3.3: Cloud whitening & brightening

Australia is apparently considering some local testing of the idea.

Update 2021: Australia is doing work on marine cloud brightening to forestall damage to the Great Barrier Reef.

¹ This comes with pro's and cons. If something goes wrong you can stop the effects very quickly but it means that you need to be doing this relatively continuously.
A distinct way to potentially increase atmospheric albedo would be to “spray seawater into low-lying marine cloud formations thereby whitening clouds and increasing their albedo”. This could probably be done with a fleet of on the order of hundreds to a few thousand specialized wind (sailing) or solar-powered boats. The Scientific American article concludes that this method might be comparatively benign, locally controllable, inexpensive and easy to shut off¹ (but with consequences for localized weather effects that may be hard to predict)! And it is just spraying seawater, not any exotic chemical.

This idea was originally proposed by Latham in 1990 in a commentary in the journal Nature, and refined somewhat through interaction with Salter and others. The basic idea is to alter the droplet size inside clouds:

effect of big and small droplets on albedo
One project in this area is based at the University of Washington. From their paper, “Early studies suggest that it might be possible to offset a doubling of CO2 globally by brightening 10%–30% of marine clouds”. The scheme is also very fast to switch on and off, since any given cloud system only lasts a matter of days.

The authors emphasize that preliminary studies of such “Marine Stratus Brightening” (MSB) could also serve to help pin down key parameters involved in aerosol-cloud interactions, which are a key uncertainty in climate models generally. Thus research in this area is valuable for the fundamentals of climate science even without reference to geoengineering per se.

The Lawrence paper concludes that this science is needed:

Or in other words, we don’t really know how Marine Stratus Brightening will go but it seems likely that it would lead to cooling above our target for radiative forcing geoengineering of 0.6 W/m^2

the limited knowledge about key microphysical and dynamical processes involved results in a large uncertainty in the maximum cooling that could be achieved via MSB, with estimates ranging from 0.8 to 5.4 W/m^2, i.e., likely well above RFGref”.

To illustrate just how much our knowledge of aerosol-cloud interactions remains in flux, consider this article from Science in Feb 2019, which states:

Accepting the large sensitivity revealed in this study implies that aerosols have another large positive forcing, possibly through the deep clouds, which is not accounted for in current models. This reveals additional uncertainty that must be accounted for and requires a major revision in calculating Earth’s energy budget and climate predictions. Paradoxically, this advancement in our knowledge increases the uncertainty in aerosol cloud–mediated radiative forcing. But it paves the way to eventual substantial reduction of this uncertainty.”

The Leeds IAGP project has done some more detailed simulations of at least one version of the Latham-Salter cloud whitening idea, and uncovered some potential problems, concluding:

“Our simulations found three issues that reduced the efficacy of the spraying mechanism: only certain clouds were susceptible to spraying at certain times of day; many of the sea-salt particles coagulated and rained out before they reached the cloud; and the particle plume generated by the moving ship had a tendency to sink rather than rise to cloud level (due to the evaporation of water from the generation of sea-salt). No doubt many of these obstacles would be surmountable, but development and testing take time.”

The 2015 National Academies report on solar radiation management concluded the following in terms research needs and potential problems, but overall seemed to judge it feasible that it could make a sufficiently large impact on the planet’s energy balance if this research is resolved:

Or briefly, we know very very little about how any of this would work in real life in terms of feedback effects or second order consequences

“Research beyond the use of computational models is needed to address some of the key open questions on the potential for marine cloud brightening to be useful for albedo modification purposes. The reason is that the uncertainties of cloud susceptibility, scale-up, and feedbacks are not sufficiently understood to be included with confidence in models.… The committee is specifically aware of a lack of knowledge about (a) impacts on ocean circulations, (b) consequences to ecosystems due to significant reductions in sunlight reaching the surface where MCB is operating, (c) interactions of MCB with dominant modes of interannual variability like ENSO and the Pacific Decadal Oscillation (PDO), and (d) the nature of the remote impacts to precipitation like that found in the U.K. Met Office model discussed previously (Jones et al., 2013). These processes are all likely to operate at longer timescales and be sensitive to forcing on larger space scales and should also be explored.”

The Lawrence paper notes that cost estimates for both stratospheric aerosols and Latham-Salter style cloud whitening could be <$100B. In other words within some people’s personal net worth. This clearly presents both positive and negative aspects.