Carbon budget for 1.5°C at current emissions levels will run out in six years – new research
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Even if humanity wanted to have a 50-50 chance of limiting global warming to 1.5 degrees Celsius, we could still emit only 250 gigatons (billion metric tons) more of CO2. According to calculations in our new paper published in Nature Climate Change, this effectively gives the world just six years to reach net zero. The global level of emissions is currently 40 gigatons of CO2 per year. And, since this figure was calculated from the beginning of 2023, the time frame may actually be closer to five years. Our projections are updated with new climate data, including an assessment published by 50 leading climate scientists in June and several key figures reported by the Intergovernmental Panel on Climate Change (IPCC) in August 2021.
The amount of CO2 that can be emitted while still remaining within a certain level of warming is called the “carbon budget”. The carbon budget concept works because the rise in Earth’s global average surface temperature has increased in a linear manner with the total amount of CO2 emitted by people since the Industrial Revolution. The other side of this equation is that, broadly speaking, warming stops when CO2 emissions stop: in other words, at net zero CO2. This explains why net zero is such an important concept and why so many countries, cities and companies have adopted net zero targets. We have revised the residual carbon budget downwards from the 500 gigatonnes reported by the IPCC since the beginning of 2020.
Some of this revision is simply timing: three years and 120 gigatons of CO2 emissions later, the world is nearing the 1.5°C threshold. The reforms we have made to the method of calculating the budget adjustment have further shrunk the budget balance. Cleaning the air Along with CO2, humanity emits other greenhouse gases and air pollutants that contribute to climate change. We adjusted the budget to take into account the projected warming caused by these non-CO2 pollutants. To this end, we used a large database of future emissions scenarios to determine how non-CO2 warming relates to total warming.
Some of the warming caused by greenhouse gases is offset by cooling aerosols such as sulphates – air pollutants that are emitted along with CO2 from car exhaust and furnaces. Nearly all emissions scenarios project a reduction in future aerosol emissions, even if fossil fuels are phased out or CO2 emissions continue unabated. Even in scenarios where CO2 emissions increase, scientists expect stricter air quality laws and cleaner combustion. In its most recent report, the IPCC updated its best estimate of how much air pollution cools the climate.
As a result, we expect future declines in air pollution to contribute more to warming than previously estimated. This reduces the remaining 1.5C budget by approximately 110 gigatonnes. Other updates we made to the carbon budget methodology reduce the budget even further, such as including estimates of permafrost melting that were not included in earlier estimates. All is not lost It is important to emphasize that many aspects of our carbon budget estimates are uncertain. The balance of non-CO2 pollutants in future emissions scenarios may be as influential on the remaining carbon budget as different interpretations of the climate response.
We also don’t know for sure whether the planet will actually stop warming at net zero CO2 emissions. On average, evidence from climate models suggests this will happen, but some models see substantial warming continuing for decades after reaching net zero. If the heat increases after net zero, the budget will be further reduced. It is because of these uncertain factors that we quote a 50/50 chance of limiting temperature rise to 1.5°C at 250 gigatons of CO2. A more risk-averse assessment would report a two-in-three chance of staying below 1.5°C with a remaining budget of 60 gigatonnes – or one and a half years of current emissions. Time is running out to limit global warming to 1.5C above pre-industrial levels. While we have revised the remaining carbon budget, the message of the earlier assessment remains unchanged: dramatic reductions in greenhouse gas emissions are necessary to stop climate change.
It looks unlikely that we will limit temperature rise to 1.5°C, but that doesn’t mean we should give up hope. Our update has revised the budget down to 2°C relative to the IPCC’s 2021 estimate, but by a smaller amount – from 1,350 to 1,220 gigatonnes, or 34 to 30 years of current emissions. If current national climate policies are fully implemented (admittedly, this is an optimistic scenario), this could be enough to keep temperatures below 2 °C. The risks of tipping points such as the loss of the Amazon rainforest increase as temperatures rise – sometimes exponentially – but 1.5 degrees Celsius is not in itself a hard threshold beyond which climate chaos becomes rampant. With effective action on emissions, we can still limit maximum temperature rise to 1.6°C or 1.7°C, with a view to keeping temperatures well below 1.5°C in the long term. This is a goal worth moving towards.
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