Senior scientists have warned that severe disruption to the Gulf Stream ocean currents crucial in controlling global climate must be avoided “at all costs,” senior scientists have warned. This week, the alert follows the revelation that the system is at its weakest ever recorded.
Past collapses of the giant network have seen some of the most extreme impacts in climate history, with western Europe particularly vulnerable to a descent into freezing winters. A significantly weakened system is also likely to cause more severe storms in Europe, faster sea-level rise on the east coast of the US, and increasing drought in the Sahel in Africa.
The new research worries scientists because of the massive impact of global warming on the currents and the unpredictability of a future “tipping point.”
The currents bring warm Atlantic water northwards.
The currents that bring warm Atlantic water northwards towards the pole, where they cool, sink and return southwards, are the most significant control on the northern hemisphere climate outside the atmosphere. But the system, formally called the Atlantic Meridional Overturning Circulation (Amoc), has weakened by 15% since 1950, thanks to melting Greenland ice and ocean warming, making sea water less dense and more buoyant.
This represents a massive slowdown – equivalent to halting all the world’s rivers three times over or stopping the most significant river, the Amazon, 15 times. Such weakening has not been seen in at least the last 1,600 years, which is as far back as researchers have analyzed so far. Furthermore, the new analyses show the weakening is accelerating.
“From the study of past climate, we know changes in the Amoc have been some of the most abrupt and impactful events in the history of climate,” said Prof Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research in Germany and one of the world’s leading oceanographers. They showed some of the new research. During the last Ice Age, winter temperatures changed by up to 10C within three years in some places.
“We are dealing with a system that in some aspects is highly non-linear, so fiddling with it is very dangerous because you may well trigger some surprises,” he said. “I wish I knew where this critical tipping point is, but that is unfortunately just what we don’t know. We should avoid disrupting the Amoc at all costs, which is another reason we should stop global warming as soon as possible.”
A collapse in the Amoc would mean far less heat reaching western Europe and plunge the region into severe winters. The scenario is depicted significantly in the movie The Day After Tomorrow, and a widespread collapse of deep-sea ecosystems has also been seen in the past.
But as the Amoc weakens, it might increase summer heatwaves. That is because it takes time for the northern waters to cause cooling over the adjacent lands. However, the calmer waters affect the atmosphere in a way that helps warm air flood into Europe from the south, a situation already seen in 2015.
This week’s new research showed that Greenland’s massive ice cap is melting at the fastest rate for at least 450 years. This influx will continue to weaken the Amoc into the future until human-caused climate change is halted, but scientists do not know how fast the weakening will be or when it will reach the point of collapse.
“Many people have tried to check that with computer models,” said Rahmstorf. “But they differ greatly because it depends on a very subtle balance of density – temperature and salinity distribution in the ocean. We are not able to model this with any confidence right now.”
“We are hoping to make some headway somehow, but I have been in this area for more than 20 years now, and we still don’t understand why the models differ so much in the sensitivity of the Amoc,” he said.
However, Rahmstorf said the international climate deal agreed in 2015 offers some hope if its ambition is increased and achieved: “If we can keep the temperature rise to well below 2C as agreed in the Paris agreement, I think we run a small risk of crossing this collapse tipping point.”