Will The Day After Tomorrow happen sooner than we think?
A Nature paper this week suggesting an early collapse of a key ocean current system raised fears of a 'Day after Tomorrow'-style catastrophe. That's overplayed, but the reality is still very worrying
Could the collapse of an ocean current system really tip us back into the ice ages?
Hollywood blockbuster “The Day After Tomorrow”, released nearly 20 years ago, depicted North America in the grip of a sudden new ice age.
The movie followed Dennis Quaid as he made a dangerous cross-glacial trek to rescue Jake Gyllenhaal from flood-ravaged and frozen New York after the collapse of the North Atlantic circulation system or ‘AMOC’ (Atlantic Meridional Overturning Circulation) triggered climate chaos.
So it’s not surprising that people have been alarmed by a paper in Nature Communications this week suggesting the AMOC could, in reality, collapse around the middle of this century – even as early as 2025 – with potentially devastating consequences for the North Atlantic region.
A breakdown of these crucial systems of ocean currents is one of the most serious climate ‘tipping points’ and a topic much discussed and disputed amongst climate scientists. Last year the IPCC said a full collapse was “very unlikely” within this century. (Although, in IPCC-speak ‘very unlikely’ means a less than 10% chance, which is still quite high for an event with such devastating consequences).
Until recently, an imminent collapse of the system has been treated largely as science fiction, as in “The Day After Tomorrow” and its ilk.
And like the movie, a lot of this week’s coverage of the new research has dramatised the effects, taking some liberties with the likely speed and scale of impacts, creating confusion about how concerning the research really is.
Many media reports have equated an AMOC collapse with a complete collapse of the wind-driven Atlantic Gulf Stream, with which it overlaps. That said, it is the AMOC that transports the bulk of the heat through the ocean system, making it more important for climate than the Gulf Stream.
The AMOC repeatedly collapsed and restarted during the ice ages between 115,000 and 12,000 years ago, and if it collapsed again - or became much more unstable - there would definitely be drastic effects, even if far from the exaggerated CGI depictions of the movie.
What happens if we tip over this ‘tipping point’?
These effects include: severe disruptions to rainfall that billions of people depend on for growing crops in India, South America and West Africa; a sharp reduction in temperatures (in the region of 2-5˚ C) across Europe and much of the Northern Hemisphere; more severe storms; and a rise in sea-levels along the Atlantic coast of North America. It has also been shown to increase the variability of weather in the Pacific, interacting with ENSO (the El Nino Southern Oscillation).
But it is important to note that while scientists largely agree that there are increasing signs of instability or ‘flickering’ in the AMOC, signalling that a tipping point of some sort is approaching, the science is far from settled on the likely timing and scale of any ‘collapse’. Many recommend caution when interpreting the results of the latest study from researchers Peter Ditlevsen and Susanne Ditlevsen at the Niels Bohr Institute, University of Copenhagen.
For instance, Professor Andrew Watson at the Global Systems Institute, University of Exeter suggests that while the study highlights that the North Atlantic circulation is showing signs of instability, which might indicate that a collapse of the overturning could occur, it could also be less dramatic, “not a full-scale shutdown but a change in the sites of deep water formation, for example”.
Other scientists have pointed out the authors' own acknowledgement of the large uncertainty in their conclusions and that they cannot rule out a slowing rather than a collapse of the AMOC in the reported interval period of 2025-2095.
‘The risk is now higher than the IPCC’s 10% estimate’
At the same time opinions appear to be shifting away from the more conservative IPCC conclusion toward higher concern, with this latest paper the third in a row to suggest a transition may be closer than previously believed.
In a commentary for RealClimate, Professor Stefan Rahmstorf reviews the latest paper within the context of previous studies, concluding that while the timing of a critical AMOC transition is still highly uncertain, the evidence increasingly suggests that the risk is higher than the IPCC’s 10% estimate of something happening this century, and is very much a matter of concern over the next few decades.
Rahmstorf points out that the AMOC is weaker now that at any time in the past millennium and that the long-term weakening trend has anthropogenic origins.
So what is the AMOC and how does it work?
The AMOC is a large system of ocean currents that work as a conveyor belt to circulate water within the Atlantic Ocean, sending warm water north and cold water south. It is driven by differences in temperature and salinity (salt content) of water. As ice sheets in Greenland and North Antarctica melt, driven by the rising temperatures of climate change, they have deposited more than half-a-trillion tonnes of fresh water into the Northern Atlantic since 2000. That equates to six icy Olympic swimming pools worth of water every second.
Lack of salinity makes the water lighter, which disrupts the normal pattern of cold, dense water sinking kilometres deep into the ocean and slowly spreading southwards, as warm, heated water wells up in tropical latitudes and travels northward at the surface.
This warm surface water combines for a time with the gulf stream ocean current that runs up along the Eastern coast of the United States from the Gulf of Mexico and is driven largely by trade winds, before peeling off towards the northern Atlantic, carrying the bulk of the heat. This conveyor belt of warm surface water moving towards Europe affects weather and rainfall patterns making Europe warmer than its latitude alone would suggest and bringing rainfall to key crop growing areas in the Northern hemisphere.
How concerned should we be?
In short, very.
That’s because even a partial collapse of the Atlantic circulation would have serious implications for eco-systems and for humanity.
While there are questions over the conclusions of this latest report, there is a growing body of evidence of instability in these current systems, increasing the pressure to act on emissions. It all adds further support to the OECD’s recent call for unprecedented climate action to avoid the catastrophic impacts associated with climate system tipping points such as this.
Ka kite ano
Cathrine.
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Thank you for making this so rationale and understandable. This issue alone should be making governments, scientists, professors and other teachers use their positions to educate the mainstream. A good set of talking points to use in everyday conversations with the doubters would also be useful. This column will certainly help me around this specific topic - very much appreciated.
What drives me crazy in the mainstream response to reports like this one is that the uncertainty element is treated as a mitigating factor -- "well, clearly this may not happen, so..." The uncertainty is one of the scariest aspects of the research! When a serious article like your one can include the line "even as early as 2025", we're at the cliff's edge.