C.J.G. (Jon) Morris
School of Earth Sciences, The University of Melbourne, Victoria, Australia
The results from a recent study in Melbourne have found that the central business district (CBD) and industrial suburbs (IS) retain stored heat from the previous day until sunrise the following morning. This explains why these areas are consistently warmer at night than the outer suburbs and surrounding rural environments. Now climatologists are carefully examining the climate record from Australia's towns and cities to find out whether urban areas have influenced our understanding of the way our climate has changed. The question being asked is, `Have urban areas contributed to the observed trend of warming temperature and influenced our knowledge of global warming?'
The warm nights are due to what has been called the Urban Heat Island (UHI) effect, which is the result of two main features of urban areas. First, buildings, roads and paved surfaces store heat during the day, which is then released slowly over the evening due to the thermal properties of the surface materials and the building geometry which traps the heat stored during the day. The second contributing factor to the UHI is due to the artificial heat released into the urban atmosphere by combustive processes from vehicles, industrial activity and the heat that escapes from commercial and domestic air conditioning.
Using a dataset complied from twice daily observations from 1985 to 1994 from weather stations maintained by the Victorian Environmental Protection Authority and the Australian Bureau of Meteorology, results indicate that just prior to sunrise the CBD and IS are on average 4.0 deg C warmer during the summer months and 3.2 deg C warmer during the winter months. Interestingly over the 10 year period the summer months indicated that the CBD was the warmest area, whilst during the winter months the industrial area of Dandenong was 0.3 deg C warmer than the CBD.
These values for the UHI represent its average intensity during all of the weather events between 1985 and 1994. The values have been adjusted to a reference level to account for the effects of topography. The study has also found that the UHI is most pronounced when the wind speed in the CBD and at the airport is less than 3 m/s. On some occasions when there is little or no cloud and wind speeds below 1.5 m/s the heat island may be as high as 10 deg C around midnight. During very windy evenings the heat normally retained by the urban area is dispersed more easily which results in a smaller difference in temperature between the CBD and the outer suburbs.
The research of Melbourne's UHI is part of a larger project which is investigating the influence of urban areas on our knowledge of climate change. The global warming issue has been well documented in the media. Knowledge of the observed temperature warming has been derived from a variety of sources, the most important of which is the land based observation network of weather stations maintained by the Australian Bureau of Meteorology. One of the problems with the observed data is that some of it is from weather stations that have been located in the centre of small urban areas.
Whilst large urban areas such as Melbourne have been excluded from the dataset used to determine climate change, the UHI in the small towns may still cause a unrealistic warming in the temperature records used to determine climate change. The research into Melbourne's UHI has been used to more fully understand the physical processes that occur in small towns. The problem with the temperature records from small towns is if climatologists use these records to examine climate change then what they may actually be seeing is how the urban area has warmed as the town has grown, rather than how the larger scale climate has changed.
Whilst climatologists now think that the warming in the temperature record from some small urban areas is partly the result of the UHI, this is not evidence that Australia's climate has remained unchanged rather than warmed over the past 100 years. Average minimum temperatures from many stations over most of Australia have shown an increase of between 0.1 deg C and 0.3 deg C per decade since 1951. Whilst some temperature records from small towns do not represent the large scale climate, it is unlikely to have any major impact upon our estimates of temperature warming over Australia. This is because there are numerious other weather stations located in remote areas such as lighthouses and regions far removed from urban areas that still indicate a warming temperature trend. The research currently taking place in Melbourne and a limited number of small towns in south eastern Australia aims to improve the quality of these particular town's climate records by accounting for the UHI effect in their temperature record and the confidence climatologists have in accurately monitoring and assessing climate change.
This research is currently being undertaken in the School of Earth Sciences at The University of Melbourne, Melbourne, Victoria, Australia.