Considering that we have already warmed 0.8°C, we can’t risk another 1°C more — a challenging goal since the Earth will warm another 0.6°C even if we stop all carbon dioxide emissions tomorrow. Nonetheless, James Hansen et al. very much believe the goal is achievable if we act quickly and focus on all greenhouse gases, not just carbon dioxide. (I share this view, and indeed they cite a Science article I coauthored on emissions reductions.)
The 1992 UN Framework Convention on Climate Change (signed by President Bush’s father and ratified unanimously by the Senate) identified an objective–”to achieve stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” But it never answered the obvious question– really the central question of the century — what is that “safe” level?
The nation’s top climatologist is nothing if not prolific. He and nearly four dozen co-authors answer the question in a masterful article for Atmospheric Chemistry and Physics, “Dangerous human-made interference with climate.”
They look at the “potential criteria for dangerous climate change assuming that humanity wants to preserve planetary conditions similar to those in the period of civilization.” They are especially concerned about the risks posed by an ice-free Arctic, tropical storm intensification, “the potential for accelerating sea level rise [from the disintegration of the Greenland and Antarctic ice sheets], and future positive feedback from methane release.” They warn:
If the [additional] warming is less than 1°C , it appears that strong positive feedbacks are not unleashed, judging from recent Earth history. On the other hand, if global warming gets well out of this range, there is a possibility that positive feedbacks could set in motion climate changes far outside the range of recent experience.