Discussion

It follows from the discussion in Section 10.6, particularly items 3 and 4, that although two separate sets of GCM computations (Section 10.4) agree in concluding that this cloud seeding scheme is in principle powerful enough to be important in global temperature stabilization, there are important clearly defined gaps in our knowledge which force us to conclude that we cannot state categorically at this stage whether the technique is in fact capable of producing significant negative forcing. There are also currently unresolved technological issues (Salter et al., Chapter 11).

If it is found that the unresolved issues defined in Section 10.6 (especially item 4) do not yield the conclusion that the cloud albedo seeding technique is much weaker than is estimated from the GCM computations, we may conclude that it could stabilize the Earth's average temperature TAV beyond the point at which the atmospheric CO2 concentration reaches 550 ppm but probably not up to the 1000 ppm value. The corresponding amount of time for which the Earth's average temperature could be stabilized depends, of course, on the rate at which the CO2 concentration increases. Simple calculations show that if it continues to increase at the current level, and if the maximum amount of negative forcing that the scheme could produce is -3.7 Wm-2, TAV could be held constant for approximately a century. At the beginning of this period, the required global seawater dissemination rate dV/dt (iff3 = 1) would be approximately 0.14 m3 s-1 initially, increasing each year to a final value of approximately 23 m3 s-1.

Recent experimental studies of both the indirect and direct aerosol effects involving data from the MODIS and CERES satellites (Quaas et al. 2006, 2008) have led to a study by Quaas & Feichter (2008) of the quantitative viability of the global temperature stabilization technique examined in this chapter. They concluded that enhancement (via seeding) of the droplet number concentration in marine boundary-layer cloud to a uniform sustained value of 400 cm-3 over the world oceans (from 60° S to 60° N) would yield a short-wave negative forcing of -2.9 Wm-2. They also found that the sensitivity of cloud droplet number concentration to a change in aerosol concentration is virtually always positive, with larger sensitivities over the oceans. These experimental results are clearly supportive of our proposed geoengineering idea, as is the work of Platnick & Oreopoulos (2008) and Oreopoulos & Platnick (2008), which also involves MODIS satellite measurements.

Further encouraging support for the quantitative validity of our global temperature stabilization scheme is provided by the field research of Roberts et al. (2008) in which, for the first time, the enhancement of albedo was measured on a cloud-by-cloud basis, and linked to increasing aerosol concentrations by using multiple, autonomous, unmanned aerial vehicles to simultaneously observe the cloud microphysics, vertical aerosol distribution and associated solar radiative fluxes. In the presence of long-range transport of dust and anthropogenic pollution, the trade cumuli have higher droplet concentrations, and are on average brighter, the observations indicating a higher sensitivity of radiative forcing by trade cumuli to increases in cloud droplet concentrations than has been reported hitherto. The aerosol-cloud forcing efficiency was as much as -60Wm-2 per 100 per cent cloud fraction for a doubling of droplet concentrations and associated increase in liquid water content; the accompanying direct top of the atmosphere effect of this elevated aerosol layer was found to be -4.3 Wm-2.

Our view regarding priorities for work in the near future is that we should focus attention on outstanding meteorological issues outlined earlier in this chapter, particularly in Section 10.6, as well as technological ones described in Chapter 11. At the same time, we should develop plans for executing a limited-area field experiment in which selected clouds are inoculated with seawater aerosol, and airborne, ship-borne and satellite measurements are made to establish, quantitatively, the concomitant microphysical and radiative differences between seeded and unseeded adjacent clouds and thus, hopefully, to determine whether or not this temperature stabilization scheme is viable. Such further field observational assessment of our technique is of major importance.

Advantages of this scheme, if deployed, are that (i) the amount of cooling could be controlled - by measuring cloud albedo from satellites and turning disseminators on or off (or up and down) remotely as required, (ii) if any unforeseen adverse effect occurred, the entire system could be switched off instantaneously, with cloud properties returning to normal within a few days, (iii) it is relatively benign ecologically, the only raw materials required being wind and seawater, and (iv) there exists flexibility to choose where local cooling occurs, since not all suitable clouds need to be seeded.

A further positive feature of the technique is revealed by comparing the power required to produce and disseminate the seawater CCN with that associated with the additional reflection of incoming sunlight. As determined in Chapter 11, approximately 1500 spray vessels would be required to produce a negative forcing of -3.7 W m-2. Each vessel would require approximately 150 kW of electrical energy to atomize and disseminate seawater at the necessary continuous rate (as well as to support navigation, controls, communications, etc.), so that the global power requirement is approximately 2.3 x 108 W. Ideally, this energy would be derived from the wind. The additional rate of loss of planetary energy, resulting from cloud seeding, required to balance the warming caused by CO2 doubling would be AF.Ae = -1.9 x 1015 W. Thus, the ratio of reflected power to required dissemination power is approximately 8 x 106. This extremely high 'efficiency' is largely a consequence of the fact that the energy required to increase the seawater droplet surface area by four or five orders of magnitude - from that existing on entry to the clouds to the surface area achieved when reflecting sunlight from cloud top -is provided by Nature.

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