In 2016, world leaders signed the Paris Agreement, a legally binding treaty setting long-term goals for countries to reduce their greenhouse gas emissions.
Countries that signed the treaty agreed to limit warming to “well below” 2°C and to pursue efforts to limit warming to 1.5°C above preindustrial levels. The United Nations’ Intergovernmental Panel on Climate Change (IPCC) has defined those thresholds as an average over a 20-year period. Using this multidecadal average helps ensure that the trends in warming are due to human intervention rather than year-to-year natural variations in temperature.
Humanity will know for certain if we’ve completed the 20-year period at 1.5°C—and therefore reached the Paris Agreement limit—only in hindsight. But two modeling studies published in Nature Climate Change signal that we’ve probably entered the 20-year window already—and potentially crossed the midpoint, reports Eos. Only drastic emissions cuts can keep the world below the threshold, according to the study authors.
June 2024 was the twelfth consecutive month that the world’s monthly average surface temperatures reached or surpassed the 1.5°C mark, according to data from the Copernicus Climate Change Service and Berkeley Earth. Global temperatures were hotter than scientists expected in both 2024 and 2023.
“If we stay in this current policy situation, the first year at 1.5°C is signaling that we’re already in this 20-year period,” said Emanuele Bevacqua, a climate scientist at the Helmholtz Centre for Environmental Research–UFZ in Leipzig, Germany, and an author of one of the new studies.
A Window of Warming
Scientists use models, or simulations, to project future climate conditions. These models include historic climate observations such as temperature, precipitation, and ocean chemistry, as well as scientists’ understanding of how Earth systems work. One of the most familiar climate models is the Coupled Model Intercomparison Project Phase 6 (CMIP6), published by the World Climate Research Programme.
CMIP6 includes different warming scenarios based on different potential policy pathways. The SSP2-4.5 scenario, known colloquially as the “middle of the road” scenario, most closely resembles current climate policy trajectories (SSP stands for Shared Socioeconomic Pathways).
In one of the new studies, Alex Cannon, a climate scientist at Environment and Climate Change Canada, analyzed the likelihood that 12 consecutive months at or above 1.5°C would occur within the 20-year window during which Earth would surpass the Paris Agreement limit. He found that in CMIP6 projections under the SSP2-4.5 scenario, there was a 76% likelihood of such a result.
“In most models, long-term warming beyond the [Paris Agreement] threshold occurs before we see 12 months above that threshold,” Cannon said.
In the second study, Bevacqua and his colleagues performed a similar analysis, finding the probability of Earth having entered the 20-year window to be greater than 99%.
Both studies underscore what scientists have known for years—that Earth is speeding toward the 1.5°C threshold. “We already know a version of this,” said Robert Rohde, a climate scientist at Berkeley Earth, a climate research nonprofit. “That we are knocking on the door of 1.5°C, and that we’ll be there within a few years.” Rohde was not involved in the new studies.
The authors of both studies emphasized that their analyses do not mean that Earth’s temperatures have already breached the Paris Agreement threshold.
But having the near certainty that Earth has entered the 20-year window is useful from an adaptation perspective, Bevacqua said. If Earth has entered that window, humanity will probably start experiencing the effects scientists have predicted, such as worsening and more frequent droughts, heat waves, wildfires, flooding, and heavy precipitation.
Unaccounted-For Forcing
Twelve consecutive months of global temperatures observed at or above 1.5°C was surprising, Cannon wrote, because the CMIP6 models indicated that such a long stretch of high temperatures would be unusual as early as 2024.
The discrepancy between observed temperatures in 2024 and model expectations may be due to climate warming events, or forcings, that occurred after CMIP6 was published, such as the Tonga eruption in 2022, shipping regulation changes in 2020, and the impacts of the COVID-19 pandemic, Cannon wrote.
The impact of global events not incorporated into CMIP6 is expected to be small, however, said Rohde. Global temperatures in the past few years are “a little bit on the high side” but still “within the realm of what models are expecting,” he added.
U.S. Withdrawal and a Call to Action
President Donald Trump’s announced withdrawal from the Paris Agreement—which, per the treaty’s terms, would not take effect for 12 months—may slow efforts to reduce emissions from the United States, but the withdrawal doesn’t necessarily change Earth’s long-term temperature trajectory, Rohde said.
“The lack of leadership from the U.S. is a negative.…At the same time, the U.S. is unlikely to turn back the clock on its own progress,” for example, by drastically increasing coal use, as a result of pulling out of the agreement, he said.
“The rest of the world,” Rohde continued, “will hopefully keep on with the progress we’ve been making.”
“A year above 1.5°C is not the time for despair but a call to action,” Bevacqua said.
This story originally appeared on Eos and is part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.
