SRM: Marine cloud brightening (MCB)

So marine cloud brightening brings SRM into the tropopause.  It consists of smaller scale, regional changes in albedo by increasing the concentration of cloud micro-droplets in localised marine cloud systems, basically making clouds whiter and therefore reflect more solar radiation.

So how would it work...?

Increased micro-droplet concentration can occur by increasing the concentration of particles that act as cloud condensation nuclei (CCN); though if too large these particles will increasing precipitation, thus reducing micro-droplet concentration.  Subsequently to minimise research and engineering costs, naturally occurring CCN have been proposed in the form of sea salt - a highly abundant and readily, not to mention freely, available resource!  

The complicated bit about implementing this would be distribution, how do we go about injecting sea salt into the lower atmosphere? This could be possible via aircraft or being projected from ocean vessels.  Though, due to very short residence time (<10 days), it has been calculated that 1500 ocean vessels, each with 28 billion nozzles, distributing >50 cubic meters of sea water droplets per second,  would be required to counteract a doubling of natural levels of atmospheric CO2.  Thats a lot of energy required in a time when we are supposed to be making cutbacks! Fortunately, those clever scientists (Salter et al., 2008), concerned about further exacerbating CO2 emissions, have proposed a low carbon option in the form of 'Flettner rotor ships' (see below); these are wind-powered, unmanned, remote controlled ships that can be moved with the seasons to desirable locations (allowing targeted cooling!) whilst spraying sea water droplets into the lower atmosphere. However, the plans aren't as comprehensive as they may seem, the technology required between drawing on sea-water and it's effective utilization in cloud seeding is far from complete, the intricacies involved are beyond the needs of this blog, but they would sufficiently prevent efficient and effective implementation in the near future.  The overall feasibility the  Flettner vessels is still unknown.

The capacity for MCB to mediate climate warming is limited to the doubling CO2, beyond this this method could only feasibly be used to stabilise climate.  As such, in the case of a climate emergency (which at our current increasing rate of CO2 emissions will hopefully occur  beyond doubling natural levels of CO2), this method is unlikely to be a considered option in returning us to pre-industrial temperatures, though it could be used to keep temperatures below a critical threshold.

Cost

There has been little in the way of analysing cost-effectiveness of this method, but ocean water is far from expensive so that reduces costs significantly.  It has been suggested that the fleet of Flettner vessels could cost in the region of £2 billion (Salter et al., 2008), which depending on the cost of maintenance could make them a cost-effective long-term investment in comparison to the high annual costs of the previous SRM methods.  But, how much additional investment is required for research and development of the appropriate technology?

Environmental impacts 

So lets begin with the positives:

• Global temperature reduction: Various models indicate that the potential to increase albedo will be substantial enough to balance the radiative forcing of doubling CO2 concentrations (Latham et al., 2008; Rasch et al., 2009; Jones et al., 2010 ... to name but a few!).  As such it is possible for us to counteract global warming, excellent this is a good start!  Latham et al. (2011) have established that such temperature reductions will also address ice loss in the poles.
• Targeted regional temperature reductions: Unlike stratospheric aerosols, which are difficult to confine regionally due to stratospheric circulation, this method could enable us to address just those areas most susceptible to the effects of global warming, should we be hesitant to implement engineering on a global scale!  Latham et al. (2008) were able to demonstrate, through modelling, that focussing on cloud modification in the North Atlantic we may be able to reduce summer ice retreat.  
• Reduce hurricane spawning and intensification: The links between extreme weather and climate change are still being explored (Check out Andy's blog for more on this!).  It is still being established whether intensification of hurricanes is a likely symptom of increased sea surface temperature due to global warming.  Due to the potential for regional application of cloud brightening, it would be possible to focus cooling sea surface temperatures in areas of hurricane spawning and intensification.

... If only it was that simple! If you play with clouds, you inevitably affect precipitation.

 Now the negatives;

• Precipitation patterns: despite different models using different strategies (location, intensity, season) the message is largely the same, that precipitation will largely decline and patterns will change.  The amazonian basin is subject to significant precipitation reductions which are essential for the productivity of the rainforest, though it is difficult to gauge the impact as the magnitude of precipitation decline is imprecise.   
• Localised changes in albedo:  Though previously put as an advantage, this could also be a huge disadvantage due to the interconnected and non-linear nature of the planet's climate and weather systems.  Little is understood about how this would firstly affect regional climate, but also how these changes may perturbate the rest of the global system.  
• Unbalanced effects: Highlighting the non-linear nature mentioned in the precious point, the link between temperature reduction and resolving loss of polar ice is somewhat out of sync (Latham et al., 2008).  Correcting global temperatures doesn't fully resolve ice loss at the poles (possibly partially due to reductions in precipitation), similarly, the forcing required at each the poles appears to also be different. 
• Not directly an environmental impact, but knowledge and modelling limitations make it difficult to fully anticipate the extent of these impacts.  Cloud seeding not only increases cloud albedo, but it will also reduce precipitation as well as increasing cloud cover and longevity, the relationships and climatic responses of which are little understood.  As such they are not incorporated into modelling and the regional and global effects are unknown.   These undesirable effects fo cloud seeding are likely to exacerbate the negative impacts on global and regional climate. 

Conclusion

Much like the other techniques this has its pros and cons. It lacks the ability for immediate implementation, the climatic intricacies appear to be little understood and it may not have the capacity to fully rectify anthropogenic climate warming.  We don't have significant examples to draw on to really understand the effects (such as volcanic eruptions and stratospheric aerosols) and much more research is needed to really gauge the feasibility of this method as the current models are significantly lacking.  Jones et al. (2011) compare the modelled impacts of sulphate aerosols and MCB ... it looks as though sulphate aerosols comes out as the overall winner (but I'll save that for the next blog!)