Urban Heat Island vs. Anthropogenic Greenhouse Effect

  This week's post is largely a review of chapter three in the book The City and the Coming Climate. Brian Stone introduces a study undertaken by his Urban Climate Lab to distinguish global warming due to greenhouse gases from the effects of the urban heat island.
 Stone (2012) takes us back to the year 1999. The Midwestern United States experienced a heat wave which led to the death of several hundreds of people. Figure 5 displays the temperature difference between the city of Chicago as well as the metropolitan area of St. Louis and their respective surrounding rural areas. The graph shows a mean difference of 1.1°C (2°F) to 1.7°C (3°F) for the week before the peak of the heat wave. During the last week of July 1999, the urban heat island intensified significantly. The average rural-urban temperature difference for St. Louis climbed to 2.8°C (5°F) and even up to 3.3°C (6°F) in Chicago for that week at the height of the heat wave. The temperature difference therefore doubled! Hence, urban areas have the potential to exacerbate extreme rural warming trends on the regional scale.

Figure 5: Urban-Rural Temperature Difference for 1999 Heat Wave, source: Website 1

  Since cities make up not more than 3% of the global land surface (Stone, 2012) the effect of the urban heat island is largely neglected in global warming research. An increase in atmospheric greenhouse gases has led to increasing global temperatures after the onset of industrialisation. Weather stations around the world confirm this trend. Many of these weather stations are located in major cities. They are statistically adjusted to mirror rural conditions, the urban heat island effect is therefore artificially excluded. However, focussing only on warming due to greenhouse gases greatly underestimates warming on a regional scale in metropolitan areas. As mentioned above urban centres exacerbate the underlying warming due to greenhouse gases. Stone (2012) identifies a lack of knowledge on urban climates.
  More people have died from heat stress than all other forms of extreme weather. It is therefore mandatory to get more data on the places half of the world's population lives in and where temperatures increase at a higher speed than anywhere else: our cities. Stone (2012) presents a study by the Urban Climate Lab in Atlanta, Georgia. The first problems arise when areas have to be identified as urban. Here, the night light intensity as measured from space (see Figure 6) is combined with population density data to distinguish an urban (bright lights) from a periurban (dim) and a rural place (dark).

Figure 6: Light Intensity at Night, source: Website 2

The urban heat island in the 50 most populous American metropolitan areas were monitored during a five-decade period, dating back to 1960 (see Stone, 2007 & Stone et al, 2012). Figure 7 gives an account of the rural (grey line) and urban (black line) temperature development from 1961-2010 in the US. The horizontal line at 0°F is the rural temperature average for this time period. The rural long term increase of about 0.8°C (1.4°F) is equivalent to global surface warming due to rising greenhouse gas concentrations in the atmosphere. The vertical distance between the grey and the black line represents the urban heat island. Stone et al (2012) have found an urban mean rate of warming over the 50-year period of 0.24°C (0.43°F) per decade. Rural areas warmed at a pace approximately one third less: 0.16°C (0.29°F) per decade. However 14 out of 50 cities did not show an intensifying urban heat island. These cities were largely older industrial cities with low sprawl indicies. Please read last week's post to find out about the relationship between a large sprawl of a city and its urban heat island. When only the 36 cities with a growing urban heat island are considered, the rate of warming is 0.31°C (0.56°F) and therefore double the rate of rural warming.

Figure 7: Urban-Rural Temperature Difference Over Past 50 Years, source: Stone et al (2012)

  The urban heat island effect in the majority of US metropolitan areas has the same warming potential as the rise in greenhouse gases. Major cities may therefore continue to warm twice as fast as rural areas. Heat-related deaths could still increase and will most likely do so in the megacities of the global south. An increase of concrete and asphalt in urban settings must be balanced with more green spaces to let the city breathe.

To look out for:
Beijing and Tokyo have been warming at rates five times greater than their rural surroundings in the past 50 years. In following posts I want to move away from the US for a bit and consider other continents and their urban heat islands. Architecture to counteract unwanted effects of urban warming are also still to come...

References

Stone, B. Jr. (2012), The City and the Coming Climate - Climate Change in the Places we live in. Cambridge University Press

Stone, B.Jr., Vargo, J., Habeeb, D. (2012) Managing climate change in cities: Will climate action plans work?, Landscape and Urban Planning, Vol. 107, No. 3, pp. 263-271

Stone, B.Jr. (2007) Urban and rural temperature trends in proximity to large U.S. cities: 1951-2000, International Journal of Climatology, Vol. 27, pp. 1801-1807

Website 1: http://mrcc.isws.illinois.edu/living_wx/heatwaves/heatwave1999.html (Midwestern Regional Climate Center, accessed 31/10/2012, 14:58)

Website 2: http://apod.nasa.gov/apod/ap001127.html (NASA, accessed 31/10/2012, 15:38)