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Read more: “Climate change: What we do – and don’t – know“

We can’t say how much Earth will warm over the coming years unless we know how much more greenhouse gas will end up in the atmosphere.

The biggest uncertainty is human. Were we to cut our emissions drastically tomorrow, CO₂ levels might not rise much beyond 400 parts per million. But that is improbable: only a few countries – and not the biggest emitters such as China and the US – are promising cuts on anything like the scale needed, and the credibility of such promises is undermined by those same countries building more coal-fired power stations. Our current emissions trajectory is close to the worst-case scenario of the . If we continue on this path, CO₂ levels could by 2100 – or perhaps .

“Were we to cut emissions drastically tomorrow, C0₂ levels might not rise much more. That is improbable”

The second uncertainty is Earth’s response. So far about a third of our CO₂ emissions have been sucked out of the atmosphere, mainly by the oceans. . Currently, rising CO₂ levels are driving global warming, but in the past CO₂ levels have naturally risen in response to rising temperatures. We do not know why exactly, but the reduced solubility of CO₂ in warm water and changes in biological activity have been suggested as reasons. If such mechanisms kick in, even bigger cuts in emissions will be needed to limit warming.

There are also vast quantities of greenhouse gases locked away in , in peat bogs and undersea methane hydrate deposits. We don’t know how big these stores are. Nor do we know how much permafrost will melt, or how much peat will dry out and decay, or whether the seas will get warm enough to trigger the release of methane – an even more potent greenhouse gas than CO₂ – from the hydrates.

Because these risks are unquantifiable, IPCC scenarios largely ignore them. In the worst case, CO₂ levels could continue to climb even if we make drastic cuts in emissions. The longer we delay action, the less likely it is to be effective.

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