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Keeping the warming of the planet to under 1.5°C as per the Paris Agreement will be critical for biodiversity and human flourishing, but unless carbon dioxide emission are drastically reduced and fast, we only stand a 50% chance of doing so before 2030, say scientists at Imperial College London.

The Paris Agreement aims to limit global temperature increases to well below 2°C above preindustrial levels, but we are fast using up our existing global carbon budget for that with less than 250 gigatonnes of CO2 left in it.

This is according to the researchers, who warn that if global carbon dioxide emissions remain at 2022 levels of about 40 gigatonnes a year, the planetary carbon budget for an increase of 1.5°C will be exhausted by 2029.

Meanwhile, the carbon budget that gives us a 50% chance of limiting warming to 2°C is around 1,200 gigatonnes. This means that if carbon dioxide emissions continue at current levels the 2°C budget will be exhausted by 2046.

This means that the budget is less than previously calculated and has halved since 2020 “due to the continued increase of global greenhouse gas emissions, caused primarily from the burning of fossil fuels as well as an improved estimate of the cooling effect of aerosols, which are decreasing globally due to measures to improve air quality and reduce emissions,” they explain in a study published in the journal Nature.

“Our finding confirms what we already know: we’re not doing nearly enough to keep warming below 1.5°C,” stresses Robin Lamboll, a research fellow at the Centre for Environmental Policy at Imperial College London.

“The remaining budget is now so small that minor changes in our understanding of the world can result in large proportional changes to the budget. However, estimates point to less than a decade of emissions at current levels,” Lamboll explains. “The lack of progress on emissions reduction means that we can be ever more certain that the window for keeping warming to safe levels is rapidly closing.”

The stark update on the global carbon budget is consistent with the latest UN Climate Report, from 2021, which noted that  there was a one in three chance that the remaining carbon budget for 1.5°C could be smaller than presumed.

“This shows the importance of not simply looking at central estimates, but also considering the uncertainty surrounding them,” says Joeri Rogelj, director of Research at Imperial College London’s Grantham Institute.

Calculating the remaining carbon budget can be fraught as a number of variables are involved in it, including warming from gasses other than carbon dioxide. The new research “has an updated dataset and improved climate modelling compared to other recent estimates, characterising these uncertainties and increasing confidence around the remaining carbon budget estimates,” the scientists note.

“The strengthened methodology also gave new insights into the importance of the potential responses of the climate system to achieving net zero. ‘Net zero’ refers to achieving an overall balance between global emissions produced and emissions removed from the atmosphere,” they add.

Still, uncertanties remain as to how the global climate system will respond in the years just before net zero is achieved. “It is possible that the climate will continue warming due to effects such as melting ice, the release of methane, and changes in ocean circulation,” they explain.

“However, carbon sinks such as increased vegetation growth could also absorb large amounts of carbon dioxide leading to a cooling of global temperatures before net zero is achieved.”

Be that as it may, the research underlines the urgency of cutting our emissions.

“At this stage, our best guess is that the opposing warming and cooling will approximately cancel each other out after we reach net zero,” Lamboll says.

“However, it’s only when we only when we cut emissions and get closer to net zero that we will be able to see what the longer-term heating and cooling adjustments will look like,” the scientist adds.

“Every fraction of a degree of warming will make life harder for people and ecosystems. This study is yet another warning from the scientific community. Now it is up to governments to act.”

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