SETI Re: [acc-list] Fate of Earth's biosphere


Larry Klaes (lklaes@bbn.com)
Fri, 04 Jun 1999 17:04:01 -0400


>X-Authentication-Warning: clarke.lac.usp.br: mail set sender to owner-acc-list@clarke.lac.usp.br using -f >Date: Thu, 3 Jun 1999 15:43:25 -0700 (PDT) >From: Bill Wheaton <waw@ipac.caltech.edu> >X-Sender: >X-Sender: waw@clarke >To: Rob McCann <rmccann@mail.arc.nasa.gov> >cc: acc-list@clarke.lac.usp.br >Subject: Re: [acc-list] Fate of Earth's biosphere >Sender: owner-acc-list@clarke.lac.usp.br > >On Wed, 2 Jun 1999, Rob McCann wrote: > >> >> Fellow listers: >> > >.... > >> miles) than to Venus's orbit (67 million miles). Presumably, as the energy >> output of the Sun has increased, this inner boundary has been creeping ever >> outward. The "serious compromise" to the Earth's biosphere noted by Adams >> will occur when it crosses the Earth's orbit, and the oceans boil away. >> >> My question: does anyone on this list know when that is projected >> to happen? I seem to recall reading estimates of as short as 250 million >> years, and as long as a billion. >> >> The shorter estimate, in particular, is interesting. If so, then >> in the context of geological time, man has appeared quite late in the >> intricate dance that is the story of life on Earth, very close to the final >> curtain call. > >Rob & All, > >Yet soon enough, it would seem. > >I checked some stellar models, > >( see http://obswww.unige.ch/~schaerer/evol/Evol_grids.html ) > >and find a 1% solar luminosity increase per 100 Myrs is probably about >right for us just now. I think changes in the Earth's orbit (due to >perturbations by the other planets) could cause changes of at least this >order on the same time scale. My guess is planetary biology (? Gaia, I >suppose) could regulate things to accommodate changes several times >larger. > >In saying that, I implicitly assume the whole system is basically pretty >stable, not precariously balanced. This is probably the really critical >question, and I'm pretty sure we are far from knowing that for certain yet >in any detail. > >Except for one thing: given the huge natural insults the planet has surely >endured over cosmic time, how could it have remained habitable otherwise? > >Lately the case has been advanced that there was a period about 600 Myr >ago when the whole Earth was frozen solid, oceans and all: "Snowball >Earth". That would have been just before the Cambrian explosion of >complex multicellular life, but after the big microbiological advances: >sex, oxygen, cells with nuclei, etc. I think it had to do with the >configuration of the continents in their drift at that that time, and its >effect on ocean circulation and climate. Still highly controversial, of >course. > >Just this week I've been wondering what happens when the Sun wanders >through one of those dense cosmic gas and dust clouds in its 225 Myr orbit >around the Galaxy. It must pull in a lot of gas and junk, and that has to >affect its mass, its structure, and its luminosity, on a pretty short time >scale. > >There is just no end of gruesome possibilities; yet here we are! Laughing >and scratching! > >Venus gets a little over twice the flux we do. That seems to be too much, >at least! But I myself would be surprised to learn the oceans would boil >in less than 1 Gyr. And I think life could hang on until at least that >point -- after all, there are bacteria that do OK in 80-110 C water. > >Pathologically optimistic, as usual! > >Bill >



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