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"We know that [the Wilkins Ice Shelf] has been completely or very stable since the 1930s and then it started to retreat in the late 1990s; but we suspect that it's been stable for a very much longer period than that," he told BBC News.

That's an interesting ... telling graph. Can you give the link to the source?

WASHINGTON (AP) - The president's new science adviser said Wednesday that global warming is so dire, the Obama administration is discussing radical technologies to cool Earth's air.

John Holdren told The Associated Press in his first interview since being confirmed last month that the idea of geoengineering the climate is being discussed. One such extreme option includes shooting pollution particles into the upper atmosphere to reflect the sun's rays. Holdren said such an experimental measure would only be used as a last resort.

"It's got to be looked at," he said. "We don't have the luxury of taking any approach off the table."

Holdren outlined several "tipping points" involving global warming that could be fast approaching. Once such milestones are reached, such as complete loss of summer sea ice in the Arctic, it increases chances of "really intolerable consequences," he said.

Twice in a half-hour interview, Holdren compared global warming to being "in a car with bad brakes driving toward a cliff in the fog."

At first, Holdren characterized the potential need to technologically tinker with the climate as just his personal view. However, he went on to say he has raised it in administration discussions.

Holdren, a 65-year-old physicist, is far from alone in taking geoengineering more seriously. The National Academy of Science is making climate tinkering the subject of its first workshop in its new multidiscipline climate challenges program. The British parliament has also discussed the idea.

The American Meteorological Society is crafting a policy statement on geoengineering that says "it is prudent to consider geoengineering's potential, to understand its limits and to avoid rash deployment."

Last week, Princeton scientist Robert Socolow told the National Academy that geoengineering should be an available option in case climate worsens dramatically.

But Holdren noted that shooting particles into the air—making an artificial volcano as one Nobel laureate has suggested—could have grave side effects and would not completely solve all the problems from soaring greenhouse gas emissions. So such actions could not be taken lightly, he said.

Still, "we might get desperate enough to want to use it," he added.

Another geoengineering option he mentioned was the use of so-called artificial trees to suck carbon dioxide—the chief human-caused greenhouse gas—out of the air and store it. At first that seemed prohibitively expensive, but a re-examination of the approach shows it might be less costly, he said.

And your point is?

The president's new science adviser...

I'd say they have already been doing it.

The effects of the global recession will be to drastically reduce CO2 emissions. Are we getting cooler yet?

Please elucidate

Cloud seeding is a short term expedient to alter the weather rather than an attempt to enginerr changes in the climate

Have you stopped using your car yet?

I’d have thought that by altering the weather you could alter climate, and if they were looking at cloud seeding in 1946, who’s to say they don’t know how to alter the climate.

T'other way round, surely - by altering the climate you alter the weather. Weather is the result of climate not the cause.

They certainly do know how to alter the climate by climate engineering. The problem is that they aren't entirely sure what the change would be

Maybe they "have" tried it, and that's why we are seeing such large changes in the weather over the last few decades. I also started to wonder if all those underground testing of nuclear bombs have any effect on the earth, where does all that energy go once it's been released?

I can assure you that that there are no large mirrors in orbit around earth

Atmospheric rather than underground testing of nuclear weapons is more likely to affect the climate by raising dust. But in global terms the energy involved is quite small