And it gets worse every year. The number of nights when temperatures stay above 25 Celsius (77 Fahrenheit) in Tokyo has doubled over the last 30 years, while average temperatures have shot up by 2.9 degrees C over the last century. Relief, however distant, could be on the way.

At the behest of the Construction Ministry, the panel has drawn up a plan that would use a network of buried pipes, and water pumped from the sea, to cool things down. "In the very best conditions, certain areas could in theory become as much as 2.6 degrees Celsius cooler," said Yujin Minobe, a ministry planner.

The huge air-conditioning systems currently used to cool buildings get rid of the heat they take out of the structure by venting it into the outside air, raising temperatures still further and creating a "heat island" phenomenon in large cities.

Under the plan, this heat would be transferred to water in large underground tanks, and the water then pumped through a six-km (3.7-mile) network of underground pipes to a cooling plant on the Tokyo waterfront.

There the heat from this water would be transferred to cooler sea water before the then-cooled water was pumped back through the underground pipes. The sea water, now warmed, would be released into the waters of Tokyo Bay.

COSTLY PLAN Minobe said the plan would cover some 123 hectares (304 acres) in the centre of Tokyo, including the Marunouchi business district and the posh Ginza shopping area, and would initially cost around 41 billion yen ($344 million).

"Savings on reduced energy usage would eventually help pay for this," he said. Officials quoted in the English-language Japan Times said energy savings would total more than 1 billion yen a year, meaning the system would pay for itself in a bit over 30 years.

However, Minobe said many problems remained with the plan, which has only been under discussion since April last year. One of the most serious problems is whether warmer water being returned to Tokyo Bay would damage the fragile marine ecosystem, a point Minobe said still required more study.

He said the average temperature cut is likely to be only around 0.4 degrees. "I'm not even sure people would be able to feel that difference," he said. Any such plan, however, would likely produce a gleam in the eyes of Japan's huge construction industry, known for its propensity for public works projects. Although several are decried as wasteful, public works projects have long been used by the government in attempts to stimulate the economy.