The best argument against a runaway greenhouse on Earth is the observation that it hasn't happened yet. Most of Earth's history has had higher CO2 levels than today without triggering a Venus-like runaway. The Earth has been as high as ~2000 ppmv CO2 in the last 150 million years, and ~5000 ppmv during the last 500 million. Even if you burn all the declared fossil fuel reserves on Earth you'd have a hard time finding enough to go much past 1000 ppmv, so it is quite unlikely that we could reach a state that exceeded the natural variation during the recent geologic past. Hence, history indicates that the climate avoids a Venus-like runaway under the foreseeable concentrations humans are likely to create.
On very long time scales, some evidence suggests CO2 concentrations may have been as high as 1 bar at 3.5 billion years ago, though at that time the atmosphere would have been very different and solar luminosity only 70% of modern. -Robert Rohde On Jan 2, 11:02 am, Michael Tobis <[email protected]> wrote: > I have always believed that the "runaway greenhouse" was not possible > on earth, but from my point of view that is hearsay; I've never seen > the calculations. > > Venus is cloud covered and this has not prevented a runaway greenhouse there. > > The idea that increased column humidity necessarily leads to either > increased cloud cover or increased precipitation is not correct in > itself; you have to appeal to the complexities of the climate system. > > (Current evidence is strong that column precipitation increases much > more slowly than column humidity; this has important implications for > the large scale circulation. I don't know what the projections are for > clouds and wouldn't entirely trust them. All of this, though, presumes > conditions much less catastrophic than Hansen is discussing.) > > I think it's an unfair summary to say that Hansen is actually > predicting a Venuslike state for the earth; he is simply speculating > upon it under a reporter's questioning. He may be political enough to > be reluctant to say "a runaway greenhouse won't happen". But he didn't > say it will. > > Amid all the gloom (which I think and hope is a bit excessive in what > we see of the Hansen interview), I remain amused by the common > grammatical form of earth scientists, the "second person planetary" as > in "But with continued rapid increase in greenhouse gases, you could > melt the ice sheets in less than a century". Who, me? > > mt -- 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
