But now, researchers estimate, in a new paper published Thursday, that the number has been cut in half — thanks to three years of high emissions and new scientific understanding of how particles of air pollution, known as aerosols, affect the climate.
“This is unprecedented,” Piers Forster, one of the authors of the paper and a climate scientist at the University of Leeds, said at a news conference in Bonn, Germany.
The shift is due in part to the fact that, despite the brief dip in emissions during the coronavirus pandemic, carbon dioxide emissions from burning coal, oil and gas are still sky-high — and even increased slightly last year.
At the same time, researchers have also updated their climate models to account for new scientific findings. As the world phases out fossil fuels, pollution from aerosols has already begun to decline. That’s good for air quality and human health — but aerosols also cool the planet slightly by reflecting sunlight. Scientists updating climate models to account for this fact have realized that the planet may have a much smaller carbon budget than originally hoped.
“We have a quite significant update of the remaining budget for 1.5 degrees,” Joeri Rogelj, one of the authors of the paper, said at the news conference. “Only 250 gigatons of CO2 left.”
Other experts say it’s very possible that the remaining carbon budget has been winnowed down to almost nothing.
“It’s possible — perhaps likely — that we’ve already exhausted much of the remaining 1.5C carbon budget,” Rob Jackson, a professor of earth system science at Stanford University who was not involved in the study, said in an email. “The Earth is warmer today than it was five years ago, and we’re speeding toward 1.5C with no hint of a decline in greenhouse gas emissions.”
It’s important to note that 1.5 degrees Celsius was always a highly ambitious climate goal — early on, some scientists questioned whether it was even possible to phase out fossil fuels fast enough to reach it. But in the past five years it has become a key focus of climate activism, even as some experts have warned that it is now essentially out of reach. Scientists now estimate that the world is likely to pass that temperature mark sometime in the next 10 years.
Researchers don’t think that catastrophic tipping points will trigger exactly at 1.5 degrees Celsius, but they do know that every tenth of a degree makes warming and the damage it causes worse.
The new paper shows that progress toward the 2 degree Celsius goal has also taken a hit. In the past three years, the researchers estimated, the carbon budget for 2C has narrowed by about 15 percent. And temperatures are already over 1 degree Celsius higher than in preindustrial times.