Study Warns of 'Irreversible Impacts' From Overshooting 1.5°C, Even Temporarily

Just over a month away from the next United Nations climate summit, a study out Wednesday warns that heating the planet beyond a key temperature threshold of the Paris agreement—even temporarily—could cause "irreversible impacts."

The 2015 agreement aims to limit global temperature rise this century to 1.5ºC, relative to preindustrial levels.

"For years, scientists and world leaders have pinned their hopes for the future on a hazy promise—that, even if temperatures soar far above global targets, the planet can eventually be cooled back down," The Washington Post detailed Wednesday. "This phenomenon, known as a temperature 'overshoot,' has been baked into most climate models and plans for the future."

"The earlier we can get to net-zero, the lower peak warming will be, and the smaller the risks of irreversible impacts."

As lead author Carl-Friedrich Schleussner said in a statement, "This paper does away with any notion that overshoot would deliver a similar climate outcome to a future in which we had done more, earlier, to ensure to limit peak warming to 1.5°C."

"Only by doing much more in this critical decade to bring emissions down and peak temperatures as low as possible, can we effectively limit damages," stressed Schleussner, an expert from Climate Analytics and the International Institute of Applied Systems Analysis who partnered with 29 other scientists for the study.

The paper, published in the peer-reviewed journal Nature, states that "for a range of climate impacts, there is no expectation of immediate reversibility after an overshoot. This includes changes in the deep ocean, marine biogeochemistry and species abundance, land-based biomes, carbon stocks, and crop yields, but also biodiversity on land. An overshoot will also increase the probability of triggering potential Earth system tipping elements."

"Sea levels will continue to rise for centuries to millennia even if long-term temperatures decline," the study adds, projecting that every 100 years of overshoot could lead seas to rise nearly 16 inches by 2300, on top of more than 31 inches without overshoot.

The scientists found that "a similar pattern emerges" for the thawing of permafrost—ground that is frozen for two or more years—and northern peatland warming, which would lead to the release of planet-heating carbon dioxide and methane. They wrote that "the effect of permafrost and peatland emissions on 2300 temperatures increases by 0.02ºC per 100 years of overshoot."

"To hedge and protect against high-risk outcomes, we identify the geophysical need for a preventive carbon dioxide removal capacity of several hundred gigatonnes," the authors noted. "Yet, technical, economic, and sustainability considerations may limit the realization of carbon dioxide removal deployment at such scales. Therefore, we cannot be confident that temperature decline after overshoot is achievable within the timescales expected today. Only rapid near-term emission reductions are effective in reducing climate risks."

In other words, as co-author and Climate Analytics research analyst Gaurav Ganti, put it, "there's no way to rule out the need for large amounts of net negative emissions capabilities, so we really need to minimize our residual emissions."

"We cannot squander carbon dioxide removal on offsetting emissions we have the ability to avoid," Ganti added. "Our work reinforces the urgency of governments acting to reduce our emissions now, and not later down the line. The race to net-zero needs to be seen for what it is—a sprint."

While the paper comes ahead of COP29, the U.N. conference in Azerbaijan next month, co-author Joeri Rogelj looked toward COP30, for which governments that have signed the Paris agreement will present their updated nationally determined contributions (NDCs) to meet the climate deal's goals.

"Until we get to net-zero, warming will continue. The earlier we can get to net-zero, the lower peak warming will be, and the smaller the risks of irreversible impacts," said Rogelj, a professor and director of research for the Grantham Institute at Imperial College London. "This underscores the importance of countries submitting ambitious new reduction pledges, or so-called 'NDCs,' well ahead of next year's climate summit in Brazil."

The U.N. said last November that countries' current emissions plans would put the world on track for 2.9°C of warming by 2100, nearly double the Paris target. Since then, scientists have confirmed that 2023 was the hottest year in human history and warned that 2024 is expected to set a new record.

The study in Nature was published as Hurricane Milton—fueled by hot waters in the Gulf of Mexico—barreled toward Florida and just a day after another group of scientists wrote in BioScience that "we are on the brink of an irreversible climate disaster. This is a global emergency beyond any doubt. Much of the very fabric of life on Earth is imperiled."

Those experts emphasized that "human-caused carbon dioxide emissions and other greenhouse gases are the primary drivers of climate change. As of 2022, global fossil fuel combustion and industrial processes account for approximately 90% of these emissions, whereas land-use change, primarily deforestation, accounts for approximately 10%."