Essay by Eric Worrall
All bad weather is our fault…
Gulf stream could collapse as early as 2025, study suggests
A collapse would bring catastrophic climate impacts but scientists disagree over the new analysis
The Gulf Stream system could collapse as soon as 2025, a new study suggests. The shutting down of the vital ocean currents, called the Atlantic Meridional Overturning Circulation (Amoc) by scientists, would bring catastrophic climate impacts.
The new analysis estimates a timescale for the collapse of between 2025 and 2095, with a central estimate of 2050, if global carbon emissions are not reduced. Evidence from past collapses indicate changes of temperature of 10C in a few decades, although these occurred during ice ages.
“I think we should be very worried,” said Prof Peter Ditlevsen, at the University of Copenhagen in Denmark, and who led the new study. “This would be a very, very large change. The Amoc has not been shut off for 12,000 years.”
The abstract of the study;
Warning of a forthcoming collapse of the Atlantic meridional overturning circulation
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. Predictions based on observations rely on detecting early-warning signals, primarily an increase in variance (loss of resilience) and increased autocorrelation (critical slowing down), which have recently been reported for the AMOC. Here we provide statistical significance and data-driven estimators for the time of tipping. We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions.
If the AMOC collapse occurred it would likely be really bad, temperatures in Europe and North America could plunge, and Europe and North America would likely experience ice age like conditions which could last hundreds, or even thousands of years.
An AMOC collapse is believed to have caused the Younger Dryas, an abrupt return to Northern Hemisphere ice age conditions which occurred 12,900 years ago, which lasted over a thousand years.
But AMOC collapse is a bit like end of snow predictions or ice free Arctic predictions. Climate models predict an Atlantic meridional overturning circulation collapse, so it must happen, right? I mean, we’d never expect climate scientists to perhaps mistake a natural cycle for a dramatic one way shift?
The scientists in the body of the study above admit continuous monitoring of the AMOC only started in 2004, which seems a pretty short baseline to make long term forecasts. But there have been other attempts to reconstruct the recent history of the AMOC, which have produced far less conclusive results.
The evolution of the North Atlantic Meridional Overturning Circulation since 1980
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate through its transport of heat in the North Atlantic Ocean. Decadal changes in the AMOC, whether through internal variability or anthropogenically forced weakening, therefore have wide-ranging impacts. In this Review, we synthesize the understanding of contemporary decadal variability in the AMOC, bringing together evidence from observations, ocean reanalyses, forced models and AMOC proxies. Since 1980, there is evidence for periods of strengthening and weakening, although the magnitudes of change (5–25%) are uncertain. In the subpolar North Atlantic, the AMOC strengthened until the mid-1990s and then weakened until the early 2010s, with some evidence of a strengthening thereafter; these changes are probably linked to buoyancy forcing related to the North Atlantic Oscillation. In the subtropics, there is some evidence of the AMOC strengthening from 2001 to 2005 and strong evidence of a weakening from 2005 to 2014. Such large interannual and decadal variability complicates the detection of ongoing long-term trends, but does not preclude a weakening associated with anthropogenic warming. Research priorities include developing robust and sustainable solutions for the long-term monitoring of the AMOC, observation–modelling collaborations to improve the representation of processes in the North Atlantic and better ways to distinguish anthropogenic weakening from internal variability. The Atlantic Meridional Overturning Circulation (AMOC) has a key role in the climate system. This Review documents AMOC variability since 1980, revealing periods of decadal-scale weakening and strengthening that differ between the subpolar and subtropical regions.
One thing which is missing from today’s forecasts of imminent AMOC collapse is a large body of fresh water which could be the potential trigger for the collapse. The Younger Dryas collapse in Northern Hemisphere temperatures was believed to have been caused by disruption to ocean currents which occurred when a gigantic glacial lake sitting on the North American and Canadian ice sheet abruptly discharged thousands of cubic miles of water into the Atlantic Ocean, though there is evidence a lot of fresh water may have ended up in the Arctic Ocean.
Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean
The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change1,2,3,4. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broecker et al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas 1, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”2. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the North Atlantic Ocean.
Read more: https://www.nature.com/articles/nature08954
If say the Greenland Ice Sheet were to form a gigantic glacial lake comparable to Lake Agassiz, that would be a major cause for concern. Such a lake could potentially deliver a devastating impulse of fresh water to the far North, potentially triggering a replay of the environmental catastrophe which some believe caused the Younger Dryas mini ice age.
But to my knowledge, no similar glacial lake exists in today’s world.
In the absence of a credible source of a fresh water impulse on the scale of Lake Agassiz, the lack of clear understanding of exactly what happened 12,900 years ago, short evidential baselines, and the absence of firm observational evidence of a looming AMOC collapse, lets say I’m not about to lose sleep over claims we are approaching an imminent major climate tipping point.
For more information on climate uncertainties and risks, click here.