Can't we just make the clouds brighter?

The next solar radiation proposal under scrutiny will be marine cloud brightening. This idea has been up in the air (get it?) since 1999 when it was first proposed by James Latham, a climatologist at the National Centre of Atmospheric Research. He proposed seeding marine stratocumulus clouds with seawater aerosols generated at the ocean surface (Latham et al. 2012) to result in additional cloud condensation nuclei within the cloud. These generated particles are held 1000 metres into the air, to help increase cloud reflectivity. Marine stratocumulus clouds cover over a quarter of the ocean surface and are hundreds of metres cubed in volume with albedos ranging between 0.3 and 0.7 (Hanson, 1991). Therefore by increasing the reflectivity of these clouds, it reduces a large proportion of incoming solar radiation to Earth and prevents excess warming induced by anthropogenic greenhouse gases.

In fact - did you know that clouds are already reflecting more than the amount of solar radiation which has been captured by anthropogenic carbon dioxide (Mims, 2009)? Ergo, Latham's idea utilises and optimises the current functionality of clouds to our advantage to reduce the induced impacts of climate change, and encourage a greater reflectivity of sunlight through the artificial generation of a seawater mist to the clouds in our atmosphere.

Source: Giphy

So how can we generate this additional seawater mist?

Many have debated different ideas; from collisions between air-saturated jets of water, a hydraulic equivalent of a photomultiplier, vibrating piezoelectric vapourisers, or to simply forcing water through 0.8 micron diameter holes in a wafer-thin sheet of Silicon. Over 1.5 billion holes would be required for a silicon slice only 20cm in width (Mims, 2009). This is an obstacle in itself- what is the most appropriate method? Silicon was suggested as the material of choice but its ability to withstand high pressure in experiments has been discouraging. There is also a need for uniformity in size of the water droplets, as to not run the risk of larger heavier droplets falling as rain prior to elevation of the mist (Mims, 2009).

Let's say hypothetically that the ability to generate a seawater mist is achieved- how are we to elevate it to 1000 metres high into the air?

Well, Stephen Salter from Edinburgh University has been engineering a concept of wind-powered, remote-controlled, unmanned Flettner vessels (Salter et al. 2008), otherwise known as 'albedo yachts' (see image below). The Flettner motors can transform wind energy into thrust to generate lift of the seawater particles upwards (Mims, 2009). Over 1500 of these vessels would need to be deployed worldwide and transforming 30 litres of saltwater per second in order to correspond to the rate of increasing carbon dioxide concentrations, but not without a hefty construction cost of between $3.2-4.8 billion (Mims, 2009)!

Source: UCAR

However, this idea has caused concerns, particularly with regards to changes in precipitation patterns. Plenty of General Circulation Models (GCMs) have been produced - including that by Latham himself, which all suggest that deployment of marine cloud brightening will cause a dramatic decrease of precipitation in the Amazonian Basin, and both the Hadley Centre and the UK MET Office in particular, suggested that desertification could result in the Amazon rainforest, as cooler temperatures, as a consequence of less incoming solar radiation, in the South Atlantic will result in less evaporation and therefore a reduction in excess of 1mm per day (Latham et al. 2012) of precipitation in the Amazon (Mims, 2009). If this were to occur, this would have huge implications on biodiversity- the rainforest is home to 40,000 plant species alone (WWF, 2014) and that's not even including birds, mammals, reptiles, fish, or amphibians! As well as social and economic consequences of livelihood and loss of homes.

Models have similarly suggested that under conditions of double the current carbon dioxide levels in the atmosphere, full seeding of marine cloud brightening would reduce precipitation in the Amazon, North America and South East Asia, but would however increase precipitation in Africa and Australia (Latham et al. 2012)! Is this a fair compromise to make? The influence of marine cloud brightening will definitely result in a global environmental change and affect the entirety of the global population.

Source: Eating jellyfish

So should maritime cloud brightening be given the go ahead? A complexity (as always) resolves in its application on the large scale- can we ensure success? Can we ever be certain that it will work? What if there are unintended consequences? Another trouble with this proposal is that it meddles with the dynamics of clouds, of which the microphysical processes are not yet even fully understandable within science. With so much uncertainty, how can we trust that the generated seawater mist will add to cloud condensation nuclei concentrations without a glitch? The problem of rainfall patterns similarly suggests corrections and certainty before deployment.

Rasch et al. (2009) have similarly suggested that an increase in planetary albedo cannot and will not compensate for increasing greenhouse gases especially in relation to consequences such as ocean acidification which could destroy many marine ecosystems. It merely provides extra time for us to ponder over future uncertainties and hopefully come up with an appropriate mitigation response.

However, the problem of time is that it always runs out.

Till next time!

S xx