While the Tokyo Metropolitan Government and researchers race to gather information on the “heat island” effect in central Tokyo, the possible impact of the phenomenon on the waters of Tokyo Bay is an issue that has been largely ignored.
Urban climatology experts estimate that half of the heat released from the metropolis — from power plants, heat-retaining concrete structures and other sources — is released into the air and the remainder into Tokyo Bay.
According to Hideaki Nomura, who works at the Ocean Research Institute at the University of Tokyo, a gradual warming trend was observed in the bay between around 1950 and 1975. In this respect, Nomura cited a paper by now retired researcher Sanae Unoki, formerly of the Institute of Physical and Chemical Research in Saitama Prefecture.
Unoki, who acknowledged that urban heat could be warming the bay, veered toward the belief that any rise in temperature might be due to the vagaries of the Kuroshio current. He concluded, however, that it is impossible to tell either way.
“The problem is you cannot differentiate (between the impact of urban heat and that of the Pacific Ocean),” Unoki said. “It also takes much more heat to warm water than air, so an effect on the bay would be less likely than on the atmosphere.”
Nomura attributed the lack of any followup research based on Unoki’s findings to the dearth of experts dealing with Tokyo Bay, the voluminous data that would have to be analyzed and the monumental effort it would entail.
“It is important information and I would like to analyze it if I could find the time,” he said, adding that he hopes to tackle the issue next year.
According to Unoki’s data, the water temperature at a Tokyo Bay measuring site that has since been reclaimed rose around 2.5 degrees between 1950 and 1975. The site in question was near where Tokyo Disneyland stands today.
“Often, measuring points and depths have changed, as some have been reclaimed, so before you can analyze the data you need to determine which are valid. You need to consult a map while doing it,” Nomura said.
Moreover, the payback would be small and would take around a year to arrive, he estimated.
Other researchers say that less-comprehensive data compiled since the 1970s show no consistent trends.
“The data we have are monthly samples dating from the 1970s,” said Haruo Ando, a senior researcher at the Tokyo Metropolitan Research Institute for Environmental Protection.
“But different parts of the bay show increases while other parts show decreases.”
Furthermore, marine temperatures are complicated by land, Ando said. The land masses of Tokyo, Kanagawa and Chiba prefectures exact a heating effect on the inner bay in the summer and a cooling effect in the winter.
Meanwhile, temperatures at the mouth of the bay vary along with fluctuations in the current.
Unfortunately, neither the metropolitan government nor the Japan Coast Guard keep thorough and regular records on the bay’s water temperature, so there is no way of knowing whether or not the average temperature is rising.
Most researchers seem to think that it is.
“Tokyo Bay is getting warmer, that is for sure,” said Takashi Ishimaru, a plankton specialist at Tokyo University of Fisheries in Tokyo. “Whether that is because of waste water or global warming is difficult to tell.”
Even if the bay as a whole is not warming, isolated patches may well be.
The number of green mussels common to the warmer waters of Southeast Asia as well as swimming blue crabs appear to be multiplying in Tokyo Bay.
The shells of green mussels were first found off the coasts of Tokyo and Chiba Prefecture in 1985, with live mussels being observed near the effluent cooling water pipes of power plants along Tokyo Bay the following year and in Yokohama a year later, Ando said.
Samples of the shellfish — which must live in temperatures above 25 degrees to survive — have since been discovered in metropolitan marine animal surveys in areas between the mouth of the Sumida River and Haneda airport every year, with the exception of 1988.
This is near the area that fisherman Toshitsugu Ito of the Ota Fisheries Cooperative knows best.
“These are crabs that are found more in the south,” he said. “They say they came along with tanker boats.”
Ito has also seen evidence that the blue swimming crab is carving out its own niche — possibly at the expense of the common swimming crab.
“The common swimming crab used to overwhelmingly outnumber the blue swimming crab; now their numbers are almost equal,” he lamented. “The blue swimming crab does not sell for much.”
Ito and others believe that the crabs and the shellfish hitched rides in ships’ ballast water.
He also thinks that the warming of the bay is another factor behind the influx of tropical creatures.
“No parts of the rivers freeze and the sides of the boats don’t ice over anymore either,” he said.
Although officials of the Tokyo Metropolitan Government take exception to the argument that Tokyo Bay is getting warmer, both sides of this divide agree that there is a lack of information.
Water temperature measurements are merely one part of water quality surveys that are taken monthly and are unsuitable for long-term comparisons, explained Yoshito Mishima of the metropolitan government’s environment department.
Moreover, any statistical increase in tropical foreign sea creatures could be attributed to the robustness of foreign species or the vagaries of a particular survey, he said.
“It is hard to say whether they just happened to be caught or are really increasing,” Mishima said. “But looking at the statistics on green mussels over the past 10 years, they do appear to be increasing.
“Higher water temperatures could be the reason for this, but alternatively it could be that they have adapted to a cooler water temperature and increased. I think that the latter possibility is more likely.”