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A visualisation of Atlantic Ocean currents based on sea surface temperature data<\/p>\n
NASA\/Goddard Space Flight Center Scientific Visualization Studio<\/p>\n<\/p>\n
A potentially catastrophic collapse of the Atlantic Ocean currents that control Europe\u2019s climate may already be inevitable. Based on model simulations, researchers estimate that there is a 10 to 23 per cent chance that such a collapse is locked in.<\/p>\n
\u201cThere is a significant probability that we\u2019re already committed to collapse, and we can\u2019t change that even now,\u201d says Phil Holden<\/a> at the Open University, UK.<\/p>\n The Atlantic Meridional Overturning Circulation (AMOC) carries warm, salty water from the tropics into the North Atlantic Ocean, where it cools, sinks and then returns south. This circulation regulates the climate across Europe, Africa and the Americas.<\/p>\n Recently, there have been signs that this vital current system is weakening<\/a>, including by slowing in some areas<\/a>, partly because the melting of the Greenland<\/a> ice sheet caused by climate change is making the salty water less dense, so that it sinks more slowly.<\/p>\n Some scientists have suggested that the AMOC could collapse<\/a>, plunging Europe into near-Arctic conditions and weakening monsoon systems around the world. One\u00a0recent study\u00a0found the AMOC could cross a tipping point within decades<\/a>, but it is difficult to say how likely this is.<\/p>\n \u201cThe AMOC collapse has just been so intangible,\u201d says Holden. \u201cSo far there\u2019s been no firm quantification of whether it is going to happen or when it is going to happen.\u201d<\/p>\n \u201cThere\u2019s a lot of uncertainty, and a lot of different opinions among the expert scientific community,\u201d says Tim Lenton<\/a> at the University of Exeter, UK.<\/p>\n To get a better idea of what might happen to AMOC, Holden, Lenton and their colleagues ran 21 computer simulations with varying rates of Greenland ice melt and emissions peaking at different dates, at 10-year intervals from 2005 through to 2135. The team assumed that, after the peak, greenhouse gas emissions would fall to net zero over 35 years while the melt rate of Greenland ice would stay constant. Each simulation ran for a total of 300 years.<\/p>\n The models indicate that under very conservative assumptions \u2013 emissions peaking in 2025 and the Greenland ice sheet adding just 54 millimetres to sea-level rise by 2100 \u2013 there is a 10 per cent chance that the collapse of the AMOC is already inevitable. The researchers defined this as when the circulation would only occur at lower latitudes, and when the overturning current that brings heat to the high latitudes has stopped.<\/p>\n If we don\u2019t start on the path to net zero until 2100, the probability of collapse rises to 80 per cent, the model predicts.<\/p>\n Under less conservative assumptions, with melting ice from Greenland adding 274 mm to sea-level rise, as is projected by 2100<\/a>, the probability that\u00a0we are already committed to collapse is 23 per cent.<\/p>\n Even when a collapse becomes inevitable, it would take a long time to happen. In the simulations, the average delay between the year in which the world becomes committed to collapse and the collapse coming to pass was 84 years, with the earliest collapse occurring around 2060.<\/p>\n \u201cThis idea of talking about committed collapse, rather than when the collapse actually happens, frames it in a way that\u2019s quite interesting for risk management,\u201d says Till Wagner<\/a> at the University of Wisconsin-Madison, but he is cautious about extrapolating to the real world. \u201cI think there\u2019s fairly good evidence that there\u2019s going to be a weakening, but the actual larger-scale dynamical outcome is still very much up in the air.\u201d<\/p>\n Jonathan Baker<\/a> at the Met Office, the UK\u2019s national weather service, says the simulations provide a valuable way to explore how the AMOC responds to different scenarios, but the low resolution of the model used means it isn\u2019t as sensitive as some other climate models, which might influence its estimates of risk.<\/p>\n Many state-of-the-art climate simulations can compute the globe in grids of 1\u00b0 of latitude and longitude, which takes huge amounts of computational resources and time to run long-term simulations. The model used in this study uses 5\u00b0 grids, but this low resolution was a deliberate choice, says Lenton.<\/p>\n \u201cThere isn\u2019t the compute power, or no one\u2019s had the ability to do this exercise with a higher-resolution model,\u201d he says. This does mean the probabilities of the risk estimates could be different at higher resolution, he says, but recent research on the AMOC using a higher-resolution model<\/a> indicates that, if anything, this might raise rather than lower the estimates.<\/p>\n \u201cFurther work using multiple climate models and comparison with the wider body of evidence will be important before drawing reliable conclusions about the magnitude of future AMOC collapse risk,\u201d says Baker.<\/p>\n If there is a chance the world is already committed to AMOC collapse, as the model suggests, this should provide extra incentive to cut emissions, says Lenton. This is because the model indicates that the probability of AMOC collapse rises sharply if net zero is delayed. If emissions continue unabated for 10 extra years beyond the point of commitment, the actual collapse would happen faster \u2013 after 57 years, on average, rather than 84.<\/p>\n \u201cWhat the model is saying to me is \u2018let\u2019s do everything in our power to get to net zero as quickly as possible to try to keep this probability down at the 10 per cent level\u2019,\u201d says Lenton.<\/p>\n