The Guardian – 25 July, 2023
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.
Amoc was already known to be at its weakest in 1,600 years owing to global heating and researchers spotted warning signs of a tipping point in 2021.
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 indicates changes of temperature of 10C in a few decades, although these occurred during ice ages.
Other scientists said the assumptions about how a tipping point would play out and uncertainties in the underlying data are too large for a reliable estimate of the timing of the tipping point. But all said the prospect of an Amoc collapse was extremely concerning and should spur rapid cuts in carbon emissions.
Amoc carries warm ocean water northwards towards the pole where it cools and sinks, driving the Atlantic’s currents. But an influx of fresh water from the accelerating melting of Greenland’s ice cap and other sources is increasingly smothering the currents.
A collapse of Amoc would have disastrous consequences around the world, severely disrupting the rains that billions of people depend on for food in India, South America and west Africa. It would increase storms and drop temperatures in Europe, and lead to a rising sea level on the eastern coast of North America. It would also further endanger the Amazon rainforest and Antarctic ice sheets.
“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 Amoc collapsed and restarted repeatedly in the cycle of ice ages that occurred from 115,000 to 12,000 years ago. It is one of the climate tipping points scientists are most concerned about as global temperatures continue to rise.
Research in 2022 showed five dangerous tipping points may already have been passed due to the 1.1C of global heating to date, including the shutdown of Amoc, the collapse of Greenland’s ice cap and an abrupt melting of carbon-rich permafrost.
The new study, published in the journal Nature Communications, used sea surface temperature data stretching back to 1870 as a proxy for the change in strength of Amoc currents over time.
The researchers then mapped this data on to the path seen in systems that are approaching a particular type of tipping point called a “saddle-node bifurcation”. The data fitted “surprisingly well”, Ditlevsen said. The researchers were then able to extrapolate the data to estimate when the tipping point was likely to occur. Further statistical analysis provided a measure of the uncertainty in the estimate.
The analysis is based on greenhouse gas emissions rising as they have done to date. If emissions do start to fall, as intended by current climate policies, then the world would have more time to try to keep global temperature below the Amoc tipping point.
The most recent assessment by the Intergovernmental Panel on Climate Change concluded that Amoc would not collapse this century. But Ditlevsen said the models used have coarse resolution and are not adept at analysing the non-linear processes involved, which may make them overly conservative.
The potential collapse of Amoc is intensely debated by scientists, who have previously said it must be avoided “at all costs”.