Global warming impacts will be irreversible if the world gets too hot


Climate scientists to world: There are no do-overs to the damages caused by the planet’s rising temperatures

When it comes to climate change, the journey matters as much as the destination. That’s one of the major lessons from the sobering report released Monday by the Intergovernmental Panel on Climate Change, which warns that some dire impacts of warming will be irreversible if the world exceeds certain temperature thresholds.

In other words, the final temperature the world settles on if it succeeds weaning itself from fossil fuels is crucial, but how it gets there will determine how much humanity suffers along the way. From sea level rise to the loss of coral reefs, some of the damage is forever.

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“Just as there is no magic number of cigarettes we can smoke before we damage our lungs, so too every additional ton of carbon carries with it an increasingly heavy cost,” said Katharine Hayhoe, chief scientist for the nonprofit Nature Conservancy and a professor at Texas Tech University.

Climate scientists refer to this problem as “overshoot” — the idea that the planet’s average temperature might climb more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels before dipping back down to that level in response to cuts in greenhouse gas emissions. Arriving back at lower temperatures at some point later this century, which the IPCC’s various projections suggest is a relatively likely outcome, would be something to celebrate — but it is becoming increasingly clear that a lot will be lost along the way.


The new IPCC report makes clear that the strategy is less workable than previously thought. Overshoot, the authors wrote, “will result in irreversible impacts on certain ecosystems.” In other words, just turning the temperature back down won’t undo the damage.

“Overshoot has definitely been gaining importance and relevance in this report,” said Simone Lucatello, a co-author of the IPCC report and a professor at the Dr. José María Luis Mora Research Institute in Mexico City. This increasing interest is thanks to a deluge of science in recent years demonstrating the potential for some of those irreversible effects, including snow and ice loss in polar and glacial ecosystems and similarly catastrophic damage in other sensitive ecosystems.

“It means that [with] overshoot and major global warming levels, adaptation can be become more difficult,” Lucatello said.

Beyond 1.5 degrees of warming, damage to most warm-water tropical reefs and to kelp forests — both remarkably biodiverse and crucial marine ecosystems — would likely undergo “irreversible phase shifts,” according to the report. In other words, they turn into something else, or die out entirely, and it will take a very long time to get them back. Global sea level rise and ocean acidification — the process by which the ocean absorbs carbon dioxide emissions, changing its chemistry and harming marine life and people who depend on it — will be irreversible “for centuries or longer.”

Unlikely pathways

Scientists have begun to examine overshoot scenarios in more detail because exceeding the 1.5 degree threshold seems more and more likely to happen. Robert Lempert, an IPCC co-author and director of the Frederick S. Pardee Center for Longer Range Global Policy and the Future Human Condition at the Rand Corporation, said that many potential scenarios “rely” on overshoot to eventually bring the temperature back down to 1.5 degrees — because there has been no indication that the world could cut emissions fast enough to avoid it.


In a future the IPCC refers to as the “very low greenhouse gas emissions scenario,” there would likely be a temporary overshoot of no more than 0.1 degrees Celsius, potentially small enough to avoid some of those catastrophes. But that will be a tough path to follow.

Computer modeling studies suggest that making that scenario a reality would require the world to cut its greenhouse gas emissions 45 percent below 2010 levels by 2030, just eight years from now. But those emissions are projected to rise in the coming years rather than fall. Other more likely emissions pathways, even those that eventually do bring the planet back down below 1.5 degrees, result in more overshoot than many ecosystems and people can handle. And those impacts will fall disproportionately on the developing world and those living in poverty everywhere.

Importantly, scientists still don’t know all the climate change impacts that will fall under this category. “Some impacts are insufficiently understood to be able to project at exactly what level of global warming they will become irreversible,” said Camille Parmesan, a professor of oceans and human health at the University of Plymouth in the U.K. and an IPCC co-author. “But we can say that they become increasingly more difficult to reverse as warming continues.”

Action versus fatalism

The report’s underlying message is that rather than relying on future technology to help lower the planet’s temperature, cutting emissions today is even more important than previously thought. In some scenarios, the IPCC imagines the use of carbon dioxide removal (CDR) will be crucial to bring temperatures down, a technology that does exist but is in its relative infancy and is considered prohibitively expensive.

“The potential for irreversible damage due to an overshoot beyond 1.5 degrees raises the importance of near-term emissions reductions” compared with using CDR later this century, Lempert said. Pulling CO2 out of the sky would be great, but a bit less great if all the corals are gone.

Hayhoe has warned against the sense of fatalism that can arrive with such dire reports as the IPCC periodically puts out — and the increasing clarity around overshoot adds weight to the idea that the bleak reports should engender a push for rapid action rather than a throw-up-your-hands resignation.

“What is at risk is not the planet itself,” Hayhoe said. “It’s our human systems (and many other living things that share our home) that are vulnerable to changes that are occurring at a pace that far exceeds anything we have experienced over the course of human civilization to date.”

  • Dave Levitan
    Dave Levitan

    Climate Reporter

    Dave Levitan is a climate reporter for Grid where he focuses on interconnected stories about climate and science, and politics shaping action around both.