Some catastrophic changes to the climate can still be headed off

Climate change has already touched every corner of the planet and will continue to reshape the human experience for centuries to come, its impacts intensifying as warming grows, scientists warn.

The 2.0 degrees Fahrenheit (1.1 degrees Celsius) the planet has warmed since the preindustrial period has pushed Earth toward irreversible change, some of which is unavoidable. But decisive action to cut emissions quickly and thoroughly—keeping total temperature rise as low as possible—can greatly reduce the risks of crossing further dangerous thresholds that would put the planet even more at risk, according to a massive new Intergovernmental Panel on Climate Change (IPCC) report released Monday.

“In order to stabilize climate, we have to stop emitting immediately, full stop,” says Charles Koven, one of the report authors and a climate scientist at the Lawrence Berkeley National Laboratory in California. 

Risk of irreversible change have become clearer

Earth’s temperatures have increased more or less steadily for decades, neatly in tandem with rising greenhouse gases. The basic rule of thumb is simple: the more carbon dioxide emitted, the warmer it gets, and that relationship will continue, the report says.

But scientists have known for over 30 years that there are thresholds in the climate system which, if crossed, could drastically reshape the world as we know it—causing changes that are irreversible on human timescales. Pushing ice sheets in Greenland and Antarctica past certain points, for example, can drive them into self-reinforcing declines that would continue even if emissions stopped tomorrow.

"We play Russian roulette with climate [and] no one knows what lies in the active chamber of the gun," groundbreaking climate scientist Wally Broecker wrote in 1987.

Since then, reams of research have shown that many of these outcomes could occur at lower global temperature changes than anyone expected and that some may have already begun. Though the exact thresholds are still uncertain, some could be triggered within the 1.5 to 2°C warming range, the warming limits suggested in the 2015 Paris Agreement.

The new report says the planet could warm by about 2.5°F (1.4°C) above preindustrial temperatures by 2100 under the most ambitious pathway to reduce emissions, or more than 7.2°F (4°C) in the least ambitious.

Even at the low end of that range, shifts that can’t be taken back could occur in all corners of the planet: the icy parts, the oceans, the land, and the atmosphere. But the risks become much greater and harder to escape with more warming.

“The more we push the climate system from the state that it’s been in for the last several thousand years, the greater the odds we pass thresholds we can only poorly project,” says Bob Kopp, an author of the report and climate scientist at Rutgers University.

Some of these changes have very local effects. The loss of local mountain glaciers, for example, can deeply affect communities that depend on them for water. Others, like melting of the major ice sheets, have global impacts. Many are self-reinforcing: For example, wildfires are more likely to burn in dry, hot conditions made common by climate change. As they burn, they release carbon into the atmosphere, exacerbating planetary warming and making future fires more likely—a pattern that is all too familiar today.

What’s scary, says Koven, is that “there are thresholds we can cross and not know we’ve crossed them until they’re passed.” That highlights the importance of doing whatever possible to keep far from the theoretical limits.

Here, we explain some of the potentially irreversible changes that can still be avoided with decisive action.

We can still avoid catastrophic losses in Earth’s biggest ice reserves

Melting ice from both Greenland and Antarctica is already fueling faster sea level rise than at any point in the last 3,000 years, threatening billions of coastal dwellers worldwide. Greenhouse gas emissions have locked us into continued rise for centuries to come, but the speed and seriousness of that lock-in are still well within our control, the report says.

It finds that sea levels could rise by just over 1.6 feet (0.5 meters) by 2100 if emissions are cut aggressively, or two to three feet if emissions continue to increase. But under the worst-case scenarios—and if Antarctica’s tipping points are surpassed—that number could be as high as about six feet.

The scariest forecasts only kick in if the ice sheets cross critical thresholds, after which physics would dictate their continued decline—but “you can definitely reduce the possibility of that happening by reducing emissions,” says Baylor Fox-Kemper, one of the authors of the report and an oceanographer at Brown University.

West Antarctica alone is home to enough ice to raise global sea levels by about 10 feet if it all melted away, and its geology makes that possibility a real concern. The region is shaped like a bowl: The rock deep underneath the massive ice sheet sits below sea level. The ocean is prevented from spilling into the bowl by the ice sheet itself, which drapes over the rim and out into the ocean like a convex lid. But if that lid is broken or even pushed slightly back from the rim, ocean water can spill down the side of the bowl, gnawing away at the ice from below, likely speeding the sheet’s demise.

There is evidence that the inevitable decline could be triggered once Earth has warmed somewhere between 1.5 to 2°C above preindustrial temperatures, and some scientists think there are signs the process is already underway, adding great urgency to reducing emissions.

Ice at the northern pole could flip into a new, dangerous state too. It is particularly vulnerable already, since the Arctic is warming at about twice the world average, the report says.

The Greenland ice sheet, which would raise global sea levels about 24 feet if it disappeared, is shrinking faster than at any time in the past 350 years and is on track to exceed melt rates for the last 12,000 years. In a single super-hot day in late July, enough water slipped from its surface to cover Florida in two inches of water.

One of the key feedback loops that could hasten its demise works like this: Blasting summer sun melts the bright white snow that collects on the ice sheet, exposing denser, darker ice below and sometimes creating meltwater pools. The darker ice and water soaks up more heat, causing more melting, which leads to more meltwater, round and round in a destructive cycle. The summer shrink problem will only get worse as the ice sheet gets smaller and shorter: As it loses height, its surface gets closer to sea level, where the air is substantially warmer, further speeding the demise.

Climate change-warmed ocean waters also gnaw at some of the ice sheet’s edges, causing more big chunks to break off. More ice slides downhill to replace the lost chunks, leading to more chunks falling off, and so on. It’s like pulling a gumball out of a machine: The others rush to be the next to come out.

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Greenland’s ice won’t disappear tomorrow. Scientists estimate it would take over 1,000 years to disintegrate completely, and potentially thousands of years longer if we manage to cut emissions quickly. But once the process passes certain thresholds, which some groups estimate could happen at around 2.7°C of warming or maybe even less, the demise is unlikely to be reversible. That means the ice will continue to disappear for centuries, even if temperatures stabilize.

Despite that, “we shouldn’t throw our hands up,” stresses Twila Moon, a climate scientist at the National Snow and Ice Data Center in Colorado. “How much more emissions we put into the atmosphere, how much we let it warm—that will really influence the rate of change.”

Limiting warming to 1.5°C, recent work has found, would cut sea level rise this century in half.

A crucial ocean current could slow down

Dangerous shifts in a major ocean current that controls weather around the Atlantic basin could also become permanent with unchecked climate change, the report finds.

Water constantly pumps through the world’s oceans, ferrying heat, carbon, and much more around the planet. In the Atlantic Ocean, a part of that giant, powerful ocean conveyer belt carries warmth northward as it flows along the western side of the basin. That warmth affects everything from everyday weather in the United States and Europe to sea levels along the East Coast and rainfall patterns in Africa.

But climate change is already slowing the current. The speed of the water is partly controlled by how dense it is when it gets by Greenland, where it usually cools quickly and plunges into the deep ocean like a ball rolling down a hill. But the water hitting that sink point is getting warmer, and Greenland’s melting ice is pouring fresh water into it too—both things that make it less dense and less able to sink, which decelerates the whole conveyer belt. Research suggests it has slowed roughly 15 percent since the middle of the 20th century, and is now moving more sluggishly than at any point in the last 1,000 years.

An even more complete collapse is possible. In the past, the conveyer belt ground to a crawl and maybe even a halt, leading to abrupt chill and wholesale reshaping of weather and rainfall patterns around the Atlantic basin.

The new IPCC report reasserts that such an earth-altering slowdown is very possible, though unlikely before 2100. Continuing decline, which would likely extend for centuries, could move Europe and Africa’s major rainfall patterns south from their present locations, weaken the monsoons that now swing annually through tropical Africa and Asia, drive an extra foot or more of sea level rise along the U.S. East Coast, and more.

No one knows exactly where the current’s dangerous thresholds lie. “All the elements to get it to go the wrong way are there,” says Paola Cessi, an oceanographer at the Scripps Institution of Oceanography in California. “And if we keep going as we are going, we certainly will get there eventually.” But strong climate action can still reverse the decline, forestalling or even avoiding the worst impacts.

Permafrost could disintegrate

The Arctic is home to 9 million square miles of permafrost, soils that stay frozen year-round. These soils hold huge quantities of dead organic material—safe and inert while it’s frozen. But when permafrost thaws, the dead stuff turns into greenhouse gases—the super-potent greenhouse gas methane, as well as carbon dioxide. More carbon is trapped in these soils than is currently in the atmosphere.

But the Arctic is warming faster than the rest of the planet, destabilizing permafrost and slowly leaking its carbon into the atmosphere, which contributes to further warming and more melting. A special interim IPCC report published in 2019 suggested that the feedbacks could intensify around 5.4°F (3°C) of warming, but the process will continue no matter what if temperatures rise further, says Koven.

“We expect these processes to operate as a kind of positive feedback—destabilizing the climate system and make it that much harder to meet our climate goals,” he says. But strong emissions cuts could slow or even reverse the carbon emissions from the permafrost ecosystem, preventing the worst of the feedback effects.

Amazon forest could tip into savannah

Today, the Amazon rainforest does something remarkable: It makes its own water.

Rain sweeps into the eastern part of the forest from the Atlantic Ocean. Trees use it and breathe it back out, where it recondenses into new clouds, which move along with the westward breeze, raining as they go and continuing the cycle. A single water molecule can be recycled five times along the width of the rainforest.

But deforestation, forest degradation, and climate change itself interrupt that process, says David Lapola, a researcher at the University of Campina in Brazil, spurring a transition from rainforest plants to ones that prefer drier conditions and triggering a lasting change in the entire ecosystem.

Dry-adapted species protect their water more, breathing out and returning less to the air above, disrupting the rain cycle and leading to further drying. Already, dryland species are taking over parts of the southeastern Amazon.

The Amazon holds about 150 to 200 billion tons of carbon, about 15 percent of the remaining carbon budget suggested by this IPCC report in order to have a 50 percent chance of staying below 2°C of warming. Losing water would mean losing much of that stored carbon, Lapola explains.

It’s still not clear exactly where the critical threshold lies. One study suggests losing 40 percent of the forest or crossing 7.2°F (4°C) of warming, could cause permanent, irreversible change. Others think it could take even less. Rampant deforestation—estimates suggest almost 20 percent of the forest has been cleared—and inexorable warming are bringing the prospect too close for comfort.

“20 years ago we were expecting this would happen, but we originally thought it would happen in 2050 or after,” says Lapola. But now, when he looks at the reality, it’s clear that “we may have been conservative compared to what we are observing.”

The list goes on. It's time for action.

These are only a few of the irrevocable changes that we can expect if the planet’s climate warms much further, the report says. Major shifts in monsoons; amplifying ocean warming, acidification, and loss of oxygen; increases in extreme heat at the limit of human habitability—climate change leaves no corner of the planet untouched.

And because each extra bit of warming will have much bigger impacts than the bit before, the worst impacts can be avoided with strong action. For example, today a heat wave that would have historically occurred once every 50 years is now about five times as likely; at 2°C warming it will be 14 times as likely; but in a 4°C warmer world, it would be 40 times as likely, the report finds.

It is now a moral imperative to prevent those extra risks, says Tim Lenton, a climate scientist at the University of Exeter who has been warning of irreversible climate change for years.

“We need to act like we are in a climate emergency,” says Lenton. “People have now woken up and said, ‘Damn, the scientist weren’t bluffing,’ but now 30 years later here we are. It’s the action that counts now.”