The issue of whether global warming is man-made or not seems to me to be largely irrelevant. If its warming, we're in the poop.
Mass extinction of a variety of different species is a change humans won't respond well to.
I think if I was sufficiently paranoid about this, I'd become a prepper. Survivalist skills might be very handy if the global economy collapses because of environmental change.
However I also have belief in Western institutions and think it would take a massive population drop for these not to persevere.
If I were an alien species out to quietly change the planet's climate in preparation for colonisation, I think I'd do it more quickly by dropping a comet on the Amazon rainforest.
