Land Use and Geo-Engineering - Introduction to Carbon Capture and Sequestration - page 536

Section 4
Geo-engineering: solar
radiation management
In the effort to reduce global warming, an alternative to the geo-
engineering techniques that reduce the CO 2 levels in the atmosphere,
which we focused on in the previous section, is to reduce the amount of
sunlight that is reaching the earth. We have seen in Chapter 2 that a
doubling of the CO 2 concentration would correspond to a radiative forc-
ing of 4 W/m 2 . The temperature increase can therefore be compensated
if we are able to reduce the solar radiation by an equivalent amount. The
methods that have been proposed include:
Refl ecting part of the sunlight back into space : to achieve a reduction of
4 W/m 2 we need to reduce the incoming sunlight by 1.8%. The idea is to
bring giant refl ectors into orbit around the earth that would refl ect about
2% of the incoming sunlight. The size of these refl ectors would need to
be on the order of a million square kilometers [11.15]. Clearly, building
such a refl ector is not a short-term solution. However, if such a refl ector
could be built, it is one of the few options that would give an instantane-
ous reduction of the global temperature.
Increase the concentration of aerosols in the atmospher e: in Chapter 2
we saw that the effect of large eruptions of volcanos is a reduction of
global temperatures through increasing levels of sulphate aerosols in the
stratosphere, which increases the stratosphere's natural albedo effects.
The idea is to mimic this cooling effect from volcanos by introducing
hydrogen sulphide (H 2 S) or sulphur dioxide (SO 2 ) into the stratosphere as
gas [11.16, 11.17], where it is converted into sulphate particles with char-
acteristic sizes on the order of several tenths of a micron.
Albedo effect at the surface of the earth : in Chapter 2, we also saw that
the earth refl ects part of the radiation from the sun. This refl ection
depends on the nature of the surface (see Table 11.4.1 ). To compensate
for the 4 W/m 2 , we would need to increase the average refl ection of the
earth's surface from ~107 to ~111 W/m 2 . This would imply an increase in