(iv) Examples of future work
Readers will recognize that the results shown earlier, while containing interesting and encouraging pointers, represent just a start on a range of CFD comparisons that will need to be made. As a first step, the computations need to consider a complete rotor rather than just the section between one disc and the next. This will enable the exploration of end effects and the possible variation with height of the wind velocity to be examined as well as potential interference effects
Figure 12. Mean lift coefficients for a bare cylinder and a cylinder with discs over a bare cylinder for a range of rotation rates. Measurements from Reid . (Online version in colour.)
between the rotors (if, as in figure 9, a multi-rotor vessel is chosen). The effect of heeling of the ship (even by only around 2–3◦) on the rotor’s aerodynamic performance also needs to be examined. As a final example of issues requiring examination, we note the possible effects of the top disc of the rotor on the behaviour of the salt-water spray discharge. One may well wish to cause the spray to spread as quickly as possible to minimize the risk of droplet collisions (which would create a larger than optimal size of droplets). It is known that imparting swirl to the spray will do that. However, that will lead to a reduction in the droplets’ vertical velocity, which, on its own, may reduce the proportion of salt particles reaching the cloud base. Such competing effects and their consequences need to be considered in the next phases of this research.
6. A limited-area experiment to explore the fundamental processes involved in marine cloud brightening
Before any geoengineering scheme based on SRM could be implemented, it first must fulfil the following criteria: (i) it can deliver the desired agent by which solar radiation will be scattered to space (sulphate particles in the case of a stratospheric aerosol and increases in sea spray aerosol in the case of cloud brightening); (ii) it can deliver the desired radiative response; and (iii) the undesirable climatic responses to geoengineering perturbations are minimal; certainly, they should be no worse than those associated with changes induced in the climate system from the inadvertent human activity that geoengineering is aiming to mitigate. The last of these cannot be tested by experiment for any of the SRM methods without full implementation lasting multiple years and carries a risk of substantial negative impacts. This was argued by Robock et al. , who focused upon geoengineering through stratospheric sulphur injection, currently considered to be one of the most feasible schemes . Robock et al.  further argued that it is impossible to fully field test geoengineering schemes without significant modification to the climate system owing to non-local climate responses.
While we agree that large-scale field testing of any geoengineering scheme is indeed inseparable from deployment, small-scale field testing will be necessary to make significant progress in understanding the feasibility of geoengineering schemes . An attractive aspect of MCB in terms of field testing is that, because aerosol particles in the MBL are extremely short-lived (typically a few days) compared with their stratospheric counterparts (1–2 years), perturbations to the radiative budget from MCB are inherently localized. This is not the case with stratospheric sulphur injection. This essentially means that it is possible to conduct a useful test of MCB (with minimal climate impacts) over a limited area that includes testing of TOA radiative responses in addition to the testing of injection methodologies and dispersion, etc. This is in contrast to stratospheric sulphur geoengineering, in which case, as Robock et al.  correctly argued, it would be extremely difficult to measure either an effect on the Earth’s radiation budget or maintenance of the aerosol in the stratosphere using only a small number of injections that might constitute a field test.
The stratospheric sulphur injection scheme has so far been considered one of the most viable schemes, not least because previous volcanic eruptions such as Pinatubo in 1991 have provided significant data against which model predictions of the radiative effects of sulphate particles in the stratosphere can be tested and validated [61,62]. Unlike stratospheric aerosols, many of the basic processes linking tropospheric aerosols, clouds, precipitation and radiation underpinning the cloud-brightening scheme are rather poorly understood . Given that the influence of human activity on such processes has been proposed to make a substantial contribution to the radiative balance [63,64], it is imperative that basic knowledge of aerosol–cloud interactions is improved substantially, regardless of the viability of cloud brightening as a geoengineering scheme.
Inadvertent human-induced changes to regional aerosol particle burdens have been used to investigate these processes in regions of stratocumulus in the past [65,66], though large natural variability and co-dependency of processes has to date limited progress towards full understanding. Also, emissions from the stacks of ships have been used to study aerosol–cloud interactions , but single plumes of this type can provide only limited information as plumes are narrow and entrainment and mixing are often dominant. A limited-area field experiment that provides a substantial and detectable perturbation above the background on spatial scales that are detectable from space could therefore offer a unique way to probe aerosol–cloud–precipitation interactions, and their influence on radiation. It would also enable new knowledge on aerosol influences on climate to be gained.
An analogy can be drawn between improving knowledge of aerosol–cloud interactions through a limited-area perturbation experiment and previous experiments conducted to investigate the control of micronutrients (notably iron) on the drawdown of carbon by marine biological systems. A number of experiments have been conducted, which have deliberately added iron to the ocean to improve knowledge of ocean biological carbon cycling. These have substantially improved knowledge of nutrient limitation on oceanic primary production, its subsequent control on plankton communities and how this impacts experiments to develop knowledge of the fundamental processes are seen as crucial to furthering the understanding of the Earth system and are critical before any consideration is given to large-scale deliberate attempts at carbon sequestration by such means . A major concern is that larger scale experiments may have significant impacts on ocean ecosystems. A key point is that a limited-area field experiment to study aerosol–cloud interactions using artificially generated aerosol from sea spray can be carried out without any climatically damaging effects as the lifetime of atmospheric aerosol in the MBL is of the order of a few days at most. Such experiments therefore offer a valuable contribution to climate science and should not be viewed as solely a means of validating the cloud-brightening scheme.
Here, we present an initial framework for the testing and implementation of such experiments. We propose a set of field tests to critically assess the efficacy of MCB over a limited area. The tests are de minimus with respect to their climate effects, as we shall discuss later. The tests involve three phases, with increasing logistical complexity, each of which is designed to test one or more important components of the cloud-brightening scheme. Each involves the introduction and monitoring of controlled aerosol perturbations from one or more ship-based seeding platforms up to a limited area of approximately 100 × 100km2. A suite of observational platforms of increasing number and complexity, including aircraft, ships and satellites, will be required to observe the aerosol plume and in the latter experiments the cloud and albedo responses to the aerosol perturbations. These include the necessary cloud physical and chemical processes that determine theefficacy of the cloud-brightening scheme and are central to the broader questions of aerosol–cloud interactions. Multi-scale modelling work will be carried out to simulate/predict the cloud responses. The modelling work will be used to drive quantitative hypothesis testing for the field tests, and will be used to test our understanding of, and ability to simulate, aerosol–cloud interactions on the regional scale.
The proposed experiments are on a similar scale and complexity to those being routinely conducted by the international research community through interagency cooperation.2 Such integrated inter-agency collaboration will be necessary to deliver a limited-area field test of aerosol–cloud–precipitation interactions generated by a sea spray generation system. The field testing would need to be conducted in an open and objective manner, in accordance with the Oxford principles of geoengineering governance . Further, they should be sufficiently small to not have inadvertent climate impacts, and certainly within an internationally agreed ‘allowed zone’  to be determined through consultations between high-level international scientific organizations and other potential stakeholders. We return to this point later.
2For example, the variation in the American monsoon system (VAMOS) ocean cloud atmosphere land study (VOCALS) was developed to improve understanding of the southeast Pacific coupled ocean–atmosphere–land system on diurnal to inter-annual time scales. A large component of VOCALS centred around a large-scale regional experiment to investigate the interactions between aerosol, cloud and precipitation across a strong pollution gradient in a region dominated by the largest and most persistent stratocumulus cloud sheet on the planet . The field experiment involved the use of five aircraft and two research ships, operating in the region between 70 and 80Wat a longitude of around 20S for a period of around four to six weeks and received multi-agency and multi-national support.
The recommended approach is to test any sea spray generation method, its effect on the cloud system and subsequent radiative impacts through a series of field trials of increasing complexity and expense. The first phase is to establish the ability of a full-size spray generation system to deliver sea spray particles of the correct size and number in such a way that they become mixed throughout the depth of the boundary layer. The second phase would be to use a single system to investigate cloud responses. Because it involves a different suite of cloud measurement instruments and a more complex array of platforms, phase 2 would commence only after the spray system and dispersion has been tested (requirements for success are provided below), and we anticipate that several attempts at phase 1 will be required to refine the spray generation methodology. The third phase would be to conduct a multi-source limited-area experiment at the 100 × 100km2 scale. Such a strategy assesses viability at each stage without incurring unnecessary risk or expense.
(a) Field phase 1: injection and dispersion of particles
Technology to create the large number of small particles that can act as CCN on which cloud droplets will form will need to be field tested to ensure that the delivery mechanism (here termed injection) can deliver particles in sufficient quantity and of the appropriate size into the MBL, and to study the dispersion of the aerosols throughout the MBL.
The seeding technology should be deployed on a ship or barge platform in a marine region favourable for MCB. Only a single aircraft fitted with state-of-the art aerosol measurement technology would need to be deployed to sample the aerosol plume as a function of the distance downwind of the injection source.
This study does not need to be carried out in the remote ocean boundary layer and could be located near to the coast for convenience during the early stages of testing of the engineering system. Studies of diesel-burning commercial shipping indicate that a single source will generate a plume that is typically
10km wide at a distance of 100km downwind . The aircraft would be used to examine the physical and chemical characteristics of particles (size distribution, chemical composition and cloud-forming properties) close to the injection source and to examine how these particles disperse in the boundary layer with distance downwind. Tracer technology should be used to unequivocally identify the plume and hence record if concentrations of sea salt are undetectable from the background. No attempt should be made in phase 1 to study the cloud responses to the aerosol plume. To do so would significantly increase the complexity and cost of the experiment and would represent a risk were any given generation scheme to fail to deliver the required perturbation. Modelling activities in this phase should focus on the examination of the processes associated with the formation of particles and their modification in the stack, and on aspects of the dispersion and mixing of aerosols throughout the boundary layer downwind of the source.
Phase 1 would be considered successful if the aerosol concentrations measured approximately 100km downwind of the spray are sufficient to result in significant increase in aerosol concentration and enhanced CCN burden. From the parcel modelling studies described in §4, there is a need to increase the cloud droplet concentration from background values of perhaps 50–100 cm−3 to values of 200–400 cm−3, which previous estimates suggest requires a sprayer source rate of approximately from 1015 to 1016 particles per second [4,5]. Particles with salt masses greater than approximately 10−16 kg are optimal for seeding (see §4). Wang et al.  used a single source generating 1016 particles to seed a domain of 60 × 120km2 and obtained significant albedo enhancements in simulations of non-precipitating stratocumulus. Measurement of CCN concentrations within an approximately 10km wide plume that are consistently several hundred cm−3 would constitute a successful phase 1 trial.
(b) Field phase 2: single source cloud responses
Once the injection and dispersion technology has been tested and the aerosol plume characterized, the next stage is to examine the cloud responses to a single injection source. The cloud response to a single source will take the form of a ship track (albeit a deliberately produced one). Ship tracks are commonly observed features in regions of marine stratocumulus [72–75] and are associated with small particles emitted from large, commercial, diesel-burning ships . There are existing field observations of ship tracks (e.g. the Monterey area ship track experiment in 1994; ). Figure 13 shows a schematic of the scale of such a plume. Ship tracks from commercial ships are typically 300km in length and approximately 10km wide a few hours downwind of the emitting ship .
Measurement both of the aerosol characteristics below the cloud and of the cloud physical processes should be made with multiple aircraft platforms. The goal would be to test the sensitivity of the cloud microphysical properties to the perturbations in the plume and to contrast these with the surrounding unseeded clouds under a range of conditions. Once again, releasing a tracer from the spray generation system would provide a useful identification of the plume position. Combinations of volatile organic carbon compounds with varying chemical lifetimes can be used to not only identify the plume but also determine its photochemical age, and these can be identified online using modern mass spectrometric or online chromatographic methods.
Success in phase 2 tests would require ship tracks that are readily detectable both as increased cloud droplet concentrations and reduced droplet sizes from the aircraft flying in the cloud layers, and from space using visible and near infrared satellite imagery. Particular emphasis would be placed upon trying to quantify and understand the extent to which the liquid water contents in the seeded clouds remain unchanged in the seeded area, or whether they decrease as some satellite measurements appear to indicate .
Modelling work would be conducted with both process-scale cloud models (see §§3 and 4) and climate models, to test the observed cloud microphysical and macrophysical responses. These modelling studies would also be used to quantitatively predict the outcome of introducing multiple injection sources, which is the key task in phase 3 testing.
(c) Field phase 3: multiple source limited-area experiment
In the third phase of the proposed field trials, multiple (between 5 and 10) injection sources (figure 13) would be used to create a line (of order 100km long) of injection sources approximately perpendicular to the mean wind. The plumes from these sources would disperse and would create a single broad perturbed area extending from the source line several hundred kilometres or more downwind. At such scales, the changes in the cloud-filled boundary layer as a result of the doping by particles should be detectable from space if the radiative impact is significant.
Figure 13. Schematic of the proposed phase 2 and 3 field testing to evaluate the cloud responses to (a) a single-seeded plume; (b,c) multiple-seeded plumes. Examination of ship tracks from commercial ships  tells us that the plumes spread quasi-linearly with time at a rate of approximately 2kmh−1 , which for typical wind speeds of 5–10ms−1 is a width of approximately 6–12km at a distance of 100 km downwind of the source (a). For phase 3 testing, 5–10 ships (six shown in the example here) would be spaced approximately 10km apart to generate a single plume 50–100km wide at a distance of 100km downwind (b). This broad plume and its surrounding unperturbed cloud would be sampled in the crosswind direction by stacked aircraft as discussed in the text (c). (Online version in colour.)
Multiple observational platforms should be used to study: (i) the aerosol physical and chemical properties below the cloud inside and outside the seeded area; (ii) the cloud microphysical, structural and dynamical response; and (iii) the cloud albedo response. Measurements should be made at different distances downwind of the source line. Aircraft flights at stacked levels below cloud, in cloud and above cloud would be complemented by a research ship that would continuously sample the air at a variety of distances downwind of the source line (figure 13c). Control experiments could be performed in two ways: (i) spatial control would involve contrasting the seeded area with the surrounding region and (ii) temporal control would involve temporal modulation of the source strength, perhaps with a 6 hour on–off frequency. The required duration of the entire field test would probably be one to two months, which would permit perhaps 15–20 aircraft case studies under different meteorological conditions and under different background aerosol regimes. Sufficient temporal control modulation would be available on these time scales to provide adequate constraints for model studies. The albedo response to the aerosol perturbations would be quantitatively determined using a combination of airborne and satellite remote sensing. One of the research aircraft would be dedicated to remote-sensing measurement of the shortwave and longwave radiation field above the clouds. Perturbations of several tens of Wm−2 are expected, which should be readily detectable compared with the background control cloud either side of the perturbed region. Process scale and climate modelling should be performed to quantitatively test the MCB hypothesis. This should involve studies designed to calculate the expected magnitude of the albedo perturbations as a function of the seeding strength and meteorological conditions and to compare these with the observations. In addition, the effects of seeding on the cloud dynamical fields and on the precipitation they produce both need to be determined using state-ofthe- art cloud physics and aircraft radar/LIDAR remote-sensing measurements. These would be used to examine the effects on precipitation as a function of distance downstream of the source. On the basis of ship track studies , the radiative effects of the seeding are likely to become indistinguishable from the background cloud within 200–300km downwind of the source, which suggests that precipitation impacts are also likely to be confined to within this distance from the source.
Passive, inert tracers would also be released from the ships (as is the case in all other phases of the work) to provide a control to examine how the particle size distributions are modified with distance downstream. Relative falls in the concentrations of particles with respect to the tracers would provide unprecedented information about the lifetime of the spray particles in the MBL. These would also have the benefit of providing unique data on the cloud top entrainment processes upon which ship track responses are now thought to be critically dependent [27,28].
Phase 3 success would comprise changes in reflected solar radiation within the seeded area of several tens of Wm−2, because this kind of change would be required in regions of marine stratocumulus to offset the radiative forcing due to anthropogenic greenhouse gases. An experiment that did not produce
brightening in excess of 10Wm−2, given increases in aerosol particles that parcel models demonstrate to be sufficient (§4), would be considered unacceptable, because it is almost inconceivable that this could be made to generate brightening deliver changes greatly in excess of 100Wm−2 would be termed successful because in this case the seeding could be scaled back to produce results of the desired magnitude.
Because MCB is aimed at brightening marine stratocumulus clouds, it would be natural to pursue an experiment in a region that frequently experiences this type of cloud. Further, because the increase in albedo owing to the addition of a quantity of additional CCN is greatest for regions with low background
concentrations [80,81], it would make most sense to conduct our proposed MCB test in one of the quasi-permanent sheets of marine stratocumulus, and sufficiently far from continental pollution influences that the radiative susceptibility is high. The northeastern or southeastern subtropical Pacific Ocean would be excellent choices. In addition, the field tests should be conducted sufficiently far upstream of landmasses so that the aerosol loading is able to return to normal background values by the time the advected air masses reach landfall. The larger Pacific Ocean basin would perhaps be more appropriate in this regard, although with typical aerosol lifetimes in the boundary layer of 1–2 days, either the Atlantic or the Pacific Basins would be suitable without due concern.
(e) Climatic impacts of marine cloud-brightening field testing
To ensure that the climatic effects of our proposed MCB field experiments are negligible, we argue here that they will satisfy two important and stringent criteria:
(i) The experiments do not cause detectable climatic responses inside or outside the region defined to be part of the experiment.
(ii) The experiment does not cause damage to the ecosystem.
We argue that detectable climatic responses of repeated MCB would require the SST to be lowered by several tenths of a Kelvin over the 100 × 100km2 area. Phase 3 has the greatest potential impact. The proposed experiment would be conducted over a period of perhaps two months (to ensure a sufficient
number of flights). To allow sufficient time for aerosols to disperse and to impact the low clouds and the radiation field, for each of the 15–20 flights, the spray generation system would need to be operated for perhaps a 6–12 hour period. The mean perturbations to the TOA solar radiation needed in regions of marine stratocumulus to produce a sufficient global response to counter anthropogenic greenhouse gas warming is of the order of 20–40Wm−2 (see fig. 3 in Latham et al. ). Because solar radiation is zero at night, daytime mean values of 40–80Wm−2 are needed. Atmospheric absorption changes and longwave perturbations are expected to be small, and so, in a two-month period, the mean perturbation to the surface net radiation budget from 20 instances lasting 12 hours during daylight would be approximately 10Wm−2. For an oceanic mixed layer depth of 50 m, which is typical in regions of subtropical marine stratocumulus , a net radiative perturbation of this magnitude would lead to a cooling of the SST of approximately 0.25K over the 100 × 100km2 experimental domain. To assess the detectability of such a systematic cooling, we can compare this number with fluctuations in SST typically associated with similarly sized ocean mesoscale eddies that are comparable in magnitude . Large mesoscale eddies are common over the subtropical oceans where MCB experiments would be likely to take place . It would therefore be difficult to detect impacts of MCB experiments on SST against the backdrop of natural oceanographic variability. In addition, it is difficult to argue that such a small perturbation to the SST over a region on the scale of a mesoscale ocean eddy can produce a significant climate impact. Nevertheless, it would be responsible for conducting high-resolution regional climate model simulations prior to conducting the field trials in order to provide assurance that climatic responses to such perturbations would indeed be negligible. Evidence of detectable remote impacts from the simulations would be sufficient to prevent field testing.
Further, it is difficult to conceive of significant ecosystem impacts of the experiment. The SST changes are small, and because the salt used to generate the aerosol particles originates and is returned to the ocean surface fairly locally, salinity and other nutrients are not significantly impacted. Changes in the level of illumination at the sea surface are relatively small (several times smaller than they would be for full-scale deployment), but further work will be needed to understand fully the potential ecosystem impacts .
The multi-faceted research described in the preceding sections and conducted by our rather amorphous ‘team’ of scientists and technologists can be summarized as follows.
Several GCM studies ([5,6] and Jones et al. [7,8,10]) yield the conclusion that—subject to satisfactory resolution of all of a number of important issues, described earlier—MCB could produce a globally averaged negative forcing of significance. A detailed study by Korhonen et al.  predicts appreciably lower forcing, and this study outlines possible reasons for this disparity. Our GCM modelling confirms the results of studies by Jones et al. , which show that MCB could produce unacceptable rainfall reduction in the Amazonian region of South America. However, Jones et al.  show that this reduction could be circumvented by not seeding in a particular area. This study also provides some new results regarding the influence of MCB on sea-ice thickness. Our high-resolution cloud modelling underlines earlier work on the complexities of marine stratocumulus clouds, and shows how the negative forcing produced by cloud seeding is sensitive to both cloud characteristics and seeding strategy. Cloud parcel modelling provides estimates of the ranges of sprayed sea water droplet sizes and salt masses that would be effective for cloud droplet activation, as a function of cloud characteristics. This information is required for the development of the spray generators/disseminators for cloud seeding. Current work on one possible spray system—electrohydrodynamic spray fabrication — is described, while an alternative system involving microfabrication lithography was presented in Salter et al. . More testing of both techniques is required. This earlier (2008) work also provided detailed information on an updated version of unmanned, satellite-guided, wind-powered Flettner-rotor vessels, which could be the vehicles from which the spray droplets would be disseminated, if MCB was ever to be deployed. Here, we present CFD studies of possible instabilities in Flettner rotors. Finally, we summarize current thinking regarding a possible three-stage quantitative field study of MCB, designed to determine whether cloud seeding with sea water aerosol can increase cloud albedo, and, if so, to what degree and under what circumstances. This study—which we envisage would be performed on a spatial scale of about 100 × 100km2, and is not designed to examine possible effects on climate—should also yield useful fundamental information on these climatologically important clouds. As already stated, deployment of MCB should never occur unless approved by the relevant international authority, and shown, via intensive modelling studies, to have no likelihood of significant adverse consequences.
It is unclear whether deployment of the MCB geoengineering technique would be warranted, even if the climate-change problem reached such a drastic stage that some form of intervention was deemed to be required. GCM modelling by three independent groups, using three different models [6,7,10], indicates that, if it functioned as assumed in the modelling, it could—roughly—stabilize the Earth’s average surface temperature and maintain current levels of polar sea-ice cover at approximately current values for some decades, at least up to the carbon dioxide-doubling point, where the required negative forcing for full compensation is approximately −3.7Wm−2. The computations of Korhonen et al. , discussed in §2, yield significantly lower values of negative forcing. This disparity may result from the usage of appreciably different values of natural (no-seeding) CDNCs, N0 (see §§1 and 2) or possibly the vertical velocity field values used in their simulations were too small. In practice, it may be possible to reconcile these disparate results by increasing the dissemination rate of sea water aerosol assumed in the Korhonen study—which we believe would be feasible technologically. However, as discussed in §4, marine stratocumulus clouds are much more complex than has been implicitly assumed in this modelling, and considerably more fundamental research into these clouds is required before we can establish whether our assumptions are justified to an acceptable degree. Also, we have not yet established—for all situations of interest—quantitative values for the fraction of spray droplets generated at or near the ocean surface that enter the bases of the clouds above. Nor have we succeeded to date in developing a sea water spray-production system that meets our requirements as to droplet size and spray rate. Finally, we have not yet thoroughly examined the (possibly adverse) ramifications of deployment of the technique. No case for deployment would exist unless it was established that all such deleterious effects of significance could be remedied. We need constantly to keep in mind that, while some areas may benefit from MCB geoengineering, there may well be regions where the response is significantly detrimental. If so, and if this situation could not be corrected, deployment of MCB would not be justified.
Two advantages of MCB, in principle, are that (i) the sprays could be switched off immediately, with essentially all of the sea water droplets returning to the ocean within a few days, and (ii) because, for some decades, not all suitable clouds would need to be seeded in order to produce sufficient negative forcing to balance the carbon dioxide increase, there exists, in principle, flexibility to confine the seeding to selected cloudy areas which produce no adverse consequences or reduce them to acceptable levels. However, item (i) just mentioned above could prove to be a serious disadvantage, because MCB is more vulnerable to attack than other leading SRM techniques, the spray vessels being located around the oceans. If all or some significant fraction of the fully deployed vessels were destroyed or otherwise rendered unworkable a rapid rise in temperature would be initiated, with concomitant changes in weather patterns and other adverse consequences. This would be true whether the vessels were powered by the wind or by burning fossil fuel.
If MCB proves to be viable, and deployment of an SRM scheme necessary, optimal beneficial cooling might be produced if it was used in concert with another possibly viable technique (e.g. stratospheric sulphur seeding , or microbubble ocean whitening ). In the former case, for example, the primary cooling could be supplied by the stratospheric scheme, with beneficial adjustments being made by MCB, which can function in a more localized manner. It may even prove possible and useful to create localized warming via seeding, to optimize this fine tuning.
Other issues that might be addressed by exploiting the initially localized cooling of oceanic surface waters that we hope would be produced by MCB (and/or the microbubble technique) are coral reef protection and hurricane weakening. In the latter case, it may prove possible to cool oceanic waters in the regions where hurricanes spawn. This would probably require continuous seeding over several months, culminating in the hurricane season. Also, it may prove possible to produce sufficient polar cooling to maintain existing sea-ice cover by seeding specially selected cloudy regions of much smaller total area than considered in our study .
Bala et al.  found that when MCB was used in a carbon dioxide-doubled environment the cooling associated with cloud seeding was a maximum in the two polar regions, compensating roughly for the preferential warming resulting from the additional carbon dioxide. Our own modelling (§2) has produced similar results. A comprehensive series of model inter-comparisons is urgently required in order to optimize and better quantify our understanding and assessment of MCB. We must also conduct a parallel programme of fundamental research into the associated cloud physics and chemistry, aerosol properties and transport, meteorology, etc.
As mentioned earlier, Bala et al.  also found that, if all suitable clouds were seeded, MCB would cause a decrease in globally averaged rainfall, but a net increase in rainfall over land. They surmised that this latter effect occurred because the cooling produced by MCB set up air circulations that brought moist air from ocean to land.
If satisfactory resolution of all significant problems associated with MCB, identified earlier, were to be achieved, and a need for its deployment was deemed to exist, it would be necessary to make an informed decision as to the type of vessel to be used for spray dissemination. Seeding from aircraft is one possibility. Alternatively, in principle, nuclear-powered vessels could be used. However, Salter et al.  focused attention on wind-powered, unmanned, satellite-guided Flettner ships, and it was estimated that about 1500 spray vessels, each consuming about 150kW (derived from the wind), would be required to produce the globally averaged negative forcing of −3.7Wm−2 required to balance carbon dioxide doubling. Flettner ships have the advantages of low cost, high manoeuvrability and low carbon footprint. A conventionally, powered ship might consume about 1MW; so, for both types of vessel, the ratio of the rate of planetary radiative loss to required operational power is very large (in the range from 105 to 107). It follows that considerations of energy efficiency, desirable though that is, should not dictate the selection of type of spray vessel. Latham et al.  pointed out that the main reason that this ratio is so high for MCB is that Nature provides the energy required for the increase of surface area of newly activated cloud droplets by four or five orders of magnitude as they ascend to cloud top and reflect sunlight.
The earlier mentioned arguments are based on the assumption that current GCM modelling is reasonably accurate. However, if it transpires that estimated albedo change/droplet flux ratio values are seriously inflated because, for example, of significant overestimation of the fraction of disseminated sea water particles that rise into the clouds, this issue would need to be reassessed. Other factors then to consider include the levels of pollution produced by spray vessels and the energy they consume. It is also to be noted that, during the decades leading to carbon dioxide doubling, the amount of negative forcing required of MCB would be correspondingly less, as would the emissions (which would be very low for wind-powered Flettner vessels). Definitive statements on these issues must wait on further research, on all fronts covered in this article.
We are grateful for the detailed and extremely helpful comments provided by the two reviewers of this study. We are deeply appreciative of the important role played by Kelly Wanser in the development of our research activity. We are grateful to Graham Feingold for helpful comments. We are grateful for the use of NERC, NCAS, HECToR supercomputer resources. Support for elements of this research was provided by the Fund for Innovative Climate and Energy Research, FICER, at the University of Calgary. This does not constitute endorsement of deployment in any form of cloud albedo modification by the funding agency. Part of this work was performed at the Stanford Nanofabrication Facility, supported by National Science Foundation grant no. ECS-9731293.
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