Osaka, like much of Japan, is highly vulnerable to natural disasters. From wind and rain typhoons to earthquakes and tsunamis, the Japanese government spares no effort on flood defence. Sea level rise means scientists must reevaluate their current strategies

Earth.Org has mapped what severe flooding could look like in Osaka by 2100. 

Osaka has been an important economic hub in Japan since the year 300, and is the second largest metropolitan area in the country today with 20 million inhabitants. According to a 2008 OECD report, Osaka is one the most vulnerable port cities in the world, risking USD $200 billion in damage from coastal flooding. 

Sitting on low-lying flatlands, Osaka was reminded of its exposure to paralysing floods on September 4th, 2018 as typhoon Jebi made landfall. 11 people were killed and 700 more injured while a massive oil tanker crashed into a bridge, and the Kansai international airport was flooded.  

The Japanese are no strangers to natural disaster, having dealt with some of the worst earthquakes, tsunamis and heavy flooding. Their flood defence systems are carefully built based on scientific models, but these must be updated to keep up with recent advances. The dykes and flood gates on the Yodo River, which runs through the city, can withstand a 1-in-200 year flood (whose intensity has a 0.5% chance occurring in any given year). However, sea level rise is changing the baseline, and will eventually shift a 1-in-200 to a 1-in-100 and so on. 

Japan wrote a law in 2015 requesting municipalities to prepare for 1-in-1000 year scenarios, before the latest reports predicting a 1 to 2 metre rise in sea level. The costs will be huge, but the alternative could cost even more, adding the financial to the human cost. Earth.Org has modelled what severe flooding would look like in Osaka by 2100, to illustrate the extreme necessity of prevention. 

Sea Level rise 2100 Osaka

Sea level rise projections by 2100 for two scenarios with the amount of rise in meters indicated (mild = 2m; extreme = 4m). Percentage and total population displacement indicated bottom right.

Methodology

Global mean sea level is projected to rise by 2m at the end of this century. However, in order to determine local sea level rise (SLR), one has to take into account local coastal flood levels which could be 2.8m above Mean Higher-High Water (MHHW) at extreme forecasts. These local levels bring variability to the projected SLR from 1m to 6.5m (eg. Rio vs Kolkata).

The SLR scenarios used in this study are based on the forecasts from Climate Central – Coastal Risk Screening Tool  with the following parameters:

Sea level Projection Source: is from two highly cited journals by Kopp et al., estimating SLR mainly due to ocean thermal expansion and ice melt. The mid-range scenario projected 0.5-1.2m of SLR based on different representative concentration pathways (RCP) defined by the IPCC. While the pessimistic scenario added more mechanisms of ice-sheet melting, estimating SLR at 1m-2.5m in 2100, with projection of 10m SLR at 2300.

Coastal Flooding: More frequent coastal flooding is a direct impact of sea-level rise. Based on the Global tides and surge reanalysis by Muis et al., (2016), it is estimated that the extreme coastal water level could be from 0.2 – 2.8m over mean level. While in extreme cases like China and the Netherlands it could experience 5-10m of extreme sea levels. Here, the coastal local flood level is added on top of the projected SLR.

Pollution Scenario: allows to choose the RCP, the greenhouse gas concentration trajectory defined by the IPCC.  The mild level is based on RCP4.5, of 2°C temperature rise; while Extreme level is based on RCP 8.5, of 4°C temperature rise.

Luck: applies to the the baseline SLR, defined in the “Sea level projection” section, upon which we add flooding. “Mild” refers to the mid-range scenario of 0.5-1.2m, and “extreme” to the pessimistic scenario of 1-2.5m. We used the high-end value of each scenario (mild = 1m; extreme = 2.5m).

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