This may be slightly off the topic but I can't find a better place to pursue it.
I just watched a couple of lectures by Dr. Peter Ward, astrobiologist, the co-author (with astronomer Donald Brownlee) of
Rare Earth. One is a TED talk he gave in Feb of 2008 (
http://www.ted.com/index.php/talks/peter_ward_on_mass_extinctions.html), the other a lecture in Boston last September (
http://www.youtube.com/user/pangeaprogress#p/c/5CD8F71DDA841C3D/0/loWhbfwh3Hc). Both cover a lot of the same matter, but from different points of view; there was enough in them for several threads, and I'll consider that in the next week or so. The one I want to get into here is his view on global warming and global cooling.
Ward sees that Earth is in the middle of it's halcyone days of animal life. It started about 500 mya with multicellular creatures) and will end in about 400 my from now when CO2 reaches 10 ppm in the atmosphere due to eventual carbonation within rock and all plant life dies (I'll not go into that right now; later on that). However, he points out that a natural death is not the only possibility; there is also the possibility of accidental death, such as a major strike (he mentions a Hale-Bopp-sized comet as an object that will wipe animal life out). However, the one he wants to explore is the possibility of an end to the interglacial. which could lead to major mass extinctions planet wide. He doubts humans would die out, but civilization as we know it may well. Further, he thinks that it may lead to a resurgence of bacterial life which could poison he atmosphere with hydrogen sulfide. He claims that this has happened before, and that mammals will die if the concentration reaches 200 ppm. (There is some neat things that may happen at lower concentrations; again, later.)
He claims (semi-facetiously) an anti-gaia hypothesis in which bacteria will use H2S to try to reclaim the Earth from multi-cellular life. H2S is stored in the fossils of bacteria that live around the black smokers in the ocean deeps. He hypothesizes it has been a major case of deaths at all the major extinction events. It arises when atmospheric CO2 rises because of trapp volcanic events. Then a miracle occurs (ie, it's not clear what happens here how the CO2 or atmospheric heat causes the H2S to be injected into the atmosphere in the lectures) and H2S also arises and keeps multicellular life from arising. This is his explanation for why it took 3 billion years for multicellular life to get its act together, and what the killer is at the major extinction events.
He sees global warming as the trigger that could easily bring to an end the current interglacial and result in the concentration of H2S increasing in the atmosphere, and it will be deleterious to us and all animal life. As sort of a freebie, he points out that the record here on Eaerth is that at no time when there was a CO2 concentration of 1000 ppm or above that any ice was present at either pole. That means a 240' rise in sea levels between now and the time when the concentration gets that high, which he estimates (from other scientists he's talked to) from 100 to 300 years from today.
This is a simple exposition of his lectures, and I don't know yet where they might be strong or weak as theories. I do think it is pretty silly that we should be straining so hard to fight our way into either heating or cooling; either way, it seems to me, will be a looser in the long run, and perhaps the short.
