Hansen mentioned the Venus syndrome in his Bjerknes Lecture he gave at AGU in December 2008:
http://www.columbia.edu/~jeh1/2008/AGUBjerknes_20081217.pdf What he said is this: "The Earth’s climate becomes more sensitive as it becomes very cold, when an amplifying feedback, the surface albedo, can cause a runaway snowball Earth, with ice and snow forming all the way to the equator. If the planet gets too warm, the water vapor feedback can cause a runaway greenhouse effect. The ocean boils into the atmosphere and life is extinguished. The Earth has fell off the wagon several times in the cold direction, ice and snow reaching all the way to the equator. Earth can escape from snowball conditions because weathering slows down, and CO2 accumulates in the air until there is enough to melt the ice and snow rapidly, as the feedbacks work in the opposite direction. The last snowball Earth occurred about 640 million years ago. Now the danger that we face is the Venus syndrome. There is no escape from the Venus Syndrome. Venus will never have oceans again. Given the solar constant that we have today, how large a forcing must be maintained to cause runaway global warming? Our model blows up before the oceans boil, but it suggests that perhaps runaway conditions could occur with added forcing as small as 10-20 W/m2. There may have been times in the Earth’s history when CO2 was as high as 4000 ppm without causing a runaway greenhouse effect. But the solar irradiance was less at that time. What is different about the human-made forcing is the rapidity at which we are increasing it, on the time scale of a century or a few centuries. It does not provide enough time for negative feedbacks, such as changes in the weathering rate, to be a major factor. There is also a danger that humans could cause the release of methane hydrates, perhaps more rapidly than in some of the cases in the geologic record. In my opinion, if we burn all the coal, there is a good chance that we will initiate the runaway greenhouse effect. If we also burn the tar sands and tar shale (a.k.a. oil shale), I think it is a dead certainty." ________________________________ From: "[email protected]" <[email protected]> To: globalchange <[email protected]> Sent: Sat, 2 January, 2010 16:29:22 Subject: [Global Change: 3383] Re: Hansen on runaway warming > That type of runaway is not possible on the earth, because if the > oceans boiled away the earth would be completely covered in cloud. > This would raise the albedo from 0.3 to 0.9 and the planet would cool! I thought William had somewhere stated that even with an atmosphere out of pure water vapour there could be clear sky areas without clouds. I cannot find that statement in the group archives so maybe I mixed things up somehow. Anyway, is Hansen being misquoted here? Tbe review of his book makes it sound, as if runaway global warming of the ocean boiling variety is a certainty, as opposed to a near impossible event. Let me add a few other thoughts: The oceans are a few km deep. 10m of water column is a bar. If the oceans were to evaporate, atmospheric pressure would rise by of the order of a bar for ever 10m of water. At atmospheric pressure water boils at 100C, at 30 bar it's more like 250C. I've done some order of magnitude calculations, and, 100m of perfectly insulated air column can be heated by 200 W/m2 to the surface temperature of the sun in a few weeks and melting the world's ice would take a few decades. To get through the first 10 m (doubling atmospheric pressure and raising world temperature to above 100C) would also take a few decades under these extreme assumptions (all the world's insolation going towards evaporating water, the world being perfectly insulated against heat loss). So, to get to 100C in 200 years should take a forcing of the order of 50 to 100 W/m2 by my reckoning. -- You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange -- You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange
