

The second paper is “Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years” David J. R. Thornalley et al. Nature April 11, 2018.

My summary:

Thornalley and his co-authors looked into the deeper history of this Atlantic Meridional Overturning Circulation. Their team used “proxies” to look further back in time. They had temperature-based proxies and other data (core samples) that recorded the velocities of currents running across the deep floor off the coast of Labrador, Canada. They also saw evidence that the AMOC got weaker toward the 1950’s. However, that does not yet represent the full overturning circulation, because the data comes from a limited location. Still, it helps confirm evidence of the current weakening since the 1950’s, likely the weakest at least in the last 1,000 years (according to that paper). I ask Levke Caesar how we know that new record in 1,000 years has been set.

Their temperature-based proxy also showed that there is less heat being transferred from the tropics to the high latitudes in this century.


According to these two studies, the change in the Atlantic overturning brought on by human-induced warming is greater than anything seen in the last 1,000 years.

Perhaps this explains why there is a region of the North Atlantic is one of the few places on Earth getting colder, even as the rest of the world warms generally. The “sub-polar” gyre is south of Greenland. There are other factors, such as cold meltwater entering from melting Greenland glaciers and sea ice – but again, that is partly what the paper led by Levke is all about. This meltwater is the actual force that is slowing down the Atlantic overturning circulation!

Increasing precipitation is also a factor. The global hydrologic system is being disrupted as the planet warms. Not only can a warmer atmosphere carry more water vapor (which has to fall as either snow or ice) – but changes in the atmosphere predict that a swath of northern Canada, Scandinavia and Russia will become wetter, even as other parts of the world become drier. More on that in a future interview. In any case, from all these sources (precipitation, and glacier runoff) it is the fresh water flux that is hindering the sinking of sea water, due to a change of salinity. Thus the overturning is weakened by a measurement of 3 “Sverdrups” – which is a 15% reduction of the whole current since 1950.

Levke tells us that climate models indicate that a reduction in the AMOC could change the winter storm track in Europe, and could possible intensify those storms but more research is needed.

A lot of countries have been researching the Atlantic Meridional Overturning Current – and with good reason. The Americans have a program to monitor AMOC. Another is called RAPID, operated with British funding and with the Hadley Centre in the UK. Both the RAPID program and this paper find the same reduction in AMOC.


Levke tells us the cold in the North Atlantic can “favor an air pressure distribution that spans around the whole globe at the high northern latitudes – and this air pressure distribution can channel warm air into Europe from [unknown] regions, and it would persist for quite some while and you can actually have a heat wave.

While the overturning circulation is known to be weakening, climate models cannot agree on how fast. But Caesar says there is a breaking point, where that ocean heat exchange could collapse. We just don’t know where that point is.

Levke Caesar’s next project is to study the link between the reduction in AMOC and extreme weather events. Scientifically she is excited about what she will find, but also a little afraid of the possible results.

All of us can find more relevant papers, and the debate within climate science about the meaning of the new paper here, in this discussion at the RealClimate site.

Paul Beckwith video posted this YouTube video about changes in AMOC on April 17, 2018. It’s a very helpful explanation, with maps and graphs, by Paul: “Global Ocean Circulation: AMOC runs Amok”


Here is a good article from University College London titled “Atlantic circulation that helps warm UK is at its weakest for over 1500 years“, published April 12.



Forget about the 1.5 degree warming “aspiration” at Paris in 2015. There have already been two conferences of international scientists talking about 4 degrees warming and beyond. A new paper investigates what that could mean for Europe, and it is devastating. Southern Europe could become “a desert”, and all European agriculture is threatened by long-lasting and frequent droughts. We are talking about 740 million people.

The paper published in Nature Climate Change is titled “Anthropogenic warming exacerbates European soil moisture droughts“. Our guest is the lead author, Dr. Luis E. Samaniego, Senior Researcher at the Helmholtz Centre for Environmental Research – UFZ.

Dr. Luis Samaniego

Frankly, this paper becomes very technical, quickly. In my opinion, it is not for public consumption. And yet it contains warnings that really draw out our drastic choices about the future.



Negative consequences can be expected for the Mediterranean region, where the drought regions could expand from 28 percent of the area in the reference period to 49 percent of the area in the most extreme cases.

The number of drought months per year will also increase significantly in Southern Europe: ‘In the event of a three-degree warming, we assume there will be 5.6 drought months per year; up to now, the number has been 2.1 months. For some parts of the Iberian Peninsula, we project that the drought could even last more than seven months,’ says UFZ hydrologist Dr Luis Samaniego, one of the two main authors of the study.


His colleague Dr Stephan Thober, who co-wrote the study as the second main author, added: ‘A three-degree temperature rise also means that the water content in the soil would decline by 35 millimeters up to a depth of two metres. In other words, 35,000 cubic metres of water will no longer be available per square kilometre of land.’

This corresponds roughly to the water deficit experienced during the drought period that prevailed in the summer of 2003 throughout much of Europe. If the three-degrees scenario takes place, drought events of this intensity and extent could therefore occur twice as often in years ahead and become the normal state in many parts of Europe. In future, droughts would even far exceed this normal state; the impact on civil society and the economy would be severe.”


If, on the other hand, global warming increases by only 1.5 degrees Celsius, only 3.2 months of drought could be expected annually in the Mediterranean region and there would be a decline in the water content in the soil of about eight millimetres.”


Conversely, in the Baltic states and Scandinavia, the projected increases in precipitation triggered by the global warming would even cause the drought-affected area to shrink by around three percent. For Germany as well, the warming would only have relatively minor consequences — with one constraint: ‘Here too, summers would be drier in the future than has been the case so far,’ says Thober.

The UFZ researchers also stress that humankind can react to the expected spreading of droughts. “The effects of global warming can be reduced in part with some technical adjustments. However, these are costly,” says Samaniego. The more certain way would be to implement the climate protection objectives of the Paris Agreement and thus reduce the negative effects on terrestrial droughts in Europe.


At three degrees, Iberia (Spain mainly, Italy, and Eastern Europe have much longer droughts. They are more frequent in Iberia, France, Italy, and around the Mediterranean (Yugoslavia and Greece). Essentially 3 degrees is a disaster for southern Europe.

Note that some parts of the Iberian Peninsula are projected to experience more than seven drought months per year under the 3 K warming level (Fig. 2r). These events may no longer be considered droughts, given that they occur half of the time.”

What happens to the vegetation? Will fires burn off the last Iberian forests, and agricultural trees like olives groves?

At 3 degrees, in the UK and Ireland droughts last longer, but cover about the same area as historically, and the frequency is not much more than historical records.

The paper also talks about increasing carbon dioxide emissions during a drought. It says:

Europe emitted an amount of CO2 that corresponds to the amount that is normally sequestrated in four years during the 2003 drought event.

Luis explains why a drought can increase CO2 emissions so drastically. It presents a positive feedback, where higher temperatures lead to more CO2 emissions from the land.

The paper does not delve into the social impacts. For example if centuries-old farming regions, and families, turn to dust… Or wildfires wipes out whole regions repeatedly… there may be fierce competition for remaining water rights, leading to social friction.

Will cities need to ration water most of the year? Will citizens accept that? The paper considers agriculture, but less about cities. For example, will some Mediterranean cities face the same water crisis as Cape Town South Africa, every year? The Cape Town water crisis also brought out a deep-running social inequality.

When the authors talk about migration, that is already a huge hot-button issue in Europe. Will more and more Iberians migrate within Europe. Will Europeans tend to migrate north, into Germany and France, while North Africans increase their migration into the newly desertified regions? This whole issue could re-draw the map of Europe!

I have been waiting for the stage of science where we can focus on continents, and even parts of continents, with accurate predictions. Here we have it for European water supplies, for agriculture, and for society. The costs of a transition away from fossil fuels are big. The costs of adapting – even to produce food – are very large. The cost of doing nothing is disaster for our civilization, our species, and all species.

We cannot allow rampant climate change to happen. That is the message and purpose of this program. Please support Radio Ecoshock with a $10 a month “membership” or a donation of any amount. Radio stations do not pay for this program. Your financial help is essential to keeping this program on the air.

I’m Alex. Thank you for listening, and thank you for caring about our future.