New research suggests that climate change will worsen the urban heat island effect as this century progresses, as a combination of accelerated urban growth and global warming pushes city temperatures much higher than surrounding rural areas, especially at night. Based on a new model developed by the UK’s Hadley Centre, some of the findings included:
>The effect is most extreme in dry, subtropical regions, where the differences between night-time and daytime temperatures are greatest.
>Regions of high population growth will coincide with regions of high urban heat island potential, most notably in the Middle East, the Indian sub-continent, and East Africa.
>Warming and extreme heat events due to urbanisation and increased energy consumption are simulated to be as large as the impact of doubled CO2 in some regions.
>Climate change increases the disparity in extreme hot nights between rural and urban areas (up to 5.6C in some areas).
“If you’ve been exposed to hot temperatures during the day and you expect relief over night, that becomes increasingly difficult as temperatures at night get warmer,” says Betts. “We have to prepare to live in a warmer world.” (quoted here)
Health Effects
UHIs have the potential to directly influence the health and welfare of urban residents. Within the United States alone, an average of 1,000 people die each year due to extreme heat. As UHIs are characterized by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. Research has found that the mortality rate during a heat wave increases exponentially with the maximum temperature, an effect that is exacerbated by the UHI. The nighttime effect of UHIs can be particularly harmful during a heat wave, as it deprives urban residents of the cool relief found in rural areas during the night.
Research in the United States suggests that the relationship between extreme temperature and mortality varies by location. Heat is more likely to increase the risk of mortality in cities at mid-latitudes and high latitudes with significant annual temperature variation. For example, when Chicago and New York experience unusually hot summertime temperatures, elevated levels of illness and death are predicted. In contrast, parts of the country that are mild to hot year-round have a lower public health risk from excessive heat. Research shows that residents of southern cities, such as Miami, tend to be acclimated to hot weather conditions and therefore less vulnerable to heat related deaths.
Increased temperatures and sunny days help lead to the formation of low-level ozone from volatile organic compounds and nitrous oxides which already exist in the air. As urban heat islands lead to increased temperatures within cities, they contribute to worsened air quality. UHIs also impair water quality. Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Rapid temperature changes can be stressful to aquatic ecosystems.
About the heat island effect
An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak.
Seasonally, UHI is seen during both summer and winter. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat. Waste heat generated by energy usage is a secondary contributor.
As population centers grow they tend to modify a greater and greater area of land and have a corresponding increase in average temperature. The lesser-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.
Increases in the death rate during heat waves has been shown to increase by latitude due to the urban heat island effect.
Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increases the length of growing seasons, and decreases the occurrence of weak tornadoes. The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams, which stresses their ecosystems (source: wikipedia)
