Heat islands and global warming

A weather station located in a large urban area will consistently show higher temperatures than a weather station located in the open countryside. If the temperature record is being used to detect signs of climate change, this is not necessarily important, because both sets of readings will change together. They will be in step, so that any warming or cooling trend should appear in both.

There is a problem, however, where a city spreads to engulf a weather station in a formerly rural area. The record from that station will then show a rise in temperature that has nothing to do with genuine climate change, but is caused by the urban heat island effect. Climatologists searching for evidence of climate change must remove this bias from the record.

They have been quite successful at this in many places. Where there are two reporting stations in the same area, one in an urban and the other in a rural location, their records can be compared. If the urban station reports a rise in temperature that is not detected at the rural station, the rise is attributed to the heat island effect and is ignored.

The method is more difficult to apply in areas that have become urbanized only recently. This is because a period of between 10 and 15 years must elapse before the heat island warming becomes evident against the natural variation in temperature. During this time the station may record a rise in temperature that cannot be clearly identified as due to urbanization. There is no way to remove this bias and therefore recent temperature trends have to be adjusted by an estimated amount. Studies have shown that urbanization has produced an apparent increase in temperature of approximately 0.09°F (0.05°C) in the period 1900-90 and the IPCC climate scientists have allowed a margin of error to produce an estimate of a heat island effect of 0.22°F (0.12°C) by 2000. This is an estimate, however, and some critics feel it is too low. They maintain that cities have spread faster and wider than the IPCC allows and that urban heat islands are contributing more than 0.22°F (0.12°C) to the warming trend.

Scientists know that any village with a population of more than about 1,000 will have a heat island, raising the average temperature to 3.6-4.5°F (2-2.5°C) above that of the surrounding countryside, and that even an out-of-town retail park will produce one. The magnitude of the effect is directly linked to the size of the population and can reach a maximum of almost 22°F (12°C).

A study based on measurements of infrared radiation by the Advanced Very High Resolution Radiometer carried on the NOAA-9 satellite and

Heat islands and global warming

similar data from the NOAA-14 satellite revealed the scale of the urban heat island effect over the city of Houston, Texas. The data showed that between 1987 and 1999 the nighttime surface temperature at Houston increased by 1.46-1.49°F (0.81-0.83°C) and that the rate of increase was proportional to the 30-percent increase in population during this period. The warming was due to the urban heat island effect and not to any wider change in the climate.

Urban heat islands are real. They do increase the temperature above that of their rural hinterland and it may be that their influence on global temperatures is being underestimated.

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