Increasing flood risk and wetland losses due to global sea-level rise: regional and global analyses
Introduction
The balance of scientific evidence now suggests that anthropogenic emissions of greenhouse gases are having a discernible effect on the earth's climate (Houghton et al., 1996). These effects are expected to intensify in the 21st Century with a range of climatic effects, including an acceleration in global sea-level rise (Warrick et al., 1996). Regional and global perspectives on the potential impacts of climate change are required for a range of purposes, including communicating the likely implications of different climate change scenarios to a non-specialist audience, examining the costs and benefits of different combinations of mitigation–adaptation policies, and identifying regions where collective action could be beneficial (Nicholls and Mimura, 1998). Given that 21% the world's population already live within 30 km of the coast (Gommes et al., 1997) and these populations are growing at twice the global average (Bijlsma et al., 1996), the potential impacts of sea-level rise are an important focus for such assessments.
The DETR-funded Fast Track Programme has examined the potential regional and global impacts of climate change on terrestrial ecosystems, human health, water resources, food supply and coastal areas (Parry et al., editorial, 1999). This paper presents details of the coastal analysis, which uses two models for improved analyses of the potential impacts of global sea-level rise scenarios for:
For the purposes of this analysis, all other climate factors are assumed to be constant. However, it is recognised that both regional variations in sea-level rise and changes in surge characteristics could have important influences on these impacts (cf. Warrick et al., 1996). In addition to climate change, increases in population and the standard of flood protection (using gross national product per capita (GNP/capita) as an “ability-to-pay” parameter) are considered. This allows the climate change scenario to be imposed upon a world that is evolving without climate change (i.e., an evolving reference scenario). In both cases, relative and absolute impacts are evaluated for 2025, 2055 and 2085, representing the 2020s, 2050s and 2080s, respectively.
The Second Assessment report of Working Group II of the Intergovernmental Panel on Climate Change (IPCC) concluded that accelerated sea-level rise due to greenhouse gas-induced changes of climate could have important impacts on coastal populations and ecosystems (Bijlsma et al., 1996). According to Hoozemans and Hulsbergen (1995), about 200 million people lived in the coastal flood plain (defined as beneath the 1 in 1000 year storm surge elevation) in 1990. In the developed world, people in such locations are generally protected from flooding by structural measures such as dikes and flood barriers. However, many people in such locations in the developing world are subjected to regular flooding with consequent disruption and economic loss, and at the extreme, severe loss of life as occurred in Bangladesh in 1970 and 1991 (see Nicholls et al., 1995a).
In the 21st century, global sea-level rise will raise flood levels and hence increase flood risk (Hoozemans et al., 1993; Hoozemans and Hulsbergen, 1995; Bijlsma et al., 1996). The number of people who experience flooding will also be affected by other factors such as increasing populations within the coastal flood plain. As already noted, coastal populations are already large and growing rapidly, often in urban settings (Nicholls, 1995a). Subsidence (which produces a local to regional relative sea-level rise) also enhances coastal flooding and in certain geological settings it is often exacerbated by human activity (Holzer and Johnson, 1985). Osaka, Tokyo, and Shanghai have subsided several metres during the 20th Century due to excessive groundwater withdrawal, and similar problems are now recognised in other large coastal cities such as Tianjin, Jakarta and Bangkok (Nicholls, 1995a). Such changes are expected to continue into the 21st Century. However, these increases in flood risk can be offset or even reversed if flood protection of these vulnerable populations is upgraded, or other approaches to flood management are implemented. Such changes are already happening without any consideration of sea-level rise and climate change — they are simply an adaptation to present climate variability. For example, the incidence of coastal flooding in the United Kingdom has declined substantially during the 20th Century (compare Steers, 1953; Steers et al., 1979). Similar trends are apparent in other developed countries. It is useful to distinguish such changes from adaptation to global sea-level rise induced by climate change, which would involve additional action.
Coastal wetlands (collectively comprising saltmarshes, mangroves and intertidal areas) could experience substantial losses given sea-level rise (Hoozemans et al., 1993; Bijlsma et al., 1996). These areas are highly productive and provide a number of important functions such as flood protection, waste assimilation, nursery areas for fisheries and nature conservation. Therefore, wetland loss has a high human cost. This is not widely perceived and wetland areas are already declining: about 1% of the global coastal wetland stock is lost each year, primarily by direct human reclamation (Hoozemans et al., 1993). Significant losses are likely to continue without climate change, but they will be exacerbated by sea-level rise.
Section snippets
Previous studies
The Global Vulnerability Assessment (or GVA) was conducted to provide a first worldwide estimate of socio-economic and ecological implications of accelerated sea level rise (Hoozemans and Hulsbergen, 1995; Hoozemans et al., 1993). It used the IPCC Common Methodology (IPCC CZMS, 1992). In consideration of data and modelling constraints, among others, the GVA was limited to the impacts of sea-level rise on three elements of the coastal zone:(1) coastal flooding, including (a) population at risk
Methodology
Building on the earlier global analyses of Hoozemans et al. (1993), the potential impact of sea-level rise is investigated for (1) coastal flooding and (2) coastal wetland losses. The coastal flood model is adjusted to better reflect the existing risk of flooding due to storm surges and how it will increase with sea-level rise. For coastal wetlands, a dynamic non-linear model of losses is developed, including uncertainties which are expressed as a range. The results are presented for the 2020s,
Flood model validation
An important, but difficult step is model validation. In the present case, there is only limited information with which to compare the new model results. Nicholls (1995b) supported Hoozemans et al. (1993) for both people in the hazard zone and the losses of wetlands given a 1-m rise in sea level. However, average annual people flooded was not validated.
Six national studies: Egypt (Delft Hydraulics et al., 1992); Germany (Sterr and Simmering, 1996; Ebenhoeh et al., 1997); Guyana (Kahn and Sturm,
Response surfaces for sea-level rise
Before examining the implications of the HadCM2 and HadCM3 scenarios, it is useful to examine the broad properties of the new flood model. Fig. 9 shows global estimates of people in the hazard zone, average annual people flooded and people to respond assuming an instantaneous rise in sea level on the 1990 situation. While this is an artificial scenario, it is comparable with much earlier work at both the national and global scale (see Flood Model Validation above). Presently there are about 200
Flood risk
Collectively, these results show larger relative and absolute increases in flood risk as sea levels rise than described in earlier work (Hoozemans et al., 1993; Hoozemans and Hulsbergen, 1995; Baarse, 1995), or by the IPCC Second Assessment Working Group II report (Bijlsma et al., 1996). This reflects a more realistic estimate of the 1990 level of protection and the calculation of increased flood risk within the 1990 flood plain as sea levels rise. The latter effect is much more important than
Conclusions
The analyses presented here shows that without an adaptive response, a global sea-level rise of only 37–38 cm by the 2080s could greatly enhance the occurrence of coastal flooding and increase the decline of coastal wetlands. The impacts are not uniform around the globe and some regions will be more adversely affected than others will. For coastal flooding, the southern Mediterranean, Africa, South and South-East Asia are most vulnerable in absolute terms, while the Caribbean, Indian Ocean
Acknowledgements
This work was funded by the Global Atmospheres Division of the Department of the Environment, Transport and the Regions as part of the Fast Track Programme (Contract No. EPG 1/1/71). Other members of the Fast Track Team are thanked for useful comments. Drs. Jonathon Gregory and Jason Lowe (the Hadley Centre) calculated the sea-level rise scenarios. Both they and Richard Klein are thanked for their helpful comments on an earlier draft of this manuscript.
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