Climate change won’t be a smooth transition to a warmer world, warns the Tipping Points Report by Allianz and WWF. Twelve regions around the world will be especially affected by abrupt changes, among them the North Pole, the Amazon rainforest, and California.

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Sea level rises, unpredictable monsoons in India, Amazon die-back, and the desertification of Southwest North America (California and neighboring states) are the most significant climate change catastrophes we face, according to the report’s authors from the Tyndall Centre for Climate Change Research.

In the Southwestern U.S. the tipping point has probably already been passed. The scientists now predict that levels of aridity last seen in the 1930s Dust Bowl will have become the norm by mid-century.

However, in some cases the report offers reassurance. Permafrost melt in Eastern Siberia could release greenhouse gases CO2 and methane. But it would require an extreme 9 degrees Celsius surface warming for the system to tip. Claims that the release of greenhouse gases trapped in the permafrost will lead to runaway global warming are “grossly exaggerated,” the authors conclude.

Modern glaciers, such as those making up the Greenland and Antarctic ice sheets, are capable of undergoing periods of rapid shrinkage or retreat, according to new findings by paleoclimatologists at the University at Buffalo. The paper, published on June 21 in Nature Geoscience, describes fieldwork demonstrating that a prehistoric glacier in the Canadian Arctic rapidly retreated in just a few hundred years.

The proof of such rapid retreat of ice sheets provides one of the few explicit confirmations that this phenomenon occurs.

Should the same conditions recur today, which the UB scientists say is very possible, they would result in sharply rising global sea levels, which would threaten coastal populations.

“A lot of glaciers in Antarctica and Greenland are characteristic of the one we studied in the Canadian Arctic,” said Jason Briner, Ph.D., assistant professor of geology in the UB College of Arts and Sciences and lead author on the paper. “Based on our findings, they, too, could retreat in a geologic instant.”

Hawking said he was “very worried about global warming.” He said he was afraid Earth “might end up like Venus, at 250 degrees centigrade [482 degrees Fahrenheit] and raining sulfuric acid.”

That being said, slow onset runaway change is clearly better than a fast onset variety. It would give us a chance to see if anyone’s geoengineering idea actually works.

“Looking on the bright side?”

It seems that there are various conceptions of just how far climate needs to run in order to qualify as ‘runaway.’

This video uses the term to describe a temperature rise consistent with the projection for business-as-usual emissions from the Hadley Centre (5.5 to 7.1 degrees by 2100).

The Permian-Triassic Extinction event definition is intermediate. It would certainly be horrific, and almost certainly mean the end of humans. We also know it is possible on Earth, though the original may have been kicked off by volcanic activity or an extraterrestrial collision.

The Venusian example is about as extreme as you can get: permanent, massive, probably irreversible recomposition of the atmosphere. The article linked above saying runaway climate change is impossible on Earth is referring to this definition.