The most common position among climate change analysts is that we need to stabilize the atmospheric concentration of carbon dioxide somewhere between 450 and 550 parts per million (ppm). That is, for instance, the target range endorsed by Nicholas Stern. It is also thought by many to be compatible with the EU goal of generating less than two degrees Celsius of temperature increase, though that is only really plausible at the low end.
In recent Congressional testimony, James Hansen, director of the NASA Goddard Institute for Space Studies, argued that we actually need to cut concentrations from the present 385ppm to 350ppm or less. Basically, his argument is that even stabilization at the present level would have unacceptable consequences: both directly, in terms of impacts on physical and biological systems, and by kicking off feedback loops that will further worsen things. The distinction between the numbers may seem abstract to those not familiar with climate policy, but the practical differences between stabilizing between 550, 450, or 350ppm are massive. Each scenario requires that emissions peak at a different date, and that they fall more or less rapidly afterwards. Even staying below 450ppm requires that global emissions peak within 10-15 years, and that they fall to a small fraction of present levels by 2050.
If accurate, the 350ppm target invalidates a great deal of climate change planning. The general view is that we still have a cushion for additional emissions, to be split up between developed and developing countries. The former would lead the way, showing the latter how they can also do so once they reach a somewhat higher level of affluence. Getting back to 350ppm in a reasonable amount of time requires much more aggressive cuts, universally. It would also require that India and China move to a low-carbon economy long before any significant proportion of their population has reached Western levels of affluence.
Personally, I hope Hansen’s most recent testimony is not as prescient as that he gave twenty years ago. If we need to get the planet on a rapid path towards 350ppm, the disjoint between what is physically necessary and what is politically possible is far wider a chasm than has hitherto seemed to be the case.