The amount of greenhouse gases already released into the atmosphere by humans will likely warm our planet beyond the limits set out under the Paris climate agreement, alarming new research which recalculates previous climate models warns.
Future warming from greenhouse gases that have already been emitted is called “committed warming”.
“It arises because when you emit greenhouse gases into the atmosphere, it takes a very long time to fully realise the warming,” said Andy Dessler, a co-author of the report.
“The ocean has an enormous amount of heat capacity and it takes decades or centuries for it to warm up in response to those emissions.”
And as fossil fuels continue to be burnt, the level of future warming humans are committing the planet to is still continuing to rise.
“We have few ways to avoid this committed warming,” he said in a video summarising the findings of the report.
Previous estimates for the Earth's committed warming suggested an additional 1.3C was on the way, but the new report suggests that number could be over 2C.
In particular the paper looks at how cloud cover over Antarctic areas could change in future. Clouds reflect sunlight back into space, helping to keep the planet cool, but if cloud coverage drops, energy from the sun is readily absorbed by the Earth, warming it up.
“One way to estimate committed warming is to assume that changes in the future will pretty much follow changes in the past. In particular we can estimate how clouds have changed in the past as the climate warms – a process known as the cloud feedback, and assume that future changes will be similar. This yields an estimate for committed warming of 1.4C [above pre-industrial levels],” said Professor Dessler.
This is similar to the previous estimates for committed warming.
“What this means is that we are committed to 1.4C of warming above pre-industrial, just due to emissions which have already taken place – future emissions will cause warming above this committed baseline.”
But he said: “The main conclusion of our paper is that assuming future changes in the climate system will follow past changes is a bad assumption.”
He said this is because historical data reveals that over the 20th century, the slowest area of warming has been in the Southern Ocean, around Antarctica.
But this is an area of very high heat capacity – it takes a long time to warm it up, and global warming has not been going on long enough to have had a significant impact on weather patterns here.
“The existence of cold present day sea-surface temperatures in these regions, while the overlying atmosphere is warming due to global warming, favours the buildup of low clouds over the region," he said. "These clouds reflect sunlight back into space and tend to cool the planet.”
But when the Southern Ocean does eventually warm up, the likely result will be the clouds are more readily burnt off, allowing more sunlight through to be absorbed by the Earth, and giving us additional warming, Professor Dessler said.
This has not happened during the 20th century, and is therefore not in the datasets which have previously been used to calculate likely future warming, and have potentially therefore underestimated possible global temperature rises.
“When we account for this process, we find that committed warming has a most-likely value of around 2.5C above pre-industrial ... this is just from emissions that have already occurred,” he said.
“Obviously this exceeds the Paris agreement limits. While I would not categorise this as good news, it is not game over for the climate.
"It’s important to realise this committed warming is a very slow process because it requires warming regions of the planet that are very slow to warm, therefore it could take centuries for the bulk of this committed warming to occur.
“On the other hand if we continue to emit greenhouse gases at the rate we currently are, we will blow through the 1.5 and 2C limits possibly within a few decades.”
He added: “This means our work is consistent with the conclusion we need to reduce emissions as quickly as possible - if we do that, we will still break 2C, but it will be hundreds of years in the future.
“If we can get emissions to net zero soon, we can stay below 2C for a very long time, giving us more time to adapt."
The research is published in Nature Climate Change.
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