At the current rate, humanity will release into the atmosphere the emissions that lead to warming above 1.5ºC in just six years. In other words, if carbon dioxide emissions are not rapidly reduced, the world has a 50% chance of reaching 1.5ºC in a shorter period than previously reported, according to a study led by researchers from the Imperial College London (United Kingdom) published in the journal Nature Climate Change.
Researchers warn that if carbon dioxide emissions remain at 2022 levels (about 40 billion gigatonnes annually), the carbon budget will be exhausted around 2029, leaving the world facing a 1.5°C warming above pre-industrial levels
It does not mean that exceeding 1.5ºC will occur in 2029 (since the temperature increase is not linear, and numerous interacting factors are involved, including the natural variability component); but in the atmosphere the conditions will exist for that temperature to be reached.
The study is the most up-to-date and comprehensive analysis of the global carbon budget. The carbon budget is an estimate of the amount of carbon dioxide emissions that can be emitted by keeping global warming below certain temperature limits.
The Paris Agreement aims to limit global temperature rise to well below 2°C above pre-industrial levels and pursue efforts to limit it to 1.5°C. The remaining carbon budget is routinely used to assess global progress towards these goals.
The new study estimates that for a 50% chance of limiting warming to 1.5°C, less than 250 gigatonnes of carbon dioxide remain in the global carbon budget.
Researchers warn that if carbon dioxide emissions remain at 2022 levels (about 40 billion gigatonnes annually), the carbon budget will be exhausted around 2029, leaving the world facing a warming of 1.5°C above pre-industrial levels.
The finding means the budget is smaller than previously estimated and has been roughly halved since 2020 due to the continued rise in global greenhouse gas emissions, caused mainly by the burning of fossil fuels, as well as an estimated improved cooling effect of aerosols, which are declining worldwide due to measures to improve air quality and reduce emissions.
According to Robin Lamboll, a researcher at the Center for Environmental Policy at Imperial College London and lead author of the study, these results "confirm what we already know: we are not doing enough to keep warming below 1.5ºC."
"The remaining budget is now so small that small changes in our understanding of the world can lead to large proportional changes in the budget," he adds. However, estimates point to less than a decade of emissions at current levels.
"The lack of progress in reducing emissions means that we can be increasingly confident that the window to keep warming at safe levels is rapidly closing," he warns.
Dr Joeri Rogelj, Director of Research at the Grantham Institute and Professor of Climate Science and Policy at the Center for Environmental Policy at Imperial College London, emphasizes that "this update of the carbon budget is expected and fully consistent with the latest Report of the UN on Climate".
"That 2021 report already highlighted that there was a one in three chance that the remaining carbon budget for 1.5ºC would be as small as our study now indicates," he comments. "This shows the importance of not simply looking at central estimates, but also take into account the uncertainty surrounding them.
The study also found that the carbon budget for a 50% chance of limiting warming to 2°C is about 1,200 gigatonnes, meaning that if carbon dioxide emissions continue at current levels, the central budget for 2 ºC will be exhausted in 2046.
There has been a lot of uncertainty in calculating the remaining carbon budget, due to the influence of other factors, such as warming by gases other than carbon dioxide and the ongoing effects of emissions that are not accounted for in the models.
The new research used an updated data set and improved climate modeling compared to other recent estimates, published in June, characterizing these uncertainties and increasing confidence around estimates of the remaining carbon budget.
The strengthened methodology also provided new data on the importance of the possible responses of the climate system to achieve net zero, that is, the achievement of a global balance between global emissions produced and emissions removed from the atmosphere.
Based on the study's modeling results, there remain large uncertainties about how various parts of the climate system will respond in the years immediately prior to achieving net zero.
The climate may continue to warm due to effects such as melting ice, the release of methane and changes in ocean circulation. However, carbon sinks, such as increased vegetation, could also absorb large amounts of carbon dioxide, causing global temperatures to cool before net zero is reached.
Dr Lamboll says these uncertainties further underline the urgent need to rapidly reduce emissions. "Right now, our best estimate is that opposing warming and cooling will roughly cancel each other out after we reach net zero," he notes. "However, only when we reduce emissions and get closer to net zero will we be able to see what warming and cooling adjustments will look like in the longer term.”
"Every fraction of a degree of warming will make life more difficult for people and ecosystems. This study is one more warning from the scientific community. It is now up to governments to act," he concludes.
