Front Lines; September 2008; Scientific American Earth 3.0; by John P. Holdren, Peter G. Brewer and James Barry, Robert M. Metcalfe, Majora Carter, Kenneth G. Cassman, Sheldon Krimsky; 10 Page(s)

The ongoing disruption of the earth¿s climate by manmade greenhouse gases is already well beyond dangerous and is careening toward completely unmanageable. Under midrange projections for economic growth and technological change, the planet¿s average surface temperature in 2050 will be about two degrees Celsius (3.6 degrees Fahrenheit) higher than its preindustrial value. The last time the earth was that warm was 130,000 years ago, and sea level was four to six meters higher than today. No one knows how long it will take sea level to ¿catch up¿ with such an increase; it could be several centuries, or it could be less.

Even with uncertainties, there is reason to believe that tipping points into unmanageable changes will become much more probable for increases larger than two degrees C. To achieve a better-than-even chance of not exceeding that figure, human emissions must start to decline soon, falling to about half of today¿s level by 2050 and further thereafter.