Sunday, August 30, 2015

Rising seas mean less bridge clearance

One of those things that seem obvious in hindsight.

After leaving its berth on a canal in Tokyo’s Shinagawa district, a yakatabune roofed pleasure boat operated by Funayado Nakakin very narrowly cleared the underside of the Tennosu Bridge as it slowly passed under it. The young boatman’s head cleared the underside of the bridge, which was just 2.7 meters above the surface, by just a few centimeters.

“We really have to watch out for this bridge,” he said nervously.

Kazuo Yamada, a boatman belonging to another yakatabune operator, says he has been navigating the same canal for 20 years. He says about twice a year, during the high tides of summer and autumn, he is unable to pass under the Tennosu Bridge, which forces him to forgo operating boat trips. Higher temperatures in summer cause the seawater to expand and consequently push up the canal’s water level.

But in the past 10 years there have been days outside of summer and autumn when the water level has been too high for the boat to pass under the bridge. Yamada now cancels about 10 boat trips a year.

“The rising water levels in recent years are abnormal,” the 52-year-old president of Nakakin said.

According to the Japan Meteorological Agency, the average yearly water level in Tokyo Bay rose by about 15 centimeters between 1951 and 2013, while the rise measured at 16 other locations across the country in the past 50 years is also around 15 centimeters.


In the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), experts concluded that the world’s sea level has risen almost consistently for about 110 years to 2010, with a world average increase of 19 centimeters. At the end of this century, scientists forecast an additional global rise of 63 centimeters — with a rise of as much as 70 to 80 centimeters forecast in the seas around Japan.

The melting of ice caps, especially in Antarctica, are believed to be causing rising global water levels, while the hotter temperatures, which cause seawater to expand, also contribute. Changes in water levels vary slightly between regions due to the effects of currents and other factors. However, the meteorological agency maintains that “because the world’s oceans are all connected, there is a risk that the trend of rising [sea levels] will continue in waters around the country as well.”


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