IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

3.5.2 How will climate change affect flood damages?

Future flood damages will depend heavily on settlement patterns, land-use decisions, the quality of flood forecasting, warning and response systems, and the value of structures and other property located in vulnerable areas (Mileti, 1999; Pielke and Downton, 2000; Changnon, 2005), as well as on climatic changes per se (Schiermeier, 2006). Choi and Fisher (2003) estimated the expected change in flood damages for selected USA regions under two climate-change scenarios in which mean annual precipitation increased by 13.5% and 21.5%, respectively, with the standard deviation of annual precipitation either remaining unchanged or increasing proportionally. They used a structural econometric (regression) model based on time series of flood damage, and population, wealth indicator, and annual precipitation as predictors. They found that the mean and standard deviation of flood damage are projected to increase by more than 140% if the mean and standard deviation of annual precipitation increase by 13.5%. The estimates suggest that flood losses are related to exposure because the explanatory power of population and wealth is 82%, while adding precipitation increases the explanatory power to 89%. Another study examined the potential flood damage impacts of changes in extreme precipitation events using the Canadian Climate Centre model and the IS92a emissions scenario for the metropolitan Boston area in the north-eastern USA (Kirshen et al., 2005b). They found that, without adaptation investments, both the number of properties damaged by floods and the overall cost of flood damage would double by 2100 relative to what might be expected with no climate change, and that flood-related transportation delays would become an increasingly significant nuisance over the course of the century. The study concluded that the likely economic magnitude of these damages is sufficiently high to justify large expenditures on adaptation strategies such as universal flood-proofing for all flood plains.

This finding is supported by a scenario study of the damage due to river and coastal flooding in England and Wales in the 2080s (Hall et al., 2005), which combined four emissions scenarios with four scenarios of socio-economic change in an SRES-like framework. In all scenarios, flood damages are predicted to increase unless current flood management policies, practices and infrastructure are changed. For a 2°C temperature increase in a B1-type world, by the 2080s annual damage is estimated to be £5 billion as compared to £1 billion today, while with approximately the same climate change, damage is only £1.5 billion in a B2-type world. In an A1-type world, with a temperature increase of 2°C, the annual damage would amount to £15 billion by the 2050s and £21 billion by the 2080s (Hall et al., 2005; Evans et al., 2004).

The impact of climate change on flood damages can be estimated from modelled changes in the recurrence interval of present-day 20- or 100-year floods, and estimates of the damages of present-day floods as determined from stage-discharge relations (between gauge height (stage) and volume of water per unit of time (discharge)), and detailed property data. With such a methodology, the average annual direct flood damage for three Australian drainage basins was projected to increase by a factor of four to ten under conditions of doubled atmospheric CO2 concentrations (Schreider et al., 2000).