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| Fig. 1: Standard asphalt pavement absorbs heat and creates a mirage. (Source: Wikimedia Commons) |
Have you ever tried walking barefoot on an asphalt road or shingled roof during a hot sunny day? If so, you probably don't plan on doing it again because it's searing hot! The uncomfortably high surface temperature of traditional urban surfaces, i.e. roads and roofs, is usually due to a material characteristic called "albedo." The albedo of a material quantifies the proportion of incident solar radiation that is reflected after striking the surface, on a scale of 0 to 1. An albedo of 1 is the conceptual equivalent of a mirror. Standard roofs and roads have a small albedo value, less than 0.25 and usually closer to 0.15. [1] Thus, a large fraction of sunlight is absorbed and converted to heat in the material. These hot materials warm the environment around them, whether that is air or another structure, such as the roof of a building. Hence, an increase in albedo values of urban materials on a global scale would result in a larger portion of total incoming solar radiation being reflected and could therefore mitigate the effects of global warming. [1]
According to a research work from the Lawrence Berkeley National Laboratory, pavements and roofs make up more than 60% of total surfaces in urban areas. Specifically, roofs account for about 25% and pavements account for roughly 35%. [1] Since roads and roofs occupy a major fraction of urban surfaces, changing the albedo of these surfaces will noticeably modify the total effective albedo in urban locations. As of 2008, reflective tiles, shingles, and coatings can be purchased to raise roof albedo up to 0.6. Additionally, a light- colored concrete used instead of standard asphalt can raise road albedo to about 0.3. [1] Assuming all roads and roofs are updated to high-albedo materials, the total effective albedo in urban settings would be increased by 0.1. Since urban areas make up conservatively 1% of total land area on Earth and land makes up 30% of Earth's surface area, this materials improvement would result in an effective increase of 3 × 10-4 in the Earth's albedo! [1]
Of all solar radiation incident on Earth, roughly half is absorbed by the surface to become heat. The remainder of incoming solar radiation is either reflected or absorbed by the atmosphere, or reflected by the surface at the time of arrival. [1] Concurrently, Earth and its atmosphere release energy into space, creating an equilibrium between energy in and out. A radiative energy imbalance imposed on Earth by anthropogenic activities is called radiative forcing. A radiative forcing will primarily cause Earth's surface temperature to change until the radiative energy imbalance in the climate is resolved. [2] An increase in surface albedo can result in a negative radiative forcing, while detrimental effects such as an increase in atmospheric CO2 concentrations raises radiative forcing. [1] Intuitively, negative radiative forcing decreases net absorbed energy from the sun, and results in lower surface temperatures at equilibrium. Globally, raising surface albedo of roofs and roads would decrease radiative forcing levels by 0.044 W/m2. [1] Considering the surface area of Earth (5.1 × 1014 m2), this is a significant decrease. In fact, it was hypothesized that this would offset the radiative forcing effects for 12 billion tons of carbon emitted into the atmosphere. [1] The Intergovernmental Panel on Climate Change predicted global warming will reach 1.5°C between 2030 and 2052. [3] Given the radiative forcing effect of carbon on global temperature, postulated to be 0.175°C per 100 billion tons of carbon, raising global albedo could thus counteract global warming by 0.021°C. [4] This is a very small number compared to 1.5°C but it still suggests that switching out surface materials can have a realizable passive improvement on climate change. Acknowledgement should be made that the calculations discussed above were preliminary in nature, as stated by Akbari et al. [1]
Despite the low direct temperature improvement calculated above, replacing surface materials can still benefit climate in other ways. Heat absorbed by urban surfaces is also responsible for the so-called urban heat island phenomenon, which refers to the increased ambient temperature of urban areas compared to surrounding, less-populated areas. [5] The urban heat island effect occurs as natural vegetative cover becomes replaced by dry, low-albedo, and thermally massive materials during urbanization. [6] The observed temperature difference due to urban heat islands can be quite significant depending on solar exposure in the area. [1,5,6] For example, in Athens, Greece where urban heat islands are well-documented, temperature differences upwards of 6°C have been observed. [5] This effect can of course be mitigated in part by replacing traditional surfaces with highly reflective (high albedo) roofs and pavements. Surface temperature reductions of more than 12°C have been measured on higher albedo pavements. [6] An analysis of tens of large-scale applications and studies discussed by Kyriakodis and Santamouris indicate that implementation of cool, high albedo materials may reduce peak and average urban ambient temperatures (not just the surface) by up to 2.5°C and 1.0°C, respectively. [5] This prediction has been supported by recent analysis of 15 large-scale cool material projects using reflective pavements yielding very similar results. [5]
Implementing high albedo surfaces in place of standard materials can contribute to mitigating the effect of global warming in various ways. Higher albedo directly lowers temperature, as discussed above. However, such materials can also provide indirect benefits. For instance, urban heat islands lead to increased air conditioner use, which produces excess heat and indirectly emits CO2 via the generated electricity required to cool the air. Implementing cool urban surfaces, thus lowering urban heat island behavior reduces these effects. While high-albedo surfaces seem promising for combating global warming, diligence must be observed to ensure that the environmental impacts of implementation do not exceed the benefits. [7]
© Aaron Scherr. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. Akbari, S. Menon, and A. Rosenfeld, "Global Cooling: Increasing World-Wide Urban Albedos to Offset CO2," Climatic Change 94, 275 (2009).
[2] Radiative Forcing of Climate Change (National Academies Press, 2005).
[3] V. Masson-Delmotte et al., "Global Warming of 1.5°C;," Intergovernmental Panel on Climate Change, October 2018.
[4] H. D. Matthews and K. Caldeira, "Stabilizing climate Requires Near-Zero Emissions," Geophys. Res. Lett. 35, L04705 (2008).
[5] G. E. Kyriakodis and M. Santamouris, "Using Reflective Pavements to Mitigate Urban Heat Island in Warm Climates - Results From a Large Scale Urban Mitigation Project," Urban Climate 24, 326 (2018).
[6] A. Mohegh et al., "Modeling the Climate Impacts of Deploying Solar Reflective Cool Pavements in California Cities," J. Geophys. Res. 122, 6798 (2017).
[7] M. Pomerantz, "Are Cooler Surfaces a Cost-Effect Mitigation of Urban Heat Islands?," Urban Climate 24, 393 (2018).
