An important system of currents in the Atlantic Ocean could collapse this century under the pressure of human-driven climate change, triggering “a major tipping element in the climate system” that will have devastating impacts on people and ecosystems, reports a new study.
The new research reveals a far grimmer view of the North Atlantic’s future than the most recent predictions from the Intergovernmental Panel on Climate Change (IPCC) and underscores the urgency of transitioning away from the consumption of fossil fuels to curb the greenhouse gas emissions that are driving warming global temperatures.
The Atlantic meridional overturning circulation (AMOC) acts like a huge conveyor belt that delivers warm water from the equator to the poles and brings cold water back southward to the equator. In the process, the AMOC heats up Europe and cools the tropics, providing milder and more stable climate conditions in both regions.
The AMOC has been circulating since the end of the last ice age some 12,000 years ago, though scientists have found evidence that earlier collapses of the system set off sudden temperature extremes. In recent years, the AMOC has shown signs that it is weakening, leading the most recent IPCC report to conclude that it is on track to collapse sometime after the dawn of the 22nd century.
Now, physicist Peter Ditlevsen and mathematician Susanne Ditlevsen, who are siblings and researchers at the University of Copenhagen, have presented new evidence that the AMOC may collapse decades sooner.
The pair “predict with high confidence the tipping to happen as soon as mid-century,” with a 95 percent chance of occurring anytime between 2025 to 2095, assuming a “business-as-usual” future in which greenhouse gas emissions are not reduced, according to a study published on Tuesday in Nature Communications.
“The AMOC is an extremely important part of the global climate,” Susanne Ditlevsen told Motherboard in an email. “Previous studies have found early warning signals that the AMOC is weakening. However, no study has given strict statistical confidence to these findings, and the timing has not been determined.”
“We asked ourselves if it would be possible to provide reliable predictions for the timing of a possible collapse based on observations by using advanced statistical methods. And indeed, to our own surprise, this was possible,” she added.
In a follow-up call with Motherboard, Ditlevsen noted that the IPCC report is based on complex models with a dizzying array of global climate parameters. In their study, the Ditlevsens aimed to hone in on the specific fate of the AMOC by extracting so-called “fingerprints” of the system’s strength from a 150-year dataset of sea surface temperatures. This specialized technique suggested that the AMOC is much closer to collapse than implied by the IPCC’s models, raising alarm bells about the future of the North Atlantic.
“We had expected to confirm what was found in the IPCC report, so we were actually quite surprised,” Ditlevsen said in the call. “We did not expect to find such an early indication of collapse. It's not a completely different result because [the IPCC] also predicts a collapse in the models, but it's an earlier result.”
If the AMOC does collapse in the coming century, it will unleash severe disasters for the North Atlantic region. Without this circulation of hot and cold water, the higher latitudes will get colder while the tropics will get warmer, a shift that will have unpredictable and destabilizing effects on the region.
“We would probably get a climate in Western Europe more like the climate in Alaska,” Ditlevsen said over email. “Other consequences could be that the heat transported northward by the AMOC stays in the tropics, heating these even more—on top of global warming. The larger temperature difference between subtropics and mid-latitudes will increase the strength of the jet-stream and could lead to intensified storminess.”
“How fast the change will be, when the collapse is reached, is not really known,” she continued. “Climate models give different estimates from decades to centuries. The problem is that we have not seen a collapse of the AMOC in the past 12.000 years. The collapses and restarts seen in the paleo-climatic record (ice-cores) from the last glacial period were extremely abrupt” including “estimated 10-15 degrees in a decade, whereas today we might expect a decline of 5-8 degrees (compared to 1.5 degrees in the past century).”
While it’s frightening to consider that we might cross this dangerous threshold within our lifetimes, Ditlevsen emphasized that the collapse of the AMOC could potentially be averted if humanity is able to rapidly transition away from fossil fuels. In the meantime, she and her colleagues will continue monitoring AMOC observations to refine their predictions of its possible collapse.
“The closer you are to the tipping point, the more accurate our predictions will be,” Ditlevsen said. “In five years, we will be much better at saying if we were wrong, of course, and we could have waited five years with putting [the study] out, but on the other hand, we think that we have an obligation to say something.”
“Of course, I hope we are wrong,” she concluded. “I sincerely hope we are wrong.”