A level of no more than 350 ppm is still feasible, with the help of reforestation and improved agricultural practices, but just barely - time is running out.

Several times in Earth's history, rapid global warming occurred, apparently spurred by amplifying feedbacks. In each case, more than half of plant and animal species became extinct. New species came into being over tens and hundreds of thousands of years. But these are time scales and generations that we cannot imagine.

As species are exterminated by shifting climate zones, ecosystems can collapse, destroying more species.

...99 percent confident that the world really was getting warmer and that there was a high degree of probability that it was due to human-made greenhouse gases.

The five-year mean global temperature has been flat for the last decade

The urgency derives from the nearness of climate tipping points.

The climate is nearing tipping points. Changes are beginning to appear and there is a potential for explosive changes, effects that would be irreversible, if we do not rapidly slow fossil-fuel emissions over the next few decades.