A Karamojong woman washes clothes at a small pool of water in a dried-up riverbed in the semi-arid savannah region of Karamoja, in northeastern Uganda, Oct. 18, 2017. Credit: Adelle Kalakouti | AP

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 traveling 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 climate accord are kept — runs right off a cliff.

[One of the most worrisome predictions about climate change may be coming true]

Hothouse Earth is not very hospitable to human life. Hundreds of millions or even a billion or two would probably survive, but the damage to agricultural systems would be so extreme that billions more would die. (The authors don’t say this, of course. Putting it into words is too “alarmist” — but the people who actually have to think about these contingencies, like the military in the developed countries, know it very well.)

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 or two of these feedbacks could cause enough additional warming to set off others, like a row of toppling dominoes, and take us up to those lethal temperatures within this century.

Yet, the role of these feedbacks in the system 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, and definitely not part of the public debate. Why not?

[The Atlantic Ocean’s circulation hasn’t been this sluggish in 1,000 years. That’s bad news.]

When you make a statement in science, you have to be able to prove it, generally with hard numbers and testable predictions. The hard numbers simply weren’t available yet — and

if you go public without that evidence, you will be torn to pieces by your scientific colleagues (who are also your rivals, of course).

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 for the Intergovernmental Panel on Climate Change’s efforts to get the warming under control. (Nobody said publicly how they arrived at that number, but it was because the scientists thought that a 2 degrees C temperature rise 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 just as bad as the scientists feared. We have already passed the point where a return to the stable climate of the past 14,000 years is possible, and we are on course for hothouse Earth.

[Greenland is melting faster than at any time in the past 450 years (at least)]

The best we can do is try to stabilize the warming at or just below 2 degrees C, and that will not be possible without major human interventions in the climate system. The “stabilized Earth” is not a natural stopping place: staying there would require “deep cuts in greenhouse gas emissions, protection and enhancement of biosphere carbon sinks, efforts to remove CO2 from the atmosphere, [and] possibly solar radiation management.”

You will notice that geo-engineering (“solar radiation management”) is already part of the package, and that it will be down to human beings to manage the entire ecosystem to keep it “stable.” As 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.

Gwynne Dyer’s book “Climate Wars” was published in 2010. Unfortunately, almost every word in it is still true.

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