Heat island Effect Goes Underground
7 Sep, 2007 12:21 pm
While global warming is considered a serious contemporary environmental issue, discussion of the phenomena has been limited primarily to issues above the ground or near the ground surface. However, underground temperatures are also affected by surface warming.
In addition to global warming, the “heat island effect” due to urbanization creates subsurface thermal anomalies in many cities. The recently published paper “Combined effects of urbanization and global warming on subsurface temperature in four Asian cities” by Makoto Taniguchi, Takeshi Uemura and Karen Jago-on , a team of RIHN (Research Institute for Humanity and Nature, Kyoto, Japan) in Vadose Zone Journal (Soil Sci. Soc. Amer.) revealed how much the heat island effect goes to underground.
Underground temperature is used to reconstruct past climate change because the signature of changes in ground surface temperature is preserved in underground thermal regime (Huang et al., 2000, Taniguchi et al., 2003). Recent air temperature in cities has increased by global warming and heat island effects due to urbanization. Studies on the effects of heat from urban areas on subsurface temperature were limited, and analyses of the timing of the start of urbanization using underground temperature data are also limited.
In their research, underground temperature profiles measured in four Asian cities (Tokyo, Osaka, Seoul, and Bangkok) were analyzed to infer depths of thermal disturbance and associated surface warming due to combined global warming and urbanization (heat island effects). Asian cities are extremely vulnerable due to rapid increases in population. The magnitude of surface warming is largest in Tokyo (2.8 ℃), followed by Seoul (2.5 ℃), Osaka (2.2 ℃), and Bangkok (1.8 ℃). Comparisons between theory and observations show that the mean depth of deviation from the regional geothermal gradient in each urban area may be one of the indicators of the history of urbanization in each city. In other word, the depth of deviation may show the development stage of the city (i.e. how old the city is). The mean depth of deviation from the steady thermal gradient, which is approximately 140 m in Tokyo, 80 m in Osaka, and 50 m in Seoul and Bangkok, indicates the time from the start of the additional heat from urbanization. These results agreed qualitatively with air temperature records in the cities during the last 100 years.
The heat island effect on underground temperature is an important global groundwater quality issue, because it may alter the groundwater systems chemically and microbiologically. Measurement of underground temperature data provides important information for understanding the joint effects of urbanization and global warming on groundwater systems.
Taniguchi M., et al, Combined effects of urbanization and global warming on subsurface temperature in four Asian cities, Vadose Zone Journal (Soil Sci. Soc. Amer.), August 2007
Huang, S., H. N. Pollack, P. Y. Shen, Temperature trends over the past five centuries
reconstructed from borehole temperatures. Nature, 403, 756-758, 2000.
Taniguchi, M, Shimada, J., Uemura, T., Transient effects of surface temperature and groundwater flow on subsurface temperature in Kumamoto Plain, Japan. Physics and Chemistry of the Earth, 28, 477-486, 2003.