What's Up With The AMOC?
The Atlantic Meridional Overturning Circulation is part of the global ocean circulation and important to the weather and climate of the planet. It is at risk of collapse or slowing, and possibly soon.
A couple of days ago, 44 climate scientists signed and presented a letter to the Nordic Council of Ministers.
Their message:
You can find the entire letter here!
Given this and the dire scientific research that has been coming out, I wanted to talk about the AMOC and the latest scientific research on this significant circulation pattern.
On a side note: It’s interesting to me because this letter came out as I was writing this post…
Maybe it’s just my circles that keep talking about the AMOC (Atlantic Meridional Overturning Circulation) lately… or maybe it’s because I’m constantly looking at the latest research that I see AMOC everywhere and have been thinking about it a lot.
But whatever the reason, I wanted to share this sooner than later!
And even if you haven’t heard about the AMOC before, I’m here to give you all the details in this edition of “What’s Up…”.
What is the AMOC?
To dive into this topic, we need an explanation of what the Atlantic Meridional Overturning Circulation even is.
And to do this, we need this piece of knowledge: the oceans around the world are constantly circulating through currents.
There are localized currents and currents that occur close to shore or on the surface. And there are large ocean currents.
Globally the oceans are constantly moving, and move as the Global Conveyor Belt.
Video credit: NOAA and NASA Goddard Space Flight Center
The Global Conveyor Belt is a combination of surface conditions and thermohaline currents. When you break this apart, we can understand the term a bit better!
Thermo = temperature and haline = salinity.
Basically, cold, salty, dense waters sink deeper to the bottom of the ocean. Meanwhile, warm, less dense waters stay on top.
And therefore the thermohaline currents power the global conveyor belt of water.
The Route AMOC Takes
When it comes to the AMOC, this is the circulation that occurs across the Atlantic Ocean and is part of the global thermohaline circulation.
It brings water from north to south and back again. And it has both surface and deep water currents and is a complex multi-step route with various extensions, paths and changes.
A Super Simplistic Explanation of The AMOC’s Multi-Step Route
I’m going to put it simply when I explain the AMOC and the general route it takes. And to do this, we are going to start near the United States.
There, warm salty waters travel north along the U.S. East Coast as the Gulf Stream, travelling from the Florida Straits to the Grand Banks off Newfoundland. The Gulf Stream meanders as it goes, there are various eddies and extensions as well as warm and cold water interact.
The Gulf Stream then merges with the North Atlantic Current which flows towards northwestern Europe across the Atlantic. It is driven by topography and does meander as it goes. And as temperatures change the North Atlantic Current breaks in various directions including into the Canary Current, the Irminger Current and the Norwegian Current (depending on location).
As this warmer water reaches the North Atlantic it loses heat to the atmosphere (which spoiler alert: keeps the air in this region aka Europe warmer). Near Greenland, Iceland and northern Europe, in the Labrador and Norwegian Seas, the water cools and some of it freezes (when seawater freezes it is almost entirely water, increasing the salt concentration around it. This colder and saltier water then sinks (downwells) becoming part of the North Atlantic Deep Water (NADW).
It is then that the water starts to move back south through the Atlantic towards Antarctica where it feeds into the Antarctic Circumpolar Current.
Some of it upwells though and returns to the surface. A surface current west of Africa moves back north, through the Gulf of Mexico and then again merges into the Gulf Stream.
Again, this is a very simplified version of this part of the global conveyor belt, but it gives you the information you need.
How Does AMOC Influence The Climate Globally?
The AMOC regulates temperatures and weather across continents by redistributing heat between the equator and the poles. This is very important, and the reason that scientists are concerned about it!
This circulation influences both regional and global climate patterns in several key ways. First, the Gulf Stream and North Atlantic Current carry warm waters from the tropics to Europe, preventing much of Western Europe from experiencing the harsher winters typical of similar latitudes in North America. This heat makes Western Europe’s winters milder than places like Canada or Russia, which are at comparable latitudes.
In the tropics, the AMOC helps to cool the region by transporting heat north. Without this heat redistribution, tropical regions would experience more extreme warming and temperatures.
At the poles, the AMOC helps keep Arctic and North Atlantic waters cooler, which slows ice melt and stabilizes sea levels.
Additionally, the AMOC has been shown to play a role in stabilizing monsoon systems in Africa and South Asia by maintaining a consistent heat balance in the oceans. And also has impacts on the Atlantic hurricane season…
So What is Going on with the AMOC?
Well, more and more research is coming out that there could be a slowing or “collapse” of the AMOC, and that it could happen sooner than later.
Some people tout collapse as “The Day After Tomorrow”. But that wouldn’t exactly be what happens.
This major issue starts with the fact that the Arctic is warming two to four times as fast as the rest of the planet. And as it warms, there is more ice melt, more freshwater in the region and generally warmer conditions.
This means that the ocean water moving in this region due to the AMOC mixes with more freshwater, is warmer and freezes less.
So the water in this region that would normally be cold, salty and dense, becomes less salty and less dense. This means that it sinks slower or not at all.
And remember from above… the water needs to sink (downwelling) and move south into the deeper ocean.
So if the water sinks slower or not at all, then it could slow or stop AMOC completely.
And if that happens, well it would have major impacts on the climate around the world, particularly in regions in the North Atlantic like Europe.
What Are Scientists Saying?
It is easiest to show you what the science is saying when it comes to this topic with some quotes from the research. And there has been a lot of research, especially lately!
First of all, I want to show you what the scientists wrote in their letter to the Nordic Council of Ministers:
Given the increasing evidence for a higher risk of an AMOC collapse, we believe it is of critical importance that Arctic tipping point risks, in particular the AMOC risk, are taken seriously in governance and policy. Even with a medium likelihood of occurrence, given that the outcome would be catastrophic and impacting the entire world for centuries to come, we believe more needs to be done to minimize this risk.
The impacts particularly on Nordic Countries would likely be catastrophic, including major cooling in the region while surrounding regions warm (Figure) . This would be an enlargement and deepening of the ‘cold blob’ that already has developed over the subpolar Atlantic Ocean, and likely lead to unprecedented extreme weather. While the impacts on weather patterns, ecosystems and human activities warrant further study, they would potentially threaten the viability of agriculture in northwestern Europe.
This comes after a large body of research on the matter has been published, but they also call for more research. This letter again comes from some of the biggest names in climate.
In a paper published in Arvix titled Probability Estimates of a 21st Century AMOC Collapse which can be found here, scientists noted:
Based on the reanalysis products, we next determine probability density functions of the AMOC collapse time. The collapse time is estimated between 2037-2064 (10-90% CI) with a mean of 2050 and the probability of an AMOC collapse before the year 2050 is estimated to be 59 ± 17%.
There is always verification that happens, and this study is still being looked at by other scientists in the space. But based on their findings, we could see a 42-76% chance of AMOC collapsing before 2050.
In another study published in Nature titled Warning of a forthcoming collapse of the Atlantic Meridional Overturning Circulation which can be found here, scientists noted:
The Atlantic meridional overturning circulation (AMOC) is a major tipping element in the climate system and a future collapse would have severe impacts on the climate in the North Atlantic region. In recent years weakening in circulation has been reported, but assessments by the Intergovernmental Panel on Climate Change (IPCC), based on the Climate Model Intercomparison Project (CMIP) model simulations suggest that a full collapse is unlikely within the 21st century. Tipping to an undesired state in the climate is, however, a growing concern with increasing greenhouse gas concentrations.
This study notes that a full collapse is unlikely, but an undesired state is a growing concern.
Nature also saw another published article titled Observed fingerprint of a weakening Atlantic Ocean overturning circulation which can be found here. Scientists in this study noted:
Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century.
From this research, there is evidence that the AMOC is weakening and we have seen it already happening.
And one last paper I’m going to note was published in Science, titled Exceeding 1.5°C global warming could trigger multiple climate tipping points. You can find the excerpt here and they noted a lot of uncertainty on at which temperature we are going to see impacts/collapse:
Given the wide range and lack of agreement amongst models, as well as potential complexities from sensitivity to freshwater forcing location, rate-dependence, and a possibly ‘fractal’ threshold, we assign low confidence to this threshold range.
For AMOC collapse timescales, we adopt the 50y time horizon observed in models showing abrupt hysteretic-type collapses, with a lower bound of ~15y based on extreme forcing and an upper bound of 300y based on the more gradual AMOC collapses observed in many models. We assess this timescale range as having medium confidence given their basis in multiple different models.
What can we do?
If we want to stop ourselves from passing climate tipping points and disrupting major climate systems on the planet we need urgent climate action to reduce emissions.
We also need to prepare for what could happen if the AMOC collapses… and take valuable and urgent action to do just that.
Thank you for your gentle clarity here!