Climate
Urban Albedos

Text ... Report (2009) about Global Cooling and the impact world-wide urban albedos might offset impact of CO2 and global warming

Burgess COMMENTARY

Peter Burgess

This is text (partial) from the following Springer Nature webpage related to a paper written in 2009 https://link.springer.com/epdf/10.1007/s10584-008-9515-9?shared_access_token=LjOMPG7AcRqPOCboVG3bWfe4RwlQNchNByi7wbcMAY6bE23FsEUmzlsK35BTYJqPX8_1NNKbs7jCtqJV7jnbhOin78eQjr59uxeiKQ9AE8Fg-lmwd9hbEGAnl05-0CGoECpS3F9NpHXSomYpR2MCdkbYaN_sczCpQTCMIBejC5A= Climatic Change (2009) 94:275–286DOI 10.1007/s10584-008-9515-9 Global cooling: increasing world-wide urban albedos to offset CO2Hashem Akbari ·Surabi Menon ·Arthur RosenfeldReceived: 29 January 2008 / Accepted: 4 September 2008 / Published online: 20 November 2008© Springer Science + Business Media B.V. 2008 Abstract Increasing urban albedo can reduce summertime temperatures, resultingin better air quality and savings from reduced air-conditioning costs. In addition,increasing urban albedo can result in less absorption of incoming solar radiation bythe surface-troposphere system, countering to some extent the global scale effects ofincreasing greenhouse gas concentrations. Pavements and roofs typically constituteover 60% of urban surfaces (roof 20–25%, pavements about 40%). Using reflectivematerials, both roof and pavement albedos can be increased by about 0.25 and 0.15,respectively, resulting in a net albedo increase for urban areas of about 0.1. Ona global basis, we estimate that increasing the world-wide albedos of urban roofsand paved surfaces will induce a negative radiative forcing on the earth equivalentto offsetting about 44 Gt of CO2emissions. At ∼$25/tonne of CO2,a44GtCO2emission offset from changing the albedo of roofs and paved surfaces is worth about$1,100 billion. Furthermore, many studies have demonstrated reductions of morethan 20% in cooling costs for buildings whose rooftop albedo has been increasedfrom 10–20% to about 60% (in the US, potential savings exceed $1 billion per year). Our estimated CO2 offsets from albedo modifications are dependent on assumptions used in this study, but nevertheless demonstrate remarkable global cooling potentials that may be obtained from cooler roofs and pavements. 276 Climatic Change (2009) 94:275–286 Table 1 Urban fabricSource: Rose et al. (2003) Metropolitan Areas Vegetation Roofs Pavements Other Salt Lake City 33.3 21.9 36.4 8.5 Sacramento 20.3 19.7 44.5 15.4 Chicago 26.7 24.8 37.1 11.4 Houston 37.1 21.3 29.2 12.4 1 Introduction In many urban areas, pavements and roofs constitute over 60% of urban surfaces(see Table 1; roof 20–25%, pavements about 40%) (Akbari et al. 2003; Roseetal 2003; Akbari and Rose 2001a, b). Many studies have demonstrated buildings cooling-energy savings in excess of 20% upon raising roof reflectivity from an existing10–20% to about 60%. We estimate a U.S. potential savings in excess of $1 billion per year in net annual energy bills (cooling-energy savings minus heating-energy penalties). Increasing the albedo of urban surfaces (roofs and pavements) can reducethe summertime urban temperature and improve the urban air quality (Taha 2001,2002;Tahaetal.2000; Rosenfeld et al. 1998; Akbari et al. 2001; Pomerantz et al.1999). The energy and air-quality savings resulting from increasing urban surface abedos in the U.S. alone can exceed $2 billion per year.Increasing the urban albedo results in reflecting more of the incoming global solar radiation and countering to some extent the effects of global warming. Here wequantify the effect of increasing the albedo of urban areas on in terms of CO2emission offset. 2 Estimating global urban areas Figure 1 lists the area densities for the 100 largest metropolitan areas of the world (Demographia 2007). The median area density is about 430 m2 per urban dweller. The 100 largest metropolitan areas (with a total population of 670 million) comprise about 0.26% of the Earth land area. Assuming that about 3 billion people live in urban areas, the total urban area of the globe is estimated at about 1.2% of theland area. As an independent verification for the estimate of urban areas, we used the data from Global Rural–Urban Mapping Project (GRUMP) Urban Extent Mask (CIESIN 2007). The Urban Extent Mask combines National Oceanic and At-mospheric Administration measurements of nighttime lights with the US Defence Mapping Agency Digital Chart of the World’s Populated Places to assess the geographic extents of rural and urban areas (Balk et al. 2004). Equal-area sinusoidal projection of the 30-arc-sec urban extent mask indicates that of the Earth’s 149 million km2 of land area, 128 million km2 is rural and 3.5 million km2 is urban. The 3.5 million km2 of urban land represents 2.4% of global land area and 0.7% of global surface area. Most of the 17.5 million km2 of unclassified land lies in Antarctica (14 million km2) or Greenland (2.2 million km2). The GRUMP estimate of 2.4% is twice the estimate of 1.2%.1Furthermore, the analysis from McGranahan et al.(2005) shows that the urban areas account for 2.8% of the land area. We expect thatthe GRUMP estimate is closer to reality, since the population densities in the world’s 100 largest cities are probably higher than in other urban areas. In our calculations,we conservatively assume that urban areas are 1% of the land area. Climatic Change (2009) 94:275–286 277 Fig. 1 Area density for the 100 largest cities in the world; 670 million people livein these cities. Source:Demographia (2007) Get GIMP image 3 Potentials for urban albedo change Rose et al. (2003) have estimated the fractions of the roof and paved surfaceareas in four U.S. cities. The fraction of roof areas in these four cities varies from20% for less dense cities to 25% for more dense cities. The fraction of paved surfaceareas varies from 29% to 44%. Many metropolitan urban areas around the world areless vegetated than typical U.S. cities. For this analysis, we consider an average areafraction of 25% and 35% for roof and paved surfaces, respectively. Akbari and Konopacki (2005) have reviewed the solar reflectance of typicalroofing materials used on residential and commercial buildings in many U.S. regions.A solar-reflective roof is typically light in color and absorbs less sunlight than aconventional dark-colored roof. Less absorbed sun light means a lower surfacetemperature, directly reducing heat gain from the roof and air-conditioning demand.Typical albedo values for low- and high-albedo roofs can be obtained from the coolroofing materials database (CRMD 2007). The albedo of typical standard roofingmaterials ranges from 0.10–0.25; one can conservatively assume that the average albedo of existing roofs does not exceed 0.20. The albedo of these surfaces can beincreased to about 0.55 to 0.60. For the sloped-roof residential sector, available highly reflective materials arescarce. White asphalt shingles are available, but have a relatively low albedo ofabout 0.25. Although it can be argued that white coatings can be applied to shinglesor tiles to obtain an aged albedo of about 0.5, this practice is not followed in the field. Some highly reflective white shingles are being developed, but are only in theprototype stage. Recently, one U.S. manufacturer has developed and marketing cool-colored fiberglass asphalt shingles with a solar reflectance of 0.25. Some reflectivetiles and metal roofing products with greater than 50% reflectivity are also available.Conversely, highly reflective materials for the low-slope commercial sector are onthe market. White acrylic, elastomeric and cementatious coatings, as well as whitethermoplastic membranes, can now be applied to built-up roofs to achieve an agedsolar-reflectance of 0.6. 278 Climatic Change (2009) 94:275–286 Table 2 Albedo modificationscenarios Source: Akbari et al. (2003) Surface-type Albedo changeHigh Low This study Residential roofs 0.3 0.1 0.25 Commercial roofs 0.4 0.2 0.25 Pavements 0.25 0.15 0.15 Pomerantz et al. (1997, 2000a, b) and Pomerantz and Akbari (1998) have documented the solar reflectance of many standard and reflective paved surfaces including paving materials such as chip seal, slurry coating, light-color coating. They reportthat the solar reflectance of a freshly installed asphalt pavement is about 0.05. Aged asphalt pavements have a solar reflectance between 0.10–0.18, depending on the typeof aggregate used in the asphalt mix. A light-color (low in carbon content) concrete can have an initial solar reflectance of 0.35–0.40 that will age to about 0.25–0.30. Akbari et al. (2003) provide estimates for two scenarios for potential changes inthe albedo of roofs and paved surfaces (See Table 2). Based on these data, we assumethat roof albedo can increase by 0.25 for a net change of 0.25 × 0.25 = 0.06. The pavement albedo can increase by 0.15 for a net change of 0.35 × 0.15 = 0.04. Hence, the net potential change in albedo for urban areas is estimated at 0.10. Increasing the albedo of urban areas by 0.1 results in an increase of 3 × 10−4 in the Earth’s albedo. 4 Methodology to estimate urban albedo effects on radiative forcing Changing albedo of urban surfaces and changing atmospheric CO2concentrationsboth result in a change in radiative forcing. In these calculations, using the existingdata, we will first estimate the increase in radiative forcing from increasing theatmospheric CO2by 1 tonne (Section 5). Then through a simple model, we estimatethe decrease in radiative forcing by increasing the albedo of roofs and paved surfacesin the urban areas (Section 6). A simple comparison of these two radiative forcingsallows us to relate the changes in the solar reflectance of urban surfaces to thechanges in the atmospheric CO2content. We note that there are several underlying assumptions/approximations made in our calculations that may affect the projections of CO2 offsets from increasing urban albedos, when equating urban albedo effects with that of CO2 on radiative forcing. These are listed as follows: 1. Any discussion of atmospheric CO2 involves a time series of physical effects, e.g.,sequestration in land or ocean. Here we are interested only in the short-termeffect (25–50 years). We ignore any time dependence and economics. 2. Using the existing short-wave radiation balance models for the earth–atmosphere system, our calculations for radiative forcing changes implicitlyaccount for the effect of multiple scattering and absorption of radiation withinthe atmosphere caused by the increased reflectance from urban surfaces. The calculations are performed for the entire globe combining the effects of cloudsand atmospheric scattering and absorption into two components—atmosphericabsorption and atmospheric reflection. The cloud cover over the oceans istypically higher than over the land. Meanwhile, the atmospheric absorption overthe land in some urban areas may be higher because of increased absorption of radiation by some aerosol species such as black carbon that may increasethe heating within the atmospheric layer, especially in areas where black carbonconcentrations are relatively high. The relative effects of these factors are hard toestimate without using a detailed radiative transfer model coupled to a chemicaltransport model (outside the scope of this work), and may to some extent beoffset by the effects of local climate on smog formation. For example it has beenshown that reflective surfaces in general result in cooler urban temperatures, inturn slowing the formation of smog and decreasing the urban boundary layerthickness. Observations and simulations of smog concentration versus ambienttemperature have also shown that cool urban surfaces have a dramatic effect inreducing urban smog (Taha 2005, 2008a, b). Also, most non-metallic surfaces(independent of their solar reflectance) absorb over 90% of the incoming UVlight (Levinson et al. 2005a, b) and thus, we do not expect any UV-related effecton photochemical urban smog because of reflective urban surfaces. 3. In most urban areas, residential and suburban neighborhoods constitute the ma-jority of the surfaces (the fraction of areas with tall buildings are fairly small). Alimited analysis for the effect of shading of roofs by trees and adjacent buildingsshows that shadows from all sources reduce the annual incidence of sunlighton residential roofs by about 10–25%, depending on tree cover (Levinson et al.2008). This tends to reduce the equivalent potential of cool surfaces by a similar 10–25%. 5 Radiative forcing from atmospheric CO2 To estimate the radiative forcing associated with CO2, we consider four differentsources as indicated in Table 3.Hansenetal.(1997a)estimatea2× CO2adjustedtop-of-atmosphere (TOA) radiative forcing (RF) of 4.19 W m−2. In a more recent study, Hansen et al. (2005) estimate an adjusted RF of 3.95 ±0.11 W m−2. This yields a RF of 0.93 kW/tonne of CO2. Estimates used in the IPCC (2007), are based onMyhre et al. (1998) who use RF [W m−2] =5.35 ln(1 + C/C) =5.35 ×ln(2) to obtain a RF of 3.71Wm−2. This yields a RF of 0.88 kW/tonne of CO2. Also using Myhre’sequation and estimating the marginal RF at the current atmospheric concentration of 385 ppmv, we estimate a RF of 0.91 kW/tonne of CO2{(5.35/385) [W m−2/ppmv]×(5.1 × 1014) [m2] × (0.128) [ppmv/Gt] × (10−9) [Gt/t] × (10−3) [kW/W]}. In thesecalculations, we estimate that 1 Gt of CO2 increases the atmospheric CO2 concentration by 0.128 ppmv (see Table 3, Note c). Note that the three methods used yieldalmost similar results; hence, we will use an average of 0.91 kW/tonne of CO2for theremainder of these calculations. When equating the forcing between CO2 and urban albedo changes, we consider CO2that is in the atmosphere at a given time. Hence, we do not explicitly considerthe time dimensions for the two forcings (urban albedo change and forcing fromCO2). We also note the approach of Matthews and Caldeira (2008) who attempt tocalculate the effective forcing from CO2on a decadal to century time scale. Usingan earth system model with the carbon cycle, Matthews and Caldeira (2008) founda 0.175◦C temperature increase for every 100 GtC emitted and estimate that every100 GtC is ∼5% of the climate sensitivity (3.5◦Cby2× CO2) in the model they use.Thus, they suggest that, on the decadal to century time scale, the effective radiative forcing from CO2 after accounting for carbon cycle effects is ∼5% ×3.7 W m−2×5.1 ×1014m2/(100 GtC) = 0.26 kW/tonne of emitted CO2.Here3.7Wm−2is the IPCC radiative forcing for 2 × CO2.IPCC(2007) estimates that only 55% of the emitted CO2 stays in the atmosphere. Then Mathews and Caldeira’s estimates of effective radiative forcing is 0.26/0.55 = 0.47 kW/tonne of atmospheric CO2. This value is considerably lower than the range in RF per tonne of CO2 obtained prior, but nevertheless indicate how these values may differ depending on the model and assumptions used. 280 Climatic Change (2009) 94:275–286 Table 3 Radiative forcing at the top of the atmosphere (TOA) and CO2 equivalence Row Item Value 1a. 2 × CO2TOA radiative forcing (RF)—Hansen et al. estimatea3.95 W m−21b. 2 × CO2TOA radiative forcing (RF)—IPCC estimateb3.70 W m−22. Increase in atmospheric concentration by doubling CO2275 ppmv 3. Increases in atmospheric concentration by adding 1Gt of CO2c0.128 ppmv 4. Increase in atmospheric CO2by doubling 2.15 × 1012tonneconcentration [Row 2/Row 3] 5. Surface area of the Earth 5.1 × 1014m26a. Total radiative forcing on theEarth [Row 1a × Row 5], Hansen RF 2.01 × 1015W6b. Total radiative forcing on theEarth [Row 1b × Row 5], IPCC RF 1.89 × 1015W7a. TOA radiation change per tonne ≈ 0.93 kW/tonne CO2of atmospheric CO2[Row 6a/Row 4], Hansen RF 7b. TOA radiation change per tonne of atmospheric ≈ 0.88 kW/tonne CO2CO2[Row 6b/Row 4], IPCC RF 7c. Incremental TOA radiation change per tonne of atmospheric ≈ 0.91 kW/tonne CO2CO2(at current concentration of 385 ppmv)using Myhre equation 7d. TOA radiation change per tonne of atmospheric CO2, ≈ 0.91 kW/tonne CO2 average of Row 7a–7c7e. TOA radiation change per tonne of emitted CO2≈ 0.26 kW/tonne CO2(long-term) using Caldeira’s estimated aHansen et al. (2005) bIPCC (2007) cThe current atmospheric CO2concentration is about 385 ppmv corresponding to about 3,000 GtCO2(Trenberth et al. 1988). At current conditions, we estimate an increase of 385/3,000 = 0.128ppmv/Gt of CO2 dMatthews and Caldeira (2008). Note that the radiation change is estimated per tonne of emitted CO2 6 The effect of changing urban albedos on global radiative forcing Hansen et al. (1997a) have estimated an adjusted top of the atmosphere RF of−3.70 W m−2for increasing the albedo of ‘Tropicana’ by 0.2. We estimate thatTropicana is 22% of the land area (a major portion of land area between 22◦Sto30◦Nand20◦–50◦Eindicatedin Fig.1 of Hansen etal.(1997b)); or about 1/16th of the global surface. For the reflected surfaces, the radiative forcing per 0.01 increase in albedo as estimated by Hansen et al. (1997a)is−2.92 Wm−2 of Tropicana land. This estimate appears to be high. Alternatively, as shown in the left side of Fig. 2,Kiehl and Trenberth (1997) estimate that of the total global average of 342 W m−2incident short-wave solar radiation, 77 W m−2(22.5%) is reflected by the atmosphere(that include clouds, aerosols, and the atmosphere), 67 W m−2(19.6%) is absorbedby the atmosphere (again including clouds, aerosols, and the atmosphere), and168 W m−2(49.1%) is absorbed by the earth’s surface (see Fig. 2). Kiehl and Trenberth also estimate that the reflected radiation at TOA from the surface is30 W m−2(8.8%) net. All these include the effect of multiple scattering by the earthatmosphere-and-surface system. We use the Kiehl and Trenberth data and a simplesingle-path model to estimate the sensitivity of RF at TOA to the solar reflectance atthe surface. Let f be the fraction of radiation absorbed by the atmosphere (including theclouds) either for incoming or reflected shortwave radiation. Of the 265 W m−2net(342–77 W m−2) incident shortwave radiation 265 f Wm−2(Term A: absorptionof in-coming solar radiation) is absorbed by the atmosphere. The atmosphericabsorption of the reflected shortwave radiation from the earth’s surface is estimatedat 30 f/(1 − f)Wm−2(Term B: absorption of reflected solar radiation). The sumof Term A and Term B is equal to total short-wave atmospheric absorption of67 W m−2(i.e., 265 f+ 30 f/(1 − f) = 67). Solving this quadratic equation yieldsf = 22.1%. Hence, we estimate an incident short-wave radiation of 265 ×(1 − f) =206 W m−2on the surface. This value is within the range (169–219 W m−2) given in Hatzianastassiou et al. (2005) based on estimates from several studies. However,Hatzianastassiou et al. (2005) themselves estimate that only 172 W m−2of short-waveradiation reaches the earth surface. This would require that about 50% of the SWradiation is either absorbed or reflected by the atmosphere. Using a simple scaling,we recalculate f = 26%. Then we calculate a change in RF per 0.01 change in solar Table 4 Radiative forcing of changing roofs and pavements albedo and their equivalent CO2 offset Row Item Value 1. Average RF for an albedo increase of 0.01 (see Section 6) // 1.27 Wm−2 2. Atmospheric CO2equivalent for 0.01 albedo increase of urban area [Row 1/Row 7d Table 3] // -1.40 kg CO2m−2 of urban area 3. Fraction of emitted CO2that remains in the atmosphere // 0.55 4. Emitted CO2equivalent offset for 0.01 increase in albedo of urban surface (based on IPCC estimate of RF)[Row 2/Row 3]a // −2.55 kg CO2m−2of urban area 5. Proposed change in the solar reflectance of roofs // 0.25 6a. Emitted CO2 offset for increasing roof albedo by 0.25 // −64 kg CO2m−2 of roof area 6b. Cool roof area to offset 1 tonne of emitted CO216 m2 7. Proposed change in the solar reflectance of pavements 0.15 8a. Emitted CO2 offset for increasing pavement albedo by 0.15 −38 kg CO2m−2of paved area 8b. Cool pavement area to offset 1 tonne of emitted CO2 26 m2a For comparison, using Matthews and Caldeira’s methodology, we estimate an emitted CO2 equivalent offset of −4.90 kg CO2m−2reflectance of the surface to be (1 − f)×172/100 =−1.27 Wm−2. We use this lower estimate for the remainder of our calculations (see Table 4,Row1).We note that our calculations apply for the average cloud cover over the earth.The RF for albedo change is a strong function of the cloud cover. The larger the cloudcover, the lower the RF resulting from changes in surface albedo. Thus, dependingon the location of urban areas considered, the RF values would need to be adjustedfor variations of local cloud cover with respect to the average, which is not consideredin our calculations.Using the estimated kW RF per tonne of atmospheric CO2in Table 3,wecalculateaCO2equivalency of −1.40 kg of CO2per m2of urban areas for a 0.01 change inalbedo (see Table 4,Row2).IPCC(2007) estimates that only 55% of the emittedCO2stays in the atmosphere. Using the IPCC RF equivalent for cool roofs with aproposed albedo change of 0.25, the emitted CO2offset is then estimated at −64 kgCO2per m2of roof area (i.e., 16 m2of cool roof area to offset 1 tonne of emittedCO2).2For cool pavements with a proposed albedo change of 0.15, the emitted CO2offset is equal to −38 kg CO2per m2of pavement area (i.e., 26 m2of cool paved areato offset 1 tonne of emitted CO2).7 Global cooling: CO2equivalenceWe estimate that urban areas are at least 1% of the Earth’s land area or about 1.5×1012m2(see Table 5). The roof area is 3.8 × 1011m2. The paved surface area is5.3 × 1011m2.WecalculateaglobalRFof−4.4× 10−2Wm−2by cool roofs and coolpavements. We then estimate the global emitted CO2offset potentials for cool roofsand cool pavements to be in the range of 24 Gt of CO2and20GtofCO2, respectively,giving a total global emitted CO2offset potential range of 44 Gt of CO2.This44Gt2A cooler roof on a typical new house with a roof area, including garage, of about 180 m2offsets theemissions of more than 10 tonnes of CO2 284 Climatic Change (2009) 94:275–2869 Conclusions Using cool roofs and cool pavements in urban areas, on an average, can increase the albedo of urban areas by 0.1. We estimate that increasing the albedo of urbanroofs and paved surfaces will induce a negative radiative forcing of 4.4×10−2Wm−2equivalent to offsetting 44 Gt of emitted CO2. A 44 Gt of emitted CO2offsetresulting from changing the albedo of roofs and paved surfaces is worth about$1,100 billion. Assuming a plausible growth rate of 1.5% in the world’s CO2-equivalent emission rate, we estimate that the 44 Gt CO2-equivalent offset potentialfor cool roofs and cool pavements would counteract the effect of the growth in CO2-equivalent emission rates for 11 years.We emphasize that these calculations and estimates are preliminary in nature.Converting to cool urban surfaces does not address the underlying problem ofglobal warming, which results from the emissions of greenhouse gases and absorbingparticles. The problem of global warming emissions must be resolved by developingand implementing a complete portfolio of measures to reduce GHG emissions.We also note that the cool urban surfaces, particularly cool roofs, yield significantenergy savings and hence a reduction in GHG emissions. The global cooling effectof increasing urban solar reflectance is an added effect that is quantified here.Acknowledgements This work was supported by the California Energy Commission (CEC)through its Public Interest Energy Research Program (PIER) and by the Assistant Secretary forEnergy Efficiency and Renewable Energy under Contract No. DE-AC02-05CH11231. We thankDr. Ken Caldeira for enlightening us about radiative forcing and Dr. M. MacCracken for extensivecomments and discussions. Additional thoughtful comments from G. Franco, M. Jacobson, andJ. Sathaye helped improve this paper. R. Levinson helped us with the analysis of urban areas, usingthe GRUMP (Urban Extension) data. References Akbari H, Rose LS (2001a) Characterizing the fabric of the urban environment: a case study ofmetropolitan Chicago, Illinois. 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