Hi Andrew,
I've been reading through Hansen's Bjerknes Lecture slides (plus text) that you refer to, and he mentions the possibility of thermal runaway in connection with continuing burning coal. So I've been looking up quantities of carbon in various forms and the effect of methane release. Clathrates were not implicated in rapid warming periods, coming out of the last glacial period. It's possible that they were formed before the Ice Ages. At the thermal maximum, of the Eocene, from 1,500 to 2000 gigatons of carbon were released into the atmosphere [1] and the CO2 level reached around 3500 ppm. Much of that carbon went into products of erosion, but some went into plants whose anaerobic decomposition created methane, to be trapped in permafrost or methane clathrates. So it's possible that release of all the methane trapped in the Arctic could conversely lead to CO2 levels much above the present. The problem is that the methane has a GWP (global warming potential) way above CO2, especially if released over say 20 years when GWP = 72. Global inventory of clathrates is estimated at 500-2500 gigatonnes carbon (with 400 Gt C in Arctic permafrost), compared to 5000 Gt C for other fossil fuel reserves and 700 Gt C in the atmosphere [2]. So only a fraction of the potential methane needs to be released quickly (i.e. over a few decades) to cause global warming to completely swamp anthropogenic global warming. The possibility of thermal runaway of Venus proportions is basically due to water vapour being a potent greenhouse gas. As temperature increases, I wonder whether climate forcing from increased water vapour is normally offset by increased cloud cover and its albedo. But there would come a point when the cloud cover approaches 100%, and then the greenhouse effect takes over until the oceans boil. Has anybody studied this possibility? As Hansen says: The Earth is truly the goldilocks planet! Not too hot, not too cold. We have to be proactive for it to remain so. Cheers, John [1] http://en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Thermal_Maximum [2] http://en.wikipedia.org/wiki/Methane_clathrate Andrew Lockley wrote: > There's a brief mention in the article John kindly forwarded to Jim > Hansen's opinion that the earth will go the way of Venus, desolate and > devoid of life. Scary stuff, which I'd dismiss out of hand if it > wasn't for Hansen's reputation. It's a new one on me, and I hope some > people on this list can comment. > > The only reference I could find to Hansen's views was this link > http://www.columbia.edu/~jeh1/2008/AGUBjerknes_20081217.pdf > > I'd love to hear more. If this is a serious risk, we need to be > making a lot more fuss. > > A > > 2009/10/3 John Nissen <[email protected]>: > >> "It is not hard to imagine that if we fail to get a grip on the climate, >> civilisation will collapse altogether." >> >> http://www.newscientist.com/article/mg20427282.900-earth-will-be-ok-but-for-us-its-not-so-good.html >> >> >> >> >> >> >> --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. 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/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
