There is some good discussion occasionally at the environmental threads of POForums and Malthusia. The former seems to experience waves of denialists that occasionally wash through, but most of the time the discussion is more sane (except for some persistently cranky posters, but all threads have those.) The latter has mostly managed to keep out straight-up denialists, but has done so by now closing itself to new members/contributors, except by invitation. Let me know if you want in there and I'll see what I can do.
Anyway, here are the addresses:
http://peakoil.com/forums/environment-f45.htmlhttp://www.malthusia.com/viewforum.php?f=4The other standards are, of course, climate progress--mostly policy/media stuff; and realclimate--more scientific, but tends to wander annoyingly off topic in the open thread and get into personality wars, imho:
http://thinkprogress.org/climate/issue/http://www.realclimate.org/Others that get less traffic are:
climate central
climate state (prokaryote's site)
climate sight
skeptical science (that is
real, not fake, skepticism--one of the best debunking sites)
tamino's "open mind" blog, desdemona despair (gotta love that one), and stoat "taking science by the throat" are not specifically devoted to climate, but often cover stories on it or do good analysis.
(You shouldn't have any trouble finding most of those with a simple search.)
I'm sure there are many others that have just flown out of my brain, or that I have never heard of. But these are the ones I frequent the most.
ETA: Oh, and The Oil Drum used to have regular good discussions on the subject, but, sadly, they are no longer:
http://www.theoildrum.com/node/10059 )