The extensive damage left by Typhoon Hagibis that swept central and eastern Japan, bringing record-breaking rainfall that destroyed embankments on more than 50 rivers in seven prefectures and flooded nearby areas, is yet another indication that extreme weather is becoming more frequent and catastrophic, overwhelming anti-disaster measures built on past experience.

National and local governments need to fundamentally review their defense against torrential rains and flooding in light of the changing extreme weather conditions, which are feared to grow more deadly as climate change deepens.

The full extent of the damage from the deadly typhoon, the most powerful to hit Japan in decades, is still unclear days after it swept through huge swaths of the country. More than 80 people in 12 prefectures were left dead or missing — many of them drowned when their neighborhoods were flooded after river embankments in their areas collapsed following the massive rainfall.

In more than 100 locations in the Tohoku and Kanto regions, the amount of rain that fell in a 24-hour period Saturday hit record highs, and more than 10,000 houses were flooded. According to an analysis by the National Research Institute for Earth Science and Disaster Resilience, areas along the Chikuma River in Nagano Prefecture and the Abukuma River in Miyagi and Fukushima prefectures, where the flood damage was the most severe, suffered an extremely rare amount of torrential rain that is believed to happen only once every 100 years.

Anti-flooding steps are formulated on the basis of experience with past disasters. Dams and reservoirs are created to adjust the flow of water in river systems and embankments are built to prevent rivers from overflowing in the event of the kind of torrential rain that occurs only once in decades. The flooding that took place simultaneously over such broad areas across the country makes it clear that what used to be classified as extreme weather is occurring much more frequently, and that defensive measures against flooding are falling short.

Possibly due to the effects of global warming, typhoons are becoming more powerful and bringing much heavier rain. Normally, typhoons gradually lose their force as they head northward and enter areas with lower ocean temperatures. However, Hagibis appeared to maintain its brute strength as it hit central and eastern Japan at a time when temperatures in the sea around the country was 1 to 2 degrees higher than normal. Experts warn that further rises in global temperatures will increase the chances of powerful typhoons and torrential rains. Officials and others involved in efforts to defend against flooding and mitigate their damage need to take the changing weather conditions into account and respond accordingly.

The required efforts going forward will include beefing up defensive infrastructure, such as dams and river embankments, to make them capable of withstanding greater amounts of rainfall. However, there will be budgetary constraints that prevent the quick completion of such work throughout the country. Worst-case scenarios that assume flooding will be inevitable in the event of torrential rains must be anticipated and steps taken to get residents in risky areas to evacuate before disasters hit. That will require reviewing evacuation plans for people who are particularly vulnerable in times of disaster, such as the elderly and hospital patients.

This time, the government took extra steps to warn the nation of the anticipated disaster. Three days before the typhoon was forecast to hit, the Meteorological Agency called on people to take precautions and evacuate promptly when necessary. One day before it hit, the agency warned that the typhoon could cause downpours equal in scale to the storm that hit the Kanto region and the Izu Peninsula in 1958 and left more than 1,200 people dead. Still, many of the residents living alongside the flooded rivers drowned or were stranded because they did not evacuate in time. Whether the disaster alerts issued before Hagibis hit actually led people to take action to ensure their safety must be scrutinized.

Longer-term efforts to defend against natural disasters such as more powerful typhoons should include measures to raise residential areas and roads to prevent flooding or to tighten land-use regulations to prevent the building of houses and public facilities such as hospitals in high-risk areas. As we learned from the lessons of the 2011 Great East Japan Earthquake and tsunami, we need to prepare for worst-possible scenarios to ensure people’s safety when large-scale disasters strike.

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