In this post I want to sketch the history of urban climatology biefly and go into further detail on how urbanisation alters climate. Last week I purchased the book The City and the Coming Climate - Climate Change in the Places we live in written by Brian Stone Jr. who is the director of the Urban Climate Lab at Georgia Tech (http://www.urbanclimate.gatech.edu/). This week's post will be heavily influenced by his book as it is dedicated solely to urban climates and one of the very few publications on the topic.
The urban heat island effect had already been recognised in the Roman Empire. During the reign of Nero, commercial activity in the capital flourished. Roads were widened to accomodate increasing traffic. It became apparent that these roads got hotter during summer. It was then recommended that streets should be made narrow and houses high for shade to reduce air temperatures.
It was not until 1818 though that the term 'urban heat island' was first used by Luke Howard. The chemist recorded 3°C warmer air temperatures in London compared to its rural surroundings (Stone, 2012). He described 'an artificial excess of heat' in the city and linked it to industrial and domestic heat production (Jankovic et al, 2012).
The field of urban climatology experienced an important development in the 1960s. In the USA a series of 'new towns' were built from scratch to promote affordable housing with more social interaction. James W. Rouse designed Columbia, Maryland with innovative neighbourhood patterns whereby he clustered housing around public schools and small commercial districts. Furthermore, biking and walking trails were designed to reduce vehicle travel. The town was to be built in a rural area with low land prices. The urban climatologist Helmut Landsberg saw an opportunity to understand the urban heat island better and started to measure air temperature with the start of construction in Columbia. In the beginning, he recorded a temperature difference of 1°C between the emerging town and its surroundings (Stone, 2012). In the next six years the population of Columbia grew rapidly from 1000 to 20 000. The urban-rural temperature difference after these six years was up to 7°C. This was the first time the urban heat island effect was so obviously proven.
Still today, research is undertaken to understand the effect of urban sprawl on climate. Stone et al (2010) used a sprawl index for 53 major American cities and climate data of Extreme Heat Events (EHEs). An EHE is defined as any day where the minimum, maximum or average air temperature is above the 85th percentile of the total temperature range in the years 1961 - 1990 in that city. Studies have shown that this is the threshold for a multitude of heat-related hospital admissions. Stone et al (2010) found that rapidly developing and sprawling cities (Phoenix or Atlanta) are more likely to experience extreme heat events than slower growing more compact cities (Boston or Chicago, see Figure 4).
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| Figure 4: Relationship between sprawl index and mean annual increase in frequency of Extreme Heat Events (EHEs), source: Stone (2012) |
The rapidly growing megacities in the developing world have a sprawl index much higher than the above mentioned American cities. The Urban Heat Island will contribute to high levels of warming in these already climate change vulnerable regions. The next blog post will examine the influence of the urban heat island in regional climates compared to the warming due to greenhouse gases.
Jankovic, V. & Hebbert, M. (2012), Hidden Climate Change - Urban Meteorology and the Scales of Real Weather, Climatic Change, Vol. 113, pp. 23-33
Stone, B. Jr., Hess, J., Frumkin, H. (2010), Urban Form And Extreme Heat Events: Are Sprawling Cities More Vulnerable to Climate Change Than Compact Cities?, Environmental Health Perspectives, Vol. 118, pp. 1425-1428
Tino I really enjoyed reading your blog, its a new concept to me. You mentioned that narrow roads and tall buildings would reduce the temperature. But don't you think narrow roads may lead to increased congestion causing road blocks and leading to more emissions/noise pollution?
ReplyDeleteHi Tanvi, thanks for your comment!
ReplyDeleteYou are absolutely right, narrow roads and high buildings reduce the potential for wind to carry away polluted air due to traffic or domestic heat production. The town structure in Mediterranean countries, for example, was not laid out for motorised traffic. This most certainly impacts people's health today.
In terms of the urban heat island itself, studies which researched the impact of the apparent CO2 dome over a city found out that the higher amounts of greenhouse gases over an urban area do not contribute to its warming significantly. For the city of Phoenix they found a warming of only 0.1°C due to its CO2 dome. (see Balling et al (2001), Does the urban CO2 dome of Phoenix, Arizona contribute to its heat island?, Geophysical Research Letters, Vol. 28, pp. 4599-4601).
During hours of sunshine places in the shadow of tall buildings do not heat up that much. Highly sprawling cities with large surface areas of impervious structures exposed to the sun are more prone to heat up. The urban heat island has its greatest impact in the evening and night hours, when all that stored heat is released from underground.
I hope this anwers your question somewhat.