"After years of resisting efforts to define a dangerous level of warming in international climate discussions, the United States joined with the rest of the world’s major industrial powers on Wednesday in a (non-binding) pledge to avoid warming the planet beyond a threshold long favored by European governments and many climate campaigners as a no-go zone.
The chosen danger zone, derived from a host of scientific studies over the last two decades, lies 2 degrees Celsius (3.6 degrees Fahrenheit) beyond the planet’s average temperature in 1850 or so. (That translates to about a 2 degree Fahrenheit warming from today’s global average, by some ways of measuring, of about 59 degrees.)
But, given the persistent lack of clarity on how much the world will warm from a certain buildup of greenhouse gases and the divergent views around the world on what an ideal climate is in any case, is this threshold meaningful or useful? (And of course there’s the question of whether it’s an utterly wishful goal given emissions trends and energy options, as Richard Black explores on his BBC blog.)"
Andrew C. Revkin writes in his Dot Earth blog for the New York Times July 9, 2009.