Discussion

Before drawing conclusions, it is first necessary to address a few shortcomings of the data set and data analysis techniques. First, in the replication of Mackey et al.’s experiment, it is important to realize that many of the sources of error associated with their data set apply to this study including the atmospheric effects of light haze on albedo and temperature and the unclear relationship between remotely-sensed surface temperature and on-ground air temperature. Mackey et al. address these issues heavily in the discussion section of their paper and one should refer to it for a clarification of these issues.

Another major shortcoming is that the building file obtained from the City of Chicago includes all of the city’s buildings as of 2010 while the temperature and albedo change data sets document changes happening from 1995 to 2009. Accordingly, the discrepancies between the time frames of the two data sets may result in some error. However, since all new reflective roofs occurring with new construction and renovation are included as buildings in the 2010 shape file, this error should be small for the type of analysis that this study is employing. The only buildings that would generate error for this type of analysis are the new buildings constructed between 2009 and 2010, which are few in number and should not heavily alter analyses over the whole city.

Perhaps the greatest error resulting from this study’s methodologies occurs as a result of counting any 30-meter pixel with a building footprint in it as a “building pixel.” This results in pixels that are actually mixed in reality with many types of materials to be counted fully as building roofs for the purpose of this study. This error was seemingly necessary since over 95% of buildings in Chicago cover an area smaller than a 30-meter pixel and an analysis that isolated only pixels fully enclosed by building footprints or even 50% enclosed would not be representative of the previously-observed changes over the whole city.

The assumption that all albedo changes in these mixed pixels have resulted from roof changes could be especially dangerous if albedo increases occurring in the city were the result of something occurring directly adjacent to buildings. For example, the re-paving of sidewalks to be more reflective, the replacement of dense vegetation around buildings with lighter grass, or the drying out of dark moist soil around buildings could all increase the albedo of these mixed pixels. However, given that the width of a sidewalk is typically very small in relation to a 30-meter pixel and the intense cooling associated with albedo increase (noted in the first 4 figures) would not exist if dense vegetation or moisture had been removed, it is probably safe to rule out these alternate explanations as the drivers of the large albedo changes. Accordingly, the hypothesis that the changes are related to roof alterations seems the most compelling explanation considered, although there may be other explanations that the author has not considered yet.

Arguably, the most valuable conclusion that can be derived from this study is the quantitative justification that the large albedo increases occurring in Chicago between 1995 and 2009 are likely the result of new reflective roofs. The second part in which albedo increases are assessed in terms of building characteristics seems less helpful, mainly because of the weakness of the correlations and because the conclusion that can be drawn from them is fairly obvious: that larger, flatter buildings tend to be optimal for increasing the albedo of 30-meter pixels. Future research might focus on the relationship between albedo changes and social criteria describing the inhabitants of the buildings, such as building occupant income, age and race.

For the time being, the cause of the albedo increases seems to be identified and, at this point, it may be worthwhile for the city of Chicago to consider intensifying its reflective roof codes or starting a white roofs campaign. Perhaps even a “white city” or “cool city” campaign to complement the “green city” vision it has been marketing itself with might be a worthwhile agenda that could reduce the city’s heat island while preparing it for the climate changes to come.

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Contents:

Discussion