Tokyo is becoming more of a jungle every year -- meteorologically speaking. As metropolitan temperatures continue to climb annually, there are signs that temperate Tokyo is becoming more tropical.

Tokyo's average temperature has jumped 2.9 over the past century -- outpacing global warming by about a factor of five, experts say. Meanwhile, the number of "tropical nights," during which the temperature fails to slip below 25, has climbed steadily and the number of hours the city bakes in 30-degree heat or more each summer has doubled in less than 20 years.

But this is old hat. The heat island phenomenon has been understood for more than a century and at least one paper was written about it by a researcher in Tokyo as early as 1930. What is new is that Tokyo has gotten so hot that it creates its own weather, hosts subtropical flora and fauna, and -- some say -- poses a threat to the health of Tokyo residents as well as Tokyo Bay.

On a typical summer day, the "heat island intensity," or the difference between the temperature in the heart of Tokyo and that of the suburbs, often shows central Tokyo to be 2 to 3 degrees hotter than surrounding areas.

"But this is even greater at night and in the winter; it is just that it is cooler at these times so people don't notice it as much," said climatologist Takehiko Mikami. "In the winter, central Tokyo can be 7 or 8 degrees hotter than the suburbs."

From summer 1998 to summer 1999, heat island intensity averaged 5.2, the Tokyo Metropolitan University professor said. Tokyo now hosts subtropical plants and animals typically found in subtropical climates.

Trees such as the "shuro," a hemp palm tree indigenous to warmer climes such as southern China that can only survive if the ground does not freeze, have put down roots around Tokyo, such as around Meiji Shrine. Likewise, birds like the "inko," a type of bright green parakeet, began to appear in Tokyo around a decade ago and now number around 700, experts say.

But while heat islands exist wherever there are people bustling about en masse, Mikami said that Tokyo's has some unique twists.

For one, the hottest area is not always the city center hub of human activity. When the wind blows in off of Tokyo Bay in the middle of the day, it often carries hot air generated near Otemachi inland toward Nerima Ward, Mikami said.

A second unique feature is Kanpachi Gumo, or Loop Number 8 Clouds. The Tokyo Bay breeze clashes with a similar cool wind blowing in from Sagami Bay. Add the city's urban thermal energy and you have a recipe for clouds that crown this road, Mikami said.

While this may be an interesting phenomenon for meteorologists, for drivers it can mean wet and dangerous conditions.

"In the summer at around 2 or 3 p.m., when the heat island is centered around Kanpachi near Nerima and Suginami wards, you get the strongest updrafts. So the polluted air from central Tokyo gathers there and acts as a seed for high rain clouds," Mikami said.

Add a third -- more fickle -- sea breeze that blows in from the Pacific Ocean over the Kashimanada coast and you get wind from three directions creating a towering mass of thunderclouds. This can translate into violent bursts of rain of sometimes more than 100 mm per hour in very limited areas, enough to inundate and threaten the city with flash floods.

Nerima and Suginami wards have been known to get pounded with more than 100 mm of precipitation per hour while neighboring areas such as Nishi Tokyo receive little more than a sprinkle.

However, within the heat island of Tokyo are oases of green, such as the Imperial Palace and parks. Mikami has documented the cooling effects of Shinjuku Gyoen on the surrounding area, showing that after the sun goes down cool air seeps out and chills the urban area within about 100 meters of the park.

Mikami and other experts warn that if urban warming continues, there is a possibility that mosquito-borne tropical diseases such as malaria could gain a foothold in the city. "Tokyo is a very big city with a lot of anthropogenic energy," said Toshiaki Ichinose, a senior research scientist specializing in urban environmental systems at the National Institute for Environmental Studies.

Research indicates that the three big causes of urban warming are an increase in heat emissions from human activity, a reduction of green spaces and water surface area, and heat retention in surfaces such as concrete and asphalt, Ichinose said.

But "thermal pollution" is not released solely into the atmosphere. Ichinose's calculations show that nearly the same amount of heat released into the air is poured, in the form of warm water, from waste-water power plants, bathrooms and kitchens into Tokyo Bay.

This could have dire consequences for the bay, as warmer temperatures could change the composition of plankton and wreak havoc on Tokyo Bay's ecosystem, maybe even killing it, warn Ichinose and others.

In terms of health threats, the number of victims of heat stroke in Tokyo has nearly tripled since 1985, jumping in correlation to temperature and the number of tropical nights, according to a government study released in early August.

Some experts have detected changes in red blood cells at high temperatures, and the number of stroke victims could jump in tandem with temperatures.

In addition, high temperatures exacerbate air pollution, especially in the winter when a "dust dome" is created and air stagnates in the city. The heat island effect obstructs the ocean breeze, preventing it from flushing out hot air and cooling the city.

Some experts, including Ichinose, are calling for Tokyo to create "wind paths" to let the ocean wind cool the metropolis.

Tokyo's record temperature of 39.1 was logged in August 1994. However, the mercury could be pushed over 40 in the next decade, said Yasunobu Ashie, a member of the environment research group at the Building Research Institute in Tsukuba, Ibaraki Prefecture.

"If trends in energy consumption, vegetation disappearance and floor space observed over the past decades continue and the meteorological conditions (of Aug. 3, 1994) recur, our model shows most of central Tokyo will hit 40," he said.

Ashie has also helped to develop moisture-retaining building materials from stone dust and sludge to be placed on roofs or used for roads. He contends that a policy mix of using these materials and boosting vegetation is needed to address the heat island effect. The Tokyo Metropolitan Government has passed legislation mandating that buildings of more than 1000 sq. meters contain greenery comprising at least 20 percent of their area.

In addition, this summer it started experiments with grass parking lots and water-retaining road materials.

The Environment Ministry has requested 40 million yen to streamline Ashie's codeveloped Urban Climate Simulation System model for use by municipalities as well as evaluate the benefits of certain measures.

Interest in the heat island rises and falls with the temperature, but hang on to your air conditioners, because Tokyo's urban jungle is only going to get more tropical.