Solar Radiation Management: Land Based SRM

The previous carbon dioxide removal methods have all dealt with the root cause of climate change. However, solar radiation methods (SRM) take a somewhat different approach; dealing with the effects. Rather than removing greenhouse gases they propose to cool the planet by reducing the amount of solar insolation (radiation from the sun) reaching the Earth’s surface. These methods tend to be cheaper and act quicker than CDR methods but would not address issues such as ocean acidification which directly arise from greenhouse gases.  

The main SRM (Rodas, 2007)

Several methods fall under the SRM umbrella, with the altitude of the method affecting the efficiency. The Royal Society (2009) suggests an SRM in space would need to divert 1.8% of solar insolation in comparison to SRM’s at the Earth’s surface which would need to divert 2% of solar insolation. However, land based SRM is probably the least controversial out of all the SRM.

How could the solar insolation be reduced from the land?

It is more accurate to describe the 2% decrease in solar insolation as being reflected instead of diverted, because this hints at the method; albedo (reflectivity) modification. The overall global surface albedo would need to be increased from 0.31 to 0.32, which does not sound like a lot. This essentially involves making surfaces lighter so they reflect more and absorb less solar insolation, thereby reducing warming. However, as soon as you start look at the Earth’s surface as separate components i.e. those which could be modified and those which can’t, problems become apparent. The infographic shows a general division of the Earth into land and water. Some land surfaces can be modified but the colour of the oceans is difficult to change, and clearly there is a lot of ocean.

Approximate surface of the Earth by water and land mass (Alastair, 2016)

So what could be modified?

A study by Irvine et al. (2011) into the climatic effects of land based SRM grouped the potential land surfaces for albedo modification into three; desert, urban and cropland. The table highlights the values they used from other literature as the inputs for their climate model.  

	Area of Earth's surface	Potential albedo increase
Desert	2%	0.8
Urban	0.29%	0.2
Cropland	3.1%	0.04

Polyethylene-aluminium could be used to cover deserts to produce the largest albedo increase. However, this method is also deemed the riskiest because the local cooling effect caused by the changes in albedo over a large area is sufficient to dramatically change large scale atmospheric patterns. Most notably, reducing precipitation and soil water availability across much of the globe. Therefore, it is dismissed by Irvine et al. (2011) as a potential geoengineering technique, based on the range of unwanted climatic changes produced by their model.

Conversely, urban and agricultural surfaces are more spread out, so the effects aren’t as concentrated over specific areas. Urban surfaces could be lightened by whitening rooves and pavements,, which act to reduce the urban heat island effect. However, this is partially offset by the increased household heating during winter months in response to cooler temperatures. Preferential planting of lower albedo crops could also lower regional surface temperatures, but would not have a huge impact on global temperatures.

Albedo values for different urban surfaces (EPA,2006)

Although these methods are deemed inexpensive to deliver, I do not feel they are sufficient to reduce temperatures, reflecting the conclusions of Irvine et al. (2011). Future research into ocean albedo modification may be more promising in terms of combatting climate change but it is also likely the negative consequences could be higher than changing the land.