A white roof future

In 2009, Nobel prize-winning scientist and the former US Secretary of Energy, Steven Chu, spoke in London at a meeting on climate change. His message was simple: paint your roof white.

It's a novel and intriguing idea, and in the excerpt of his talk embedded below, he claims that a worldwide whitening of roofs and roads is capable of removing as much carbon dioxide as removing every car for 11 years. A study conducted by Akbari et al. (2012) estimated that light-coloured roofs, pavements and roads could increase urban albedo by about 10%, and if undertaken globally could offset 130 - 150 billion tonnes of CO2, equivalent to removing every car for 50 years.

It shocked me that such a seemingly easy procedure like painting a roof could have such a significant impact, so I was eager to investigate the potential of this method and how suitable it was for urban areas in colder environments.

How can painting a roof white help to combat global warming?

Figure 1: Approximate albedos of various urban surfaces. (Source: https://weather.msfc.nasa.gov/urban/urban_heat_island.html).

As Figure 1 illustrates, the reflectivity of roofs varies immensely. Roofs and roads together account for roughly half of urban areas and contribute to the Urban Heat Island (UHI) effect by preventing evaporation and absorbing sunlight. The idea is simple: increase the albedo (solar reflectance) of these roofs and roads, which in turn will reflect more incoming solar radiation and hence tackle global warming (Akbari et al., 2008).

White roofs are also capable of reducing energy use through air conditioning by up to 40% in some climates and through reducing the impact of the UHI effect, could also provide better air quality and comfort as well as mitigating the UHI contribution to global warming (Akbari et al., 2008).

How effective is it?

There is currently much debate within the scientific community about the effectiveness of white roofs. Most notably, a modelling study by Jacobson and Ten Hoeve (2011) estimated that the UHI effect might contribute to 2-4% of global warming, in comparison to 79% from greenhouse gases and 18% from dark particulates. The study suggested that a worldwide conversion to white roofs could cause a net warming of the Earth due to less hot air rising resulting in fewer clouds being formed, as well as the increased surface reflectance creating an increase in sunlight absorbed by dark pollutants like black carbon. Jacobson and Ten Hoeve (2011) suggested that attaching photovoltaic panels to roofs would be a much better alternative, but their study on white roofs did not account for any reduction in electricity use for cooling.

However, later research such as the previously mentioned Akbari et al. (2012) study disagree with Jacobson, and a more recent study by Sproul et al. (2014) comparing roofs in the United States suggested that white roofs are the most economic to install and are three times more effective than green roofs (vegetated) at achieving global cooling.

What about in cold climates?

Within the research, there is a problem that the studies are often undertaken in hot climates (the bulk of research coming from the Lawrence Berkeley Laboratories) and do not assess their full environmental consequences. More recent research suggests that net negative environmental impacts can occur in colder climates due to a large heating penalty occurring from white roofs; that is, the resulting increase in heating required from high solar reflectance (Cubi et al., 2015).

The website of Steven Chu's former department, the U.S. Department of Energy, now explicitly states that cool roofs can increase energy costs in colder climates and acknowledge other potential problems such as increased susceptibility to the accumulation of moisture.

Regardless, the idea is limited in its potential impacts by the fact that less than 1% of the Earth's surface is urban. It appears to be an easy, inexpensive, effective strategy when applied to particular hot and dry climates but lacks the potential to be scaled globally as a significant geoengineering technique. 

ITCZ Cause and Effect

I recently had a lecture about Black Carbon (BC), a component of fine particulate matter which is produced from the incomplete combustion of organic matter. As well as climate change, BC is responsible for detrimental impacts on human health, transporting pollution, and damaging stone buildings. Figure 1 provides a summary of the global climate system effects of BC.

Figure 1: A summary of the climatic impacts of black carbon. (Source: Bond et al., 2013).

Of particular interest to me was the effect of BC in shifting the Intertropical Convergence Zone (ITCZ) to the north. Studies have shown that this happens as a result of BC strengthening the Hadley cell in the northern hemisphere but weakening the Hadley cell in the southern hemisphere (Wang, 2007; Jones et al., 2007). 

Just a few days ago, a study led by Dr Anthony Jones of the UK Met Office was published in Nature, examining how stratospheric aerosol injection (SAI), a type of SRM, could increase the frequency of hurricanes by shifting the ITCZ (Jones et al., 2017). Figure 2 illustrates the modelled impact of SAI on hurricane / tropical cyclone frequency, which has a large dependency on the hemisphere which SAI is undertaken. 

Figure 2: Modelled hurricane / tropical cyclone frequency in response to stratospheric aerosol injection (SAI) for years of geoengineering indicated by black lines between 2020-2070. Including no geoengineering (purple), annual SAI in southern hemisphere (red), annual SAI in northern hemisphere (blue), and annual global SAI (turquoise). (Source: Jones et al., 2017).

Essentially, preferential SAI in a single hemisphere alters sea-surface temperature gradients and shifts the ITCZ towards the opposite hemisphere. So, solar geoengineering in the south causes an ITCZ shift to the north, providing optimal conditions for hurricane formation near the United States from African easterly waves in an area in the North Atlantic known as the hurricane main development region (MDR) but would have the benefit of enhancing precipitation over the Sahel. On the other hand, solar geoengineering in the north would cause the ITCZ to shift to the south, which would increase wind shear over the MDR and reduce the number of hurricanes in the north, but would reduce precipitation over the Sahel and could cause droughts. 

Speaking with Carbon Brief, Dr Anthony Jones notes his concerns that positive regional impacts could motivate nations with greater influence to deploy solar geoengineering in a single hemisphere at the expense of nations in the other hemisphere. Both the BC and SAI studies highight the global impacts of regional actions and stress that global cooperation is crucial in tackling climate change, especially in a geoengineered world.

Geostorm (spoiler alert)

I recently succumbed to the pressure and went to watch Geostorm despite reading multitudes of bad reviews. The film itself was admittedly not great but was quite fun to watch. A satellite system known as Dutch Boy is responsible for controlling the world's climate and preventing natural disasters. During the process of handing over control of the system from the United States to an international committee, it begins freezing villages and setting towns ablaze. Later, it is found out to be the work of the US Secretary of State set on a path for world domination. If you can ignore the scientific mistakes and the installation of laser-powered death rays on Dutch Boy, the film does raise some interesting questions. What is the line between SRM research and deployment? Who should have power over SRM techniques and how should countries be represented? Should the public be consulted about the nature of geoengineering research currently being undertaken?

BECCS: The saviour of carbon geoengineering?

I recently read this article which tracks the development of Bio-energy with carbon capture and storage (BECCS) from its origins as a proposal within a doctoral thesis by Kenneth Möllersten for Swedish paper mills to benefit financially through capturing its carbon emissions and receiving credits. As mentioned in my previous post, BECCS is currently one of the most exciting and viable CDR technologies and is included in the majority of modelled pathways to achieve ‘the 2°C Scenario’ (2DS).

What is BECCS?

Figure 1: The carbon cycle involved with BECCS. (Source: http://www.bbc.co.uk/news/science-environment-26994746).

The concept of Carbon dioxide capture and storage (CCS) is to separate CO2 released from power plants or industrial sources and transport it by pipeline for storage deep underground in geological reservoirs or saline aquifers (Kheshgi et al., 2012). BECCS is the concept of using CCS at an electric power plant which uses biomass as a fuel and hence produces negative carbon dioxide emissions because the CO2 from the atmosphere is extracted by crops and stored permanently underground (Caldeira et al., 2013).

Due to the smaller size of BECCS plants in comparison to fossil fuel CCS plants, the costs associated with the CCS process are higher (Azar et al., 2006). However, Luckow et al. (2010) suggest that the large-scale utilisation of biomass could enable economies of scale to reduce the additional cost of applying CCS to biomass to only approximately 3% higher than coal.

It is estimated that through sustainably applying BECCS to one-hectare of a typical temperate forest, it is capable of removing approximately 2.5 tonnes of carbon per year (Kraxner et al., 2003). Modelling suggests that BECCS will deliver a significant improvement in the cost of achieving a 450 ppm concentration by 2100 (Azar et al., 2006) and may be necessary for achieving ambitious targets like 350 ppm concentration by 2100 (Azar et al., 2010).


Sounds fantastic, what's the catch?

The main issue with BECCS is the extent to which biomass needs to be commercialised to have a significant contribution towards the previously mentioned emissions targets. The IPCC estimates that to keep CO2 emissions below 450 ppm up to 100 exajoules (EJ) a year of biomass would need to be produced by 2030, with this figure rising to 325 EJ a year by 2100 (Clarke et al., 2014).

To provide some context for the scale of this undertaking, approximately 500 million hectares of land would be required to produce 100 EJ of biomass per year; this is equivalent to one-sixth of the area of global forests, or about 1.5 times the land area of India; in comparison, around 33m hectares of land is currently being used to produce biofuels (WWF, 2014).


With BECCS demanding such a large area of land to act as a feasible method of achieving emissions targets, there are an array of environmental and human concerns that arise. Firstly, a high demand for biofuels is capable of displacing land assigned for food production and hence increasing food prices and decreasing food security (Baier et al., 2009). Environmentally, there are concerns that unsustainable BECCS could increase CO2 emissions if forested areas are cleared to make space for biofuel production.

BECCS has not yet been developed and tested on a commercial scale, and like any other CCS technology, there is always a minor risk of CO2 leakages underground. So, is BECCS the 'saviour' of carbon geoengineering? Perhaps it is a premature saviour. Whilst it certainly has potential to be included alongside other carbon geoengineering methods in achieving emissions targets, there is much work to be done to increase its efficiency to reduce the land area required, as well as the need for a strict global regulatory framework to ensure that the biomass fuelling it is gathered in a sustainable manner.