The window for achieving the most important climate goals is even smaller than thought

Five years and change. That’s how long humans can keep pumping carbon into the atmosphere at the current rate before we’re likely to push global warming beyond the Paris Agreement’s most ambitious limit, according to new estimates released Monday by a team of climate scientists.

The calculations add weight to a bleak conclusion that many researchers already take for granted: that we are cutting emissions far too slowly to have much hope of keeping warming below 1.5 degrees Celsius, or 2.7 Fahrenheit. Human activity has already increased the Earth’s average temperature by about 1.2 degrees Celsius above pre-industrial conditions.

The most promising ways to avoid 1.5 degrees Celsius have clearly disappeared, Joeri Rogelj, a climate scientist at Imperial College London who worked on the new projections, said at a news briefing. “And honestly, they’ve been gone for a while,” he added.

Yet having an up-to-date picture of emissions and warming can still help governments figure out how to meet less ambitious climate goals, including the Paris pact’s second-best 2 degrees Celsius limit. Every additional increase in warming increases the risk of dangerous heat waves, floods, crop failures, species extinctions and forest fires.

“If we limit warming to 1.6 degrees, or 1.65 degrees, or 1.7 degrees, that’s a lot better than 2 degrees,” says Christopher J. Smith, a climate scientist at the University of Leeds, who also the calculations contributed. “We still have to fight for every tenth of a degree.”

Not so long ago, the chances seemed slightly higher. Scientists convened by the United Nations said in 2021 that we could continue emitting at current rates for about eleven more years before we would likely blow past 1.5 degrees.

Since then, however, humans have added many more billions of tons of carbon dioxide to the atmosphere, at a rate that only briefly stalled during the pandemic. After incorporating recent emissions and making other updates to their calculations, Dr. Rogelj, Dr. Smith and their colleagues arrive at a lower estimate of the amount of carbon that can still be added to the atmosphere without raising global temperatures above 1.5 degrees, an amount known as the residual carbon budget.

Scientists have long understood that global warming is directly related to cumulative emissions. But determining the precise budget for 1.5 degrees is difficult because the threshold is already so close.

“The budget for 1.5 degrees Celsius is going to be very small, so small that any change in the methodology could change the budget by a large amount,” said Pierre Friedlingstein, a climate scientist at the University of Exeter who was not involved in the new research . estimates.

One reason the final calculations, which were published in the journal Nature Climate Change, show that a smaller budget than before has to do with air pollution. Burning fossil fuels for energy can release carbon dioxide as well as small particles such as soot and sulfates. These particles are harmful to human health, but some also cool the atmosphere by blocking solar radiation.

In the new budget estimates, the researchers have included a better insight into the magnitude of this cooling effect. The result, they found, was that reducing air pollution in coming years would remove the cooling impact to a greater extent – ​​good for the lungs, bad for global warming.

Once the remaining carbon budget is spent, warming will not necessarily exceed 1.5 degrees and will remain above it. That could happen a little sooner or later, depending on natural climate cycles such as El Niño and on the extent to which societies reduce emissions of other heat-trapping gases such as methane.

This year’s extraordinary heat has allowed global warming to reach about 1.5 degrees above mid-19th century conditions by 2023. But the Paris targets are about the average climate over many years, not one year.

World leaders will meet in the United Arab Emirates next month for the final round of UN climate talks. They will discuss what is still possible to mitigate climate change, and, perhaps just as importantly, what is not.