Increasing flood risk and wetland losses due to global sea-level rise: regional and global analyses

https://doi.org/10.1016/S0959-3780(99)00019-9Get rights and content

Abstract

To develop improved estimates of (1) flooding due to storm surges, and (2) wetland losses due to accelerated sea-level rise, the work of Hoozemans et al. (1993) is extended to a dynamic analysis. It considers the effects of several simultaneously changing factors, including: (1) global sea-level rise and subsidence; (2) increasing coastal population; and (3) improving standards of flood defence (using GNP/capita as an “ability-to-pay” parameter). The global sea-level rise scenarios are derived from two General Circulation Model (GCM) experiments of the Hadley Centre: (1) the HadCM2 greenhouse gas only ensemble experiment and (2) the more recent HadCM3 greenhouse gas only experiment. In all cases there is a global rise in sea level of about 38 cm from 1990 to the 2080s. No other climate change is considered. Relative to an evolving reference scenario without sea-level rise, this analysis suggests that the number of people flooded by storm surge in a typical year will be more than five times higher due to sea-level rise by the 2080s. Many of these people will experience annual or more frequent flooding, suggesting that the increase in flood frequency will be more than nuisance level and some response (increased protection, migration, etc.) will be required. In absolute terms, the areas most vulnerable to flooding are the southern Mediterranean, Africa, and most particularly, South and South-east Asia where there is a concentration of low-lying populated deltas. However, the Caribbean, the Indian Ocean islands and the Pacific Ocean small islands may experience the largest relative increase in flood risk. By the 2080s, sea-level rise could cause the loss of up to 22% of the world's coastal wetlands. When combined with other losses due to direct human action, up to 70% of the world's coastal wetlands could be lost by the 2080s, although there is considerable uncertainty. Therefore, sea-level rise would reinforce other adverse trends of wetland loss. The largest losses due to sea-level rise will be around the Mediterranean and Baltic and to a lesser extent on the Atlantic coast of Central and North America and the smaller islands of the Caribbean. Collectively, these results show that a relatively small global rise in sea level could have significant adverse impacts if there is no adaptive response. Given the “commitment to sea-level rise” irrespective of any realistic future emissions policy, there is a need to start strategic planning of appropriate responses now. Given that coastal flooding and wetland loss are already important problems, such planning could have immediate benefits.

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.

References (58)

  • D.R. Cahoon et al.

    Estimating shallow subsidence in microtidal saltmarshes of the southeastern United StatesKaye and Barghoorn revisited

    Marine Geology

    (1995)
  • M. Hulme et al.

    Climate change scenarios for global impact studies

    Global Environmental Change

    (1999)
  • N.W. Arnell

    Climate change and global water resources

    Global Environmental Change

    (1999)
  • Baarse, G., 1995. Development of an Operational Tool for Global Vulnerability Assessment (GVA): Update of the Number of...
  • Baarse, G., Peerbolte, E.B., Bijlsma, L., 1994. Assessment of the Vulnerability of the Netherlands to Sea-level Rise....
  • Bijlsma, L., Ehler, C.N., Klein, R.J.T., Kulshrestha, S.M., McLean, R.F., Mimura, N., Nicholls, R.J., Nurse, L.A.,...
  • Bos, E., Vu, M.T., Massiah, E., Bulatao, R.A., 1994. World Population Projections 1994–1995: Estimates and Projections...
  • D.R. Cahoon et al.

    Vertical accretion and shallow subsidence in a mangrove forest of southwestern Florida, U.S.A.

    Mangroves and Salt Marshes

    (1997)
  • Carter, T.R., Parry, M.L., Nishioka, S., Harasawa, H. (Eds), 1994. Technical guidelines for assessing climate change...
  • Cicin-Sain, B, Ehler, C., Knecht, R., South, R., Weiher, R., 1997. Guidelines for integrating coastal management...
  • Day, J.W., Pont, D, Ibanez, C., Hensel, P.F., 1993. Impacts of sea-level rise on deltas in the Gulf of Mexico and the...
  • Delft Hydraulics, Resource Analysis the Netherlands and Coastal Research Institute, Egypt. 1992. Vulnerability...
  • B.C. Douglas

    Global sea risea redetermination

    Surveys in Geophysics

    (1997)
  • Ebenhoeh, W., Sterr, H., Simmering, F., 1997. Potentielle Gefaehrdung und Vulnerabilitaet der deutschen Nord- und...
  • EMF WP 14.1, 1995. Second round study design for EMF 14: integrated assessment of global climate change. Energy...
  • J.C. Ellison et al.

    Mangrove ecosystem collapse during predicted sea-level riseholocene analogues and implications

    Journal of Coastal Research

    (1991)
  • Gommes, R., du Guerny, J., Nachtergaele, F., Brinkman, R., 1997. Potential impacts of sea-level rise on populations and...
  • J.M. Gregory

    Sea-level changes under increasing atmospheric CO2 in a transient coupled ocean-atmosphere GCM experiment

    Journal of Climate

    (1993)
  • J.M. Gregory et al.

    Simulated future sea-level rise due to glacier melt based on regionally and seasonally resolved temperature changes

    Nature

    (1998)
  • T.L. Holzer et al.

    Land subsidence caused by groundwater withdrawal in urban areas

    GeoJournal

    (1985)
  • Hoozemans, F.M.J., Hulsbergen, C.H., 1995. Sea-level rise: a world-wide assessment of risk and protection costs. In...
  • Hoozemans, F.M.J., Marchand, M., & Pennekamp, H.A., 1993. A Global Vulnerability Analysis: Vulnerability Assessment for...
  • Houghton, L.G., Meira Filho, L.G., Callander B.A. (Eds.), 1996. Climate Change 1995: The Science of Climate Change....
  • IPCC CZMS, 1990. Strategies for adaptation to sea level rise. Report of the Coastal Zone Management Subgroup. IPCC...
  • IPCC CZMS, 1992. Global climate change and the rising challenge of the sea. Report of the Coastal Zone Management...
  • Kahn, M., Sturm, M.F., 1995. Assessment of the vulnerability of coastal areas to sea level rise: case study Guyana. In:...
  • Klein, R.J.T., Nicholls, R.J., 1998. Coastal Zones. In: Burton, I., Feenstra, J.F., Smith, J.B., Tol, R.S.J. (Eds.),...
  • R.J.T. Klein et al.

    Assessment of coastal vulnerability to sea-level rise

    Ambio

    (1999)
  • Klein, R.J.T., Nicholls, R.J., Mimura, N., 1999. Coastal adaptation to climate change: can the IPCC technical...
  • Cited by (0)

    View full text