Recent evidence points toward abrupt, irreversible sea level rise along the US coast

Red alerts signaling an AMOC slowdown

Stefan Rahmstorf, professor of ocean physics at Potsdam University and head of Earth System Analysis at the Potsdam Institute for Climate Impact Research, has been studying the giant conveyor belt of ocean currents called the Atlantic Meridional Overturning Circulation (AMOC) for more than 30 years. Last month, he told a live audience in Vilnius that there is "overwhelming" evidence that the AMOC is weakening and that the risk of it crossing a tipping point—a threshold beyond which amplifying feedbacks propel a shift to a different, self-sustaining state—is "much larger" than even he had thought just five years ago.

During that talk, Rahmstorf referred to René van Westen and other scientists in Utrecht as the "leading group in this topic now." Earlier this month, van Westen and colleagues uploaded a paper suggesting that the AMOC is likely to collapse sometime between 2037 and 2064, with the most likely time being around 2050. 

What does this mean and why does it matter? Just as the repricing of insurance in Florida and other states is a signal that the most liquid parts of the US financial system are beginning to reflect the risk of the rapidly changing climate—and that other elements will be affected in turn—some giant earth systems will likely change sharply before others.

Of the nine global "core" tipping elements that scientists have identified, the slowing of the AMOC is likely to be one of the first movers. When it happens, it will likely lead to higher sea levels along the East Coast of the US as water slops onshore rather than rushing by (although scientists are still trying to figure out the details of that connection). It will likely lead to a far colder Northern Europe that will periodically be swept by very hot winds from the south—meaning subject to a "crazy stormy climate" with "absolutely unprecedented weather extremes," in Rahmstorf's words. And, more generally, it will have profound implications for weather and agriculture around the globe, as hot areas get even hotter and rain patterns shift dramatically.

What is the AMOC? The AMOC is a giant complex conveyor belt of currents that is driven by a delicate balance of saltiness and temperature differences: cold, salty (so: denser) water sinks 2-3,000m down in the North Atlantic and travels down to the south as a low-lying, cold current, then rises with evaporation to the surface and rushes back (still salty) from the South Atlantic along the East Coast to the very north, where it gives off its heat to the atmosphere (thus warming Europe) and sinks again, driving itself along.

Here's a highly simplified picture from a recent article by Rahmstorf:

This graphic shows a highly simplified schematic of the Atlantic Meridional Overturning Circulation (AMOC) against a backdrop of the sea surface temperature trend since 1993 from the Copernicus Climate Change Service (https://climate.copernicus.eu/). Image credit: Ruijian Gou

It's a self-perpetuating system that has stopped abruptly in the past, according to paleoclimatologists—including when the globe warmed and huge armadas of ice slid into the north Atlantic, freshening the water there, which reduced the sinking of salty water. That meant there was no deep current rushing to the south, which meant no more heat transport to the north.

We know that the loss of that huge heat transport from the far south across the equator to the high north had swift and massive repercussions thousands of years ago. This time, during our lifetimes, global warming is happening about 20 times faster than it did at the end of the last ice age. 

Rahmstorf points to several of physical evidence in support of his claim that the AMOC is already slowing down. First, satellite data collected over the last 30 years shows the North Atlantic has warmed less than the global mean (or even cooled).

From a recent Rahmstorf article. This map plots data from the Copernicus satellite collected from 1993-2021, showing sea surface temperature (SST) trends divided by their global mean trend. White signifies the same trend as the global mean; blue is cooler, yellow is warmer. Note the warm signal next to the American east coast, signifying that the current has shifted closer to the coast and is warming.

That cold blob signals that heat transport by the AMOC to the North Atlantic is already slowing down.

Second, the water around Iceland is the least salty it's been for the last 120 years, and the water along North America is anomalously salty.

The “fresh blob” in the northeastern North Atlantic, with a corresponding salty anomaly along the North American coast and the associated time evolution. Blue indicates anomalously low salinity and brown high salinity.

Third, recent studies measuring the current next to Florida show that the last several decades have seen "robust weakening." (Here and here.)

It matters because everything is connected. Too much salt in one place; not enough in another: that's the blaring alarm signal that shifts are coming.

This AMOC tipping point, just like insurance in Florida, isn't an isolated thing: it's deeply related to what's going on with the Greenland Ice Sheet. More ice leaving Greenland means more freshwater in the North Atlantic. More freshwater in the North Atlantic means the AMOC loses enthusiasm. But it also signals that oceans around the globe may be poised to rise abruptly, because locked inside the Greenland Ice Sheet are about 24 feet of global sea level rise. And Greenland has apparently already melted enough to substantially affect the AMOC.

Even if the ice sheet takes a thousand years to melt completely, armadas of icebergs from Greenland can do a lot of damage in a matter of decades.

July 2024 photo taken from a helicopter of a river chock-full of icebergs calved by the Jakobshavn glacier, Greenland. The glacier is in the background.

The AMOC enters the narrative again here, because if it slows that means more warm water staying put in the Southern Ocean. As Johan Rockström, the director of the Potsdam Climate Impact Research, said last month, this will "lock in" more "warm saline surface water in the Southern Ocean, which can explain the rapid melt, the more rapid than expected melt of the West Antarctic ice sheet," and another potential 16 feet of global sea level rise.

The AMOC connects different tipping elements, in effect. 

Iceberg, evening, Greenland, July 2024.

None of these events will proceed in a linear fashion. That's the nature of tipping points, which shift us abruptly into different states. But we can put all the risk on the table by ensuring policymakers understand how substantial these risks are, and communicating them clearly to the public. 

There are fundamental problems right now when it comes to assessment: Rahmstorf is apparently frustrated with the quality of UN and other climate models that "don't fully include the meltwater coming from the increasingly rapidly melting Greenland ice sheet." His Utrecht colleague, van Westen, suggests it would make sense to carefully measure salt levels on the southern edge of the Atlantic in estimating the chance of an AMOC tipping point, and concludes that "the probability of an AMOC collapse before the year 2100 is very likely to be underestimated in the [current IPCC report] and needs to be reconsidered" in the next one. 

As Carolyn Kousky reminded us during a webinar this past week, risk management requires bringing together risk assessment, communication, reduction, and transfer.  So far, although there's been a good deal of focus on transferring risk through insurance vehicles along US coastlines, we haven't made enough substantial progress on any of the other three elements.

On the assessment front, the alarms are sounding. Nothing will slow these giant forces.

Glacier, iceberg river, rock wall. Greenland, July 2024. Photo: Peter Davies.

Iceberg fragment, Jul. 2024. Photo: Peter Davies.