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Environment
The amount of carbon dioxide we can still emit to have just a 50 per cent chance of limiting warming to 1.5°C is even smaller than previously thought
By Michael Le Page
A giant postcard on the Jungfraujoch, Switzerland, made up of 125,000 messages about climate change from children and young people
VALENTIN FLAURAUD/EPA-EFE/Shutterstock
To have just a 50/50 chance of limiting warming to 1.5°C, the world must keep future emissions to about 220 gigatonnes of carbon dioxide, according to the latest estimate. With annual emissions at around 40 gigatonnes and still rising, we are on course to exceed this by 2029 or earlier.
As a result, keeping that 1.5°C goal alive now requires us to hit net-zero globally by 2034 rather than by 2050, says Joeri Rogelj at Imperial College London. “There are no social or technical scenarios in the scientific literature that even describe how that would be possible.”
However, it is still feasible to limit warming to well below 2°C, says Robin Lamboll, also at Imperial College. To have a 50/50 chance of keeping warming to 2°C, the world must not emit more than 1200 gigatonnes of CO2 (GtCO2). This wouldn’t be exceeded until 2046 if emissions continue at current levels.
How much the planet warms depends largely on the amount of CO2 in the atmosphere, and that depends on how much more of it we emit. Based on this, climate scientists can calculate a carbon budget – the amount of CO2 we can still pump out before warming passes a certain level.
In 2020, the Intergovernmental Panel on Climate Change (IPCC) estimated that the budget for a 50/50 chance of staying below 1.5°C was about 500 GtCO2. Now a team that includes many of the researchers responsible for the IPCC estimate has updated the work.
It estimates that the remaining budget as of January 2023 was around 250 GtCO2. Since then there have been another nine months of emissions, so the remaining budget will already be down to roughly 220 GtCO2.
One of the main reasons for the reduction from 500 to 250 GtCO2 is simply that there have been another three years of emissions since 2020. Most of the rest is due to better estimates of the effects of aerosol pollutants.
These pollutants, such as sulphur dioxide from the burning of fossil fuels, form tiny droplets in the atmosphere that have a cooling effect by reflecting sunlight and altering cloud brightness. There has been a lot of uncertainty about the magnitude of the cooling.
Now, the uncertainties have been narrowed down to the higher side of the range. If aerosol cooling is on the higher side, there will be more warming as fossils fuels are phased out and aerosol pollution falls. That means less CO2 can be emitted if we are to limit temperature rise to a certain level.
“If Lamboll and colleagues are correct, mid-century net-zero targets are insufficient to prevent an overshoot of 1.5°C,” writes Benjamin Sanderson at the Centre for International Climate Research (CICERO) in Norway in a commentary accompanying the study.
Glen Peters, also at CICERO, estimates that global CO2 emissions from fossil fuels will be 1 per cent higher in 2023 than in 2022. While some researchers think emissions could finally start to fall in 2024, Peters isn’t confident that they will peak in 2023.
The carbon budget team doesn’t expect the world to officially pass the 1.5°C threshold until some years after the budget is exceeded, says team member Chris Smith at the International Institute for Applied Systems Analysis in Austria. That will require a temperature increase above 1.5°C averaged over multiple years, rather than just one year.
“It’s highly likely that 2023 will exceed 1.5°C,” Smith says. “But that doesn’t mean that we’ve exceeded 1.5°C in the long term.”
“If we are able to limit warming to 1.6 or 1.65 or 1.7°C, that’s a lot better than 2°C,” he says. “We still need to fight for every tenth of a degree.”
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