Window to meet Paris climate goals closing faster than we thought

(CN) — Limiting global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels, long the stated goal of global policymakers, is going to be even harder than previously thought, according to a new study published today in the journal Nature Climate Change.

There's no way to know the exact amount of greenhouse gas emissions people could emit in order to meet the 1.5 C goal. Instead, climate scientists use percentages.

The sixth assessment report produced by the United Nations' Intergovernmental Panel on Climate Change, or IPCC, set a "carbon budget" of 400 gigatons of carbon dioxide, from 2020 onwards. Staying within that budget would have given humans a 67% chance of limiting warming to 1.5 C compared to pre-industrial levels.

But the new paper, which one of its co-authors called "an extension of the IPCC's work," presents a slightly less forgiving carbon budget. The paper estimates a carbon budget of 250 gigatons of CO2, starting in January 2023, to give humanity a 50% chance of hitting the goal.

The change in estimates has more to do with an improved methodology than with mankind's behavior over the last few years — although, the authors of the paper were quick to point out at a video press conference on Thursday, humans have not cut their carbon emissions, though some countries have.

People emit roughly 40 gigatons of CO2 every year, which means we will exhaust the new budget of 250 gigatons in a little more than six years.

"It is clear that high-likelihood options for limiting warming to 1.5 degrees are gone, and they’ve been gone for a long time," said Joeri Rogelj, a Belgian climatologist working in London who co-authored the study, at a Zoom news conference Thursday.

In other words: If the world takes drastic actions to limit greenhouse gas emissions starting tomorrow, limiting warming to 1.5 degrees will still be something of a coin flip.

Rogelj added: "There is still a chance to limit warming to 1.5."

Rogelj and his co-authors stressed that their report should not be used as weight for "climate doomerism," the pessimistic belief that climate change is inevitable.

"Under the most ambitious scenario," said Rogelj, "our best estimate is that warming will be slightly above 1.5. That doesn’t mean we’re spinning out of control."

Christopher Smith, the study's co-author and a professor at the University of Leeds, said, "We don't want this to be interpreted as, 'We have six years to save the planet.'" But, he said, "We do want to underline how close we are to 1.5 degrees and the seriousness of issue."

There's nothing magical about the 1.5-degree threshold. The earth has already warmed about 1.2 degrees Celsius since 1750. Another 0.3 degrees Celsius would see 90% of the world's coral reefs die off, more severe storms and more extreme heat waves. Sea levels could rise an additional 1 to 3 feet by the year 2100. Crop yields in tropical regions would be significantly affected.

A warming of 2 degrees would be much worse, by all those metrics. Millions of people around the world would lose their homes. Millions more would be forced to move due to extreme heat and weather.

In order to give people a 50% chance of limiting global warming to 2 degrees Celsius above pre-industrial levels, the new report sets a carbon budget of 1,200 gigatons of CO2.

"If we are able to limit warming to 1.6 degrees, that’s a lot better," said Smith. "We still need to fight for every tenth of a degree."

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