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Thursday, Aug. 7, 2008
Climate change seen in recent flooding
Two recent accidents have many people worried that something's gone wrong this year, that this summer is somehow different from the summers of their childhoods.
If they've got a nagging feeling it's climate change, they may be right, experts say.
Massive downpours have been on the rise in recent years, possibly because of global warming and the "heat island" effect created by urban areas, experts said.
Localized torrential downpours can cause sudden, massive — and sometimes fatal — flooding, including along rivers and other waterways.
On Tuesday, two waterworks employees died after their five-man crew was washed away by a sudden rush of water while working in a sewer pipe in Toshima Ward, Tokyo. A third member of the group was found dead Wednesday, but the other two remain missing. The accident occurred during a localized, intense thunderstorm.
A sudden flood also killed four people along the Toga River in Nada Ward, Kobe, on July 28, after a cloudburst hit farther upriver.
The Kobe news was particularly shocking because three of the victims were children who were playing in what was supposed to be a safe water park along the river in a residential area.
Many people believe accidents like these will only rise as the effects of climate change become more evident.
According to the Meteorological Agency, the number of days in which 100 mm or more of rain fell in some part of Japan has increased 25 percent over the past 100 years.
"We believe there is the possibility that this has been caused by global warming," said Hidehiko Isobe, a researcher at the agency.
Japan has also seen a rise in the intensity of thunderstorms lasting 60 minutes or less over the past 30 years, although the agency says three decades isn't long enough to draw long-term conclusions.
The average number of torrential rains in a year (defined by the agency as those in which at least 50 mm of rain falls per hour) was 162 from 1976 to 1987, but 238 from 1998 to 2007, the agency said.
When rain falls at a rate of 50 mm to 80 mm an hour, it can be "like a waterfall" and render umbrellas useless.
It also makes driving dangerous, the agency said.
Toshio Yamagata, a professor at the Earth and Planetary Science Department at the University of Tokyo, said global warming is likely to make torrential rain common in Japanese summers.
He said this is because of the Indian Ocean Dipole Phenomenon, where the water temperature on the surface drops in the southeastern part of the ocean but rises in the west.
In conjunction with other climate factors, the dipole phenomenon destabilizes air conditions over Japan, pushes up air temperatures and causes more heavy rain to fall in the summer, Yamagata said.
Atmospheric conditions this summer are "particularly unusual and unstable because cold air from the north is likely to rise to the upper levels of the atmosphere," he said. "Together with the dipole phenomenon in the Indian Ocean, Japan is likely to see more heavy rain this summer."
In urban areas, the heat island phenomenon is another factor pushing up temperatures, Yamagata said.
Urban areas are usually covered with concrete and asphalt, which absorb heat in the daytime and release it at night. This overwhelms any cooling that may be provided by the sparse trees, plants, ponds and farmland and combines with additional heat released by human activities and machines to turn cities into sweltering concrete expanses.
The Tokyo Metropolitan Government said temperatures in the capital have risen 3 degrees over the past 100 years, blaming this in large part on the heat island phenomenon.