11.2.2.2 Technologically-varied solar radiative forcing

The basic principle of these technologies is to reduce the amount of sunlight accepted by the earth’s system by an amount sufficient to compensate for the heating resulting from enhanced atmospheric CO₂ concentrations. For CO₂ levels projected for 2100, this corresponds to a reduction of about 2%. Three techniques are considered:

A. Deflector System at Earth-Sun L-1^[4] point. The principle underlying this idea (e.g. Seifritz (1989), Teller et al. (2004), Angel (2006)) is to install a barrier to sunlight measuring about 106 km² at or close to the L-1 point. Teller et al. estimate that its mass would be about 3000 t, consisting of a 30µm metallic screen with 25nm ribs.^[5] They envisage it being spun in situ, and emplaced by one shuttle flight a year over 100 years. It should have essentially zero maintenance. The cost has not yet been determined. Computations by Govindasamy et al. (2003) suggest that this scheme could markedly reduce regional and seasonal climate change.

B. Stratospheric Reflecting Aerosols. This technique involves the controlled scattering of incoming sunlight with airborne sub-microscopic particles that would have a stratospheric residence time of about 5 years. Teller et al. (2004) suggest that the particles could be: (a) dielectrics; (b) metals; (c) resonant scatterers. Crutzen (2006) proposes (d) sulphur particles. The implications of these schemes, particularly with regard to stratospheric chemistry, feasibility and costs, require further assessment (Cicerone, 2006).

C. Albedo Enhancement of Atmospheric Clouds. This scheme (Latham, 1990; 2002) involves seeding low-level marine stratocumulus clouds – which cover about a quarter of the Earth’s surface – with micrometre-sized aerosol, formed by atomizing seawater. The resulting increases in droplet number concentrations in the clouds raises cloud albedo for incoming sunlight, resulting in cooling which could be controlled (Bower et al., 2006) and be sufficient to compensate for global warming. The required seawater atomization rate is about 10 m3/sec. The costs would be substantially less than for the techniques mentioned under B. An advantage is that the only raw material required is seawater but, while the physics of this process are reasonably well understood, the meteorological ramifications need further study.

These schemes do not affect the expected escalation in atmospheric CO₂ levels, but could reduce or eliminate the associated warming. Disconnecting CO₂ concentration and global temperature in this way could have beneficial consequences such as increases in the productivity of agri- culture and forestry. However, there are also risks and this approach will not mitigate or address other effects such as increasing ocean acidification (see IPCC, 2007b, Section 4.4.9).