Geoscience Reference
In-Depth Information
14
SLOWING CLIMATE CHANGE BY
REDUCING EMISSIONS: MITIGATION
The discussion up to now suggests that neither adaptation nor geoengi-
neering is a satisfactory solution to the threats of global warming. The
only genuine solution for the long run is to reverse the accumulation
of greenhouse gases (GHGs). This is typically called mitigation or, more
accurately, prevention.
Mitigation involves reducing the concentrations of GHG emissions.
The most important GHG is CO
2
, produced primarily by the burning of
fossil fuels. There are other long-lived GHGs, such as methane (the natu-
ral gas that heats our homes). Other GHGs are short-lived, including
particulate matter (also called aerosols). Some of these tend to cool the
globe, which complicates the picture.
It will be helpful to spell out the magnitudes here. Scientists esti-
mate that a doubling of atmospheric CO
2
would lead to an increase in
radiative forcing (roughly, heating) of 3.8 watts per square meter (W/m
2
)
of the earth's surface. This is approximately one one-hundredth of the
radiation that the earth receives from the sun. As of 2011, the total
human-caused forcings since 1750 totaled 2.4 W/m
2
.
This total is the sum of many positive and some negative numbers.
The largest single contributor in 2011 was CO
2
, which contributed
1.7 W/m
2
of heating. Other long-lived GHGs, like methane, contributed
another 1.1 W/m
2
. The contributions of CO
2
and other long-lived gases
are well measured, and we can have confi dence in these calculations.
The other contributions to warming are much more poorly mea-
sured. The most diffi cult factor to measure is aerosols. Human-caused
