Atmospheric SRM: The sky's the limit?

Solar radiation methods which involve modifying components of our atmosphere in order to reduce the incoming solar insolation is what always sprang to my mind when I first thought of geoengineering. It is potentially the most controversial, because the consequences are the least understood. If one of these methods went wrong the outcome could be devastating, with global implications. So why is it even considered? Ultimately, it would be the ‘quickest fix’ around, with no waiting for carbon to slowly sequester, or technology to be developed. Quite simply, if targets are not met and global temperatures are dangerously high, it could reduce temperatures almost instantaneously. There are two main methods; stratospheric aerosol injection and marine cloud brightening.

Stratospheric Aerosol Injection (SAI)

Aside from the fact that it sounds like a painful procedure, the idea is quite straightforward, aiming to replicate the impact of large scale volcanic eruptions, which produce global cooling. This cooling arises from the impact of aerosols in the stratosphere which reflect solar insolation before it has a chance to reach the surface. Sulphate aerosols are commonly proposed for this method, because they already exist in the stratosphere and are the main component of the volcanically reduced temperatures. The stratosphere starts approximately 10-16km above sea level and extends upwards, with the troposphere extending below this boundary. Due to constant interactions between these two atmospheric layers, the increase in stratospheric aerosols as a result of volcanic eruptions are only temporary, because the atmosphere acts to ‘flush out’ the excess. This would also occur with anthropogenic aerosol injection.

 

Natural cooling of the planet by volcanic eruptions (Ruhlman, 2011)

Longevity issues

One of the main problems with replicating volcanic emissions are the unknown climatic consequences to long term increases in stratospheric aerosols. Heckendorn et al. (2009) suggest the increased duration of aerosols would likely lead to the formation of much larger aerosols because of coagulation and condensation effects. This would result in reduced efficiency over time, because the aerosols need to be a certain (small) size. Therefore, progressively more aerosols will have to be injected into the stratosphere for the same effect.

The Heckendorn et al. (2009) study also highlights the negative impacts on the ozone layer. In their model they found that total ozone (O₃) in the ozone layer was depleted by 4.5%, despite the decrease in halogens (responsible for past ozone depletion), because of changing reactions in the stratosphere in response to extra aerosols. The ozone layer over the tropics and the poles would likely become the most depleted, negatively impacting on communities. Therefore, Preston (2012) suggests a compensation scheme would be required to offset communities which are negatively impacted.

Furthermore, if the programme had to be stopped the atmosphere would act to ‘flush out’ the aerosols, significantly increasing global temperatures. This could occur if the negative impacts start to outweigh the positive impacts or if political issues arise, especially because there are no added benefits to SAI, economic or otherwise.

Marine Cloud Brightening

In the troposphere the main type of SRM proposed is marine cloud brightening. This works on the idea that seeding marine stratocumulus clouds with sea water aerosols will increase the albedo of clouds, the areal extent and the lifespan. Stratocumulus (meaning flattened heap in latin) clouds are the most commonly occurring cloud, rarely produce precipitation and already act to cool the planet. Therefore, the aerosols need to be the right size to avoid forming precipitation and negatively impacting the cloud macrophysics. To offset the radiative forcing caused by double the atmospheric CO₂, 1,500 automated wind powered ships like those in the picture below would be required to constantly seed the clouds. However, changing the clouds will affect regional precipitation patterns and it has been difficult to quantify all the macrophysical responses to cloud seeding.

Cloud brightening ship (MacNeil, 2012)

Public Opinion

Mercer et al. (2011) surveyed public opinion of SRM and found a surprising number in favour of researching or conducting experiments for SRM geoengineering. Over the sample of 3105 people from the UK, US and Canada the general results were in agreement for future SRM use and a confidence in the ability of the scientific community to make this technology safe. Mercer et al. (2011) highlight the main reasons given for not wanting SRM are due to beliefs that the natural environment should be left alone. I found this quite surprising despite the fact that my own mistrust of geoengineering methods originally related to the idea that we shouldn’t ‘tamper’ with the environment any more than we already have.

I find my issues with geoengineering methods, particularly SRM, are more to do with the questions surrounding the uncertainties. Unfortunately, tests and experiments cannot provide conclusive results unless they are completed on a large enough scale. To obtain this scale would mean altering the global climate which crosses the line between testing and implementation. I wouldn’t dismiss atmospheric SRM on this basis but I doubt all the risks could be eliminated, although they could be reduced.