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As global greenhouse gas emissions continue to rise, some climate scientists say it’s time to start paying more attention to the most extreme, worst-case outcomes, including the potential for widespread extinctions, mass climate migration and the disintegration of social and political systems.

“Facing a future of accelerating climate change while blind to worst-case scenarios is naive risk management at best and fatally foolish at worst,” an international team of researchers wrote this week in a Perspective piece in the Proceedings of the National Academy of Sciences. 

More than half of all cumulative carbon dioxide emissions have occurred since international climate negotiations started in 1990. Global warming is accelerating and driving a steep increase of extremes like heat waves, wildfires and flooding. Most recent scientific estimates show that, under current policies, the world is headed for about 2.4 to 2.7 degrees Celsius warming by late this century.

As a result, the authors set 3 degrees Celsius warming by 2100 as a benchmark of extreme climate change. They chose that level of warming because it exceeds the current established targets of the Paris climate agreement, and because there are “substantially heightened risks of self-amplifying changes between 2 and 3 degrees Celsius warming that would make it impossible to limit warming to 3 degrees Celsius.”

Their PNAS article calls for establishing a research framework to assess the risks associated with extreme climate change and suggests that the Intergovernmental Panel on Climate Change compile a special report to give decision-makers a more accurate and realistic picture of the growing threats. The international community should also consider establishing a climate emergency brake, perhaps with a new treaty that would require an emergency response if research shows imminent, irreversible climate tipping points.

Recent scientific advances increasingly show that dangerous climate impacts are occurring faster than researchers once predicted, said Johan Rockström, director of the Potsdam Institute for Climate Impact Research. “We also have so much evidence that we are coming closer and closer to tipping points and irreversible changes,” he said.”

He said that recent research on planetary boundaries and hothouse Earth scenarios, as well as policy discussions around the world and even the outcomes of the ongoing IPCC 6th assessment, do not really address the risks of catastrophic climate change. The rapid rate of human-caused warming, he added, could be “pushing the on-buttons of irreversible trajectories at lower temperature levels than we had previously had reason to be really concerned about.”

Several elements of the planet’s physical systems are already at or very near tipping points, including tropical coral reefs, he said, which will be wiped out at 1.5 degrees of warming.

“We’re essentially there,” he said. “You could kind of say that’s victim number one and this is happening on our watch, as we speak. The second is Arctic summer ice, which is also in that range where the scientific uncertainty is so narrow that we can say with high precision that we are very close to losing it.”

Similar warning signs in other critical systems like the Amazon rainforest, ocean currents that distribute heat globally and even in the high-altitude jet stream that blows weather systems around the world increase Rockström’s worry.

“At 1.2 degrees Celsius of global mean temperature rise, you suddenly have an abrupt, unexpected amplification (of impacts) because of interactions with the tipping elements,” he said, adding that scientists have underestimated the pace of change the past 30 years.

“We’re underestimating the risk. Every time, things are happening faster than we had predicted.” 

Catastrophe, he said, will be when human interventions can no longer slow climate change, he said. ”We will just be sliding, you know, gradually just drifting off in the wrong direction in terms of sea level rise and climate niches that cannot support human life.”

Complex Systems Can Shock One Another 

“We are facing a risk and consequence issue, but have thus far been very reluctant to understand the collective scale of the consequences,” said University of Manchester climate researcher Kevin Anderson, who was not involved in the paper. “If the consequences are that great, then perhaps policy makers may … develop meaningful mitigation strategies.”

“We know the least about the scenarios that matter the most,” said lead author Luke Kemp, a catastrophic risk scholar at the University of Cambridge. “Current climate change is more rapid than the warming involved in previous mass extinction events. Previous societal crises and transformations were in response to modest, natural regional fluctuations. We now face fast, severe, global, man-made climate change.”

It’s not possible to do good risk assessment without studying low-likelihood events that have high-impact outcomes, said co-author Tim Lenton, a University of Exeter climate scientist who has focused on tipping points research for the past 15 years. After the death last week of his mentor, friend and inspiration, James Lovelock, Lenton noted that the new research fit with Lovelock’s famed Gaia hypothesis that the entire Earth functions as a self-regulating system.

“It sort of goes back to the way he was asking us to look at the climate crisis when he was writing even 20 years ago,” he said. “I’m pretty sure he would approve of this kind of approach. Not because he was a pessimistic person, he was a real optimist and a happy person. But maybe the way to be a happy person is to look hard at the risks and rule them out, or at least know what you’re playing with.”

The U.N.’s Intergovernmental Panel on Climate Change has shied away from focusing on extreme climate impacts “partly because we’ve been getting it in the neck all the time from oil-funded denialists or skeptics, pushing this message of uncertainty, uncertainty, uncertainty,” said Lenton, whose research has been cited in IPCC reports. 

The IPCC consensus process is partly political, as well, he said, which influences how the drafts get edited down and what gets included or taken out.

“As people, as societies we need a good risk assessment of climate change,” he said. “We don’t need to know just the most likely outcome, we need to look at the low likelihood, high-impact things.” 

More research needs to look at how environmental tipping points can interact with unexpected troubles like pandemics and war, he said, and researchers studying other social stressors need to consider the impact of climate change.

“We have modeling on these different things, food systems, migration, conflicts, but it seems like nobody has really put together the toolkit to take a proper look at the possibility of cascading risks, including social fragility,” he said.

“There is a collective sense, because of what we’ve just lived through and what we’re living through right now, that the world is a much more unstable and volatile place than we were brought up to think as we marched through the late twentieth  century.”

Ammunition for Doomers?

Lenton said the authors considered the fact that their paper could mistakenly be construed as feeding into the narrative that climate collapse is inevitable.

“Already some charitable trusts and foundations have decided that the game is up and that we’re in the realm of catastrophe and breakdown and we need to talk about deep adaptation,” he said. “I hope they’re wrong, but I struggle to avoid the conclusion that a lot of people are going to be harmed, and they’re going to want to move around the planet to get away from intolerable climate conditions.”

“We’re stuck in that nation-state, build-a-wall mentality that isn’t going to serve us well when the world is changing so profoundly around us,” he added. “We’ve been tied to agriculture for thousands of years, but the agriculture itself is going to have to move.”

Global warming will also pose challenges for global food security, said study co-author Kristie Ebi with the University of Washington department of global health, and a lead author of the health chapter in the Fourth National Climate Assessment for the U.S. 

“Under a doubling of carbon dioxide, in some important crop plants, you see a 10 percent reduction in protein, and about a 30 percent reduction in B vitamins,” she said. “There’s also about a 5 percent reduction in micro nutrients.”

About 800 million people in the world already experience food insecurity, she said. “About 1 billion have micronutrient deficiencies and 1.5 billion women and girls suffer from iron deficiency.

“When you start thinking about the consequences of inadequate nutrition, and what that means for brain development, what that means for education, how that intersects with all of the other risks of a changing climate, you realize that that has significant potential catastrophic consequences,” she said.

Ebi said she’s also worried that the spread of mosquitoes carrying diseases like dengue fever and yellow fever is another climate impact that could quickly spiral out of control. 

“Dengue fever is the most common viral disease carried by mosquitoes,” she said. “About 400 million people get it every year. And we know that the mosquito is changing its geographic range. As temperatures go up, mosquitoes are going to more places.”

And the spread of yellow fever could be even more catastrophic, she said.

“There’s not that many doses of the vaccine for yellow fever worldwide,” she said. “Just think about the math. How are you going to protect against a disease that could have a very high mortality rate if it shows up? That is a nightmare scenario. And, yes, COVID clearly showed we’re not ready for it.”

New climate models that can accurately show 3 million years of climate history show that, at no point during that time, has Earth come close to warming 2 degrees Celsius, Rockström said.

“It tells you a lot about what does 2.4 Celsius imply, which is the trajectory we’re following,” he said, “and it’s happening at a blink of geological time. That, to me, gives a high degree of scientific confidence that we’re facing disaster if we follow that path.”

“We don’t know exactly where these tipping points are and where we risk that the entire planet starts drifting away in the wrong direction. However, I would argue that we have enough evidence to act on the science we have now, immediately,” he said.

Lowering the risk requires drastic actions at the U.N.’s 27th Conference of the Parties climate talks in Egypt later this year, he said.

“You’d have to meet at COP 27 and ratchet up every [individual nation’s] plan, and legally lock into place plans to phase out fossil fuels, to end the use of internal combustion engines, stop all investments in coal,” he said. “We need to move faster on all the paths we know so well, but we’re moving too slowly. We’re not even bending the global curve of emissions.”

That’s left the planet on track to surpass the Paris Agreement’s goal to limit global warming to 2 degrees Celsius, and ideally, to hold it below 1.5 degrees Celsius, which pushes the planet into the danger zone for climate tipping points, he said.

“Go beyond 1.5, you go from moderate to high risk, go beyond 2, we go from high risk to catastrophic risk.”