On Jan 3, 1:41 pm, Tom Adams <[email protected]> wrote:
> On Jan 3, 9:11 am, "Robert A. Rohde" <[email protected]> wrote:
>
> > The best argument against a runaway greenhouse on Earth is the
> > observation that it hasn't happened yet.
>
> Because the Sun did not put out as much energy in the past as it does
> these days, according to Hansen in his new book.
>
> Hansen's runaway senario involves burning all the fossil fuels, even
> the tar sand, BTW.
Go back 500 million years, and solar luminosity is only about 7% lower
than now but CO2 was 20 times higher. I have trouble believing that
the small increases in solar luminosity could have left us so
precariously balanced on the edge of a knife that we'd just now be on
the brink of a Venus-like runaway. I'd also add that many points in
that same 500 million year span had deglaciated polar caps, and
average global temperatures about 10 C warmer than today. Which
further suggests that you have to have quite substantial warming
before you get outside the range of past natural variation and can
start to worry about a new kind of runaway.
I don't know that a Venus-style run away is impossible for Earth, but
I think it is so improbable (and so many other bad things would have
to happen first) that even bringing it up in a serious discussion of
global warming consequences seems misleading. It smacks of fear-
mongering, provides skeptics with a talking point that is easy to
criticize, and distracts from the quite substantial consequences of
global warming that are far more probable.
-Robert Rohde
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