While the climate targets set by the Paris Agreement are based on long-term average temperatures, a year of high temperatures could be a sign that the 1.5°C threshold has already been reached. Last month, researchers confirmed that 2024 was the first year in which global average temperatures rose more than 1.5°C above pre-industrial levels. It was a symbolic moment, given the collective global goal, set in 2015 under the Paris Agreement, of keeping long-term warming to a 1.5°C limit. But scientists were quick to emphasize that this target is based on a 20-year average temperature, so global efforts to reach it are still — at least technically — in play.
However, experts are increasingly asking whether shorter periods of high temperatures could be a sign that the world has already surpassed 1.5°C. Can we conclude that this target has already been missed?
Emanuele Bevacqua from the Helmholtz Centre for Environmental Research–UFZ in Germany and colleagues set out to investigate whether a single warm year above 1.5°C could be a sign that long-term warming will soon reach that level.
Using a combination of real-world observations and climate models, Bevacqua and his team studied the warming thresholds already exceeded between 1981 and 2014. They found that the first year that exceeded 0.6°C, 0.7°C, 0.8°C, 0.9°C and 1°C above the pre-industrial milestone fell consistently within the first 20-year period in which average temperatures reached the same thresholds.
By that measure, the first year above 1.5°C puts the world within the 20-year window that scientists use to define 1.5°C of long-term warming, the team concludes. “It’s highly likely that we’re already within the 20-year window,” Bevacqua says. “We’re very likely within the first 10 years [of that window].”
The findings match most predictions that long-term warming will reach 1.5°C by the late 2020s or early 2030s. It is a “confirmatory result” of what researchers are already predicting, says Paulo Ceppi of Imperial College London.
But monthly temperature data may tell a different, more worrying story. June 2024 was the 12th consecutive month with average global temperatures at least 1.5°C above pre-industrial levels. In a separate study, Alex Cannon of Environment and Climate Change Canada used a climate model to compare the first time global temperatures hit 1.5°C for 12 consecutive months with the time the 20-year temperature average crossed 1.5°C.
He found that in climate model simulations, a streak of 12 consecutive months above 1.5°C indicates an 80 percent probability that long-term warming of 1.5°C has already been reached, even when natural variability such as El Niño phases are accounted for. “If you go back to the real world, that would imply that there is a good probability that we have already passed the long-term threshold [of 1.5°C],” he says.
However, the conclusions are based on a climate model that assumes that Earth’s atmosphere is highly responsive to changes in CO2 concentrations. The model is also running a high-emissions scenario, notes Duo Chan at the University of Southampton, UK. “I would interpret the result with caution,” he says. Cannon notes this limitation in the study and suggests that if the model’s climate sensitivity is tempered and run under a medium-emissions scenario, the long-term crossing of the 1.5°C threshold would likely occur before 2029, in line with estimates from the wider community.
The conclusions also depend on models being able to accurately represent all drivers of warming and predict year-to-year variability in global temperature. “If the models underestimate this variability, they would overestimate the probability of having exceeded 1.5°C for a given number of months over the threshold,” Ceppi says. More research is needed to see how well climate models are simulating short-term variability, he says, especially given uncertainties about effects such as reduced atmospheric aerosols from shipping. Aerosols reflect sunlight out of the Earth’s atmosphere, so the use of cleaner marine fuels could paradoxically lead to increased warming.
Such uncertainties mean we should be cautious about overinterpreting the results of individual studies. After all, the Paris Agreement is a major political treaty, and declaring one of its main goals dead and buried would have seismic consequences. “[To answer] the question of whether or not we have exceeded the temperature levels mentioned in the Paris Agreement, we would need to have very high scientific certainty, and we don’t have that,” says Carl-Friedrich Schleussner at the Climate Analytics research institute in Berlin, who contributed to Bevacqua’s study.
Cannon says that even with the results of his research, “I don’t have enough information to say [the 1.5°C target has been breached] with any certainty.” The problem, he says, is that the climate models that predicted this scenario didn’t expect the recent string of record temperatures. “There’s a mismatch between the timing in the models and what we actually observe.”
This implies that the models are missing something that explains the recent increase in real-world warming. Most climate models don’t account for the reduction in aerosols from shipping, which is one possible explanation. Cannon says his work is unearthing “warning signs that we need to understand things better.”
Yet even though it’s too early to say whether the Paris target has been met, to some extent that’s a nitpick. “We’re moving into a 1.5°C world,” says Schleussner, with warming levels now very close to that critical threshold. “The impacts that scientists told us would happen around 1.5°C [of warming] are going to materialize.”
“The truth of the matter is that the goals of the Paris Agreement are at stake,” he emphasizes. “If we continue on our current path, we will fail.”
