Hothouse Earth is upon us and will kill many of us

We have already passed the point where a return to the stable climate of the past 14 000 years is possible.

It would be churlish to ask what took them so long. Let us be grateful, instead, that the climate scientists are finally saying out loud what they all knew privately at least 10 years ago.

What 16 of them are now saying, in an article in the Proceedings of the National Academy of Sciences, is that if we don’t soon get off the highway we are currently travelling on, we will be irrevocably committed to a “Hothouse Earth”. How soon is “soon”? Probably no more than 10 to 20 years away.

That’s the last exit.

The article has the usual low-key scientific title: “Trajectories of the Earth System in the Anthropocene.” The authors never raise their voices, but they do point out that the likeliest of those trajectories – the one we will stay on even if all the promises in the 2015 Paris Accord on climate change are kept – runs right off a cliff.

Hothouse Earth is not very hospitable to human life. Hundreds of millions would probably survive, but the damage to agricultural systems would be so extreme that billions would die. (The authors don’t say this, of course. Putting it into words is too alarmist.)

What the authors are saying is that global warming, driven directly by human emissions of carbon dioxide and other greenhouse gases, is only part of the problem. In fact, it’s the smaller part. The real threat is the unstoppable natural feedbacks, triggered by the warming that we have caused, that will take us up to the killing temperatures of Hothouse Earth.

They list 10 of them, the biggest being the loss of Arctic sea-ice, the melting of the permafrost zone, dieback in both the boreal and the Amazon forests, and changes driven by warming in the ocean circulation system. Just triggering one of these feedbacks could cause enough additional warming to take us up to those lethal temperatures this century.

The role of these feedbacks was not discussed in the scientific journals, not included in the predictions of future warming issued every four or five years by the Intergovernmental Panel on Climate Change (IPCC) and definitely not part of the public debate. Why not?

When you make a statement in science, you have to be able to prove it, but the hard numbers simply weren’t available yet.

So the climate scientists didn’t make grand assertions – but they did manage to get the threshold of 2 degrees Celsius higher global temperature adopted as the never-exceed target. (Nobody said publicly how the scientists arrived at that number, but it was because they thought that +2 degrees Celsius was about where the feedbacks would start kicking in.)

The scale and trigger-points of the feedbacks have finally been calculated, more or less, and the news is bad. We have already passed the point where a return to the stable climate of the past 14 000 years is possible.

The best we can do is try to stabilise the warming at or just below +2 degrees Celsius, and that will not be possible without major human interventions in the climate system.

Jim Lovelock, the creator of Earth System Science (Gaia), wrote 39 years ago, we may “wake up one day to find that [we have] the permanent lifelong job of planetary maintenance engineer”.

I haven’t bothered to ask Jim if we are there yet. Of course we are.