...rapid climate change? While I do believe that global climate is in a state of flux which is influenced in one degree or another by the burning of fossil fuels, is there evidence that life is _en total_ affected negatively?

74 mybp a meteor hit the earth and caused the K-T extinction in which there was a ~25% loss of biodiversity.

http://en.wikipedia.org/wiki/File:Phanerozoic_biodiversity_blank_01.png

No better example of rapid climate change can be found in earth's history. 9 million years later the slightly smaller Manson meteor impacted what is now Iowa, in this case the species extinction was negligible. Is there a threshold at which the planet experiences a cataclysmic loss of biodiversity? Do you think the projected 3-4 degree C increase in temperature will cause mass extinctions or simply a degree of discomfort? Will phenotypic plasticity prevail?