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259. Flood Risk Due to Sea-level Rise

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March 23 is World Meteorological Day. The theme for 2021—The Ocean, Our Climate and Weather — also marks the start of the United Nations Decade of Ocean Science for Sustainable Development (2021-2030).

According to the World Meteorological Organization (WMO), the ocean occupies 70% of the earth's surface and has a significant impact on the world's weather and climate. It also supports the world economy, accounting for more than 90% of world trade traffic and sustaining 40% of humanity that lives within 100 km of the coast. However, global warming is said to bring about sea level rise due to volume expansion due to glacier melting and rising water temperature, threatening the lives of people in coastal areas. During the 20th century, sea level rose by about 15 cm (1.5 mm per year on average). The Intergovernmental Panel on Climate Change (IPCC) is forecasting a rise in sea level (annual average of 3-6 mm) of 30-60 cm by 2100 in the 21st century, even if greenhouse gas emissions are reduced and global warming is kept within 2°C. However, if efforts are not made to curb greenhouse gas emissions, sea level rise can reach 60-100 cm (6-10 mm per year on average).

The effects of rising sea levels are not uniform worldwide, and regional differences are widening due to factors that are not necessarily directly linked to climate change. A paper published in Nature Climate Change in March 2021 predicts that coastal populations will experience sea-level rise four times faster than the global average. The impact of land subsidence due to sea-level rise has been seen as a local issue and has not been analyzed globally. The study demonstrates that over the past 20 years, coastal population have experienced 7.8-9.9 mm sea-level, compared to a world average sea level rise of 2.6 mm each year, which is larger than the IPCC's previous reports.

In many cities located in the delta area, groundwater use, crude oil and gas mining, and reduction of sediment accumulation by upstream dams are causing rapid land subsidence. Fifty-eight percent of the world's coastal inhabitants live in the delta, which is experiencing land subsidence. As a result, coastal inhabitants face rising sea levels 3-4 times faster than the world average. In particular, the problem of relative sea level rise is becoming more serious in South, Southeast and East Asia, which account for 70% of the world's coastal population and have huge and expanding urban areas on the subsidence delta. According to the authors, during the 20th century, some parts of Tokyo experienced land subsidence of 4 m (average of 40 mm per year), and Shanghai, Bangkok, New Orleans and Jakarta experienced land subsidence of 2-3 m (average of 20-30 mm per year). In Tokyo, Shanghai, and Bangkok, groundwater use has decreased in recent years, causing little or slowing of land subsidence, but in other cities there are no major changes that slow down land subsidence.

Rising sea levels and increasing frequency and intensity of intense typhoons and hurricanes increase the risk to low-altitude coastal areas and islands, especially at high tide. JIRCAS is pursuing a project on "Climate change measures in agricultural systems" in the delta region of Southeast Asia, developing adaptation measures and economic evaluations for disaster damage caused by extreme phenomena.



A global analysis of subsidence, relative sea-level change and coastal flood exposure, Nature Climate Change (2021). DOI: 10.1038/s41558-021-00993-z , dx.doi.org/10.1038/s41558-021-00993-z

Contributor: IIYAMA Miyuki (Research Strategy Office)

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