One more Hansen (et al.) must read — What is “dangerous” human interference with climate?

“We conclude that a CO2 level exceeding about 450 ppm is dangerous….

Both Climate Progress and the Center for American Progress strongly endorse the need for a target near 450 ppm — or a total warming from preindustrial levels not to exceed 2°C.

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.

Of those criteria, the “sharpest” is “probably maintenance of long-term sea level close to the present level, as about one billion people live within 25 m elevation of today’s sea level.” Sea levels were 25 meters higher about 3 million years ago, when global temperatures were 3°C warmer than today — a warming we will see under BAU.

Hansen et al. have an especially thoughtful discussion of how quickly sea levels could rise, a subject Hansen has discussed before. They note that during “the largest sea level rise following the last ice age … sea level rose about 20m in 400 years” or about 2 inches a year.

What does that imply if we keep on our current path?

Global warming in “business-as-usual” (BAU) climate forcing scenarios, for climate sensitivity consistent with paleoclimate data, is at least 2–3°C by 2100 (relative to 2000) and still increasing rapidly. Implications include an ice-free Arctic in the warm season, other regional climate changes outside the range of historical experience, and initiation of ice sheet changes that presage future sea level change out of humanity’s control. The Earth, in a broad sense, would be a different planet than the one that existed for the past 10 millennia.

Although “we must be close” to a “tipping point such that it is … impossible to avoid dangerous climate change,” the authors believe “we may not have passed it yet.” In particular, limiting warming to an additional 1°C probably avoids the dangerous climate cycle feedbacks and the kind of rapid sea level rise that would be all but impossible to adapt to. Therefore, scientists have a responsibility to speak out:

These stark conclusions about the threat posed by global climate change and implications for fossil fuel use are not yet appreciated by essential governing bodies, as evidenced by ongoing plans to build coal-fired power plants without CO2 capture and sequestration. In our view, there is an acute need for science to inform society about the costs of failure to address global warming, because of a fundamental difference between the threat posed by climate change and most prior global threats.

For instance, nuclear war could be initiated by the action of just one major country whereas catastrophic global warming requires only inaction by the major countires.

Thus scientists are faced with difficult choices between communication of scientific information to the public and focus on basic research, as there are inherent compromises in any specific balance. Former American Vice President Al Gore, at a plenary session of the December 2006 meeting of the American Geophysical Union, challenged earth scientists to become involved in informing the public about global climate change. The overwhelmingly positive audience reaction to his remarks provides hope that the large gap between scientific understanding and public knowledge about climate change may yet be closed.