1 THE IMPACTS OF SURFACE ALBEDO ON CLIMATE AND 2 BUILDING ENERGY CONSUMPTION: REVIEW AND 3 COMPARATIVE ANALYSIS 4 5 6 7 Xin Xu (corresponding author) 8 Research Assistant 9 Department of Civil & Environmental Engineering* 10 Phone: 617-898-8968 11 [email protected] 12 13 Jeremy Gregory 14 Research Scientist 15 Department of Civil & Environmental Engineering * 16 Phone: 617-324-5639 17 [email protected] 18 19 Randolph Kirchain 20 Principal Research Scientist 21 Institute for Data, Systems and Society* 22 Phone: 617-253-4258 23 [email protected] 24 25 *Material Systems Laboratory 26 Massachusetts Institute of Technology 27 Building E38-432 28 Cambridge, MA 02139 29 Fax: 617-258-7471 30 31 32 A Paper Submitted for Presentation at the Transportation Research Board 95th Annual Meeting 33 34 Submission Date: August 1st, 2015 35 36 37 38 Word count: 6226 words text + 7 tables/figures x 250 words (each) = 7976 words TRB 2016 Annual Meeting Paper revised from original submittal. Xu, Gregory, Kirchain 2 1 ABSTRACT 2 Rapid urbanization has changed land use and surface properties, which has an effect on regional 3 and even global climate. One of the mitigation strategies proposed to combat urban heat island 4 (UHI) and global warming as a result of urban expansion is to increase the solar reflectance (or 5 albedo) of the urban surfaces (mainly roofs and roads). Despite the observed local cooling effect 6 in many studies, the regional and global climate impacts induced by land surface change are still 7 not well understood, especially the indirect and larger-scale effects of changes in urban albedo on 8 temperature and radiation budget. Research has been growing on understanding the climate impact 9 of changing albedo, while results still demonstrate great uncertainties and variations. 10 11 This paper presents a state-of-the-art review of the existing research on the climate impacts of 12 albedo. Different analytical approaches and modeling works are synthesized and discussed. A 13 comparative analysis of results from different studies shows that the radiative forcing (RF) for a 14 0.01 increase in albedo ranges from -2.9 to -1.3, and the air temperature reduces by 0.1 °C on 15 average, indicating a cooling effect. The annual GWP savings can be up to 7 kg CO2 per square 16 meter of urban area or cool surfaces. The uncertainties in the results and limitations in the existing 17 studies are discussed. This analysis will serve as a foundation for comparing the climate impacts 18 of urban albedo. 19 20 21 Keywords: albedo, urbanization, global warming, green pavements TRB 2016 Annual Meeting Paper revised from original submittal. Xu, Gregory, Kirchain 3 1 1. INTRODUCTION 2 Anthropogenic modifications to land surface properties due to land use change have the potential 3 to change the climate. Urbanization (conversion of large areas of natural surfaces to man-made 4 impervious land), a result of population growth and economic development, is considered as one 5 of the principal human activities influencing land surface characterization and land-atmosphere 6 interaction. As humans alter the natural landscape, energy exchanges through the surface and 7 atmosphere are affected, thus influencing the local, regional, and even the global climate. 8 9 One of the land surface properties that has been changed due to urbanization is albedo, which is a 10 measure of surface reflectance defined as the ratio of solar radiation reflected by a body or surface 11 to the amount incident upon it, ranging from 0 (complete absorption) to 1 (complete reflection). 12 Roofs and pavements, which constitute about 20-25% and 29-44% respectively of typical US 13 urban surfaces (1), generally have lower albedos than their surrounding areas. These urban surfaces 14 have to be changed and maintained regularly (e.g. pavements are typically resurfaced once in a 15 decade and new roofs are installed or resurfaced every 2–3 decades) (2). Changes in the surface 16 albedo can have direct and indirect impacts on the radiative energy balance of the earth and the 17 local, regional and even global climate. Surface albedo change can affect the amount of solar 18 radiation going back to the space, thus altering the radiative balance at the top-of-atmosphere 19 (TOA), known as radiative forcing (which will be explained in the following section). Indirectly, 20 changes to the urban surface albedo also contribute to a phenomenon known as an "urban heat 21 island" (UHI) (3), due in part to the lower albedo of urban surfaces compared to their rural 22 surroundings. The higher temperature in urban areas increases the demand for cooling energy in 23 buildings in order to maintain comfort levels, and thus leads to increased emissions of greenhouse 24 gases. In addition to albedo, changes to other surface properties, such as heat capacity, thermal 25 conductivity, permeability, etc., have also influenced the urban energy balance and climate. For 26 example, impermeable surfaces like conventional roofs and pavements tend to suppress 27 evaporative cooling. As a result, absorbed solar radiation is transferred to sensible heat more than 28 to latent heat, contributing to the development of the UHI. 29 30 One strategy proposed for mitigating UHI and climate change has been to increase the solar 31 reflectance of roofs and pavements in urban areas, commonly referred to as cool roof and cool 32 pavement strategies. Cool roofs have been mandated in many states and cities over the past decades. 33 Cool pavements, however, have not been widely adopted as standard practice, but could potentially 34 contribute to the GHG reduction because of the larger percentage of urban surfaces covered by 35 pavements. 36 37 While many efforts have been made to assess the climate impacts of changing albedo, comparative 38 analyses of different modeling approaches and results from existing studies are limited. Yang et 39 al. is the only one that did a comprehensive review of potential environmental impacts (including 40 temperatures, building energy, hydroclimates, thermal comfort and air quality) of reflective 41 materials at a variety of scales (4). In addition, significant uncertainties exist in estimating the 42 climate impacts of albedo though numerical modeling and analytical approximation. Therefore, 43 this study aims to provide a summary of the state-of-art in research on quantifying the impacts of 44 albedo change on radiative energy balance and climate change using different modeling 45 approaches. Results from different studies are compared in terms of changes in temperature, 46 radiative budget and energy consumption due to albedo changes. TRB 2016 Annual Meeting Paper revised from original submittal. Xu, Gregory, Kirchain 4 1 2. IMPACTS OF ALBEDO CHANGE 2 3 Radiative Forcing 4 The term “radiative forcing” (RF) is defined as the change in net (down minus up) irradiance (solar 5 plus long-wave; in W/m2) at the tropopause or at the top of the atmosphere (TOA) due to an 6 imposed change (5). It describes any perturbation or imbalance in the radiative energy budget of 7 the Earth-atmosphere system, which has the potential to lead to climate changes and thus results 8 in a new equilibrium state of the climate system. A variety of forcing agents can cause such a 9 perturbation including greenhouse gases, tropospheric aerosols, ozone, land-use change (surface 10 albedo change), solar irradiance and aerosols from volcanic eruptions. RF is then used to estimate 11 and compare the relative strength of different anthropogenic and natural forcing agents on climate 12 change. Positive RFs represent global warming and negatives lead to global cooling. According to 13 IPCC Assessment Report 5 (IPCC AR5) (6), surface albedo change, primarily due to deforestation, 14 have induced an overall increased surface albedo and an RF of –0.15 ± 0.10 W/m2. However, there 15 is great uncertainty and spatial variability associated with this estimate due to the characterization 16 of land cover, exclusion of feedbacks, and the climate model used to simulate the RFs. Therefore, 17 the uncertainty range associated with this estimate is large and the level of scientific understanding 18 is medium-low as reported in IPCC AR5. 19 Climate Feedbacks 20 While RF is a simple and useful measure of climate impacts because it’s easily calculable and 21 comparable, it does not attempt to represent the all the climate responses other than radiative 22 balance. In fact, RF induced by changes in albedo can have a series of climate feedbacks, including 23 temperature, moisture, latent heat and sensible heat fluxes. These climate responses and feedbacks, 24 in particular through the hydrologic cycle, have the potential to offset the radiative impact of 25 albedo changes, and they are more uncertain and difficult to quantify. As a result, there is low 26 agreement on the sign of the net change in global mean temperature change induced by land use 27 change (6). For instance, Bala et al. showed that the net temperature change can be either positive 28 or negative depending on the latitude (7); Findell et al. also found a negligible impact of land use 29 change on the global mean temperature through climate simulations, although there are some 30 significant regional changes (8). 31 Building Energy Demand 32 Changes in surface albedo due to urbanization have led to higher air temperature in the urban areas 33 than their rural surroundings, known as the “urban heat island effect”. Elevated temperature in the 34 summertime results in an increase in cooling demand for buildings and excess GHG emissions 35 from producing the energy required to fulfill the needs. Akbari et al. reported that for the major 36 metropolitan areas in the U.S., peak electricity load would increase by 1.5–2% for every 1°F 37 increase in ambient temperature (9).