“GAME OVER” - The Power Surge: Energy, Opportunity, and the Battle for America's Future - page 91

the bathtub can get before the at endant risks become unacceptable.
(Looking at things in terms of temperatures is more like worrying about
how messy your bathroom might get if the tub overl owed; it's a nice
motivation but not all that useful in practice as a way of deciding when
to turn of the tap.) h ere are three popular candidates for this sort of
goal among those who spend their lives worrying about climate change:
550, 450, and 350 ppm of carbon dioxide in the atmosphere.
For the i rst couple of decades during which people started to get
worried about climate change, 550 was perhaps the biggest game in
town. Part of that was pret y arbitrary. It was popular for scientists'
models to look at what would happen if greenhouse gas concentrations
doubled from their levels prior to the industrial revolution—which
would leave the world at around 550 ppm. Interested policymakers
naturally gravitated to a similar target, and scientists also pointed to
dangerous developments that might unfold beyond that point.
Over the last decade, though, it's become more popular for many
of those who worry about climate change to talk about gunning for
a concentration of 450 ppm or less. For all practical purposes, this is
what policy discussions typically aim for, even though diplomats still
talk about trying to keep global temperatures from rising more than
two degrees. (Oi cial assessments estimate that keeping concentra-
tions below 450 would yield roughly even odds of staying below two
degrees.) Scientii c and economic papers are more and more ot en
using 450 ppm as a benchmark against which to measure develop-
ments and assess potential consequences. Studies using models and,
more important, examining the history of climate change throughout
millions of years have started to worry researchers that substantially
higher greenhouse gas concentrations would set of series of events that
are dii cult to control. Melting Arctic sea ice, for example, could trigger
higher temperatures that melt even more ice, with the cycle repeating
itself in a runaway chain that scientists call a feedback loop. Similar
feedback loops in the Amazon rainforest and in melting Siberian per-
mafrost concern researchers too. As trees in the Amazon grow, they
absorb carbon dioxide, keeping it out of the atmosphere. But if rising
temperatures kill of trees there, the rainforest will absorb less carbon,
leading to higher temperatures, more damage to the rainforest, and