Nearly 200 nations shaped their climate plans around this number: 1.5 degrees Celsius.
But that target, set seven years ago when there was less carbon in the sky, will almost certainly be overshot.
Many climate experts believe that outcome is inevitable. Global temperatures will climb higher than 1.5 degrees compared with 150 years ago, they say, though often only in private.
Such assertions stand to rupture a pillar of climate planning embraced by countries around the world. Efforts to reduce greenhouse gases are measured against that temperature target, as are estimates for adapting to the dangers of rising seas, wildfires and other perils.
It’s also the central message at the U.N. climate conference in Egypt this week, where ramping up efforts to meet the target is a priority.
That number — 1.5 C — promises to be the focus of next year’s climate talks too, even as it slips further away.
“Individually, in private, I don’t think I know of many climate scientists that think 1.5 C is possible (I could count them on a hand),” Glen Peters, a climate policy expert and research director at the Centre for International Climate and Environmental Research in Norway, said in an email to E&E News.
Some scientists now suggest that public optimism about 1.5 C gives the world false hope and could even contribute to further delays in zeroing out global carbon emissions. But that’s not a consensus viewpoint. Other experts warn that prematurely killing the target could have a chilling effect on global climate action — and cause confusion about what target the world is supposed to focus on next.
That opens up a thorny debate about when, exactly, it’s appropriate to declare the target dead — and what happens next.
The 1.5 C threshold is swiftly approaching. The world has already warmed by about 1.3 C, and studies suggest temperatures could cross 1.5 C within a decade.
World leaders, activists and some scientists say the 1.5 C target is still feasible — barely, but at least it’s technically possible. But it would require an immediate and colossal effort to bring emissions down, by at least 45 percent over the next 10 years.
It would be unlike anything seen so far. Millions of gasoline cars would likely have to disappear from roadways, fossil fuel power plants would close or be adapted to confine their carbon, and forests and wetlands would have to be protected from chain saws and development.
Then there’s this: Carbon dioxide would need to be pulled out of the sky.
Despite those challenges, the 1.5 C target continues to be the center of focus at current global climate talks in Egypt.
The 1.5 C goal “is on life support, and the machines are rattling,” said U.N. Secretary-General António Guterres at the opening ceremony of the conference in Sharm el-Sheikh, Egypt, on Monday. “We are getting dangerously close to the point of no return.”
Yet many scientists privately believe the world has already hit the point of no return. And some say it’s time to make that message public.
Last year, the prestigious journal Nature surveyed scientists who helped author the latest reports from the Intergovernmental Panel on Climate Change, the world’s leading authority on climate science. Out of the 92 anonymous respondents, the vast majority expected the world to warm by more than 1.5 C by the end of the century. Sixty percent of them predicted warming of at least 3 C.
And in the run-up to this year’s U.N. climate conference, the activist network Scientist Rebellion — an international organization of scientists advocating for stronger climate action — published an open letter calling on academics to publicly declare that overshooting 1.5 C is inevitable. So far, it’s been signed by more than 500 scientists and academics around the world.
The letter argues that proclaiming 1.5 C as still possible may inadvertently encourage polluters and policymakers to continue delaying deep carbon cuts. It presents the illusion that there’s still time to act.
“Academics cannot fix decades of delay, but we can help societies take the radical action now needed to limit even worse outcomes,” the letter states.
“I think that ‘1.5 is still alive’ is a form of hopium” — a portmanteau for false hope — “and I think hopium is very dangerous,” said Peter Kalmus, a NASA climate scientist and activist who signed the letter.
Kalmus stressed that his activism and his interview with E&E News reflect his own views and not the positions of NASA.
“False hope and narratives allow people to disengage with reality and allow them to avoid becoming climate activists,” he said. “We have to find a way to mobilize these smart people who, like all of us, are feeling overwhelmed and are looking for excuses to feel like maybe it’s not as bad as people like me are saying it is.”
If so many scientists around the world believe overshooting 1.5 C is a foregone conclusion, why aren’t more of them publicly saying so?
“It’s been hard to pinpoint why,” Peters said in an interview.
For one thing, he said, there are concerns that publicly declaring it a failure could dampen global climate action. Once the world has missed a major target, it could become easier for some people to simply give up. It’s critical to communicate that a missed target necessitates greater urgency, not less, and that every bit of additional warming that’s prevented makes a difference to the world.
At the same time, publicly suggesting that 1.5 C will be surpassed tends to be met with “pretty aggressive pushback” from people working to keep the target alive, including activists and some in the climate science community. No one wants to be accused of making a moral misstep by expressing doubts about the target.
“In a sense, you end up feeling like you’re some skeptic, or you’re giving up on small island developing states or something like that,” Peters said.
Scientists, by nature, often hesitate to push their own opinions about outcomes that are still technically possible. If it could theoretically happen — even if it’s highly unlikely — many experts don’t want to publicly rule it out.
“I think it’s simply a disconnection between what is technically possible but is not politically possible,” said Marta Rivera-Ferre, a scientist with INGENIO, a joint research institute of the Spanish National Research Council and the Polytechnic University of Valencia. Rivera-Ferre also signed Scientist Rebellion’s open letter.
“We could achieve 1.5 if we wanted,” she said. “The point is that it is the political conditions that make this not possible.”
Some climate researchers say it’s important for experts to publicly differentiate between their personal predictions and what science says is still technically achievable with enough political will.
“Scientists, as everyone else, we have our personal opinions, and certainly many of us share some disappointments with the political response to the evolution of climate science,” said Carl-Friedrich Schleussner, head of climate science at the nonprofit Climate Analytics and a scientist at Humboldt University of Berlin.
It’s clear that the world is not on track to meet its targets, he said.
“That doesn’t mean that makes the idea completely out of question, that such a response may emerge,” he said. “And I think that’s an important dimension. There’s nothing that we can say that would say scientifically this is gone or it’s dead. Scientifically, we need to say there’s a lot of remaining uncertainty around those questions.”
Yet experts like Peters argue that it’s just as important to communicate to the public how likely it is that the world will miss this target.
“I would tend to argue, if we’re going to go over 1.5 degrees, that’s important communication for people that would suffer as a consequence of that,” he said. “I do have problems with people that aren’t sufficiently frank on the difficulty of getting to 1.5 and its unlikeliness.”
The 1.5 C target wasn’t always the rallying point behind global climate efforts. Two degrees was the focal point of international climate talks for years. The idea that warming above 2 C would have dangerous consequences dates back to at least the 1970s, and it started to become a serious factor in global climate discussions in the 1990s.
But over the years, research began to suggest that global warming would have severe consequences for the planet even if it were limited to 2 degrees. And developing nations, which have historically contributed the least greenhouse gas emissions, would likely feel the worst effects.
Many of the most vulnerable countries — particularly small island nations faced with severe threats from rising sea levels — began to push for more ambitious goals.
In 2015, world leaders finalized the landmark Paris climate agreement. It calls for nations to keep global average temperatures “well below 2 C,” while “pursuing efforts” to keep them under 1.5 C.
In the years since, 1.5 C has become the global goal.
That’s not without good reason. Studies indicate that climate change is already wreaking havoc across the planet, and even small amounts of additional warming will have devastating effects.
Extreme weather events, like heat waves, droughts, wildfires and hurricanes, are already worsening around the world. Sea levels are rising and glaciers are shrinking, and some of them will likely continue to do so for years after temperatures stabilize. Some of the world’s most iconic ecosystems, from the Great Barrier Reef to the mighty Amazon rainforest, are facing irrevocable transformations. Certain coastal communities and some entire island nations are at risk of disappearing under rising oceans.
Warnings from the U.N.’s Intergovernmental Panel on Climate Change have grown increasingly dire over the years. The IPCC’s most recent assessment report, released in three installments over the course of the last year and a half, warns of cascading disasters and irreversible climate impacts.
Yet efforts to meet the Paris targets haven’t been swift enough.
A recent report from the U.N. Environment Programme found that climate policies enacted worldwide would result in about 2.8 C of warming by the end of the century. Nations have pledged more ambitious policies in the future, and many have set timelines to reach net-zero emissions. If all of these promises are met, temperatures would still rise by about 1.8 degrees.
As of today, there is “no credible path” to 1.5 C, the report says.
The report never states that missing the target is inevitable. The same is true for the most recent IPCC report. Yet the IPCC authors offered a dose of reality.
“It is almost inevitable that we will at least temporarily overshoot 1.5,” Jim Skea, an energy expert at Imperial College London and co-chair of the IPCC working group that prepared the report, said when presenting its findings in a virtual presentation in April.
The likelihood — and dangers — of overshoot
Would overshooting 1.5 C amount to a climate failure?
It’s technically possible to surpass a climate target and bring global temperatures back down later. It hinges on a concept known as “negative emissions” — using various forms of technology to suck carbon dioxide out of the atmosphere and cooling the planet in the process.
There are all kinds of theoretical proposals for accomplishing negative emissions, ranging from special machines to massive carbon-guzzling tree plantations. The problem is that most of these technologies would need to be applied at vast scales to actually work — and it’s not yet clear if that’s possible.
So if the world overshoots 1.5 C, there’s no guarantee that temperatures could be brought back down again.
But that hasn’t stopped major climate reports from suggesting it can be done. Most computer models consistent with 1.5 C involve negative emissions and at least some temporary overshoot.
The possibility of overshooting 1.5 C complicates the question of when, exactly, the world should admit the target was missed.
“The problem lies in how exactly we define the target,” said Oliver Geden, a climate policy expert at the German Institute for International and Security Affairs. “Is it never crossing 1.5? Would we be allowed to overshoot 1.5 within an IPCC definition?”
If the world eventually acknowledges that overshooting the goal is inevitable, there’s no clear consensus on what happens next. Who decides what the overshoot target should be? Who is in charge of implementing the negative emissions required to achieve it?
It will likely be years or even decades before scientists can be sure the world has fully crossed the 1.5 C threshold. There’s a lot of variability in the climate system, and the Earth’s average temperatures are likely to wiggle up and down near 1.5 C for several years before it’s clear that they’ve settled above the target.
This means the 1.5 C target will likely remain a centerpiece of international climate negotiations for years to come, even as it continues to slip away.
Experts may disagree on how to communicate the 1.5 C target. But they agree that it’s crucial for the world to keep reducing carbon emissions as swiftly as it can.
Schleussner of Climate Analytics said it’s important to keep the interests of the world’s most vulnerable places at the heart of climate negotiations. The 1.5 C target was born from the crises faced by small island states and other developing nations.
“It hasn’t been scientists — it certainly hasn’t been Western scientists — that have been calling for this,” he said. “It’s been vulnerable nations around the globe that said this is an effort we need for our very survival.”