Recarbonizing the Earth

The case for reducing emissions of greenhouse gases is more compelling than ever. But it’s also past time to begin drawing carbon out of the air.

About the time this article is published, the nations of the world will be gathered in Copenhagen to discuss ways to reduce emissions of carbon dioxide and other greenhouse gases (GHGs). In the run-up to Copenhagen, the general expectation had been that this meeting would at last chart an effective international climate-change policy to succeed the Kyoto Protocol. But expectations have moderated as 2009 progressed, and the common wisdom now is that the most likely outcome is a framework understanding with an extended working-out period to follow.

In many ways, this is all too familiar: year after year of presentations and negotiations while GHG emissions continue to rise and the scientific evidence paints an ever-more dire picture. Severe changes in the biosphere, such as the dramatic retreat of Alpine glaciers, are already occurring with atmospheric GHG concentrations at the current level of about 388 parts per million (ppm); they continue to rise about 2 ppm per year. The momentum built into the processes driving climate change virtually guarantees worse to come, even with significant cuts in emissions. To prevent the severest outcomes, it looks like we’ll have to augment whatever progress on energy emissions and forest incentives comes out of Copenhagen with new ecosystem-based initiatives to pull carbon out of the atmosphere—an effort that, in effect, will amount to recarbonizing the Earth. What GHG level should we aim for? The science is still evolving, but many important policy positions and discussions peg the acceptable upper bound at about 450 ppm, which would theoretically limit further temperature increases to an additional 1.25 degrees Centigrade above current levels (and about 2 degrees C above pre-Industrial temperatures). However, NASA’s top climate scientist, James Hansen, has been outspoken in advocating a maximum of about 350 ppm. “Humanity’s task of moderating human-caused global climate change is urgent,” Hansen and several colleagues wrote in a widely cited 2008 paper. “[T]here is a danger that human-made forcings could drive the climate system beyond tipping points such that change proceeds out of our control.” Most critically, the Antarctic and Greenland ice sheets could melt and northern permafrost zones might warm and release their methane, triggering cascading ecological catastrophes.