Radiative Forcing of Climate Change - Climate Dynamics - page 161

Two methods of expressing the influence of changes in the atmosphere's
composition are regularly used. One is the direct radiative forcing , defined as
the change in the net downward radiative flux at the tropopause due to a given
change in an atmospheric constituent. In other words, the direct radiative forc-
ing is the amount of radiative energy (W/m 2 ) added to the troposphere due to a
given change in a greenhouse gas. In calculating the direct radiative forcing, all
radiative processes and adjustments are included, in both the troposphere and
the stratosphere and for both longwave and shortwave fluxes, but all climate
parameters (e.g., atmospheric and surface temperatures, specific humidity) are
fixed—that is, the climate response is not included.
The direct radiative forcing due to the observed changes in greenhouse gases
since pre-industrial times is listed in Table 10.1 . Human activity had increased
atmospheric CO 2 concentrations by about 40% as of 2011, resulting in a direct
radiative forcing of 1.66 W/m 2 . The next largest contribution is from CH 4 , with
tropospheric O 3 close behind. Among the manufactured compounds, CFC-12
has produced the greatest perturbation to the climate system's heat balance.
A number of complications and nuances arise in calculating the overall ra-
diative effects of changes in greenhouse gases. For example, absorption bands
of atmospheric constituent gases can overlap and modify one another's absorp-
tion properties. CO 2 and H 2 O, as well as N 2 O and CH 4 , for example, have
some overlapping absorption bands.
Another measure used to quantify and compare the radiative effects of
greenhouse gas emissions is the global warming potential (GWP). GWP evalu-
ates the implications over time of releasing a unit mass of a given greenhouse
gas compared with the release of a unit mass of CO 2 . The calculation takes
into account the strength of the greenhouse gas as well as its residence time in
the atmosphere. As listed in Table 10.1, a 100-year time horizon is often used.
The GWPs of all the greenhouse gases of concern are significantly greater
than 1, meaning that their potential ability to modify climate is much greater
than that of CO 2 per unit mass . Some of this potential derives from the con-
stituent's molecular structure, which allows the component to absorb more
longwave radiation. For example, atmospheric CH 4 has lower concentrations
and shorter residence times than CO 2 , but its GWP is 25 times greater than
that of CO 2 (see Table 10.1 ). N 2 O has both a long residence time and strong
absorption, and its GWP is nearly 300. But the GWPs of the manufactured
compounds dwarf those of the naturally occurring gases. Sulfur hexafluoride
has the highest GWP because it is a powerful greenhouse gas and also has a
long residence time.
Effects of aerosols on climate are complicated, not completely known, and
highly regional. Aerosols produce direct radiative forcing on climate due to
both absorption and scattering. Increased aerosol loading can either cool or
warm climate, depending on the type of aerosol and its distribution as well
as on the albedo of the underlying surface. Cooling (negative direct radiative
forcing) is induced over dark surfaces such as oceans or forests, and warming
(positive direct radiative forcing) is induced over bright surfaces such as snow
and deserts. To complicate matters further, aerosols exert an indirect effect on
the earth's radiation budget when they interact with and modify clouds.
Calculations of the direct radiative forcing and GWP essentially convert a
given greenhouse gas emission into an energy perturbation (W/m 2 ), allowing us