Three Papers On The Role Of Surface Landscape Albedo On Radiative Forcing

There are three interesting papers on the role of the land surface in human caused climate change (h/t to Anthony Watts’s post Cooler white roofs – no complaints there).  In this case, the focus is on urban albedo.

The importance of this paper is that if such a significant effect on radiative forcing can be achieved by altering the urban albedo, a much larger radiative effect also occurs with other land use/land cover change!

The papers are

1. Surabi Menon, Hashem Akbari, Sarith Mahanama, Igor Sednev and Ronnen Levinson, 2010: Radiative forcing and temperature response to changes in urban albedos and associated CO2 offsets. Environ. Res. Lett. 5 (January-March 2010) 014005 doi:10.1088/1748-9326/5/1/014005.

The abstract reads

“The two main forcings that can counteract to some extent the positive forcings from greenhouse gases from pre-industrial times to present day are the aerosol and related aerosol-cloud forcings, and the radiative response to changes in surface albedo. Here, we quantify the change in radiative forcing and land surface temperature that may be obtained by increasing the albedos of roofs and pavements in urban areas in temperate and tropical regions of the globe by 0.1. Using the catchment land surface model (the land model coupled to the GEOS-5 Atmospheric General Circulation Model), we quantify the change in the total outgoing (outgoing shortwave + longwave) radiation and land surface temperature to a 0.1 increase in urban albedos for all global land areas. The global average increase in the total outgoing radiation was 0.5 W m–2, and temperature decreased by ~ 0.008 K for an average 0.003 increase in surface albedo. These averages represent all global land areas where data were available from the land surface model used and are for the boreal summer (June–July–August). For the continental US the total outgoing radiation increased by 2.3 W m–2, and land surface temperature decreased by ~ 0.03 K for an average 0.01 increase in surface albedo. Based on these forcings, the expected emitted CO2 offset for a plausible 0.25 and 0.15 increase in albedos of roofs and pavements, respectively, for all global urban areas, was found to be ~ 57 Gt CO2. A more meaningful evaluation of the impacts of urban albedo increases on global climate and the expected CO2 offsets would require simulations which better characterize urban surfaces and represent the full annual cycle.”

2. Oleson,K. W., G. B. Bonan, and J. Feddema (2010), Effects of white roofs
on urban temperature in a global climate model,
Geophys. Res. Lett., 37, L03701, doi:10.1029/2009GL042194.

The abstract reads

“Increasing the albedo of urban surfaces has received attention as a strategy to mitigate urban heat islands. Here, the effects of globally installing white roofs are assessed using an urban canyon model coupled to a global climate model. Averaged over all urban areas, the annual mean heat island decreased by 33%. Urban daily maximum temperature decreased by 0.6 C and daily minimum temperature by 0.3 C. Spatial variability in the heat island response is caused by changes in absorbed solar radiation and specification of roof thermal admittance. At high latitudes in winter, the increase in roof albedo is less effective at reducing the heat island due to low incoming solar radiation, the high albedo of snow intercepted by roofs, and an increase in space heating that compensates for reduced solar heating. Global space heating increased more than air conditioning decreased, suggesting that end-use energy costs must be considered in evaluating the benefits of white roofs.”

3. The third paper is

Hashem Akbari, Surabi Menon and Arthur Rosenfeld, 2010: Global cooling: increasing world-wide urban albedos to offset CO2. Climatic Change DOI 10.1007/s10584-008-9515-9

The abstract reads

“Increasing urban albedo can reduce summertime temperatures, resulting in better air quality and savings from reduced air-conditioning costs. In addition, increasing urban albedo can result in less absorption of incoming solar radiation by the surface-troposphere system, countering to some extent the global scale effects of increasing greenhouse gas concentrations. Pavements and roofs typically constitute over 60% of urban surfaces (roof 20–25%, pavements about 40%). Using reflective materials, 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. On a global basis, we estimate that increasing the world-wide albedos of urban roofs and paved surfaces will induce a negative radiative forcing on the earth equivalent to offsetting about 44 Gt of CO2 emissions. At ∼$25/tonne of CO2, a 44 Gt CO2 emission offset from changing the albedo of roofs and paved surfaces is worth about $1,100 billion. Furthermore, many studies have demonstrated reductions of more than 20% in cooling costs for buildings whose rooftop albedo has been increased from 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.”

The text of the third paper includes the text

“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 17.5million km2of 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%.1 Furthermore, the analysis from McGranahan et al. (2005) shows that the urban areas account for 2.8% of the land area.”

“Rose et al. (2003) have estimated the fractions of the roof and paved surface areas in four U.S. cities. The fraction of roof areas in these four cities varies from 20% for less dense cities to 25% for more dense cities. The fraction of paved surface areas varies from 29% to 44%.Many metropolitan urban areas around the world are less vegetated than typical U.S. cities. For this analysis, we consider an average area fraction of 25% and 35% for roof and paved surfaces, respectively.”

“The albedo of typical standard roofing materials 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 be increased to about 0.55 to 0.60.”

“In our analysis, we have estimated that available urban surfaces for potential increase of their albedo are about 1% of the land surface of the earth.Any under or over-estimation in this estimate directly scales our results.”

“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 urban roofs and paved surfaces will induce a negative radiative forcing of 4.4×10−2 Wm−2 equivalent to offsetting 44 Gt of emitted CO2. A 44 Gt of emitted CO2 offset resulting 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 potential for cool roofs and cool pavements would counteract the effect of the growth in CO2- equivalent emission rates for 11 years.”

Clearly, a message from their paper is that if a landscape albedo change of  +0.1 in urban areas, which cover ~1% of the Earth’s surface, has such a significant effect on the global radiative forcing, than a landscape albedo change of 10% of the Earth’s landscape by +0.01 would result in the same the radiative forcing effect.

As documented in Table 11-4 (page 408) in

Pielke, R.A., Sr., 2002: Mesoscale meteorological modeling. 2nd Edition, Academic Press, San Diego, CA, 676 pp.

albedo values for different landscapes are quite different. For example, a leaved deciduous forest has an albedo of about 0.20 while a cereal crop has an albedo of  about 0.25.  Thus the conversion of this type of forest for just 0.2% of the Earth’s surface would have the same radiative impact as the white roof conversion that is examined in the these papers.

Clearly, these papers provide much deserved recognition of the major role of the human management of landscape as a first order component in the climate system.

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