The carbon budget remaining to limit the climate crisis to 1.5C of global heating is now “tiny”, according to an analysis, sending a “dire” message about the adequacy of climate action. The carbon budget is the maximum amount of carbon emissions that can be released while restricting global temperature rise to the limits of the Paris agreement. The new figure is half the size of the budget estimated in 2020 and would be exhausted in six years at current levels of emissions. Temperature records have been obliterated in 2023, with extreme weather supercharged by global heating hitting lives and livelihoods across the world. At the imminent UN Cop28 climate summit in the United Arab Emirates there are likely to be disputes over calls for a phaseout of fossil fuels. The analysis found the carbon budget remaining for a 50% chance of keeping global temperature rise below 1.5C is about 250bn tonnes. Global emissions are expected to reach a record high this year of about 40bn tonnes. To retain the 50% chance of a 1.5C limit, emissions would have to plunge to net zero by 2034, far faster than even the most radical scenarios. The current UN ambition is to cut emissions by half by 2030 and reach net zero by 2050, although existing policies are far from delivering this ambition. If it was achieved, however, it would mean only about a 40% chance of staying below 1.5C, the scientists said, so breaking the limit would be more likely than not. But, they warned, every 10th of a degree of extra heat caused more human suffering and therefore keeping as close as possible to 1.5C was crucial. The new carbon budget estimate is the most recent and comprehensive analysis to date. The main reasons the budget has shrunk so markedly since 2020 are the continued high emissions from human activities and a better understanding of how reducing air pollution increases heating by blocking less sunlight. Prof Joeri Rogelj, at Imperial College London, UK, and one the study’s authors, said: “The budget is so small, and the urgency of meaningful action for limiting warming is so high, [that] the message from [the carbon budget] is dire. “Having a 50% or higher likelihood that we limit warming to 1.5C is out of the window, irrespective of how much political action and policy action there is.” He said it was “remarkable” how much risk humanity appeared willing to take with global heating. Dr Chris Smith, at the International Institute for Applied Systems Analysis in Austria, who was also part of the study, said: “Governments can control the emissions but, at the moment, they have not done so. This is why we have an ever-shrinking carbon budget. We are not saying we only have six years to solve climate change – absolutely not. If we are able to limit warming to 1.6C or 1.7C, that’s a lot better than 2C. We still need to fight for every 10th of a degree.” The study, published in the journal Nature Climate Change, used updated data and improved climate modelling compared with other recent estimates. It also used the latest figures showing that aerosol air pollution and the clouds it was helping to seed were better at blocking sunlight and limiting heating than previously thought. As a result, lower pollution in future would mean more global heating and therefore a smaller carbon emissions budget to remain under 1.5C. The analysis also looked at the 2C upper limit in the Paris agreement, which even if met would still mean a sharp increase in climate impacts from heatwaves to floods to crop losses. For a 90% chance of keeping below 2C, emissions would have to hit net zero in about 2035, the study found. Achieving net zero in 2050 would give a 66% chance of meeting the 2C target. Ben Sanderson, at the Centre for International Climate and Environmental Research in Norway, and not part of the study, said the remaining budget for 1.5C was tiny. “The [analysis] makes for uncomfortable reading for policymakers. The budget is consistent with net zero CO2 emissions being achieved in 2034. This is vastly more ambitious than current implemented global climate policies.” Dr Gabriel Abrahão, at the Potsdam Institute for Climate Impact Research in Germany, said: “There is a very real possibility that we will overshoot the 1.5C target in this decade. Thus, the public debate and the international climate negotiations should already be discussing how to return to 1.5C after an overshoot, so that [it] doesn’t end up being permanent.” The calculation of carbon budget involved significant uncertainties, the researchers said. Sanderson added: “This [updated carbon budget] illustrates that any calculation, no matter how rigorous, is subject to change with revised data and understanding.” Dr Robin Lamboll, at Imperial College London, said future revisions that increased the carbon budget were “not very likely now”. Global carbon emissions from fossil fuel burning could peak as early as this year, according to data from the International Energy Agency, meaning they could begin to fall in 2024. Lamboll said the researchers hoped to produce annual updates on the remaining carbon budgets.