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.
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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.
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.