The 1.5°C warming window could end before 2030

A study led by researchers at Imperial College London shows that there is a 50% chance that global temperatures will rise by 1.5°C by 2030 without a rapid reduction in carbon dioxide emissions. The study, published in Nature Climate Change on the 30th, is the latest and most comprehensive analysis of the global carbon budget. A carbon budget is an estimate of the amount of carbon dioxide that can be emitted while limiting global warming to a certain temperature.

The Paris Agreement aims to limit global temperature rise to well below 2°C above pre-industrial levels, and seeks to limit it to 1.5°C. The remaining carbon budget is often used to assess global progress towards these targets.

The new study estimates that there is a 50% chance of limiting warming to 1.5°C, leaving less than 250 billion tonnes of carbon dioxide in the global carbon budget.

Researchers warn that if carbon dioxide emissions remain at the level of about 40 billion tons per year in 2022, the carbon budget will be exhausted around 2029 and global temperatures will rise by 1.5°C above pre-industrial levels.

This finding implies that the budget is smaller than previously calculated and has been roughly halved since 2020 due to the continued increase in global greenhouse gas emissions (mainly caused by fossil fuel combustion) and a re-improved estimate of the cooling effect of aerosols.

The study also found that the carbon budget with a 50% probability of limiting temperature rise to 2°C is around 1.2 trillion tonnes, meaning that if CO2 emissions continue at current levels, the 2°C budget will be exhausted by 2046.

There is significant uncertainty in calculating the remaining carbon budget due to the effects of other factors, including warming from gases other than carbon dioxide and the ongoing impact of emissions not considered in the model.

The new study uses updated datasets and improved climate models to describe these uncertainties and increase confidence in the remaining carbon budget estimates, compared to other recent estimates released in June.

But according to the study's modelling results, there is still a great deal of uncertainty about how parts of the climate system will react in the years leading up to net-zero emissions.

The climate is likely to continue to warm due to effects such as ice melting, methane releases, and changes in ocean circulation, however, carbon sinks (e.g., increased vegetation growth) may also absorb large amounts of carbon dioxide, causing global temperatures to drop before net-zero emissions are achieved.