under the most optimistic assumptions about the timing and extent of global greenhouse gas emission reductions, cumulative emissions over the next few decades will result in atmospheric concentrations reaching 650 ppm of CO2-e, associated with warming of 4°C or more before the end of the century, a temperature not seen on Earth for 15 million years. It now seems almost certain that, if it has not occurred already, within the next several years enough warming will be locked into the system to set in train positive feedback processes that will overwhelm any attempts to cut back on carbon emissions. Humans will be powerless to stop the shift to a new climate on Earth, one much less sympathetic to life.Graph from David Archer via John Schellnhuber (slides here, page 11)
We moderns have become accustomed to the idea that we can modify our environment to suit our needs, and have acted accordingly for some three hundred years. We are now discovering that our intoxicating belief that we can conquer all has come up against a greater force, the Earth itself. We are discovering that humans cannot regulate the climate; the climate regulates us. The prospect of runaway climate change challenges our technological hubris and our Enlightenment faith in reason. The Earth may soon demonstrate that, ultimately, it cannot be tamed and that the human urge to master nature has only roused a slumbering beast.