FUTURE CLIMATE CHANGE - The Climate Casino - page 38

The process by which CO 2 emissions are distributed around the
planet is called the “carbon cycle.” This is an active area of research, and
many carbon cycle scientists study how carbon moves among different
carbon reservoirs. In studies done for the Intergovernmental Panel on
Climate Change (IPCC), the models on average estimated that between
50 and 60 percent of carbon emitted during the twenty-fi rst century
would still be in the atmosphere at the end of the century. There were
large differences across different models and depending upon the emis-
sions growth. 2
Before I begin the detailed discussion, let's start with a simple ques-
tion: Can it really be possible that human activities are signifi cant enough
to change the global climate? After all, humans account for but a tiny
part of global activity. To answer this question, I focus on the area that
is both best documented and most important—the rising concentration
of CO 2 in the atmosphere.
There is no question that atmospheric concentrations of CO 2 are ris-
ing. Thanks to the foresight of scientists who began monitoring atmo-
spheric CO 2 in 1958 on the big island of Hawaii, we have measurements
spanning more than fi fty years. Figure 6 plots the monthly observa-
tions from the Mauna Loa Observatory through 2012. Over that half
century, atmospheric CO 2 concentrations have risen 25 percent. 3
Are we confi dent that rising CO 2 concentrations are due to human
activity? Might they arise from natural variability? Both modeling and
measurement of historical data strongly support the view that the in-
creasing concentrations shown in Figure 6 are due to human activities.
One interesting fi nding comes from ice cores. Using these, climate scien-
tists estimate that CO 2 concentrations have ranged from 190 parts per
million (ppm) to 280 ppm over the last million years. Since current con-
centrations are now over 390 ppm, the globe is well outside the range of
concentrations experienced during the period that Homo sapiens emerged
on earth.
I mentioned above that a little more than half of emissions are esti-
mated to be in the atmosphere at century's end. What happens to the
rest of the CO 2 ? Some of the additional CO 2 may go into the biosphere
(such as trees and soils), which means that it is absorbed by plant life