Why Japan's coastal zones might be disappearing due to climate change

Climate change can cause a range of effects on coastal environments. Some of the effects are related to erosional processes such as a decrease in sediment supply, changes in the intensity and frequency of extreme events (storms and cyclones, among others), and changes in sea levels and in the wave climate. The estimation of changes due to sea level rise (SLR) and climate change is a major issue with respect to future coastal management decisions.

No one is more concerned than the Japanese, who are surrounded by seas; about 73% of Japan is forested, mountainous, and unsuitable for agricultural, industrial, or residential use, as a result, the habitable zones are mainly located in or near coastal areas, so much so that, there are growing concerns in Japan of the impact of climate change on their coastal surroundings, prompting the Japanese government to set up an Intergovernmental Panel on Climate Change (IPCC) to undertake a study on climate change, to provide future projections of coastal erosion based on representative concentration pathway (RCP) scenarios. So far, the study indicates that rising sea levels (SLR) and increasing maximum wave heights due to climate change would lead to shoreline retreat.

Japan's coasts have already undergone significant erosion due to rapid national development after World War II; future beach erosions would significantly affect areas behind the coasts where both population and property are densely concentrated. The first projection of future beach erosion all along Japanese coasts was published in 1994 by Mimura et al. (1994), who calculated that beach erosion caused by SLR would occur in values of 0.30, 0.65, and 1.00m based on the projections of the IPCC First Assessment Report (IPCC, 1990). Twenty years later, Udo and Takeda (2014) projected the rate of beach loss at SLR values of 0.1 to 1.0m using the same method as Mimura et al. (1994), further refined with a different beach data set obtained from 1/25,000 scale maps issued by the Geospatial Information Authority of Japan (GSI) (Kishida and Shimizu, 2000). In their investigation, Udo and Takeda (ibid) determined beach-loss rates of 49% for an SLR of 0.3m and 93% for an SLR of 1.0m. Yoshida et al. (2013), had projected future beach erosion using the Bruun rule (Braun, 1962) due to SLR for SRES A1B to have the greatest effect on beach erosion.

In the study by Keiko Udo and Yuriko Takeda, published in Coastal Engineering Journal on 29th May 2017, the authors projected beach losses in the 77 coastal zones throughout Japan caused by future SLR (2081 to 2100) relative to a reference period (1986 to 2005) using 21 CMIP5 models, and constructed a beach-loss curve for SLR averaged along the entire coastline of Japan. Uncertainties due to different SLR projections and sediment sizes were also taken into consideration. Finally, temporal changes in beach loss rates from 2007 to 2100 were given for the projections of GMSLR for each RCP scenario, and the histograms of mean beach width in the 77 coastal zones were projected for the future.

The beach-loss rate in the future (2081 to 2100) was projected by the Bruun rule to be 62% for the ensemble mean RCP2.6 scenario, 71% for RCP4.5, 73% for RCP6.0, and 83% for RCP8.5; the rates projected by the 21 models for RCP4.5 ranged widely from 61% (MRI-CGCM3) to 87% (MIROC-ESM). Although the effect of the spatial distribution of SLR in each CMIP5 model on beach loss rate in Japan is insignificant, the effects of differences in the SLR values among the RCP scenarios and CMIP5 models are significant. The maximum uncertainty caused by the sediment size (0.2-0.6mm) against the same SLR was 38%.

Projections of the beach-loss rate from 2007 to 2100 for GMSLR of the different RCP scenarios revealed that the rates were between 18% and 79% and differed by 60% in the near future and between 28% and 96% differed by 70% in the future. Large uncertainties were caused by the GMSLR scenario and sediment size; however, the minimum projected rate of beach loss was 18% in the near future, and this rate of loss is expected to have significant implications for coastal management. For the upper bound scenario in the near future, the projected beach width in more than half of the 77 coastal zones is 0-10 m, which would cause serious damage to coastal areas in consideration of coastal protection, environmental concerns, and beach utilization. Hence, in conclusion, the authors stated that beach loss due to SLR is an urgent issue that must be addressed through the development of better coastal management strategies to combat beach loss.