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The longer it takes to reach net zero, the greater the risk that global warming will continue for decades or millennia even after we have cut greenhouse gas emissions, according to an assessment by climate researchers.

This means we may have to emit even less carbon dioxide than we thought if we want to limit warming to, say, 2掳C, making carbon budgets even smaller than current estimates.

鈥淭he timing at which we reach net zero is also key,鈥� says at the National Centre for Meteorological Research (CNRM) in France.

Climate modellers once assumed that CO2 levels would remain roughly constant after emissions ceased. In such scenarios, the oceans would continue to warm for several decades, leading to further surface warming.

But more than a decade ago, modellers realised that in fact the oceans will continue to soak up some of the extra CO2, leading to a gradual fall in atmospheric levels and less of the sun鈥檚 heat being retained. Recent climate models suggest that, by coincidence, this cooling effect balances out the ocean warming, meaning surface warming halts within a few years of emissions stopping.

Not all climate models are the same, however. Standard climate models don鈥檛 include all feedbacks, such as how much carbon is taken up by plants on land. More comprehensive 鈥淓arth system鈥� models suggest that if, say, emissions stop when the world is 2掳C warmer than pre-industrial times, there is a 33 per cent chance that the planet would continue to warm past 2.3掳C.

Yet this is still not the full story. This is because even the Earth system models don鈥檛 include all feedbacks, such as the melting of the ice sheets on Greenland and Antarctica.

He and his team have now done a comprehensive assessment of the uncertainties over timescales of decades, centuries and millennia. They looked at 26 factors that could lead to more warming or cooling, from the loss of forests and the melting of permafrost to shrinking ice sheets and changing ocean currents.

For around a third of these factors, such as some cloud responses and the extent of sea ice loss, we have no firm knowledge and can only speculate, the researchers say.

While some of the factors are expected to cause cooling, there is a general tendency towards additional warming, says team member at Imperial College London.

鈥淧eak warming could be higher with additional impacts, damages and challenges,鈥� he says. 鈥淚t really means that reaching net zero becomes even more important as a milestone.鈥�

Additional warming doesn鈥檛 mean unstoppable warming, says Rogelj. It could be prevented by continued carbon removal after net zero, for instance.

But in a comment accompanying the paper, at the University of Pennsylvania points out that carbon removal at the scale and speed required for halting warming might not be possible. 鈥淲hat is most urgently required is a rapid phaseout of human-induced activities that produce carbon pollution,鈥� he writes.

鈥淚 think this is an important paper that emphasises that our understanding of how much the world will warm or cool after we hit net zero is still uncertain,鈥� says of Berkeley Earth.

鈥淲hile our best estimate is that warming will stop when emissions stop, it鈥檚 also possible that the world might continue to modestly warm 鈥� or modestly cool 鈥� after net zero.鈥�