Large parts of Europe in a deep freeze is more likely and closer than before caused by an abrupt shutdown of Atlantic Ocean currents, finds a new study.
The study finds a “cliff-like” tipping point is looming in the future.
An AP report said:
‘A long-worried nightmare scenario, triggered by Greenland’s ice sheet melting from global warming, still is at least decades away if not longer, but maybe not the centuries that it once seemed, finds the study published in Friday’s Science Advances (SCIENCE ADVANCES, 9 Feb 2024, Vol 10, Issue 6, DOI: 10.1126/sciadv.adk1189, https://www.science.org/doi/10.1126/sciadv.adk1189).
‘The study, the first to use complex simulations and include multiple factors, uses a key measurement to track the strength of vital overall ocean circulation, which is slowing.
‘A collapse of the current — called the Atlantic Meridional Overturning Circulation or AMOC — would change weather worldwide because it means a shutdown of one of key the climate and ocean forces of the planet. It would plunge northwestern European temperatures by 9 to 27 degrees (5 to 15 degrees Celsius) over the decades, extend Arctic ice much farther south, turn up the heat even more in the Southern Hemisphere, change global rainfall patterns and disrupt the Amazon, the study said. Other scientists said it would be a catastrophe that could cause worldwide food and water shortages.’
The AP report said:
‘“We are moving closer (to the collapse), but we are not sure how much closer,” said study lead author Rene van Westen, a climate scientist and oceanographer at Utrecht University in the Netherlands. “We are heading towards a tipping point.”
‘When this global weather calamity — grossly fictionalized in the movie “The Day After Tomorrow” — may happen is “the million-dollar question, which we unfortunately can’t answer at the moment,” van Westen said. He said it is likely a century away but still could happen in his lifetime. He just turned 30.
‘Studies have shown the AMOC to be slowing, but the issue is about a complete collapse or shutdown. The United Nations’ Intergovernmental Panel on Climate Change, which is a group of hundreds of scientists that gives regular authoritative updates on warming, said it has medium confidence that there will not be a collapse before 2100 and generally downplayed disaster scenarios. But van Westen, several outside scientists and a study last year say that may not be right.
‘Stefan Rahmstorf, head of Earth Systems Analysis at the Potsdam Institute for Climate Research in Germany, was not part of the research, but called it “a major advance in AMOC stability science.”
‘“The new study adds significantly to the rising concern about an AMOC collapse in the not too distant future,” Rahmstorf said in an email. “We will ignore this at our peril.”
‘University of Exeter climate scientist Tim Lenton, also not part of the research, said the new study makes him more concerned about a collapse.
‘An AMOC collapse would cause so many ripples throughout the world’s climate that are “so abrupt and severe that they would be near impossible to adapt to in some locations,” Lenton said.’
The AP report said:
‘There are signs showing that the AMOC has collapsed in the past, but when and how it will change in the future is still uncertain, said U.S. National Oceanic and Atmospheric Administration oceanographer Wei Cheng, who was not part of the research.
‘The AMOC is part of an intricate global conveyor belt of ocean currents that move different levels of salt and warm water around the globe at different depths in patterns that helps regulate Earth’s temperature, absorbs carbon dioxide and fuels the water cycle, according to NASA.
‘When the AMOC shuts down, there is less heat exchanged across the globe and “it really impacts Europe quite severely,” van Westen said.
‘For thousands of years, Earth’s oceans have relied on a circulation system that runs like a conveyor belt. It is still going but slowing.
‘The engine of this conveyor belt is off the coast of Greenland, where, as more ice melts from climate change, more freshwater flows into the North Atlantic and slows everything down, van Westen said. In the current system, cold deeper fresher water heads south past both Americas and then east past Africa. Meanwhile saltier warmer ocean water, coming from the Pacific and Indian oceans, pushes past the southern tip of Africa, veers to and around Florida and continues up the U.S. East Coast on up to Greenland.
‘The Dutch team simulated 2,200 years of its flow, adding in what human-caused climate change does to it. They found after 1,750 years “an abrupt AMOC collapse,” but so far are unable to translate that simulated timeline to Earth’s real future. Key to monitoring what happens is a complicated measurement of flow around the tip of Africa. The more negative that measurement, the slower AMOC runs.
‘”This value is getting more negative under climate change,” van Westen said. When it reaches a certain point it is not a gradual stop but something that is “cliff-like,” he said.
‘The world should pay attention to potential AMOC collapse, said Joel Hirschi, division leader at the United Kingdom’s National Oceanography Centre. But there is a bigger global priority, he said.
‘“To me, the rapidly increasing temperatures we have been witnessing in recent years and associated temperature extremes are of more immediate concern than the AMOC shutting down,” Hirschi said. “The warming is not hypothetical but is already happening and impacting society now.”’
ATLANTIC OCEAN IS HEADED FOR A TIPPING POINT
René van Westen, Henk A. Dijkstra and Michael Kliphuis write:
‘Superstorms, abrupt climate shifts and New York City frozen in ice. That’s how the blockbuster Hollywood movie “The Day After Tomorrow” depicted an abrupt shutdown of the Atlantic Ocean’s circulation and the catastrophic consequences.’ (René van Westen, Utrecht University; Henk A. Dijkstra, Utrecht University, and Michael Kliphuis, Utrecht University, Atlanic Ocean is headed for a tipping point − Once melting glaciers shut down the Gulf Stream, we would see extreme climate change within decades, study shows, The Conversation, Sat, Feb. 10, 2024)
‘If global warming shuts down the Atlantic Meridional Overturning Circulation, which is crucial for carrying heat from the tropics to the northern latitudes, how abrupt and severe would the climate changes be?’
The scientists write:
‘We know a lot more about the Atlantic Ocean’s circulation. Instruments deployed in the ocean starting in 2004 show that the Atlantic Ocean circulation has observably slowed over the past two decades, possibly to its weakest state in almost a millennium. Studies also suggest that the circulation has reached a dangerous tipping point in the past that sent it into a precipitous, unstoppable decline, and that it could hit that tipping point again as the planet warms and glaciers and ice sheets melt.
‘In a new study using the latest generation of Earth’s climate models, we simulated the flow of fresh water until the ocean circulation reached that tipping point.
‘The results showed that the circulation could fully shut down within a century of hitting the tipping point, and that it is headed in that direction. If that happened, average temperatures would drop by several degrees in North America, parts of Asia and Europe, and people would see severe and cascading consequences around the world.
‘We also discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation is nearing its tipping point.’
THE OCEAN’S CONVEYOR BELT
The scientists write:
‘Ocean currents are driven by winds, tides and water density differences.
‘In the Atlantic Ocean circulation, the relatively warm and salty surface water near the equator flows toward Greenland. During its journey it crosses the Caribbean Sea, loops up into the Gulf of Mexico, and then flows along the U.S. East Coast before crossing the Atlantic.
‘This current, also known as the Gulf Stream, brings heat to Europe. As it flows northward and cools, the water mass becomes heavier. By the time it reaches Greenland, it starts to sink and flow southward. The sinking of water near Greenland pulls water from elsewhere in the Atlantic Ocean and the cycle repeats, like a conveyor belt.
‘Too much fresh water from melting glaciers and the Greenland ice sheet can dilute the saltiness of the water, preventing it from sinking, and weaken this ocean conveyor belt. A weaker conveyor belt transports less heat northward and also enables less heavy water to reach Greenland, which further weakens the conveyor belt’s strength. Once it reaches the tipping point, it shuts down quickly.’
WHAT HAPPENS TO THE CLIMATE AT THE TIPPING POINT?
The scientists write:
‘The existence of a tipping point was first noticed in an overly simplified model of the Atlantic Ocean circulation in the early 1960s. Today’s more detailed climate models indicate a continued slowing of the conveyor belt’s strength under climate change. However, an abrupt shutdown of the Atlantic Ocean circulation appeared to be absent in these climate models.
‘This is where our study comes in. We performed an experiment with a detailed climate model to find the tipping point for an abrupt shutdown by slowly increasing the input of fresh water.
‘We found that once it reaches the tipping point, the conveyor belt shuts down within 100 years. The heat transport toward the north is strongly reduced, leading to abrupt climate shifts.’
DANGEROUS COLD IN THE NORTH AND THE AMAZON FOREST WILL BE VULNERABLE
‘Regions that are influenced by the Gulf Stream receive substantially less heat when the circulation stops. This cools the North American and European continents by a few degrees.
‘The European climate is much more influenced by the Gulf Stream than other regions. In our experiment, that meant parts of the continent warmed at more than 5 degrees Fahrenheit (3 degrees Celsius) per decade – far faster than today’s global warming of about 0.36 F (0.2 C) per decade. We found that parts of Norway would experience temperature drops of more than 36 F (20 C). ‘On the other hand, regions in the Southern Hemisphere would warm by a few degrees.
‘These temperature changes develop over about 100 years. That might seem like a long time, but on typical climate time scales, it is abrupt.
‘The conveyor belt shutting down would also affect sea level and precipitation patterns, which can push other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to declining precipitation. If its forest ecosystem turned to grassland, the transition would release carbon to the atmosphere and result in the loss of a valuable carbon sink, further accelerating climate change.
‘The Atlantic circulation has slowed significantly in the distant past. During glacial periods when ice sheets that covered large parts of the planet were melting, the influx of fresh water slowed the Atlantic circulation, triggering huge climate fluctuations.’
WHEN WILL WE REACH THIS TIPPING POINT?
The scientists write:
‘The big question – when will the Atlantic circulation reach a tipping point – remains unanswered. Observations do not go back far enough to provide a clear result. While a recent study suggested that the conveyor belt is rapidly approaching its tipping point, possibly within a few years, these statistical analyses made several assumptions that give rise to uncertainty.
‘Instead, we were able to develop a physics-based and observable early warning signal involving the salinity transport at the southern boundary of the Atlantic Ocean. Once a threshold is reached, the tipping point is likely to follow in one to four decades.
‘The climate impacts from our study underline the severity of such an abrupt conveyor belt collapse. The temperature, sea level and precipitation changes will severely affect society, and the climate shifts are unstoppable on human time scales.
‘It might seem counterintuitive to worry about extreme cold as the planet warms, but if the main Atlantic Ocean circulation shuts down from too much meltwater pouring in, that’s the risk ahead.’
The study report, Physics-based early warning signal shows that AMOC is on tipping course (SCIENCE ADVANCES, 9 Feb 2024, Vol 10, Issue 6, DOI: 10.1126/sciadv.adk1189), said in its results section:
To develop an early warning indicator, we performed a targeted simulation to find an AMOC tipping event in the Community Earth System Model.
Under increasing freshwater forcing, we find a gradual decrease in the AMOC strength. Natural variability dominates the AMOC strength in the first 400 years; however, after model year 800, a clear negative trend appears because of the increasing freshwater forcing. Then, after 1750 years of model integration, we find an abrupt AMOC collapse.
This result differs substantially from earlier model simulations with GCMs that have used extremely large freshwater forcing or large initial salinity perturbations.
The European climate is significantly different after the AMOC collapse, whereas for other regions only specific months undergo significant changes. The Amazon rainforest also shows a drastic change in their precipitation patterns, and the dry season becomes the wet season and vice versa. These AMOC-induced precipitation changes could severely disrupt the ecosystem of the Amazon rainforest and potentially lead to cascading tipping. The Northern Hemisphere shows cooler temperatures after the AMOC collapse, while the opposite is true for the Southern Hemisphere, although not all changes are significantly different (due to large interannual variability).
The European climate is greatly affected under the AMOC collapse. Note that the corresponding changes occur within a relatively short period (model years 1750 to 1850) and under a very small change in surface freshwater forcing. The yearly averaged atmospheric surface temperature trend exceeds 1°C per decade over a broad region in northwestern Europe, and for several European cities, temperatures are found to drop by 5° to 15°C (Fig. 3C). The trends are even more notable when considering particular months (Fig. 3B). As an example, February temperatures for Bergen (Norway) will drop by about 3.5°C per decade (Fig. 3D). These relatively strong temperature trends are associated with the sea-ice albedo feedback through the vast expansion of the Arctic sea-ice pack.
The Atlantic Ocean is a net evaporative basin, and as imposed, its surface freshwater flux increases at the same rate as the freshwater forcing before the AMOC collapse. A larger salinity transport into (and/or larger freshwater transport export out of) the Atlantic Ocean is needed to balance the Atlantic’s freshwater budget, resulting in a declining freshwater convergence.
The report said in its Discussion section:
The AMOC collapse dramatically changes the redistribution of heat (and salt) and results in a cooling of the Northern Hemisphere, while the Southern Hemisphere slightly warms. Atmospheric and sea-ice feedbacks, which were not considered in idealized climate models studies, further amplify the AMOC-induced changes, resulting in a very strong and rapid cooling of the European climate with temperature trends of more than 3°C per decade. In comparison with the present-day global mean surface temperature trend (due to climate change) of about 0.2°C per decade, no realistic adaptation measures can deal with such rapid temperature changes under an AMOC collapse.