Solar Radiation Management – a bright future?

Climate geoengineering as a set of strategies can be broadly divided into two categories: Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR). Today’s post will focus on a subcomponent of the SRM technique; its mechanisms, benefits and uncertainties.

In an already warming climate, further solar radiation inputs can cause “tipping points” to occur earlier than anticipated, such as the 2°C warming threshold (IPCC, 2013). SRM techniques aim to reduce solar insolation influx and absorption in the Earth’s atmosphere by increasing the Earth’s albedo, or reflectivity (Figure 1).

Figure 1: SRM mechanisms (The Royal Society, 2009, p23)

Sulphate aerosols

The injection of sulphate aerosols into the Earth’s stratosphere have been widely proposed due to their “global dimming” effect; generally created after volcanic eruptions (Crutzen, 2006). This effect is produced when sulphate particles ‘scatter’ solar energy back into space. Hydrogen sulphide (H₂S) or sulphur dioxide (SO₂) are overwhelmingly considered as effective ‘scatterers’ of solar energy as they are gaseous: thus less likely to clump and maintain even dispersal in the stratosphere (The Royal Society, 2009). Modelling has suggested extensive sulphate aerosol covers can prevent rapid melting of the Greenland ice sheet, and consequent sea-level rise (Irvine et al, 2009).

Stratospheric aerosols
Effectiveness	Practical & effective with no limit to effect on global temperatures. Cannot mitigate oceanic acidification	5
Affordability	Low cost of raw materials and dispersal mechanisms	5
Timeliness	Immediate dispersal within months or years possible Effects would be felt within 1 year	5
Safety	Regional and local effects on hydrology unresolved Effects on high-altitude clouds Alterations to ecosystem functioning & productivity	1

Table 1: Cost-benefit analysis of sulphate aerosol SRM technique with rankings - 1 being 'low' and 5 being 'high' (Revised from The Royal Society, 2009).

Ecological effects 

Figure 2: Summary of ecological effects caused by the sulphate aerosol SRM technique (Barrett et al, 2014) 

Atmospheric effects & governance

The immediate cooling effect of sulphate aerosols is almost certain: the 1992-93 eruption of Mt Pinatubo caused a global cooling of 0.5-0.6°C. However, land precipitation also decreased, reflecting hydrological sensitivity to volcanic and solar insolation-forcing (Gu et al, 2011). 

Current research has suggested strong teleconnections between Mt Pinatubo cooling and El Nino Southern Oscillation (ENSO). The 1992 El-Nino event, the warm phase of the ENSO, resulted in less precipitation on land, and more precipitation in the ocean (Trenberth and Dai, 2007).  Droughts prevailed in the developing global South: ranging from South America, parts of Africa, South and Southeast Asia. Countries affected included Malawi, Botswana, India, Pakistan, Iran, Colombia and the Caribbean. The event resulted in widespread famine and was regarded as the most severe event in 30 years (UNFAO, 2014). Whether geoengineering can provide climate security, at the expense of other securities, is therefore highly questionable.

The use of paleoclimatology to interpret past environments can provide valuable insight into system-responses. For example, to what extent can global solar-forcing produce localised effects in the Earth system?

Monsoon variability

Holocene variations in the Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM) strength can provide valuable insight into how systems react to global changes. Speleothem δ ¹⁸ O records, formed from drip water in caves, can provide this. For example, the ISM has shown centennial and decadal cycles, and is particularly in sync with 11 year solar cycles (Lone et al, 2014).

Short-term changes in the EASM are strongly linked to changes in solar radiation. A weakening in the system was evident during the Little Ice Age, a period characterised by glacial expansion, cool temperatures and a decrease in solar activity, from 1400-1800 AD (Paasche and Bakke, 2010; Zhang et al, 2013).  This can have a major impact on agriculture-dependent, water-scarce Asian economies where monsoons rains replenish groundwater supplies (Turner and Annamalai, 2012).

Acid Rain

The incorporation of sulphate aerosols in stratospheric clouds, resulting in acid-sulphate rain, has been of some concern (Figure 2). This has the potential to eutrophicate nutrient-poor ecosystems such as freshwater lakes, and alter ecosystem functioning (Curtis et al, 2014). However, geo-engineering advocates in the IPCC have suggested such concerns are unfounded, and that modelling shows the volume of sulphate aerosols used is not enough to induce acidic rain (IPCC,2013). With oceans already acidifying, it seems strange to enhance the process unintentionally.

After arduous negotiations surrounding acid rain and the subsequent 1979 Convention on Long-range Transboundary Air Pollution, employing techniques that could undermine environmental treaties seems regressive, personally.

Conclusion:

I have to admit; I am not completely persuaded by SRM as a mitigation for climate change. Worrisome aspects include:

1. Ascertaining the volume of aerosols needed to mitigate global warming, yet not interfere with Earth systems
2. Possible impacts on monsoon systems with potential for political conflict in water-scarce regions
3. Acid rain as an unintentional consequence causing ecosystem alterations

And yet, results have the potential to be incredibly effective, immediate and affordable.

Maybe space-based SRM techniques would be less imposing on Earth systems. This is something I will be exploring in greater depth in another post.