A vast network of ocean currents nicknamed the “great global ocean conveyor belt” is slowing down. That’s a problem because this vital system redistributes heat around the world, influencing both temperatures and rainfall.
The Atlantic Meridional Overturning Circulation funnels heat northwards through the Atlantic Ocean and is crucial for controlling climate and marine ecosystems. It’s weaker now than at any other time in the past 1,000 years, and global warming could be to blame. But climate models have struggled to replicate the changes observed to date – until now.
Our modelling suggests the recent weakening of the oceanic circulation can potentially be explained if meltwater from the Greenland ice sheet and Canadian glaciers is taken into account.
Our results show the Atlantic overturning circulation is likely to become a third weaker than it was 70 years ago at 2°C of global warming. This would bring big changes to the climate and ecosystems, including faster warming in the southern hemisphere, harsher winters in Europe, and weakening of the northern hemisphere’s tropical monsoons. Our simulations also show such changes are likely to occur much sooner than others had suspected.
The Atlantic Meridional Overturning Circulation (AMOC): what is it and why is it so important? (National Oceanography Centre)
Changes in the Atlantic Meridional Overturning Circulation
The Atlantic ocean circulation has been monitored continuously since 2004. But a longer-term view is necessary to assess potential changes and their causes.
There are various ways to work out what was going before these measurements began. One technique is based on sediment analyses. These estimates suggest the Atlantic meridional circulation is the weakest it has been for the past millennium, and about 20% weaker since the middle of the 20th century.
Since 2002, Greenland lost 5,900 billion tonnes (gigatonnes) of ice. To put that into perspective, imagine if the whole state of New South Wales was covered in ice 8 metres thick.
This fresh meltwater flowing into the subarctic ocean is lighter than salty seawater. So less water descends to the ocean depths. This reduces the southward flow of deep and cold waters from the Atlantic. It also weakens the Gulf Stream, which is the main pathway of the northward return flow of warm waters at the surface.
The Gulf Stream is what gives Britain mild winters compared to other places at the same distance from the north pole such as Saint-Pierre and Miquelon in Canada.
Our new research shows meltwater from the Greenland ice sheet and Arctic glaciers in Canada is the missing piece in the climate puzzle.
When we factor this into simulations, using an Earth system model and a high-resolution ocean model, slowing of the oceanic circulation reflects reality.
Our research confirms the Atlantic overturning circulation has been slowing down since the middle of the 20th century. It also offers a glimpse of the future.
As they travel northwards, the Gulf Stream and North Atlantic Current cool as they lose heat to the atmosphere. The waters then become dense enough to sink to depth and form North Atlantic deep water, which travels southward at depth and feeds the other ocean basins.Modified from Nature Reviews Earth & Environment (2020)
Connectivity in the Atlantic Ocean
Our new research also shows the North and South Atlantic oceans are more connected than previously thought.
The weakening of the overturning circulation over the past few decades has obscured the warming effect in the North Atlantic, leading to what’s been termed a “warming hole”.
When oceanic circulation is strong, there is a large transfer of heat to the North Atlantic. But weakening of the oceanic circulation means the surface of the ocean south of Greenland has warmed much less than the rest.
Reduced heat and salt transfer to the North Atlantic has meant more heat and salt accumulated in the South Atlantic. As a result, the temperature and salinity in the South Atlantic increased faster.
Our simulations show changes in the far North Atlantic are felt in the South Atlantic Ocean in less than two decades. This provides new observational evidence of the past century slow-down of the Atlantic overturning circulation.
The addition of meltwater in the North Atlantic leads to localised cooling in the subpolar North Atlantic and warming in the South Atlantic.https://www.nature.com/articles/s41561-024-01568-1
What does the future hold?
The latest climate projections suggest the Atlantic overturning circulation will weaken by about 30% by 2060. But these estimates do not take into account the meltwater that runs into the subarctic ocean.
The Greenland ice sheet will continue melting over the coming century, possibly raising global sea level by about 10 cm. If this additional meltwater is included in climate projections, the overturning circulation will weaken faster. It could be 30% weaker by 2040. That’s 20 years earlier than initially projected.
Such a rapid decrease in the overturning circulation over coming decades will disrupt climate and ecosystems. Expect harsher winters in Europe, and drier conditions in the northern tropics. The southern hemisphere, including Australia and southern South America, may face warmer and wetter summers.
Our climate has changed dramatically over the past 20 years. More rapid melting of the ice sheets will accelerate further disruption of the climate system.
This means we have even less time to stabilise the climate. So it is imperative that humanity acts to reduce emissions as fast as possible.
Record breaking fossil fuel production, all time high greenhouse gas emissions and extreme temperatures. Like the proverbial frog in the heating pan of water, we refuse to respond to the climate and ecological crisis with any sense of urgency. Under such circumstances, claims from some that global warming can still be limited to no more than 1.5°C take on a surreal quality.
For example, at the start of 2023’s international climate negotiations in Dubai, conference president, Sultan Al Jaber, boldly stated that 1.5°C was his goal and that his presidency would be guided by a “deep sense of urgency” to limit global temperatures to 1.5°C. He made such lofty promises while planning a massive increase in oil and gas production as CEO of the Abu Dhabi National Oil Company.
We should not be surprised to see such behaviour from the head of a fossil fuel company. But Al Jaber is not an outlier. Scratch at the surface of almost any net zero pledge or policy that claims to be aligned with the 1.5°C goal of the landmark 2015 Paris agreement and you will reveal the same sort of reasoning: we can avoid dangerous climate change without actually doing what this demands – which is to rapidly reduce greenhouse gas emissions from industry, transport, energy (70% of total) and food systems (30% of total), while ramping up energy efficiency.
This is also not surprising given that net zero and even the Paris agreement have been built around the perceived need to keep burning fossil fuels, at least in the short term. Not do so would threaten economic growth, given that fossil fuels still supply over 80% of total global energy. The trillions of dollars of fossil fuel assets at risk with rapid decarbonisation have also served as powerful brakes on climate action.
Overshoot
The way to understand this doublethink: that we can avoid dangerous climate change while continuing to burn fossil fuels – is that it relies on the concept of overshoot. The promise is that we can overshoot past any amount of warming, with the deployment of planetary-scale carbon dioxide removal dragging temperatures back down by the end of the century.
This not only cripples any attempt to limit warming to 1.5°C, but risks catastrophic levels of climate change as it locks us in to energy and material-intensive solutions which for the most part exist only on paper.
To argue that we can safely overshoot 1.5°C, or any amount of warming, is saying the quiet bit out loud: we simply don’t care about the increasing amount of suffering and deaths that will be caused while the recovery is worked on.
This article is part of Conversation Insights. Our co-editors commission long-form journalism, working with academics from many different backgrounds who are engaged in projects aimed at tackling societal and scientific challenges.
A key element of overshoot is carbon dioxide removal. This is essentially a time machine – we are told we can turn back the clock of decades of delay by sucking carbon dioxide directly out of the atmosphere. We don’t need rapid decarbonisation now, because in the future we will be able to take back those carbon emissions. If or when that doesn’t work, we are led to believe that even more outlandish geoengineering approaches such as spraying sulphurous compounds into the high atmosphere in an attempt to block out sunlight – which amounts to planetary refrigeration – will save us.
The 2015 Paris agreement was an astonishing accomplishment. The establishment of 1.5°C as being the internationally agreed ceiling for warming was a success for those people and nations most exposed to climate change hazards. We know that every fraction of a degree matters. But at the time, believing warming could really be limited to well below 2°C required a leap of faith when it came to nations and companies putting their shoulder to the wheel of decarbonisation. What has happened instead is that the net zero approach of Paris is becoming detached from reality as it is increasingly relying on science fiction levels of speculative technology.
There is arguably an even bigger problem with the Paris agreement. By framing climate change in terms of temperature, it focuses on the symptoms, not the cause. 1.5°C or any amount of warming is the result of humans changing the energy balance of the climate by increasing the amount of carbon dioxide in the atmosphere. This traps more heat. Changes in the global average temperature is the established way of measuring this increase in heat, but no one experiences this average.
Climate change is dangerous because of weather that affects particular places at particular times. Simply put, this extra heat is making weather more unstable. Unfortunately, having temperature targets makes solar geoengineering seem like a sensible approach because it may lower temperatures. But it does this by not reducing, but increasing our interference in the climate system. Trying to block out the sun in response to increasing carbon emissions is like turning on the air conditioning in response to a house fire.
In 2021 we argued that net zero was a dangerous trap. Three years on and we can see the jaws of this trap beginning to close, with climate policy being increasingly framed in terms of overshoot. The resulting impacts on food and water security, poverty, human health, the destruction of biodiversity and ecosystems will produce intolerable suffering.
The situation demands honesty, and a change of course. If this does not materialise then things are likely to deteriorate, potentially rapidly and in ways that may be impossible to control.
Au revoir Paris
The time has come to accept that climate policy has failed, and that the 2015 landmark Paris agreement is dead. We let it die by pretending that we could both continue to burn fossil fuels and avoid dangerous climate change at the same time. Rather than demand the immediate phase out of fossil fuels, the Paris agreement proposed 22nd-century temperature targets which could be met by balancing the sources and sinks of carbon. Within that ambiguity net zero flourished. And yet apart from the COVID economic shock in 2020, emissions have increased every year since 2015, reaching an all time high in 2023.
Despite there being abundant evidence that climate action makes good economic sense (the cost of inaction vastly exceeds the cost of action), no country strengthened their pledges at the last three COPs (the annual UN international meetings) even though it was clear that the world was on course to sail past 2°C, let alone 1.5°C. The Paris agreement should be producing a 50% reduction in greenhouse gas emissions by 2030, but current policies mean that they are on track to be higher than they are today.
We do not deny that significant progress has been made with renewable technologies. Rates of deployment of wind and solar have increased each year for the past 22 years and carbon emissions are going down in some of the richest nations, including the UK and the US. But this is not happening fast enough. A central element of the Paris agreement is that richer nations need to lead decarbonisation efforts to give lower income nations more time to transition away from fossil fuels. Despite some claims to the contrary, the global energy transition is not in full swing. In fact, it hasn’t actually begun because the transition demands a reduction in fossil fuel use. Instead it continues to increase year-on-year.
And so policymakers are turning to overshoot in an attempt to claim that they have a plan to avoid dangerous climate change. A central plank of this approach is that the climate system in the future will continue to function as it does today. This is a reckless assumption.
2023’s warning signs
At the start of 2023, Berkeley Earth, NASA, the UK Met Office, and Carbon Briefpredicted that 2023 would be slightly warmer than the previous year but unlikely to set any records. Twelve months later and all four organisations concluded that 2023 was by some distance the warmest year ever recorded. In fact, between February 2023 and February 2024 the global average temperature warming exceeded the Paris target of 1.5°C.
Currently we cannot fully explain why global temperatures have been so high for the past 18 months. Changes in dust, soot and other aerosols are important, and there are natural processes such as El Niño that will be having an effect.
But it appears that there is still something missing in our current understanding of how the climate is responding to human impacts. This includes changes in the Earth’s vital natural carbon cycle.
Around half of all the carbon dioxide humans have put into the atmosphere over the whole of human history has gone into “carbon sinks” on land and the oceans. We get this carbon removal “for free”, and without it, warming would be much higher. Carbon dioxide from the air dissolves in the oceans (making them more acidic which threatens marine ecosystems). At the same time, increasing carbon dioxide promotes the growth of plants and trees which locks up carbon in their leaves, roots, trunks.
All climate policies and scenarios assume that these natural carbon sinks will continue to remove tens of billions of tons of carbon from the atmosphere each year. There is evidence that land-based carbon sinks, such as forests, removed significantly less carbon in 2023. If natural sinks begin to fail – something they may well do in a warmer world – then the task of lowering global temperatures becomes even harder. The only credible way of limiting warming to any amount, is to stop putting greenhouse gasses into the atmosphere in the first place.
Science fiction solutions
It’s clear that the commitments countries have made to date as part of the Paris agreement will not keep humanity safe while carbon emissions and temperatures continue to break records. Indeed, proposing to spend trillions of dollars over this century to suck carbon dioxide out of the air, or the myriad other ways to hack the climate is an acknowledgement that the world’s largest polluters are not going to curb the burning of fossil fuels.
Over the following years we are going to see climate impacts increase. Lethal heatwaves are going to become more common. Storms and floods are going to become increasingly destructive. More people are going to be displaced from their homes. National and regional harvests will fail. Vast sums of money will need to be spent on efforts to adapt to climate change, and perhaps even more compensating those who are most affected. We are expected to believe that while all this and more unfolds, new technologies that will directly modify the Earth’s atmosphere and energy balance will be successfully deployed.
What’s more, some of these technologies may need to operate for three hundred years in order for the consequences of overshoot to be avoided. Rather than quickly slow down carbon polluting activities and increasing the chances that the Earth system will recover, we are instead going all in on net zero and overshoot in an increasingly desperate hope that untested science fiction solutions will save us from climate breakdown.
We can see the cliff edge rapidly approaching. Rather than slam on the brakes, some people are instead pushing their foot down harder on the accelerator. Their justification for this insanity is that we need to go faster in order to be able to make the jump and land safely on the other side.
We believe that many who advocate for carbon dioxide removal and geoengineering do so in good faith. But they include proposals to refreeze the Arctic by pumping up sea water onto ice sheets to form new layers of ice and snow. These are interesting ideas to research, but there is very little evidence this will have any effect on the Arctic let alone global climate. These are the sorts of knots that people tie themselves up in when they acknowledge the failure of climate policy, but refuse to challenge the fundamental forces behind such failure. They are unwittingly slowing down the only effective action of rapidly phasing out fossil fuels.
That’s because proposals to remove carbon dioxide from the air or geoengineer the climate promise a recovery from overshoot, a recovery that will be delivered by innovation, driven by growth. That this growth is powered by the same fossil fuels that are causing the problem in the first place doesn’t feature in their analysis.
The bottom line here is that the climate system is utterly indifferent to our pledges and promises. It doesn’t care about economic growth. And if we carry on burning fossil fuels then it will not stop changing until the energy balance is restored. By which time millions of people could be dead, with many more facing intolerable suffering.
Major climate tipping points
Even if we assume that carbon removal and even geoengineering technologies can be deployed in time, there is a very large problem with the plan to overshoot 1.5°C and then lower temperatures later: tipping points.
The science of tipping points is rapidly advancing. Late last year one of us (James Dyke) along with over 200 academics from around the world was involved in the production of the Global Tipping Points Report. This was a review of the latest science about where tipping points in the climate system may be, as well as exploring how social systems can undertake rapid change (in the direction that we want) thereby producing positive tipping points. Within the report’s 350 pages is abundant evidence that the overshoot approach is an extraordinarily dangerous gamble with the future of humanity. Some tipping points have the potential to cause global havoc.
The melt of permafrost could release billions of tons of greenhouse gasses into the atmosphere and supercharge human-caused climate change. Fortunately, this seems unlikely under the current warming. Unfortunately, the chance that ocean currents in the North Atlantic could collapse may be much higher than previously thought. If that were to materialise, weather systems across the world, but in particular in Europe and North America, would be thrown into chaos. Beyond 1.5°C, warm water coral reefs are heading towards annihilation. The latest science concludes that by 2°C global reefs would be reduced by 99%. The devastating bleaching event unfolding across the Great Barrier Reef follows multiple mass mortality events. To say we are witnessing one of the world’s greatest biological wonders die is insufficient. We are knowingly killing it.
We may have even already passed some major climate tipping points. The Earth has two great ice sheets, Antarctica, and Greenland. Both are disappearing as a consequence of climate change. Between 2016 and 2020, the Greenland ice sheet lost on average 372 billion tons of ice a year. The current best assessment of when a tipping point could be reached for the Greenland ice sheet is around 1.5°C.
This does not mean that the Greenland ice sheet will suddenly collapse if warming exceeds that level. There is so much ice (some 2,800 trillion tons) that it would take centuries for all of it to melt over which time sea levels would rise seven metres. If global temperatures could be brought back down after a tipping point, then maybe the ice sheet could be stabilised. We just cannot say with any certainty that such a recovery would be possible. While we struggle with the science, 30 million tons of ice is melting across Greenland every hour on average.
The take home message from research on these and other tipping points is that further warming accelerates us towards catastrophe. Important science, but is anyone listening?
It’s five minutes to midnight…again
We know we must urgently act on climate change because we are repeatedly told that time is running out. In 2015, Professor Jeffrey Sachs, the UN special adviser and director of The Earth Institute, declared:
The time has finally arrived – we’ve been talking about these six months for many years but we’re now here. This is certainly our generation’s best chance to get on track.
In 2019 (then) Prince Charles gave a speech in which he said: “I am firmly of the view that the next 18 months will decide our ability to keep climate change to survivable levels and to restore nature to the equilibrium we need for our survival.”
“We have six months to save the planet,” exhorted International Energy Agency head Fatih Birol – one year later in 2020. In April 2024, Simon Stiell, executive secretary of the United Nations Framework Convention on Climate Change said the next two years are “essential in saving our planet”.
Either the climate crisis has a very fortunate feature that allows the countdown to catastrophe to be continually reset, or we are deluding ourselves with endless declarations that time has not quite run out. If you can repeatedly hit snooze on your alarm clock and roll over back to sleep, then your alarm clock is not working.
Or there is another possibility. Stressing that we have very little time to act is intended to focus attention on climate negotiations. It’s part of a wider attempt to not just wake people up to the impending crisis, but generate effective action. This is sometimes used to explain how the 1.5°C threshold of warming came to be agreed. Rather than a specific target, it should be understood as a stretch goal. We may very well fail, but in reaching for it we move much faster than we would have done with a higher target, such as 2°C. For example, consider this statement made in 2018:
Stretching the goal to 1.5 degrees celsius isn’t simply about speeding up. Rather, something else must happen and society needs to find another lever to pull on a global scale.
What could this lever be? New thinking about economics that goes beyond GDP? Serious consideration of how rich industrialised nations could financially and materially help poorer nations to leapfrog fossil fuel infrastructure? Participatory democracy approaches that could help birth the radical new politics needed for the restructuring of our fossil fuel powered societies? None of these.
The lever in question is Carbon Capture and Storage (CCS) because the above quote comes from an article written by Shell in 2018. In this advertorial Shell argues that we will need fossil fuels for many decades to come. CCS allows the promise that we can continue to burn fossil fuels and avoid carbon dioxide pollution by trapping the gas before it leaves the chimney. Back in 2018, Shell was promoting its carbon removal and offsets heavy Sky Scenario, an approach described as “a dangerous fantasy” by leading climate change academics as it assumed massive carbon emissions could be offset by tree planting.
Shell is far from alone in waving carbon capture magic wands. Exxon is making great claims for CCS as a way to produce net zero hydrogen from fossil gas – claims that have been subject to pointed criticism from academics with recent reporting exposing industry wide greenwashing around CCS.
But the rot goes much deeper. All climate policy scenarios that propose to limit warming to near 1.5°C rely on the largely unproven technologies of CCS and BECCS. BECCS sounds like a good idea in theory. Rather than burn coal in a power station, burn biomass such as wood chips. This would initially be a carbon neutral way of generating electricity if you grew as many trees as you cut down and burnt. If you then add scrubbers to the power station chimneys to capture the carbon dioxide, and then bury that carbon deep underground, then you would be able to generate power at the same time as reducing concentrations of carbon dioxide in the atmosphere.
Unfortunately, there is now clear evidence that in practice, large-scale BECCS would have very adverse effects on biodiversity, and food and water security given the large amounts of land that would be given over to fast growing monoculture tree plantations. The burning of biomass may even be increasing carbon dioxide emissions. Drax, the UK’s largest biomass power station now produces four times as much carbon dioxide as the UK’s largest coal-fired power station.
Five minutes to midnight messages may be motivated to try to galvanise action, to stress the urgency of the situation and that we still (just) have time. But time for what? Climate policy only ever offers gradual change, certainly nothing that would threaten economic growth, or the redistribution of wealth and resources.
Despite the mounting evidence that globalised, industrialised capitalism is propelling humanity towards disaster, five minutes to midnight does not allow time and space to seriously consider alternatives. Instead, the solutions on offer are techno fixes that prop up the status quo and insists that fossil fuel companies such as Shell must be part of the solution.
That is not to say there are no good faith arguments for 1.5°C. But being well motivated does not alter reality. And the reality is that warming will soon pass 1.5°C, and that the Paris agreement has failed. In the light of that, repeatedly asking people to not give up hope, that we can avoid a now unavoidable outcome risks becoming counterproductive. Because if you insist on the impossible (burning fossil fuels and avoiding dangerous climate change), then you must invoke miracles. And there is an entire fossil fuel industry quite desperate to sell such miracles in the form of CCS.
Four suggestions
Humanity has enough problems right now, what we need are solutions. This is the response we sometimes get when we argue that there are fundamental problems with the net zero concept and the Paris agreement. It can be summed up with the simple question: so what’s your suggestion? Below we offer four.
1. Leave fossil fuels in the ground
The unavoidable reality is that we need to rapidly stop burning fossil fuels. The only way we can be sure of that is by leaving them in the ground. We have to stop exploring for new fossil fuel reserves and the exploitation of existing ones. That could be done by stopping fossil fuel financing.
At the same time we must transform the food system, especially the livestock sector, given that it is responsible for nearly two thirds of agricultural emissions. Start there and then work out how best the goods and services of economies can be distributed. Let’s have arguments about that based on reality not wishful thinking.
2. Ditch net zero crystal ball gazing targets
The entire framing of mid and end-century net zero targets should be binned. We are already in the danger zone. The situation demands immediate action, not promises of balancing carbon budgets decades into the future. The SBTi should focus on near-term emissions reductions. By 2030, global emissions need to be half of what they are today for any chance of limiting warming to no more than 2°C.
It is the responsibility of those who hold most power – politicians and business leaders – to act now. To that end we must demand twin targets – all net zero plans should include a separate target for actual reductions in greenhouse gas emissions. We must stop hiding inaction behind promises of future removals. It’s our children and future generations that will need to pay back the overshoot debt.
3. Base policy on credible science and engineering
All climate policies must be based on what can be done in the real world now, or in the very near future. If it is established that a credible amount of carbon can be removed by a proposed approach – which includes capture and its safe permanent storage – then and only then can this be included in net zero plans. The same applies to solar geoengineering.
Speculative technologies must be removed from all policies, pledges and scenarios until we are sure of how they will work, how they will be monitored, reported and validated, and what they will do to not just the climate but the Earth system as a whole. This would probably require a very large increase in research. As academics we like doing research. But academics need to be wary that concluding “needs more research” is not interpreted as “with a bit more funding this could work”.
4. Get real
Finally, around the world there are thousands of groups, projects, initiatives, and collectives that are working towards climate justice. But while there is a Climate Majority Project, and a Climate Reality Project, there is no Climate Honesty Project (although People Get Real does come close). In 2018 Extinction Rebellion was formed and demanded that governments tell the truth about the climate crisis and act accordingly. We can now see that when politicians were making their net zero promises they were also crossing their fingers behind their backs.
We need to acknowledge that net zero and now overshoot are becoming used to argue that nothing fundamental needs to change in our energy intensive societies. We must be honest about our current situation, and where we are heading. Difficult truths need to be told. This includes highlighting the vast inequalities of wealth, carbon emissions, and vulnerability to climate change.
The time for action is now
We rightly blame politicians for failing to act. But in some respects we get the politicians we deserve. Most people, even those that care about climate change, continue to demand cheap energy and food, and a constant supply of consumer products. Reducing demand by just making things more expensive risks plunging people into food and energy poverty and so policies to reduce emissions from consumption need to go beyond market-based approaches. The cost of living crisis is not separate from the climate and ecological crisis. They demand that we radically rethink how our economies and societies function, and whose interests they serve.
To return to the boiling frog predicament at the start, it’s high time for us to jump out of the pot. You have to wonder why we did not start decades ago. It’s here that the analogy offers valuable insights into net zero and the Paris agreement. Because the boiling frog story as typically told misses out a crucial fact. Regular frogs are not stupid. While they will happily sit in slowly warming water, they will attempt to escape once it becomes uncomfortable. The parable as told today is based on experiments at the end of the 19th century that involved frogs that had been “pithed” – a metal rod had been inserted into their skulls that destroyed their higher brain functioning. These radically lobotomised frogs would indeed float inert in water that was cooking them alive.
Promises of net zero and recovery from overshoot are keeping us from struggling to safety. They assure us nothing too drastic needs to happen just yet. Be patient, relax. Meanwhile the planet burns and we see any sort of sustainable future go up in smoke.
Owning up to the failures of climate change policy doesn’t mean giving up. It means accepting the consequences of getting things wrong, and not making the same mistakes. We must plan routes to safe and just futures from where we are, rather where we would wish to be. The time has come to leap.
Top image: Under a microscope, a tiny elongate poppy seed, small tan spikemoss megaspores and black soil fungus spheres found in soil recovered from under 2 miles of Greenland’s ice. Halley Mastro/University of Vermont, CC BY-NDPaul Bierman, University of Vermont and Halley Mastro, University of Vermont
As we focused our microscope on the soil sample for the first time, bits of organic material came into view: a tiny poppy seed, the compound eye of an insect, broken willow twigs and spikemoss spores. Dark-colored spheres produced by soil fungi dominated our view.
These were unmistakably the remains of an arctic tundra ecosystem – and proof that Greenland’s entire ice sheet disappeared more recently than people realize.
These tiny hints of past life came from a most unlikely place – a handful of soil that had been buried under 2 miles of ice below the summit of the Greenland ice sheet. Projections of future melting of the ice sheet are unambiguous: When the ice is gone at the summit, at least 90% of Greenland’s ice will have melted.
Results of an ice sheet model show how much of Greenland’s ice sheet survives when the ice is gone from the Camp Century (white dot), GISP2 (red dot) and DYE-3 (black dot) ice coring sites.Modified from Schaefer et al., 2016, Nature
In 1993, drillers at the summit completed the Greenland Ice Sheet Project 2 ice core, or GISP2, nicknamed the two-mile time machine. The seeds, twigs and spores we found came from a few inches of soil at the bottom of that core — soil that had been tucked away dry, untouched for three decades in a windowless Colorado storage facility.
Our new analysis builds on the work of others who, over the past decade, have chipped away at the belief that Greenland’s ice sheet was present continuously since at least 2.6 million years ago when the Pleistocene ice ages began. In 2016, scientists measuring rare isotopes in rock from above and below the GISP2 soil sample used models to suggest that the ice had vanished at least once within the past 1.1 million years.
Now, by finding well-preserved tundra remains, we have confirmed that Greenland’s ice sheet had indeed melted before and exposed the land below the summit long enough for soil to form and for tundra to grow there. That tells us that the ice sheet is fragile and could melt again.
The GISP2 ice drilling camp at the Greenland ice sheet summit has an average temperature today of minus 7 degrees Fahrenheit (minus 22 Celsius).Christine Massey, CC BYThe frozen plant remains suggest that the center of Greenland probably once looked like this dry rocky tundra, photographed in northwest Greenland.Paul Bierman/University of Vermont, CC BY-ND
A landscape with Arctic poppies and spikemosses
To the naked eye, the tiny bits of past life are unremarkable – dark flecks, floating between shiny grains of silt and sand. But, under the microscope, the story they tell is astounding. Together, the seeds, megaspores and insect parts paint a picture of a cold, dry and rocky environment that existed sometime in the past million years.
Above ground, Arctic poppies grew among the rocks. Atop each stalk of this small but tenacious herb, a single cupped flower tracked the Sun across the sky to make the most of each day’s light.
Tiny insects buzzed above mats of diminutive rock spikemoss, creeping across the gravelly surface and bearing spores in summer.
Modern rock spikemoss (left) and rock spikemoss megaspores (tan spheres, right) from the GISP2 soil sample.J.F. Clovis/Courtesy of Smithsonian Institution (left), Halley Mastro/University of Vermont (right)
In the rocky soil were dark spheres called sclerotia, produced by fungi that team up with plants’ roots in soil to help both get the nutrients they need. Nearby, willow shrubs adapted to life in the harsh tundra with their small size and fuzzy hair covering their stems.
Three fragments of wood, shown under a high-power electron microscope, right, were Arctic willow – not from grand trees but rather the remains of ankle-high shrubs, left, that characterize Greenland’s tundra today.Peter Prokosch; (left), Barry Rock/University of New Hampshire (right)
Each of these living things left clues behind in that handful of soil – evidence that told us Greenland’s ice was once replaced by a hardy tundra ecosystem.
Greenland’s ice is fragile
Our discoveries, published on Aug. 5, 2024, in the Proceedings of the National Academy of Sciences, demonstrate that Greenland’s ice is vulnerable to melting at atmospheric carbon dioxide concentrations lower than today. Concerns about this vulnerability have driven scientists to study the ice sheet since the 1950s.
In the 1960s, a team of engineers extracted the world’s first deep ice core at Camp Century, a nuclear-powered Army base built into the ice sheet over 100 miles from the northwest Greenland coast. They studied the ice, but they had little use for the chunks of rock and soil brought up with the bottom of the core. Those were stored and then lost until 2019, when they were rediscovered in a lab freezer. Our team was among the scientists called in to analyze them.
George Linkletter, working for the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory, examines a piece of ice core in the science trench at Camp Century. The base was shut down in 1966.U.S. Army Photograph
Exquisitely preserved remains of 400,000-year-old moss, on the left, and a sedge seed, on the right – found in the soil core from beneath the Greenland ice sheet at Camp Century – help tell the story of what lived there when the ice was gone.Halley Mastro/University of Vermont
Another ice core, DYE-3 from south Greenland, contained DNA showing that spruce forests covered that part of the island at some point in the past million years.
The biological evidence makes a convincing case for the fragility of Greenland’s ice sheet. Together, the findings from three ice cores can only mean one thing: With the possible exception of a few mountainous areas to the east, ice must have melted off the entire island in the past million years.
Losing the ice sheet
When Greenland’s ice is gone, world geography changes – and that’s a problem for humanity.
As the ice sheet melts, sea level will eventually rise more than 23 feet, and coastal cities will flood. Most of Miami will be underwater, and so will much of Boston, New York, Mumbai and Jakarta.
Today, sea level is rising at more than an inch each decade, and in some places, several times faster. By 2100, when today’s kids are grandparents, sea level around the globe is likely to be several feet higher.
Using the past to understand the future
The rapid loss of ice is changing the Arctic. Data about past ecosystems, like we have collected from under Greenland’s ice, helps scientists understand how the ecology of the Arctic will change as the climate warms.
When temperatures rise, bright white snow melts and ice shrinks, exposing dark rock and soil that soaks up heat from the Sun. The Arctic is becoming greener with every passing year, thawing underlying permafrost and releasing more carbon that will further warm the planet.
The authors share their research and images of the ice core drilling. Quincy Massey-Bierman/University of Vermont.
Human-caused climate change is on pace to warm the Arctic and Greenland beyond temperatures they have experienced for millions of years. To save Greenland’s ice, studies show the world will need to stop greenhouse gas emissions from its energy systems and reduce carbon dioxide levels in the atmosphere.
Understanding the environmental conditions that triggered the ice sheet’s last disappearance, and how life on Greenland responded, will be crucial for gauging the future risks facing the ice sheet and coastal communities around the world.
Paul Bierman, Fellow of the Gund Institute for Environment, Professor of Natural Resources and Environmental Science, University of Vermont and Halley Mastro, Graduate Fellow of the Gund Institute for Environment. Graduate Research Assistant in Natural Resources and Environmental Science, University of Vermont
Extreme weather is by definition rare on our planet. Ferocious storms, searing heatwaves and biting cold snaps illustrate what the climate is capable of at its worst. However, since Earth’s climate is rapidly warming, predominantly due to fossil fuel burning, the range of possible weather conditions, including extremes, is changing.
Scientists define “climate” as the distribution of possible weather events observed over a length of time, such as the range of temperatures, rainfall totals or hours of sunshine. From this they construct statistical measures, such as the average (or normal) temperature. Weather varies on several timescales – from seconds to decades – so the longer the period over which the climate is analysed, the more accurately these analyses capture the infinite range of possible configurations of the atmosphere.
Typically, meteorologists and climate scientists use a 30-year period to represent the climate, which is updated every ten years. The most recent climate period is 1991-2020. The difference between each successive 30-year climate period serves as a very literal record of climate change.
This way of thinking about the climate falls short when the climate itself is rapidly changing. Global average temperatures have increased at around 0.2°C per decade over the past 30 years, meaning that the global climate of 1991 was around 0.6°C cooler than that in 2020 (when accounting for other year-to-year fluctuations), and even more so than the present day.
A moving target for climate modellers
If the climate is a range of possible weather events, then this rapid change has two implications. First, it means that part of the distribution of weather events comprising a 30-year climate period occurred in a very different background global climate: for example, northerly winds in the 1990s were much colder than those in the 2020s in north-west Europe, thanks to the Arctic warming nearly four times faster than the global average. Statistics from three decades ago no longer represent what is possible in the present day.
Second, the rapidly changing climate means we have not necessarily experienced the extremes that modern-day atmospheric and oceanic warmth can produce. In a stable climate, scientists would have multiple decades for the atmosphere to get into its various configurations and drive extreme events, such as heatwaves, floods or droughts. We could then use these observations to build up an understanding of what the climate is capable of. But in our rapidly changing climate, we effectively have only a few years – not enough to experience everything the climate has to offer.
Extreme weather events require what meteorologists might call a “perfect storm”. For example, extreme heat in the UK typically requires the northward movement of an air mass from Africa combined with clear skies, dry soils and a stable atmosphere to prevent thunderstorms forming which tend to dissipate heat.
Such “perfect” conditions are intrinsically unlikely, and many years can pass without them occurring – all while the climate continues to change in the background. Based on an understanding of observations alone, this can leave us woefully underprepared for what the climate can now do, should the right weather conditions all come together at once.
Startling recent examples include the extreme heatwave in the Pacific north-west of North America in 2021, in which temperatures exceeded the previous Canadian record maximum by 4.6°C. Another is the occurrence of 40°C in the UK in summer 2022, which exceeded the previous UK record maximum set only three years earlier by 1.6°C. This is part of the reason why the true impact of a fixed amount of global warming is only evident after several decades, but of course – since the climate is changing rapidly – we cannot use this method anymore.
Playing with fire
To better understand these extremes, scientists can use ensembles: many runs of the same weather or climate model that each slightly differ to show a range of plausible outcomes. Ensembles are routinely used in weather prediction, but can also be used to assess extreme events which could happen even if they do not actually happen at the time.
When 40°C first appeared in ensemble forecasts for the UK before the July 2022 heatwave, it revealed the kind of extreme weather that is possible in the current climate. Even if it had not come to fruition, its mere appearance in the models showed that the previously unthinkable was now possible. In the event, several naturally occurring atmospheric factors combined with background climate warming to generate the record-shattering heat on July 19 that year.
The highest observed temperature each year in the UK, from 1900 to 2023
Later in summer 2022, after the first occurrence of 40°C, some ensemble weather forecasts for the UK showed a situation in which 40°C could be reached on multiple consecutive days. This would have posed an unprecedented threat to public health and infrastructure in the UK. Unlike the previous month, this event did not come to pass, and was quickly forgotten – but it shouldn’t have been.
It is not certain whether these model simulations correctly represent the processes involved in producing extreme heat. Even so, we must heed the warning signs.
Despite a record-warm planet, summer 2024 in the UK has been relatively cool so far. The past two years have seen global temperatures far above anything previously observed, and so potential extremes have probably shifted even further from what we have so far experienced.
Just as was the case in August 2022, we’ve got away with it for now – but we might not be so lucky next time.
First published 19 September 2023, in Environmental Research Letters 18; reproduced here under Creative Commons Attribution 4.0 licence
Authors:
Giorgia Di Capua:Magdeburg-Stendal University of Applied Sciences, 39114 Magdeburg, Germany
Stefan Rahmstorf: Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, PO Box 6012 03, D-14412 Potsdam, Germany | E-mail: [email protected]
Abstract
Extreme weather events are rising at a pace which exceeds expectations based on thermodynamic arguments only, changing the way we perceive our climate system and climate change issues. Every year, heatwaves, floods and wildfires, bring death and devastation worldwide, increasing the evidence about the role of anthropogenic climate change in the increase of extremes. In this viewpoint article, we summarize some of the most recent extremes and put them in the context of the most recent research on atmospheric and climate sciences, especially focusing on changes in thermodynamics and dynamics of the atmosphere. While some changes in extremes are to be expected and are clearly attributable to rising greenhouse gas emissions, other seem counterintuitive, highlighting the need for further research in the field. In this context, research on changes in atmospheric dynamics plays a crucial role in explaining some of these extremes and more needs to be done to improve our understanding of the physical mechanisms involved.
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A rise in weather extremes is probably the most severe early impact of rising global temperatures. Extreme weather events have a high impact on both human activities and natural ecosystems. In recent decades the world has seen an increase in heatwaves, droughts, flooding events and fire weather conditions, as expected in a warming climate. 2022 has been no exception, with record-breaking high temperatures registered across large portions of Western Europe, China and Pakistan (Hausfather 2023). In Pakistan, extreme monsoon rains and Himalayan glaciers melting away in a heat wave have flooded approximately 85 000 km2 (USAID 2022), killing more than 1700 people and causing $15 billion of damage (World Bank), while the Horn of Africa has seen an unprecedented drought that has pushed more than three million people into emergency food insecurity (NASA Earth Observatory 2022). Many of these events have been made more likely by anthropogenic warming and represent a serious threat to human society, with effects ranging from health and mortal- ity to economy and national security issues.
Even for lay persons it will be obvious that heat extremes will increase in a warming world. But it may be unexpected by how much: monthly heat extremes
that were three standard deviations above average during the baseline period 1951–1980 have already increased over 90-fold in frequency over the global land area, while the formerly near-unprecedented 4-sigma events have increased 1000-fold to affect 3% of the land area in any given month, data show (Robinson et al 2021). July 2023 was the hot- test month on record on Earth by a large margin (Copernicus 2023a), and likely the hottest for at least 10 000 years. Marine heatwaves have also doubled in the last few decades, and they are expected to see a 23-fold increase under a 2 ◦C warming scenario (Frölicher et al 2018)—the off-the charts sea surface temperatures in the North Atlantic in summer 2023 are a stern foretaste of this (Copernicus 2023b), while in Florida the marine heatwave caused wide spread bleaching turning into one of the worst events in the region on record (Dennis et al 2023).
Part of this is as expected simply by shifting a Gaussian normal distribution towards warmer values; the more extreme an event, the larger is the factor by which its likelihood increases. However, explaining the full extent of the global increase in extreme heat requires additional, dynamical effects (which we will discuss below).
And heat—especially lasting heat—is a silent killer. The death toll of the 2003 European heat wave has been estimated as ∼70 000 (Robine et al 2008), with a mortality peak in France higher than dur- ing any Covid19 wave (Ferrer and Breteau 2020). A first estimate for summer 2022 came in at ∼62 000 (Ballester et al 2023), when even in Britain temperatures soared above 40◦C for the first time in history. Temperature-related excess mortality is expected to increase with unmitigated global warming, even when accounting for a decrease in cold-related deaths (Gasparrini et al 2017).
Since early 2012, the fingerprint of climate change can be detected in any single day in the observed record. There are no more days on Earth where global weather is not significantly different from what it would be without human influence—on nearly all days even when just considering the weather pat- terns without the increase in global-mean temperature (Sippel et al 2020). Observations show that nearly the entire Earth surface has warmed since the late 19th century (except for the prominent ‘warming hole’ south of Greenland and Iceland, Rahmstorf et al 2015) and the annual global mean temperature has increased by 1.2◦C since the late 19th Century (Intergovernmental Panel on Climate Change 2021). The intensity and speed of warming differ by region, with land areas warming twice as much as the ocean surface since 1970 (Intergovernmental Panel on Climate Change 2021). Warming is more prominent in the Arctic, Eastern Europe, North Africa, the Middle East, East Asia and western North America. Key heatwave characteristics, such as frequency, duration and cumulative heat (i.e. the heat produced by heatwaves days inside a season), show increasing trends since 1950 at global scale, with stronger trends in tropical and northern latitudes. Trends for these key variables have been accelerating in the past few decades (Perkins-Kirkpatrick and Lewis 2020). Summer 2023 was no exception, with record breaking temperature recorded in Southeastern US, China, Spain, Morocco, rendered more likely by current climate change (Zachariah et al 2023).
While the world-wide rise in heat extremes is easily understood given the rise in global mean temperature, mean global rainfall and extreme precipitation trends depict a more complex relationship. Global rainfall is expected to increase as evaporation from warmer oceans increases. Indeed, the IPCC AR6 WGI (2021) reports an increase in globally averaged land precipitation since 1950, though with medium confidence given the large variability and spatial heterogeneity of precipitation. However, extreme rainfall events have shown a steep increase in the last few decades (especially in tropical regions), with 1 in 4 record breaking rainfall events being attributable to climate change (Robinson et al 2021). In August 2023, the region surrounding Beijing was hit by a severe flood event which saw the highest rainfall record of
the last 140 years (744.8 mm in less the 4 d) (Hawkins 2023) Perhaps counterintuitively, some areas even show opposite trends in mean precipitation rates and extreme rainfall events. One such example is the Indian summer monsoon system, which shows a slight decrease in its mean seasonal precipitation rates together with a three-fold increase in extreme rainfall events during the 1950–2015 period (Roxy et al 2017). Therefore, when considering the effect of climate change on extremes, it is not enough to look at trends in mean values.
Another example of that is that despite the global-mean (and often also local) rainfall increase, the frequency and severity of droughts has also increased in some regions, for a number of reasons. One overall reason is that with approximately constant relative humidity, air will contain (and at some point rain out) 7% more moisture per degree of warming, while the resupply of water via evaporation increases only by 2%–3% per degree (Allan et al 2020). The additional evaporation and rain- fall tends to end up in heavy rain rather than alleviating drought: Half of it comes down in the wet- test 6 d each year (Pendergrass and Knutti 2018), and the heaviest rainfall events increase most strongly (Fischer and Knutti 2015). Also, increasing agricultural and ecological droughts (i.e. loss of soil moisture and drying vegetation) can be caused not just by declining precipitation but also by rising temperatures causing faster evapotranspiration. In different regions, either of these effects can be the more important one (Cook et al 2018). Major droughts can also result from natural climate variability and are rare events (compared to the length of available observational data). Thus, studying their trend and, more importantly, attributing them to anthropogenic global warming is not an easy task (Cook et al 2018). Several regions in the world have shown an increase in drought risk, such as Western North America, the Mediterranean, East Southern Africa, East Asia and South Australia, at least partly attributable to anthropogenic warming, with the Mediterranean and the North-Western North America showing the highest confidence (Intergovernmental Panel on Climate Change 2021).
In the past decade, wildfire activity has produced some new extreme fires that are unprecedented regarding propagation speed, intensity, location, timing and burnt area (Descals et al 2022, Senande- Rivera et al 2022). For example, the Australian ‘Black Summer’ wildfire disaster in 2019/2020 fol- lowed Australia’s hottest and driest year on record, burned more than half of the habitat for over 1600 native species and directly caused 33 human deaths and almost 450 more from smoke inhalation (Himbrechts 2021). An increase in extreme fire weather conditions can already be detected at global scale, although trend magnitude and spatial patterns vary at regional scale (Jain et al 2022). Globally, anthropogenic global warming is projected to cause unprecedented increases in extreme fire weather risk in the 21st century (Touma et al 2021). Model experiments estimate that anthropogenic induced changes in fire weather indices have already emerged from natural variability for 22% of the burnable land area globally by 2019, while by the mid twenty-first century the emergence will reach 30%–60% (Abatzoglou et al 2019). Meanwhile, in August 2023, Hawaii has experienced its worst wildfire on record, and one of the worst in US history, where dry conditions and hurricane-force winds fueled the flames bringing utter devastation to the town of Lahaina, killing at least 93 people and thus surpassing the death toll of the Camp fire in California in 2018 (Gabbat and Anguiano 2023).
While warm extremes are to be expected due to anthropogenic global warming, cold extreme are projected to decrease in this century. Nevertheless, despite the general increase in global surface temperatures, a few regions show a cooling trend in the historical record. One such example is central Siberia, which features a cooling trend during boreal winter (Inoue et al 2012). While both natural variability and anthropogenic forcing are debated as causes of this trend (Inoue et al 2012), the cooling trend is mainly associated with an increase in cold extremes over the region. Cold air outbreaks in central Siberia and North America have been shown to result from sudden stratospheric warming events and a disruption of the stratospheric Polar vortex (Kretschmer et al 2018).
Detecting climate change signals can be challenging depending on the region and the variable selected, and attributing trends in temperature or precipitation fields to changes in thermodynamic or dynamic features of the atmosphere represent an even greater challenge (Shepherd 2014). In general, the largest portion of the change is to be attributed to thermodynamic effects. However, dynamic changes can further exacerbate thermodynamic driven changes and atmosphere dynamics and changes in weather patterns play an important role at regional scale (Rousi et al 2022). Amplified Rossby waves with preferred phase position, in particular waves with wave numbers 5 and 7, can lead to concurrent heatwaves (and crop failures) in the mid-latitudes (Kornhuber et al 2020), raising concerns about future food security. Arctic amplification, despite being more prominent in winter than summer, may also affect westerly winds, storm tracks and wave-guides in the mid-latitudes (Coumou et al 2018). Analyzing the ability of models to reproduce amplified waves 5 and 7 shows that even a small bias in upper tropospheric circulation features can have a strong impact on surface temperature and rainfall patterns (Luo et al 2022), highlighting that it is difficult for global climate models to simulate all mechanisms that contribute to making weather more extreme.
Europe has emerged as a hot-spot of heat extremes: it has seen a stronger increase in summer heat than other regions in the northern mid-latitudes. This enhanced warming has been related to dynamical changes such as an increase of double jet patterns, which could explain all of the additional rise in heat waves beyond what is expected simply by thermodynamics (Rousi et al 2022). Both observation and model experiment support the hypothesis that shrinking Arctic sea ice and reduced snow cover over northern Eurasia in spring can also contribute to increased blocking over Europe and consequent frequency of heatwaves (Zhang et al 2020). Sea surface temperature anomalies (in particular the northern Atlantic ‘warming hole’ mentioned earlier) can also reinforce heatwaves in central Europe, such as in 2015 (Duchez et al 2015).
In summary, it is now clear that global warming is already greatly increasing the number and intensity of many types of weather extremes, as has been predicted by climate science for decades. Much of this is due to thermodynamics. With that we mean that the atmosphere is warmer, which means it holds more energy and water to power extreme weather. The ocean is also warmer and can provide more energy and moisture as fuel to tropical cyclones. However, increasingly the attention of researchers has turned to dynamic effects. With that we refer to changes in circulation and stability of atmosphere and ocean. It includes changes to the jet stream, polar vortex, atmospheric planetary waves or to the Atlantic meridional overturning circulation.
Obtaining robust conclusions about changes in weather extremes requires long time series, given that extreme events are by definition rare events and are not easy to model. Nevertheless, the signal of climate change has now clearly emerged from the noise for many types of extremes. Disentangling the dynamic mechanisms is harder again and represents a current frontline of research. Driving forces behind dynamic changes are often regionally diverse temperature changes, such as Arctic amplification, enhanced land warming and sea surface temperature anomalies. Many facets of the dynamic mechanisms are still being debated in the scientific literature. Even if not everything is fully understood, researchers and journalists should not be shy to use every opportunity to educate the public about the fact that human-caused global warming is making weather extremes worse, already causing serious harm to many millions of people.
Even once global warming is stopped, we will see unprecedented extremes for a long time to come. Just think of a former once-in-5000 year event which at 1.5◦C warming may have become a once-in-50 year event. Thus, it will take many decades until we have seen all the possible extreme events a 1.5◦C warmer world has in store for us.
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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.
While Hollywood’s vision was over the top, the 2004 movie raised a serious question: 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?
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’s 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.
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.
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.
The result: Dangerous cold in the North
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 changed 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.
The annual mean temperature changes after the conveyor belt stops reflect an extreme temperature drop in northern Europe in particular.René M. van Westen
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.
So, when will we see this tipping point?
The big question – when will the Atlantic circulation reach a tipping point – remains unanswered. Observations don’t 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.
A climate model experiment shows how quickly the AMOC slows once it reaches a tipping point with a threshold of fresh water entering the ocean. How soon that will happen remains an open question.René M. van Westen
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.
This article was updated on Feb. 11, 2024, to fix a typo: The experiment found temperatures in parts of Europe changed by more than 5 F per decade.
