By Charles Scanlon BBC Tokyo correspondent

Tokyo may be located in the temperate zone, but you would never notice during the summer months.

Temperatures in the Japanese capital have been soaring into the mid and high 30s Centigrade - hotter than many of the tropical cities of south-east Asia.

Even the resilient salarymen are suffering

And it is getting worse every year. Tokyo is heating up at four times the pace of global warming - three degrees in the last century.

The phenomenon is blamed on rapid urban development since the World War II. The ever expanding sprawl of concrete traps the heat and blocks out cooling breezes.

'Heat island'

Even the famously resilient salaryman is showing signs of wilting.

"It's so much hotter than the countryside, you can feel the heat radiating off the asphalt," said one suited office worker emerging into the cauldron of the central business district.

Tokyo is an extreme example of what is known as a "heat island" - often several degrees warmer than the rural districts that surround it.

Tokyo lacks as many parks as other major cities

The growing use of air conditioners adds to the problem - pumping out huge volumes of hot air - as do the millions of vehicles circulating in the city.

Cases of heat stroke are on the rise, tropical vegetation is taking root and new types of mosquito are threatening to bring exotic diseases.

Finally the city authorities are trying to take action. They have identified a lack of green space as a key part of the problem - Tokyo has far fewer parks than most large cities in Europe and North America.

Roof gardens plan

The metropolitan government now requires all large new buildings to build roof gardens to help absorb the heat.

Even at night the temperatures remain high

City officials at Sumida ward in the east of the city are taking the lead in promoting new light weight roof gardens.

Twenty samples are laid out on the roof of the city hall - a soothing carpet of green to shield the building from the sun.

"We have so few parks and we're using up too much electricity for air conditioning," says a visitor from neighbouring Yokohama, who has come to inspect the gardens.

"I'm worried about global warming and I think more gardens will help cool things down."

Radical measures

The government wants to see gardens on 50% of all roofs in the urban area. It is also looking at proposals to install cooling pipes under the city and, to repave roads with material that absorbs less heat.

There are fears of tropical diseases such as malaria arriving

But professor of climatology Takehiko Mikami, who works with the metropolitan government, says more could be done.

"It's not enough - the best way to stop the heat island effect in Tokyo is to preserve and expand green spaces on the ground - not only on the roof top," he said.

But that does not appear to be happening. A new line of skyscrapers is going up in front of Tokyo Bay - a further obstacle to cool winds from the sea.

Cases of heat stroke are rising each year, tropical plants are becoming more common and so are new types of mosquito carrying the threat of malaria and dengue fever.

A once temperate city is now getting used to regular tropical nights, when temperatures remain above 25C.

If there is any compensation for the added discomfort it is that the winters are getting warmer as well.