There is a climate splash in Nature this week, including a cover
showing a tera-tonne weight, presumably meant to be made of
carbon (could it be graphite?), dangling by a thread over the
planet, and containing two new articles (Allen et al and
Meinshausen et al), a “News & Views” piece written by two of us,
and a couple commentaries urging us to “prepare to adapt to at
least 4° C” and to think about what the worst case scenario (at
1000 ppm CO2) might look like. At the heart of it are the two
papers which calculate the odds of exceeding a predefined
threshold of 2°C as a function of CO2 emissions. Both find that
the most directly relevant quantity is the total amount of CO2
ultimately released, rather than a target atmospheric CO2
concentration or emission rate. This is an extremely useful
result, giving us a clear statement of how our policy goals
should be framed. We have a total emission quota; if we keep
going now, we will have to cut back more quickly later. There is
uncertainty in the climate sensitivit