'Colouring' roofs

Today I would like to discuss with you 3 main types of roofs: the typical white roof, the green roof and the PV roof.  Keep on reading to find out more…

With a rapidly increasing population, and a rise in global economies, urbanisation is expanding. This increases the amount of roads and buildings, creating dark surfaces that reduce solar reflection and the Earth’s albedo.

A geoengineering method that could enlarge albedo reflectance is through painting roofs and human settlements white, bringing about a net increase in albedo of 0.1 in urban areas (The Royal Society 2009). The increase in albedo increases solar reflectance and reduces global temperatures.  This may lead to a decreasing rate of glacial ice melting and less of an impact on the environment.  Although this geoengineering process may take a few decades to be implemented worldwide, once complete, effectiveness should be highly apparent (Table 1). Furthermore, there are very low risks with the white roof method compared to other SRM processes.

 

Table 1: White Roof Evaluation Summary
Source: The Royal Society 2009

Criticism of White Roofs

Jacobson and Hoeve (2011) may disagree suggesting that local temperatures will decrease by 0.02K, however, global temperatures will increase by approximately 0.07K.  The white roofs reduce temperatures near the surface air.  However, there is a reduced sensible heat flux (i.e. heat energy transferred from the Earth’s surface to the upper atmosphere).  Consequently, due to a reduction in the sensible heat flux, cold air cannot be transported further up in the atmosphere, hence leading to a local cooling but a global warming.

Furthermore white roofs are accumulated in specific areas, causing small changes in cloud optical depths and winds.  Therefore, clouds intensify over the northern hemisphere making the area cooler and increasing surface albedo.  However in the Southern Hemisphere, temperatures increase, causing global temperatures to rise.  Hence there will be less clouds in the Southern Hemisphere leading to a decrease in precipitation events such as snow and rain (Jacobson and Hoeve 2011).   Consequently the hydrological cycle may be negatively impacted in the southern latitude.

Another disadvantage is that a high amount of carbon dioxide aerosols in the atmosphere absorb upward solar radiation from the reflection of white roofs and trap heat within the atmosphere, increasing global temperatures (Jacobson and Hoene 2011).

Lastly, white roof reflections are effective in warm areas, this causes energy efficiency when using electrical cooling appliances, saving energy (Cubi et al 2015).  However, if an area is cold and becomes cooler due to an intensification in surface albedo, such as Canada, this increases energy demand and more fossil fuels need to be burnt for electricity.  It is suggested that cold areas would consume more electricity than in warm areas, therefore increasing carbon dioxide in the atmosphere (Jacobson and Hoeve 2011).

An alternative suggestion

Cubi et al 2015 suggests an alternative method in cold climates; green roofs and Photovoltaic (PV) roofs.  Green roofs (or eco-roofs) are vegetated roofs, where organic material is grown.  This is aesthetically appealing, can create sound and heat insulation and can be a good way in preserving habitats.  Although the reflectance will be less compared to white roofs, there may be some reduction in carbon dioxide due to photosynthesis of plants.  Furthermore little maintenance is required and less heat will be needed for buildings in cold environments.  The PV roofs are photovoltaic panels placed on the roof of buildings, creating renewable energy with no carbon emissions, reflecting some solar energy and decreasing environmental impacts.  However, PV roofs, would not be sufficient during cloudy days with no sunlight for energy (Cubi et al 2015). 

Consequently, PV roofs are preferred overall, as it is the highest performing amongst all categories (Table 2).  PV roofs reduce carbon dioxide levels and save energy to a higher degree compared to any other type of roof. However, it may be argued that Green roofs are cheaper and a longer lasting method which also conserves environmental habitats to some degree.  Furthermore white roofs have significant negative impacts on ecosystems.  When modelling the white roofs process, it is suggested that the expected outcome will not be achieved, as global temperatures will increase rather than decrease.

 

Table 2: A summary of the 3 types of roof reflectance
Source: Cubi et al 2015

Thoughts

I think PV roofs would be the most beneficial in terms of reducing carbon dioxide emissions and declining climate change impacts.  Even though green roofs cannot be disregarded due to their many advantages.  In general, PV roofs and green roofs could be a success and a safe, cost-effective process.  I believe this may be the first indication of alternative small-scale geoengineering that could be successful in the long term, reduce global carbon dioxide emissions and global temperatures without many (or any) risks.  What do you think?