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We are getting into very risky territory," said Christiana Figueres, head of the UN Framework Convention on Climate Change, last week.
But she acknowledged that we may have to go there anyway.
She was talking about geo-engineering, the manipulation of the world's climate to avoid catastrophic warming.
Nobody actually wants to do that, because we don't understand the climate system well enough to foresee all the possible side-effects.
But a large number of people think that in the end we'll have to do it anyway, because we're not going to get the warming under control in time without it.
Geo-engineering might involve putting sulphur dioxide into the atmosphere (to reflect some incoming sunlight), spraying fine droplets of seawater into low-lying marine clouds to thicken them up (and reflect more sunlight), or painting the world's roads and roofs white.
There are also proposed techniques for removing carbon dioxide from the atmosphere and for slowing the acidification of the oceans.
In fact, there are dozens of proposals in all.
The topic is now on the table because 60 scientific experts are meeting in Peru this week to begin an exploration of geo-engineering options that will probably end up in the next report of the Intergovernmental Panel on Climate Change (IPCC) in 2014.
This has caused outrage in some sections of the environmental movement, and 125 organisations wrote an open letter to the IPCC head, Dr Rajendra Pachauri, condemning the whole concept.
"The IPCC ... must take great care not to squander its credibility on geo-engineering, a topic that is gathering steam precisely when there is no real progress on mitigation and adaptation," said the letter.
"International peasant organisations, indigenous peoples, and social movements have all expressed outright opposition to such measures as a false solution to the climate crisis."
Then came a sly suggestion that scientists in this field are a bunch of greedy frauds: "Asking a group of geo-engineering scientists if more research should be done on the topic is like asking a group of hungry bears if they would like honey."
This is clearly a subject that inspires passionate opposition on the left, although the geo-engineers themselves spread right across the political spectrum.
The overwhelming majority of the open letter's signatories are organisations you have never heard of - Terra-1530 Moldova, the Dogwood Alliance of North Carolina, and the Missionary Oblates of Mary Immaculate, for example.
But they include a few well-known organisations like Friends of the Earth International.
Their goal is not just to ban large-scale geo-engineering.
It is to ban even small-scale experiments in geo-engineering.
Why so angry?
Part of the problem is that there has indeed been "no real progress on mitigation and adaptation" in recent years.
And the enemies of geo-engineering are afraid that efforts to cut greenhouse-gas emissions will be abandoned in favour of just trying to hold the temperature down artificially.
I have never met a geo-engineer who thought that would work.
But there is profound suspicion of them among the Greens.
There has been a remarkable reversal of roles in environmental issues over the past century. The old left loved industry, modernity, man "conquering" nature, whereas the old right believed in tradition, conservation and preserving nature.
The new left, or large parts of it, hugs trees and romanticises peasants, while the new right, at least in the United States, denies climate change outright.
They are both wrong, and it is not an ideological issue at all.
The problem the scientists see, and many other people too, is that an industrialising world of seven billion people poses a grave threat to the very environment it depends on, notably in terms of changing the climate.
Ending greenhouse-gas emissions, reducing population, and adopting sustainable patterns of consumption are the necessary long-term responses to the threat of runaway warming, but they are not happening fast enough to avoid catastrophic changes and mass death.
At the moment, in fact, they are not happening at all.
So we had better come up with some stop-gap measures that give us more time to make the long-term changes.
That is what geo-engineering is about: holding the global average temperature down below the tipping point at 2C higher after which we get runaway heating, while we work frantically to get our emissions down and restore the self-regulating, comfortable climate that we have already destabilised.
We have not yet begun to work on that agenda seriously, let alone frantically.
On our current course, according to a study released by the Hadley Centre for Climate Prediction and Research 18 months ago, the average global temperature will be 4C higher by 2060.
If that happens, billions will probably die.
If it stays below 2C hotter, on the other hand, most of them will probably live.
So do the research on geo-engineering now: what works, what doesn't; what are the side-effects?
Do it on a small scale, in local areas, as safely as possible.
Because when we are passing through plus 2C and the famines are spreading, there will be overwhelming demands to DO SOMETHING NOW to halt the warming.
At that point, we had better already know the answers to those questions, because the technologies will then be deployed, ready or not.