Reflective surfaces (climate engineering)


Reflective surfaces, or ground-based albedo modification, is a solar radiation management method of enhancing Earth's albedo. The IPCC described GBAM as "whitening roofs, changes in land use management, change of albedo at a larger scale."
The most well-known type of reflective surface is a type of roof called the "cool roof". While cool roofs are primarily associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Painting roof materials in white or pale colors to reflect solar radiation is encouraged by legislation in some areas.
This technique is limited in its ultimate effectiveness by the constrained surface area available for treatment. This technique can give between 0.01 and 0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is small relative to the 3.7 W/m2 of positive forcing from a doubling of atmospheric carbon dioxide. Moreover, while in small cases, it can be achieved at little or no cost by simply selecting different materials, it can be costly if implemented on a larger scale.
A 2009 Royal Society report states that "the overall cost of a 'white roof method' covering an area of 1% of the land surface would be about $300 billion/yr, making this one of the least effective and most expensive methods considered." However, it can reduce the need for air conditioning, which emits carbon dioxide and contributes to global warming.

Method

As a method to address global warming, the IPCC 2018 report indicated that the potential for global temperature reduction was "small," yet was in high agreement over the recognition of temperature changes of 1-3 °C on a regional scale. Limited application of reflective surfaces can mitigate urban heat island effect.
Reflective surfaces can be used to change the albedo of agricultural and urban areas, noting that a 0.04–0.1 albedo change in urban and agricultural areas could potentially reduce global temperatures by overshooting 1.0 °C.
The reflective surfaces approach is similar to passive daytime radiative cooling in that they are both ground-based. Yet, PDRC focuses on "increasing the radiative heat emission from the Earth rather than merely decreasing its solar absorption."

Types of reflective surfaces

Cool roofs

Benefits

Cool roofs in hot climates can offer both immediate and long-term benefits, including:
Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. Therefore, the net energy saving of cool roofs varies depending on climate. However, a 2010 energy efficiency study looking at this issue for air-conditioned commercial buildings across the United States found that the summer cooling savings typically outweigh the winter heating penalty even in cold climates near the Canada–United States border, giving savings in both electricity and emissions. Without a proper maintenance program to keep the material clean, the energy savings of cool roofs can diminish over time due to albedo degradation and soiling.
A modelling study of the impacts of reductions in temperature due to cool roofs in London during the 2018 British Isles heatwave found that heat-related mortality in this period could have been reduced by 249 in scenarios where all buildings are assumed to have cool roofs installed. Using the value of statistical life, the benefits in terms of avoided deaths for cool were estimated at a saving of £615 million.
Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affect durability. High temperatures and significant variations, seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by UV and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation.
If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 gigatonnes of greenhouse gas emissions, equivalent to taking 300 million cars off the road for 20 years. This is because a white roof will offset 10 tons of carbon dioxide over its 20-year lifetime. In a real-world 2008 case study of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer, the farmers whitewash these roofs to cool their plants down.
When sunlight falls on a white roof, much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof, most of the light is absorbed and re-radiated as much longer wavelengths, which are absorbed by the atmosphere.. Findings of a study conducted by et al. from Universiti Teknologi PETRONAS and Universiti Teknologi MARA in 2021, which is based on the hot and humid climate of Malaysia, suggest that the selection of white roof tiles significantly reduces the peaks of heat conduction transfer and roof-top surface temperature as well as the values of heat conduction transfer and roof-top surface temperature throughout diurnal profiles. Contrarily, the results also reveal that it does not influence the nocturnal profiles, as a release of heat to the sky takes place throughout the night. The release of heat from the building occurs due to the absence of solar radiation, which reduces the sky temperature and enables the sky to act as a heat sink that promotes the transfer of heat from the building to the sky to achieve thermal equilibrium.
A 2012 study by researchers at Concordia University included variables similar to those used in the Stanford study and estimated that worldwide deployment of cool roofs and pavements in cities would generate a global cooling effect equivalent to offsetting up to 150 gigatonnes of carbon dioxide emissions – enough to take every car in the world off the road for 50 years.

Types

White thermoplastic membrane roofs are inherently reflective, achieving some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of white thermoplastic, for example, can reflect 80% or more of the sun's rays and emit at least 70% of the solar radiation that the roof absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation.
In addition to the white thermoplastic PVC and TPO membranes used in many commercial cool roof applications, there is also research in the field of cool asphalt shingles. Asphalt shingles make up the majority of the North American residential roofing market, and consumer preferences for darker colors make creating solar-reflective shingles a particular challenge, causing asphalt shingles to have solar reflectances of only 4–26%. When these roofs are designed to reflect an increased amount of solar radiation, the urban heat island effect can be reduced through the reduced need for cooling costs in the summer. Though a more reflective roof can lead to higher heating costs in the colder months, studies have shown that the increased winter heating costs are still lower than the summer cooling cost savings. To satisfy the consumer demands for darker colors which still reflect significant amounts of sunlight, different materials, coating processes, and pigments are used. Since only 43% of light occurs in the visible light spectrum, reflectance can be improved without affecting color by increasing the reflectance of UV and IR light. High surface roughness can also contribute to the low solar reflectances of asphalt shingles, as these shingles are made of many small approximately spherical granules which have a high surface roughness. To decrease this, other granule materials are being investigated, such as flat rock flakes, which could reduce the reflectance inefficiencies due to surface roughness. Another alternative is to coat the granules using a dual coat process: the outer coating would have the desired color pigment, though it may not be very reflective, while the inner coating is a highly reflective titanium dioxide coating.
Natural white gravel covering can be seen as an alternative option to obtain cool roofing and cool pavements.
The highest SRI rating and the coolest roofs are stainless steel roofs, which are just several degrees above ambient under medium wind conditions. Their SRIs range from 100 to 115. Some are also hydrophobic, so they stay clean and maintain their original SRI even in polluted environments.
Coated roofs
An existing roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 500 reflective coatings can be found in the Cool Roofs Rating Council.
Blue and red roofs
Researchers at the Lawrence Berkeley National Laboratory have determined that a pigment used by the ancient Egyptians known as "Egyptian blue" absorbs visible light and emits light in the near-infrared range. It may be useful in construction materials to keep roofs and walls cool.
They have also developed fluorescent ruby red coatings, which have reflective properties similar to white roofs.