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Reply #25: Well, no, that would be less precisely [View All]

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OKIsItJustMe Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Oct-20-11 04:51 PM
Response to Reply #20
25. Well, no, that would be less precisely
Heres the original question:

1. White roofs won't decrease global warming? O RLY??

Funny, I thought the purpose of them was to decrease AC bills for occupants and theoretically decrease energy usage.

Not sure anybody complained that they would do anything other than that. Nor that the heat island effect was anything more than a SYMPTOM of the problem.

To which, I replied:

5. Painting roofs white has been suggested as a method of geoengineering by increasing global albedo

See, for example:

These citations were to papers in scientific publications. (I also cited press releases about the papers, since I have found that many people prefer to read what amounts to a digest of the actual papers. Its become my general practice to post an extract of a press release, and URLs for the press release and the actual paper.)

Personally, Im no skeptic regarding the greenhouse effect, global warming, anthropogenic climate change, (or whatever we may call it tomorrow) but I never thought that painting roofs white would have as significant effect on the Earths albedo as Akbari and company thought that it would, and Akbaris work has been cited everywhere:

Ive gotten tired of reading about it again and again.

Do I just not appreciate nuance? Is that it?

Even the idea that white roofs save energy (due to less air conditioning) is bogus. It might be true for the South where air conditioning is heavily used, but it isnt for the North, where we make more use of furnaces.
GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L03701, doi:10.1029/2009GL042194, 2010

Effects of white roofs on urban temperature in a global climate model

K. W. Oleson and G. B. Bonan
Climate and Global Dynamics Division, National Center for Atmospheric Research,
Boulder, Colorado, USA

J. Feddema
Department of Geography, University of Kansas,
Lawrence, Kansas, USA


|1| 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 0.6C and daily minimum temperature by 0.3C. 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.

1. Introduction

|2| An analysis by Akbari et al. |2009| indicated that increasing the albedo of urban roofs and pavements globally could produce a negative radiative forcing equivalent to a 44 Gt CO2 emission offset. This is equivalent to offsetting the effect of the growth in CO2-equivalent emission rates for the next 11 years. It was also noted that potential energy savings may be realized due to a reduction in the amount of energy consumed by air conditioning to cool buildings. The effects of increasing the albedo of cities on near-surface climate, in particular air temperature, were not addressed.

3. Conclusions

|19| These results suggest that increasing the albedo of roofs is an effective way of reducing the urban heat island. The degree to which the heat island decreases depends on the importance of roofs relative to the rest of the urban system (i.e., building walls and canyon floor) in generating the heat island. This approach to heat island mitigation is less effective in winter at higher latitudes. At these latitudes, any benefits gained from a reduction in the summertime heat island need to be considered in the context of increased heating costs in winter. Increasing the albedo of roofs is also less effective for thin roofs constructed with high thermal admittance materials because these roofs have low storage capacities. Globally, white roofs cause a net increase in HAC fluxes due to a larger increase in space heating than decrease in air conditioning flux. As air conditioning more thoroughly penetrates the global market, however, white roofs might be expected to play a larger role in reducing this flux.

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