Window to avoid 1.5°C of warming rapidly closing

Humanity is rapidly reaching the limit for how much additional carbon can be emitted into the atmosphere to keep global warming within 1.5 °C, according to a new research.

If emissions stay at current levels, what is known as the "remaining carbon budget" will be exhausted before the end of the decade.

Dr Chris Smith, climate modeller and a research fellow in the School of Earth and Environment at Leeds, co-authored the study. He said: “This study gives the most updated measure of how much more carbon dioxide humanity can continue to put into the atmosphere to give us a fifty per cent chance of staying within the 1.5 °C threshold agreed at international climate talks.

"If we continue to emit carbon dioxide at current levels, we will exhaust that remaining 1.5°C carbon budget in just six years.

“This is not to say that we only have ‘six years to save the planet’ because 1.5°C is not a hard boundary of when climate change will suddenly become much worse. However, damages, risks and the likelihood of exceeding physical and ecological tipping points increase sharply with continued warming.

“In the end we may exceed the 1.5°C threshold but with effective action now, we could still limit warming to 1.6°C or 1.7°C. That is worth fighting for."

This latest assessment on the state of the climate threat facing the world comes in a new study – Assessing the size and uncertainty of remaining carbon budgets – published in the journal Nature Climate Change.

The paper calculated the remaining carbon budget as of January 2023. To have a 50% chance of keeping global warming within 1.5°C relative to pre-industrial levels, the budget is 250 billion tonnes of carbon dioxide.

In 2022, emissions rates were about 40 billion tonnes of carbon dioxide a year.

If carbon dioxide emissions stay at 2022 levels, the carbon budget will be exhausted by around 2029, committing the world to warming 1.5°C above pre-industrial levels.

Carbon budget halved since 2020

The finding means the budget is less than previously calculated and has approximately 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.

Dr Robin Lamboll, research fellow at the Centre for Environmental Policy at Imperial College London and the lead author of the study, said: “Our finding confirms what we already know – we’re not doing nearly enough to keep warming below 1.5°C.

“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.

"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."

Professor Joeri Rogelj, Director of Research at the Grantham Institute and Professor of Climate Science and Policy at the Centre for Environmental Policy at Imperial College London, said: “This carbon budget update is both expected and fully consistent with the latest UN Climate Report.

“That report from 2021 already highlighted that there was a one in three chance that the remaining carbon budget for 1.5°C could be as small as our study now reports.

“This shows the importance of not simply looking at central estimates, but also considering the uncertainty surrounding them.”

The Paris Agreement, adopted during international climate talks in 2015, aims to limit global temperature increase to well below 2°C above pre-industrial levels and pursue efforts to limit it to 1.5°C.

The study also examined the remaining carbon budget for a 50% chance of limiting warming to 2°C. In that case - 1,200 billion tonnes of carbon could be emitted, meaning that if carbon dioxide emissions continue at current levels, the central 2°C budget would be exhausted by 2046.

There has been much uncertainty in calculating the remaining carbon budget, due to the influence of other factors, including warming from gasses other than carbon dioxide and the ongoing effects of emissions that are not accounted for in climate models.

The new research used an updated dataset and improved climate modelling compared to other recent estimates, published in June, characterising these uncertainties and increasing confidence around the remaining carbon budget estimates.

The strengthened methodology also gave new insights into the importance of the potential responses of the climate system to achieving net zero, where there is an overall balance between global emissions produced and emissions removed from the atmosphere.

According to the modelling in the study, there are still large uncertainties in the way various parts of the climate system will respond in the years just before net zero is achieved.

The climate may continue warming due to effects such as melting ice, the release of methane and changes in ocean circulation.

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.

‘Time for governments to act’

Dr Lamboll says these uncertainties further highlight the urgent need to rapidly cut 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.

“However, it’s 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.

“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.”

The study 'Assessing the size and uncertainty of remaining carbon budgets is available at: https://www.nature.com/articles/s41558-023-01848-5