> Nevertheless a the strong argument against it, as Robert Rohde
> mentioned, is that the Earth has been up to ~3000ppm in the past and
> recovered.
Though maybe it hasn't (at least not at all recently). See:
http://www.pnas.org/content/early/2009/12/24/0902323106.abstract
http://www.nature.com/news/2009/091230/full/news.2009.1168.html
Also discussed at:
http://scienceblogs.com/islandofdoubt/2010/01/is_the_earth_even_more_sensiti.php
By the way, what I'm getting from this thread is the following:
- Reasonable GCMs do sometimes show runaway warming
- Arguments that it could never happen (because of clouds etc.) are a
bit
"handwaving" in comparison
- Runaway warming could happen if the Earth's temperature gets above
300K
- That level of warming (about 13 degrees above pre-industrial) is a
stretch,
but not impossible if we push CO2 up to 4 x pre-industrial levels,
and then
add lots of feedbacks (like methane release)
- There is a historical argument that CO2 has been very high before,
without
triggering runaway, but apparently we can't be sure about that
either.
Not very reassuring I'm afraid.
Nick
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