Increasing Surface Albedo: Reflective roofs and pavements

The objective of SRM is to reduce radiative forcing of our planet or, in other words, increase the amount of radiation reflected by the Earth. Current mean albedo (this is, the proportion of energy reflected) is 0.15 (Shepherd et al, 2009). This means we are reflecting 15% of the incoming energy, and therefore absorbing 85% of it.

This percentage of absorbed energy has increased due to the increase of greenhouse gases (GHG) in the atmosphere. So clearly the best option would be cease the burning of fossil fuels and therefore stop increasing the effect of GHG. As we are still far from reaching this ideal solution, others have been considered.

To cool the planet, the proportion of energy reflected should increase from 15% to 17% (difference of -4 W/m2) (Shepherd et al, 2009). One could think, at first glance, this is not a very important increase, and it should be easy. But, when we think that the majority of the Earth's surface is covered by ocean, and oceans' albedo is the lowest (mean 10%), then changing this last 2% by only working on land becomes a much harder job.


So one of the options considered by scientists is the option of making our planet brighter. This can be made by

• Making white every rooftop and whitening urban places in general.
• Reflective crops
• Deserts reflectors
• Reforestation
• Ocean Albedo

We will discuss these options in this and further entries.

Increasing rooftop and pavement albedo

Everyone knows the colour white is the one with the highest reflectance power.  So, the first option discussed is using high reflectance paint (bright white paint) over roof tops and pavements in urban cities.
The questions are: Would this make a change? Does it have any side effects?

In urban cities, roof top area varies from 20% to 25%, and pavement area vary from 29% to 44%, (Akbari and Rose, 2008).  Several people (Akbari and Rose, 2008, Menon et al, 2010, Oleson et al, 2010) did the experiment to see which would be the change in albedo by painting roofs and pavements white, and the all reached similar conclusions. The result is an increase in albedo of between 25% and 15% in rooftops and pavements respectively, resulting in a 10% increase in global albedo of urban cities. This number drops to half this value when the paint is "light coloured" instead of white (Akbari and Rose, 2008).

Temperature estimation in ordinary circumstances, increased albedo of 0.05, and increased albedo of 0.1. (Akbari et al., 2012)

Starting to do this implies a political decision of forcing inhabitants to paint their roofs white. Cities like California did so in 2005. However, due to aesthetic reasons, this was adapted from "white roofs" to "light coloured roofs" in 2010, and it is not considered in a lot of places because of it.


HOWEVER, these are the temperature and albedo modifications for the urban city ONLY. What about the totality of the Earth? What net consequences would these modifications bring to the total albedo? the result is a net radiative forcing change of -0.01 - 0.2 W/m2. This is significantly small, mainly because of the small percentage of urban cities. If we compare to what we said we needed (-4 W/m2), we can see it is negligible on a global sense. However, locally, it seems to have very good results.

The question now is, does it have side effects??

Of course it does, plenty...

Positive side effects: 
Painting rooftops white reduce the electricity consumed in air conditioning. These reductions where calculated by Parker et al, 1995, to be between 10 and 45% in Florida, USA. Additionally, simulations in Los Angeles by Taha, 1996 predict smog reduction of 10-20%. All these reductions would bring as a consequence great monetary savings. The final benefit involves a greater useful life for materials (Akbari et al, 2001).

Negative side effects:
The colour white can create visual discomfort that could lead to eye-illness, or even traffic accidents. Also, some roof tops are not designed to be painted, such as tiles or other materials. High albedo tiles can be created but at greater cost. Additionally, maintenance must be regularly performed for the albedo to remain high, and this also adds up to the cost of the general procedure (Akbari et al, 2001).


All this painting and manteinance makes this procedure one of the "most expensive and least effective ones" Shepherd et al, 2009.




We will discuss other options in next articles. Hope you enjoyed this as much as I did!! :D