Urban heat island

An urban heat island is a built-up area which is consistently hotter than its surroundings, particularly in Summer. Many cities exhibit a heat island effect, with several degrees between the center of the city and surrounding fields. Heat islands were first noticed in the 1800s.

Scientists believe this effect is caused mostly by the difference in albedo between the generally dark surfaces of a city - tarmac from roads, etc - and the vegetation that the city/suburbs has replaced. These dark surfaces absorb sunlight, heat up, and retain more of this heat than the suburban areas. A contributing factor is the lack of evapotranspiration from vegetation. Finally, hot air from vehicle exhausts and from industry heats up the air further.

One consequence of urban heat islands is the increased energy required for air conditioning and refrigeration. This is particularly true for cities that are already in comparatively hot climates. The Heat Island Group estimates that the heat island effect costs Los Angeles about $100 million per year in energy.

The heat island effect can be countered by using white or reflective materials to build houses, pavements, and roads. This is a long established practice in many countries. Another option is to increase the amount of well watered vegetation.

Global Warming

The urban heat island effect has a bearing on the global warming controversy. The historical temperature record records how the earth's temperature has changed over time, and ground-based thermometers are an important part, but not the sole part, of that record. Thermometers located near heat islands will record slightly higher values as a result of the heat island effect and this could potentially distort the record.

The view of the Intergovernmental Panel on Climate Change is that the effects of urban heat islands on the recorded temperature "do not exceed about 0.05°C over the period 1900 to 1990". This rests on various sources, contributing reasons being [1] (http://www.grida.no/climate/ipcc_tar/wg1/052.htm#222):

• land, sea and borehole records are in reasonable agreement over the last century. [2] (http://www.grida.no/climate/ipcc_tar/wg1/fig2-6.htm)
• the trends in urban stations for 1951 to 1989 (0.10oC/decade) are not greatly more than those for all land stations (0.09 oC/decade)
• simlarly the rural trend is 0.70 oC/century from 1880 to 1998, which is actually larger than the full station trend (0.65 oC/century)
• the differences in trend between rural and all stations are also virtually unaffected by elimination of areas of largest temperature change, like Siberia, because such areas are well represented in both sets of stations.

External links

• Learning about Heat Islands (http://eetd.lbl.gov/HeatIsland/LEARN/) rom the Heat Island Group
• Land-surface air temperature (http://www.grida.no/climate/ipcc_tar/wg1/052.htm#2221) - from the IPCC
• http://www.earthsci.unimelb.edu.au/~jon/WWW/uhi-melb.html