International climate-change negotia-
tors are focused on keeping global warming
at or below 2 °C above historical levels—
the limit beyond which the U.N.’s Inter-
governmental Panel on Climate Change
says the consequences of global warming
will become catastrophic. But even though
negotiators may have finally made some
progress on agreements to reduce emis-
sions, there is a big problem: we’re already
about halfway to the 2 °C threshold. In
October, for example, the warmest October
in 135 years of record-keeping, the global
average temperature was 1.04 °C warmer
than the preindustrial reading. It was no
aberration: 2015 is almost certain to have
been the warmest year on record, surpass-
ing the previous record set in 2014.
Even as flocks of jets began descending upon Paris for the latest talks, the delegates could see that the consequences of
global warming had been setting in fast.
Ice sheets in Greenland and Antarctica are
shrinking with unexpected speed, Arctic
sea ice is disappearing faster than forecast in computer models, and circulation
patterns over vast swaths of the planet’s
oceans are being disrupted. “The more we
learn, the more we see that these processes
are happening more quickly than we anticipated,” says Noah Diffenbaugh, a professor of earth system science at Stanford.
These trends highlight the uncertainty
of climate models and the somewhat arbitrary nature of the threshold set by the
U.N. panel: the fact is that no one really
knows how high the global average temperature will get once the accumulated
carbon in the atmosphere stays above 400
parts per million (a level it reached, on a
monthly average basis, for the first time
last March)—nor what the consequences
for humanity will be in a world that is 2 °C
hotter than it was in the preindustrial era.
And even if we do accept the goal of keeping warming below 2 °C, we still don’t
know what will have to happen to carbon
dioxide emissions to make that possible.
Here’s a smart way for us to limit carbon
emissions and keep global warming
below 2 °C.
By Richard Martin