A Human Volcano

A second Solar Radiation Management (SRM) option is to increase albedo through anthropogenically enhancing sulfate particle concentrations in the Earth’s stratosphere and troposphere.

Fossil fuel burning releases about 25PG of CO2 per year into the atmosphere leaving us with the global warming we are faced with today (Prentice et al., 2001).  Fossil fuel burning also emits 55Tg S as SO2 per year and research has shown that warming is counteracted by sulfate particles that scatter solar radiation back to space through increasing cloud albedo (Ramanathan et al., 2001).

We are faced with two main climate problems today – global warming and rising CO2 emissions. A stabilization of CO2 emissions would require an emissions reduction of around 60-80% which seems unlikely when current reductions are considered. This leaves us therefore with the option of anthropogenically enhancing the earth’s albedo through adding aerosols to cool the climate.

The conversion of SO2 into sub-micrometer sulfate particles by chemical and micro-physical processes has been observed in volcanic eruptions (Crutzen, 2006), however, it was Paul J. Crutzen who recognised the potential of these observations to alleviate global warming.

Whether it was due to him winning a nobelprize or the robustness of his work Paul Crutzen changed opinions towards SRM and geo-engineering in 2006 when he asserted that further research on ‘the feasibility and environmental consequences of climate engineering …which might need to be deployed in future, should not be tabooed’ (Crutzen, 2006) as the world may be coming closer to being characterised by conditions that would have ‘catastrophic implications for ecosystems’ (Schneider, 1996).

A loading of 1Tg S in the stratosphere yields a global average vertical optical depth of about 0.007 in the visible and corresponds to a global average sulphur mixing ratio of 1nmol/mole – 6 times more than the natural background (Albritton et al., 2001). Through looking at previous volcanic eruptions the radiative forcing caused by 1Tg S is estimated to have a cooling efficiency of 0.75 W/m² (Crutzen, 2006). The estimated cost to put 1Tg S into the stratosphere is around US $25Billion (NAS, 1992). To address climate warming therefore 1.9Tg S would be required producing an optical depth of 1.3% at a cost of US $25 -50 billion per year for residence times of 1-2 years.

At a first glance this figure may seem large, but when the benefits it will bring about are considered and expenditures such as the US $1000 billion or so that has been spent on the military in the U.S.A. is used as a comparable figure then this cost to fix climate change does not seem so high.

A doubling of CO2 would cause a greenhouse warming of 4 W/m2 meaning a sulfate loading of 5.3Tg S would be needed leaving a sizeable amount of whitening on the sky and a much bigger dent in the pockets of those funding SRM schemes.

There are also environmental risks and negative side effect involved with aerosol injections which cannot be ignored. They include

-       Effects of the stratospheric ozone – local ozone depletion has been observed as a by-product of previous volcanic eruptions.

-       Possible increases in drought severity

-       Constant injections of sulfate are required

-       The appearance of the sky is altered as it becomes much whiter

-       Can lead to acid precipitation and deposition of SO2 and sulfates which cause ecological damage

-       Pollution particles have been said to affect health (Nel, 2005)

Consequently the best way to conduct a stratospheric modification scheme has been debated.

Proposed methods include releasing an S-containing gas at the earth’s surface, using ships in remote locations, launching reflective balloons or adding other highly reflective nano particles. To achieve maximum cooling, however, the location of the particles should also be considered as residence times of sulfate particles in the stratosphere are around 1-2 years while in the troposphere they are can be as little as 1 week.

There is, however, still a lot of research that needs to be done before an albedo enhancement scheme can be deployed. In fact, Crutzen (2006) asserts that one should only be deployed when ‘there areproven net advantages’ and ‘its possibility should not be used to justifyinadequate climate policies but merely create a possibility to combatpotentially drastic climate heating’.  Despite this, I think there is something to be said about stratospheric albedo enhancement as not only are its costs more realistic that surface albedo enhancement, the climatic response of aerosol injections can be as little as six months (Hansen et al. 1992)- this is much faster than (and therefore could counteract) the rate of warming caused by CO2. Moreover, if undesired changes are observed then the sulfate injections could be stopped quite readily and the atmosphere could be left to return to its prior state. I never thought I would be in favour of humans artificially replicating anything never mind volcanos however when climate change (and the doom that entails) is the cost I feel there isn’t much to loose in developing these methods further as they can as provide something as little (or not so little if it's needed) as an escape plan that might never be used but could be, and at short notice I might add, should we ever reach dangerous levels of warming! Perhaps then, we are to become human volcanos! 

Summary of Stratospheric Aerosols (Royal Society, 2009)

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