1 CLIMATE CHANGE AND COASTAL DEFENCES IN TOKYO BAY Sayaka Hoshino 1 , Miguel Esteban 2 , Takahito Mikami 3 , Tomoyuki Takabatake 4 , Tomoya Shibayama 5 Sea level rise and an increase in typhoon intensity are two of the expected consequences from future climate change. In the present work a methodology to change the intensity of tropical cyclones in Japan was developed, which can be used to assess the inundation risk to different areas of the country. An example of how this would affect one of the worst typhoons to hit the Tokyo Bay area in the 20th century was thus developed, highlighting the considerable dangers associated with this event, and how current sea defences could be under danger of failing by the end of the 21st century. Keywords: storm surge; sustainability of coastal defences; sea level rise, typhoons INTRODUCTION Every year, Japan is attacked by a number of tropical cyclones, some of which can be very strong and cause widespread damage. Apart from wind damage, these events also generate powerful waves and storm surges, which can inundate coastal areas and lead to the destruction of property and the loss of lives. Global warming as a consequence of increasing concentrations of greenhouse gases in the atmosphere could lead to an increase in the intensity in tropical cyclones in the future, which would compound the problems already presented by sea level rise (also expected to accelerate in the course of the 21st century). Tropical cyclones need high surface sea temperatures to form (typically at temperatures of over 26°C), and use the heat from the evaporation of sea water to maintain or increase their strength. Once they move over land or into colder areas their power starts to wane, and hence it appears logical that future increases in global temperatures will increase the intensity of these events. A number of authors (such as Knutson and Tuleya, 2004, Elsner et al., 2008, Landsea et al., 2006, Webster and Holland, 2005) have carried out research using supercomputers showing how it appears likely that tropical cyclones will increase in intensity in the future. However, the IPCC reports that although there is general agreement that tropical cyclones are likely to increase in intensity there is no consensus yet on the future frequency of these events. Also, it is believed that there is a certain cyclical variability in the natural patterns of tropical cyclones, which could be due to short-scale decadal cycles such as the El Niño-Southern Oscillation, or ENSO (Mousavi et al., 2011). This short term variations make assessments of future increases in intensity difficult, and Pielke (2007) reports how nine of the leading scholars on tropical cyclones and climate change give estimates ranging from a 0 to 36 per cent increase in tropical cyclone intensity by the year 2100. More recently Knutson et al (2010) summarised all the most important work on tropical cyclone simulations, including recent research that was done using higher resolution models than those used in the work that led to the IPCC 4AR. This review appears to suggest that the intensity of tropical cyclones in the future could increase by between 2 and 11% by 2100, depending on the simulation used and the part of the world concerned. It is important to emphasize how Knutson et al. (2010) find that the higher resolution models predict higher increases in tropical cyclone intensity than those with lower resolution, and thus it is possible that the models presently employed might be underestimating potential future problems. However, an intensification of tropical cyclones is not the only potential factor that could increase the inundation of low-lying coastal areas in Japan. During the 20th century the global average sea level rose by an average of around 1.7mm per year, with satellite observations showing that since 1993 sea level has been rising at a rate of around 3mm per year, according to the 4th Assessment Report of the Intergovernmental Panel on Climate Change, or 4th IPCC. Future projections of sea level rise show that sea level could be between 0.18 and 0.59m higher than at present by the end of the 21st century. As at present CO 2 emissions continue to increase global temperatures are also likely to continue to increase, and thus it appears that a significant amount level of sea rise is inevitable unless drastic action is taken to reduce emissions. Furthermore, the IPCC 4AR notes that “atmospheric CO 2 will continue to increase 1 Waseda University, 3-4-1 Ookubo, Shinjuku-ku, Tokyo 169-8555, Japan, sayahoshi0905@goo.jp 2 Waseda University, 3-4-1 Ookubo, Shinjuku-ku, Tokyo 169-8555, Japan, esteban.fagan@gmail.com 3 Waseda University, 3-4-1 Ookubo, Shinjuku-ku, Tokyo 169-8555, Japan, t.mikami@asagi.waseda.jp 4 Taisei Corporation, Japan 5 Waseda University, 3-4-1 Ookubo, Shinjuku-ku, Tokyo 169-8555, Japan, shibayama@waseda.jp