Some prominent climate scientists, like James Hansen of NASA,
think the safe threshold for carbon dioxide in the atmosphere is
no higher than 350 parts per million. Beyond this point, they
say, we risk severe climatic consequences: melting ice caps and
glaciers, rising seas, and a sharp increase in heat waves,
droughts, crop failures and extreme weather events. Of course,
the world bid farewell to 350 p.p.m. more than 20 years ago and
is cruising steadily toward 400 p.p.m.: last year, the Mauna Loa
observatory in Hawaii observed carbon dioxide concentrations
above 390 p.p.m. for the first time. The last time carbon
dioxide levels were this high was 15 million years ago,
scientists suggest. But even as many policy makers now view
halting carbon dioxide levels at 450 p.p.m. as an ambitious if
perhaps unrealistic goal, one leading climate activist group
continues to make returning carbon levels to 350 p.p.m. its
mission. The group, called 350.org, staged an international
climate change rally last October that Fo