Increasing the reflection of solar radiation by the atmosphere

One way of increasing the reflection of solar radiation is to increase cloud albedo, particularly by whitening the low-level marine clouds that cover some one quarter of the ocean surface, mainly off the west coasts of North America, South America and Africa. All these areas are relatively dust-free and so have relatively few condensation nuclei to initiate cloud formation. An increase in the number of cloud condensation nuclei in the troposphere would allow more (and smaller) water droplets to form from a given mass of moist air. The more and the smaller the droplets, the more the incident sunlight is scattered, increasing albedo. Several ways of doing this have been proposed. If a fine spray of sea water is squirted into air unsaturated with water vapour, the water in the spray evaporates, leaving hygroscopic air-bourn salt particles. Air turbulence will carry at least some of these particles to heights where they could provide cloud condensation nuclei. Fleets of wind-powered ocean-going ships, each equipped with pumps to spray a mist of sea water into the air, have been proposed as one way of getting enough salt particles high into the cloud-forming zone. For more about the technique, its feasibility and its disadvantages, see the articles by Latham et al. and by Salter et al. (2008. Phil. Trans. Roy. Soc. A. 366, 3969-3987 and 3989-4006, respectively), where it is calculated that a fleet of 1500 vessels could give a global forcing of -3.7 Wm -2_. Alternatively, a fleet of low-flying aircraft might be used to deliver cloud condensation nuclei to the cloud base, where the air is normally supersaturated with water vapour.

Another possible way of increasing cloud albedo is to deliberately place aerosols in the stratosphere. Size is critical: to scatter short-wave radiation in the visible part of the spectrum the aerosol particles must be no more than a few tenths of a micron in size. Larger particles scatter outgoing long-wave radiation and so could contribute to the greenhouse effect. Aerosols in the stratosphere persist for a time measured in years, in contrast to the tropospheric salt particles considered in the previous paragraph, which would be rained out in a matter of days. All sorts of materials have been proposed as stratospheric aerosols, but up to now sulfate has received most attention (Rasch et al. 2008. Phil. Trans. Roy. Soc. A. 366, 4007- 4037). Sulfate (in the form of sulfuric acid) is unlikely to be used on its own, but would be formed in situ from one of a suite of sulfur-containing gases. Sulfur dioxide (SO2), hydrogen sulfide (H2S) and carbonyl sulfide (COS, the principal biological input of sulfur to the atmosphere) are all oxidized to sulfuric acid in the stratosphere. The sulfuric acid then aggregates with water to form an aerosol – see Chapter 2.20. The phenomenon is well known: large volcanic eruptions inject sulfur dioxide into the stratosphere, increasing the sulfate aerosol concentration and contributing to a cooling that can last several years. The opening of the Laki fissure in Iceland in June 1783 released roughly 120 Mt of sulfur dioxide into the atmosphere and was followed by a series of exceptionally cold winters in the Northern Hemisphere.

Calculations (see Royal Society. 2009. Geoengineering the Climate) suggest that between 1

and 5 Mt S yr-1_{would be have to be injected into the stratosphere to have the desired effect,} much less than the input to the lower atmosphere from industry, which averaged 63 Mt S yr-1

over the decade 1991-2000. All sorts of ways have been proposed to put the required quantity of sulfur into the stratosphere, among them aircraft, artillery, balloons and rockets. The cost, effectiveness and long-term consequences, particularly on rainfall distribution, are as yet unknown – but need to be thoroughly investigated (see Keith et al. 2010. Nature, 463, 426- 427) before sulfur dioxide is proclaimed as a panacea for global warming.

Of all the geoengineering possibilities discussed in this chapter, the only one that is immediately available is afforestation of land now under crops or pasture – and this, at best, could only mop up a small part of the annual input of fossil carbon dioxide to the atmosphere. The others need extensive feasibility studies, followed by research and development into the most promising, all of which could be extremely expensive and take a long time. Trapping of carbon dioxide already in the atmosphere is energy intensive and requires safe long-term sequestration of the trapped carbon dioxide. Methods based on the deflection of sunlight before it reaches the Earth’s atmosphere or on the reflection of sunlight from the Earth all have the disadvantage that carbon dioxide continues to build up in the atmosphere, with the attendant problem of ocean acidification. Deflection and reflection methods have the additional disadvantage that, should they fail for any reason, the excess carbon dioxide is still in the atmosphere and global warming would rapidly resume.

Further reading for Chapter 6

Royal Society. 2008. Phil. Trans. Roy. Soc. A Geoscale engineering to avert dangerous climate change. Special issue. 366, 3841-4056. Royal Society. 2009. Geoengineering the climate: science, governance and engineering. A report on the opportunities, realistic and otherwise.