Japanese researchers have found that the use of air conditioners in large buildings is increasing the overall temperature in big cities.

For the study, published in the Journal Applied Meteorology and Climatology, Dr Yukitaka Ohashi of Okayama University of Science and colleagues compared the summer temperatures in downtown Tokyo on weekends and that on weekdays.

The researchers noted that air conditioners dumped enough heat into the streets to raise the temperature by at least one to two degrees Celsius. They say that heat blasting from the rear ends of air conditioners is contributing to the heat island effect that makes cities hotter and their weather sometimes more severe.

"In the office areas of Chiyoda and Chuo Wards in the Tokyo metropolitan area in Japan, the waste heat resulting from the air conditioning operation is more than one-half of the sensible heat from the surface during the summer," Discovery News quoted the researchers as saying.

"In office areas in the big cities of Asian countries, this 'waste' heat from air conditioners into the urban atmosphere is significant, especially during summer," Ohashi added.

The researchers say that an AC not only moves heat from the inside to the outdoors but adds new heat to the environment also, as it is a machine that consumes power.

They reproduced real temperatures in Tokyo using two computer models coupled together -- one simulated building energy use and waste heat, while the other, air temperature in an urban environment.

The dual simulation accurately reproduced the temperature for a few days in July 2002, which were clearly warmer because of waste heat from cooled buildings.

"On the other hand, for a holiday case [in August, 2002] the waste heat from the buildings has little influence on the outdoor temperatures and can be neglected in the model," says Ohashi.

He says that to accurately predict the air temperature in any big city, meteorologists need to better understand exactly how big buildings are expelling heat.

US-based urban heat researcher Dr Stuart Gaffin of the Center for Climate Systems Research at Columbia University in New York feels that the Japanese researchers may be right in suggesting the significant contribution of air conditioning to hotter cities.

"Such heat is not fully appreciated in urban heat island discussions," he says.