SRM: In the stratosphere

One of the hottest topics in geo-engineering is that of injecting aerosols into the stratosphere, primarily sulphate aerosols.  The idea behind such proposed methods was first mentioned by Budyka in 1974 and is inspired by nature itself; the stratosphere contains a layer of aerosols that scatter solar radiation away from the earth and are therefore contributing to the planetary albedo.  There are two options for this method, engineered aerosols or sulphate particles.  Despite the potential offered by the engineered approach, sulphate particles are considered a more viable option at present as they require somewhat less research; they would be simple to implement and we already know a fair amount about the effects of dramatically increasing sulphate aerosols due to explosive volcanoes.

Volcanic climate forcing …

Volcanic eruptions project vast volumes of debris into the atmosphere; sulphur dioxide (SO2) emitted during the eruption oxidises in the atmosphere to form Sulphuric aerosols.   If larger enough eruptions can deposit SO2 into the stratosphere where it is retained long enough to subsequently increase the global albedo and reduce solar radiation reaching the surface, inducing a net cooling. Due to a combination of instrumental and palaeo- ice core records we have a good record by which to correlate volcanism with climate:

Mount Pinatubo (McCormick et al., 1995)

The infamous Mount Pinatubo eruption of 1991 produced 20 x 1012 g  (20 Tg) of SO2.  Compared with an average of 1958-91, there was a cooling of 4.3Wm-2 between 40 N and 40 S and average decline of 0.7 degrees C globally.  By 1993, however, the radiation balance was reinstated, thus the perturbation was short-lived.

Palaeo-records

However, the palaeo-record has enabled us to extend our understanding back beyond this instrumental period, during which time has been very explosive volcanism has been limited.  GISP2 ice-core records have revealed that not only are volcanic injections of sulphate aerosols capable of inducing short term climate cooling, but that the cumulative effect of frequent, large scale eruptions can result in persistently cooler global temperatures.  Such frequent and explosive volcanism is coincident with, and thus considered to have at least partially been the cause of cooling in the early Holocene (Zeilinski et al., 1994; Bradley, 2003) and the latter half of the Maunder Minimum (Bradley, 2003). As such, it can be established that, although the cooling effect of sulphate injections are relatively short-lived, continual or frequent contributions into the stratosphere can induce persistent cooler global temperatures.

It can be concluded therefore that based on current knowledge, this appears to be a highly effective option and efficient which would demonstrate results in little over a year.

How?

There are a whole variety of ideas being explored.  The main ideas include distribution into the stratosphere via planes, artillery fire and stratospheric balloons.  At present the most viable option would be that of the plane, as the military already use the necessary specification of aircraft; some modifications would be required but near immediate implementation could occur.   Other ideas, which at this stage in their development sounds a bit far-fetched to say the least, include a tall tower potentially reaching upto 114km, using balloons for support and  a space elevator running on solar power  (Robock, 2009)…

But what are the risks?

• Change in regional precipitations leading to drought in Africa and Asia
• H2SO4 deposition 
• Ozone depletion
• No more blue skies
• Less solar power
•  Environmental impact of implementation

And the cost:

Table 1. Adapted from Robock (2009).  The costs for different methods of injecting 1Tg of a Sulphur gas per year into the stratosphere

Method	Annual Cost ($)
planes	225,000,000 – 4,175,000,000
Artillary fire	30,000,000,000
Balloons	21,000,000,000 – 30,000,000,000

Conclusion

The efficiency and feasibility of this method renders it a strong possibility.  Modes of implementation are available, thus there would be no delay reliant on developing the necessary technologies.  And though the costs are large, they are substantially reduced in comparison to those proposed through space-based SRM, and are cheaper than investing in technologies as an alternative to CO2 emissions.  The next blog will be looking at the environmental risks more closely.