It is intuitive that climbing global temperatures will increase the pace of polar ice melt, which will in turn raise sea levels and flood coastal lands. Recent research reveals, however, that the scale of such flooding could be exponentially worse than previously anticipated. New data suggests that many countries, many of which are in Asia, will be transformed — literally — as coastlines are redrawn by rising waters and hundreds of millions of people will be forced from their homes. Japan will be hard hit by this development and must do more to mitigate its impact.

It has long been estimated that climate change-induced flooding would force about 80 million people from their homes in low-lying areas by the midway point of this century. Hardest hit will be the millions of people who live on small islands in the Pacific and Indian oceans that are only a meter or two above sea level and will vanish as water levels rise.

Previous studies had reckoned that “only” about 80 million people would be impacted by this change. New research by Climate Central, a United States-based nonprofit organization, indicates that a more accurate number is 300 million people and that figure could rise to 500 million by the end of this century under the worst scenarios.

The darkening outlook is not the product of pessimism about the level at which temperature increases will be stopped, but rather reflects greater accuracy in estimating land elevation and the realization that much coastal land is considerably lower than thought.

Most estimates of land elevation are taken from space shuttle-generated data, but that data set often confuses land elevation with that of anything on it, like trees or houses. Climate Central modified the radar data with airplane-deployed laser measurements, artificial intelligence and nearly two dozen other variables to generate a computer model that is thought to be more accurate than the old numbers. Tests against old estimates indicate the new data is superior, although some scientists are cautious about the results.

Climate Central’s data indicates that estimates were too low by nearly two meters globally. Asia will be especially hard-hit, with eight countries — China, Bangladesh, India, Vietnam, Indonesia, Thailand, the Philippines and Japan — accounting for about 70 percent of the people affected worldwide. Under the worst-case scenario — a temperature rise of 4 degrees — the number of Chinese affected rises threefold to 87 million, and nearly one-third of the populations of Bangladesh and Vietnam would be below the high-tide line. In Bangkok, Calcutta, Hanoi, Ho Chi Minh City, Hong Kong, Jakarta, Manila, Mumbai and Shanghai, at least 20 percent of the population would be displaced.

Japan will not be spared. The new data indicates that 27 percent of the population of Nagoya will be affected by rising sea levels and 26 percent in Osaka. The new analysis buttresses a 2017 study that concluded that Japan could lose nearly all its beaches — leaving less than 10 meters in half its coastal zones — by 2065. Moreover, scientists warn that rising sea levels are only part of the danger; there is also the heightened danger of flooding from tsunami and storms, phenomena that already lash Japan.

Mitigation technologies are well known. Already some 110 million people live below high-tide levels, protected by dikes, sea walls, levees and other flood protection systems. Visitors to Japan’s coasts know those measures are prominent features of the coastline. The new data indicates that more radical measures will be required. Some island countries will have to move their entire populations, a process that is already beginning.

Governments around the world, from Florida to Vietnam, will have to develop policies to get coastal populations to migrate. The Indonesian government’s decision to move the capital from Jakarta to a new city was prompted more by its vulnerability to flooding than the city’s awful gridlock.

While the exact number of people that will be displaced is unknown — much depends on the eventual level of global temperature increase — the evidence suggests that rising sea levels in combination with storm surges will threaten most cities on major coastlines around the world. Japan must not only be ready to protect its own threatened populations, but be ready to help those elsewhere in the world that are also at risk. Providing funds, sharing technology and offering refuge will all be part of this effort. We must begin preparing now.

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