"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.
Now, this is not really news to climate scientists.
When I was writing a book about climate change 10 years ago, I interviewed scores of them in half a dozen countries, including Dr Hans-Joachim Schellnhuber, one of the lead authors of this paper and then the director of the Potsdam Institute for Climate Impact Research in Germany (and Angela Merkel's climate adviser).
He already knew all this stuff then. Everybody did - at Potsdam, at the Hadley Centre for Climate Change in England, at the National Centre for Atmospheric Research in Boulder, Colorado, and in universities that had a serious climate research programme.
It was the point of departure, the underlying assumption of every conversation I had.
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 (IPCC), and definitely not part of the public debate. Why not?
If you spot smoke billowing out of a house, you don't wait to see actual flames, check what substances are burning, and calculate the heat of the fire. You call the Fire Department immediately.
But that's not how science works.
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 IPCC's efforts to get the warming under control.
The scale and trigger points of the feedbacks have finally been calculated, more or less.
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". The best we can do is try to stabilise the warming at or just below +2C, and that will not be possible without major human interventions in the climate system.
■Gwynne Dyer's book "Climate Wars" was published in 2010. Unfortunately, almost every word in it is still true.
