Coastal flooding and wetland loss in the 21st century: changes under the SRES climate and socio-economic scenarios

This paper considers the implications of a range of global-mean sea-level rise and socio-economic scenarios on: (1) changes in flooding by storm surges; and (2) potential losses of coastal wetlands through the 21st century. These scenarios are derived from the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES). Four different storylines are analysed: the A1FI, A2, B 1 and B2 'worlds'. The climate scenarios are derived from the HadCM3 climate model driven by the SRES emission scenarios. The SRES scenarios for global-mean sea-level rise range from 22 cm (B I world) to 34 cm (A1FI world) by the 2080s, relative to 1990. All other climate factors, including storm characteristics, are assumed to remain constant in the long term. Population and GDP scenarios are downscaled from the SRES regional analyses supplemented with other relevant scenarios for each impact analysis. The flood model predicts that about 10 million people/year experienced coastal flooding due to surges in 1990. The incidence of flooding will change without sea-level rise, but these changes are strongly controlled by assumptions on protection. Assuming that defence standards improve with growth in GDP/capita (lagged by 30 years), flood incidence increases in all four cases to the 2020s due to the growing exposed population. Then to the 2080s, the incidence of flooding declined significantly to 5 million people/ year in the B2 world, 2 million people/year in the B1 world and 1 million people/year in the A1FI world due to improving defence standards. In contrast, flood incidence continues to increase in the A2 world to the 2050s, and in the 2080s it is still 18–30 million people/year. This reflects the greater exposure and more limited adaptive capacity of the A2 world, compared to the other SRES storylines.Sea-level rise increases the flood impacts in all cases although significant impacts are not apparent until the 2080s when the additional people flooded are 7–10 million, 29–50 million, 2–3 million and 16–27 million people/year under the A1FI, A2, B1 and B2 worlds, respectively. Hence, the A2 world also experiences the highest increase in the incidence of flooding. This is true under all the realistic scenario combinations that were considered demonstrating that socio-economic factors can greatly influence vulnerability to sea-level rise. The trends of the results also suggest that flood impacts due to sea-level rise could become much more severe through the 22nd century in all cases, especially in the A1FI world. Note that impacts using a climate model with a higher climate sensitivity would produce larger impacts than HadCM3.Coastal wetlands will be lost due to sea-level rise in all world futures with 5–20% losses by the 2080s in the A1FI world. However, these losses are relatively small compared to the potential for direct and indirect human destruction. Thus, the difference in environmental attitudes between the A1/A2 worlds and the B1/B2 worlds would seem to have more important implications for the future of coastal wetlands, than the magnitude of the sea-level rise scenarios during the 21st Century.These results should be seen as broad analysis of the sensitivity of the coastal system to the HadCM3 SRES global-mean sea-level rise scenarios. While these impact estimates are only for one climate model, for both impact factors they stress the importance of socio-economic conditions and other non-climate factors as a fundamental control on the magnitude of impacts both with and without sea-level rise. The A2 world experiences the largest impacts during the 21st century, while the B1 world has the smallest impacts, with the differences more reflecting socio-economic factors than climate change. This suggests that the role of development pathways in influencing the impacts of climate change needs to be given more attention.