Climatic patterns
Where sulphate aerosol can combat
global warming they pose severe diruption global
precipitation. Higher temperatures cause more evaporation (and precipitation), however aerosol
cooling not only reduces evaporation, but
also alters global temperature gradients and therefore precipitation distributions; Goldstein et al. (2010) highlights the numerous studies predicting aerosol induced precipitation reductions by up to 10%. There would likely be disproportionate declines in some regions, putting already
hydrologically sensitive areas at further risk. Primarily studies have
indicated that climate in the tropics would be most significantly prone to such
precipitation declines, most significantly both Asian (Bollasina et al., 2011) and
African Summer monsoons would reduce in intensity. The consequences of such alterations could put billions of
lives at risk in regions where water and food supplies are already at a tipping
point.
Alternative strategies have been
proposed. One of the effects of
global warming already in motion is that of Arctic warming. Melting ice sheets and
reductions of summer ice extent are a reality, not just a projection of the
future. Some have proposed the injection of sulphates at the poles with the intention of addressing Arctic warming whilst progress is made toward CO2 mitigation, without reducing tropical
precipitation. These methods raise
as many doubts as tropical injections, though they are arguably cheaper to
implement as sulphur injections would not be required in winter (3Tg per year instead of 5Tg), significant
impacts to tropical summer monsoons appear to be unavoidable (Robock et al., 2009).
Acidification
The inevitable fallout and deposition
of sulphate aerosols has been raised as a potential issue. However, a study by Kravitz et al. (2009) has indicated that the
although this acidic deposition would increase. The midlatitudes would experience the largest absolute increase, though such level would be masked in such regions by urban deposition; comparatively the largest relative to its baseline values would be experienced in pristine
environments such as Antarctica. The magnitude in both instances is
relatively insignificant, however ... In
relation to the ocean acidification induced by CO2 over recent decades,
sulphate deposition would be 2 orders of magnitude less. Similarly, relative to the critical
loading values associated with the most sensitive terrestrial areas (Kuylenstiernaet et al., 2001), the maximum value of point deposition calculated was at least an
order of magnitude less.
Subsequently it appears that acidification in relation to sulphate
aerosols SRM would be of minimal consequence. However, the study recognises that the spatial and temporal
resolution of the study may potential disguise larger loading in smaller scaled regions, whilst the effects on terrestrial water bodies couldn’t be assessed in this study, a significant short-falling in-light of
the already declining water quality experienced globally.
Ozone
Though global surface
temperatures are cooled but sulphate aerosols, the stratosphere is in fact
warmed. Such warming has the
potential to inducing lofting of the ozone layer, raising it to altitudes that
will increase photochemical destruction of the ozone (McCormick et al., 1995). A double edged sword; by reducing solar radiation passing through
the earths atmosphere, the photochemical reactions required for ozone
production are also liable to be reduced.
This impact is further supported by studies indicating that volcanic
eruptions such as Pinatubo have shown significant reductions in the ozone (Brasseur and Granier, 1992). Tilmes et al. (2009) concluded that should
stratospheric sulphate aerosol injections be implemented that this could set
back ozone hole recovery by 30 years.
Primary productivity
With crop growth and other plants being photosynthetic it is reasonable to
assume that reductions in solar radiation would be detrimental. However, research has suggested that plant productivity would conversely increase due to increased light use
efficiency associated with
more diffuse radiation (Mercado et al., 2009). After the atmospheric sulphate loading of the Mount Pinatubo episode in 1991,
deciduous forest productivity increased by up to 23% (Gu et al., 2003). That said, should drought conditions be initiated by sulphate
injections, then plant productivity will decline in affected regions regardless
of the influence of reduced solar radiation levels.
Also...
Two other issues, not as directly
significant but important nonetheless, whitening of the skies and energy capture. Declining blue skies is not only a problem
aesthetically, but also could have severe psychological implications (Robock,2008). Whilst the energy capture point
refers to solar power, should we reduce solar passage through the atmosphere we
subsequently compromise a key source of renewable energy in some
countries, Murphy (2009) indicated that in California, peak capacity for solar
energy generation decline form 90% to 60% peak capacity in summer of 1992
proceeding Mount Pinatubo's eruption.
Depending on the
technique implemented, there will also be varying degrees of environmental
compromise associated with implementation.
Conclusions
Though some perceived problems have
been remedied by certain studies, namely ideas about plant productivity and acidification,
other problems are still remain; each of these issues has a significant weighting in opposing
implementation. The changes in
precipitation patterns could have vital implications on livelihoods, economies and biodiversity for example,
outweighing the value of diffuse radiation for primary productivity.
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