Laurens M. Bouwer

Flood risk and climate change: global and regional perspectives

Abstract A holistic perspective on changing rainfall-driven flood risk is provided for the late 20th and early 21st centuries. Economic losses from floods have greatly increased, principally driven by the expanding exposure of assets at risk. It has not been possible to attribute rain-generated peak streamflow trends to anthropogenic climate change over the past several decades. Projected increases in the frequency and intensity of heavy rainfall, based on climate models, should contribute to increases in precipitation-generated local flooding (e.g. flash flooding and urban flooding). This article assesses the literature included in the IPCC SREX report and new literature published since, and includes an assessment of changes in flood risk in seven of the regions considered in the recent IPCC SREX report—Africa, Asia, Central and South America, Europe, North America, Oceania and Polar regions. Also considering newer publications, this article is consistent with the recent IPCC SREX assessment finding that the impacts of climate change on flood characteristics are highly sensitive to the detailed nature of those changes and that presently we have only low confidence1 in numerical projections of changes in flood magnitude or frequency resulting from climate change. Editor D. Koutsoyiannis Citation Kundzewicz, Z.W., et al., 2013. Flood risk and climate change: global and regional perspectives. Hydrological Sciences Journal, 59 (1), 1–28.

Have Disaster Losses Increased Due to Anthropogenic Climate Change

Abstract The increasing impact of natural disasters over recent decades has been well documented, especially the direct economic losses and losses that were insured. Claims are made by some that climate change has caused more losses, but others assert that increasing exposure due to population and economic growth has been a much more important driver. Ambiguity exists today, because the causal link between climate change and disaster losses has not been addressed in a systematic manner by major scientific assessments. Here, the author presents a review and analysis of recent quantitative studies on past increases in weather disaster losses and the role of anthropogenic climate change. Analyses show that, although economic losses from weather-related hazards have increased, anthropogenic climate change so far did not have a significant impact on losses from natural disasters. The observed loss increase is caused primarily by increasing exposure and value of capital at risk. This finding is of direct import...

Sensitivity of global river discharges under Holocene and future climate conditions

A comparative analysis of global river basins shows that some river discharges are more sensitive to future climate change for the coming century than to natural climate variability over the last 9000 years. In these basins (Ganges, Mekong, Volta, Congo, Amazon, Murray-Darling, Rhine, Oder, Yukon) future discharges increase by 6-61%. These changes are of similar magnitude to changes over the last 9000 years. Some rivers (Nile, Syr Darya) experienced strong reductions in discharge over the last 9000 years (17-56%), but show much smaller responses to future warming. The simulation results for the last 9000 years are validated with independent proxy data.

Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

Projections of Future Extreme Weather Losses Under Changes in Climate and Exposure

Many attempts are made to assess future changes in extreme weather events due to anthropogenic climate change, but few studies have estimated the potential change in economic losses from such events. Projecting losses is more complex as it requires insight into the change in the weather hazard but also into exposure and vulnerability of assets. This article discusses the issues involved as well as a framework for projecting future losses, and provides an overview of some state-of-the-art projections. Estimates of changes in losses from cyclones and floods are given, and particular attention is paid to the different approaches and assumptions. All projections show increases in extreme weather losses due to climate change. Flood losses are generally projected to increase more rapidly than losses from tropical and extra-tropical cyclones. However, for the period until the year 2040, the contribution from increasing exposure and value of capital at risk to future losses is likely to be equal or larger than the contribution from anthropogenic climate change. Given the fact that the occurrence of loss events also varies over time due to natural climate variability, the signal from anthropogenic climate change is likely to be lost among the other causes for changes in risk, at least during the period until 2040. More efforts are needed to arrive at a comprehensive approach that includes quantification of changes in hazard, exposure, and vulnerability, as well as adaptation effects.

Understanding trends and projections of disaster losses and climate change: is vulnerability the missing link?

The recent IPCC-SREX report demonstrated for the first time comprehensively that anthropogenic climate change is modifying weather and climate extremes. The report also documents, what has been long known, that losses from natural disasters, including those linked to weather, have increased strongly over the last decades. Responding to the debate regarding a contribution of anthropogenic climate change to the increased burden from weather-related disasters, the IPCC-SREX finds that such a link cannot be made today, and identifies the key driver behind increases in losses as exposure changes in terms of rising population and capital at risk. Yet, in the presence of many uncertainties and omissions involved in studying trends in losses, the authors of the IPCC report did not exclude a role for climate change. In particular, one key uncertainty identified has been the incomplete consideration of economic vulnerability to natural hazards, defined as the propensity to incur losses in a hazardous event. Focussing on the role of vulnerability in determining today’s and future disaster loss risk, we critically review the literature on loss trends and projections, and provide context by way of a modeling case study of observed and projected losses from riverine flooding in Bangladesh. We find that research has almost exclusively focused on normalizing losses for changes in exposure, yet not for vulnerability, which appears a major gap given the dynamic nature of vulnerability, and documented evidence regarding decreases in vulnerability in many regions. One such region is South Asia, and of particular interest to us is Bangladesh, a country heavily at-risk, but also with substantial expertise regarding risk management, where we are able to show that economic vulnerability has been substantially reduced over the last decades. In order to understand future flood risk in Bangladesh, we project risk based on past reductions in vulnerability and compare it to a case where vulnerability is not considered explicitly and kept static. In the dynamic scenario, risk would still increase in absolute terms, yet at much smaller increments compared to a static vulnerability case. Thus, a key finding of our analysis is that, absent dynamic quantifications of vulnerability, studies on future losses under climatic change may overestimate future losses. Furthermore, the analysis also suggests that there are substantial benefits to gain by supporting vulnerability-reducing measures in many regions. Finally, we emphasize the need for further taking a risk-based perspective on modelling climate impacts in order to provide robust information on the costs and impacts from extremes in a changing climate.

How reliable are projections of future flood damage

Abstract. Flood damage modelling is an important component in flood risk management, and several studies have investigated the possible range of flood damage in the coming decades. Generally, flood damage assessments are still characterized by considerable uncertainties in stage-damage functions and methodological differences in estimating exposed asset values. The high variance that is commonly associated with absolute flood damage assessments is the reason for the present study that investigates the reliability of estimates of relative changes in the development of potential flood damage. While studies that estimate (relative) changes in flood damage over time usually address uncertainties resulting from different projections (e.g. land-use characteristics), the influence of different flood damage modelling approaches on estimates of relative changes in the development of flood damage is largely unknown. In this paper, we evaluate the reliability of estimates of relative changes in flood damage along the river Rhine between 1990 and 2030 in terms of different flood-damage modelling approaches. The results show that relative estimates of flood damage developments differ by a factor of 1.4. These variations, which result from the application of different modelling approaches, are considerably smaller than differences between the approaches in terms of absolute damage estimates (by a factor of 3.5 to 3.8), or than differences resulting from land-use projections (by a factor of 3). The differences that exist when estimating relative changes principally depend on the differences in damage functions. In order to improve the reliability of relative estimates of changes in the development of potential flood damage, future research should focus on reducing the uncertainties related to damage functions.

Adaptation to flood risk - results of international paired flood event studies

As flood impacts are increasing in large parts of the world, understanding the primary drivers of changes in risk is essential for effective adaptation. To gain more knowledge on the basis of empirical case studies, we analyze eight paired floods, that is, consecutive flood events that occurred in the same region, with the second flood causing significantly lower damage. These success stories of risk reduction were selected across different socioeconomic and hydro-climatic contexts. The potential of societies to adapt is uncovered by describing triggered societal changes, as well as formal measures and spontaneous processes that reduced flood risk. This novel approach has the potential to build the basis for an international data collection and analysis effort to better understand and attribute changes in risk due to hydrological extremes in the framework of the IAHSs Panta Rhei initiative. Across all case studies, we find that lower damage caused by the second event was mainly due to significant reductions in vulnerability, for example, via raised risk awareness, preparedness, and improvements of organizational emergency management. Thus, vulnerability reduction plays an essential role for successful adaptation. Our work shows that there is a high potential to adapt, but there remains the challenge to stimulate measures that reduce vulnerability and risk in periods in which extreme events do not occur.

Clarifying the Attribution of Recent Disaster Losses: A Response to Epstein and McCarthy

—HANS VON STORCH Institute for Coastal Research, GKSS Research Center, Geesthacht, Germany he December 2004 issue of BAMS contains an article warning of the threats of abrupt climate change (Epstein and McCarthy 2004, hereafter EM04). The article seeks to raise awareness of the risks of an abrupt change in climate related to human influences on the climate system, but, in doing so it repeats a common factual error. Specifically, it identifies the recent growth in economic damages associated with weather and climate events, such as Hurricanes Mitch and Jeanne and tornadoes in the United States, as evidence of trends in extreme events, arguing “the rising costs associated with weather volatility provide another derived indicator of the state of the climate system . . . the economic costs related to more severe and volatile weather deserves mention as an integral indicator of volatility.” Although the attribution of increasing damages to climate changes is but one of many assertions made by EM04, the repetition of this erroneous claim is worth correcting because it is not consistent with current scientific understandings. The rising costs of disasters are important, and so too is human influence on climate. Policy makers should, indeed, pay attention to both issues. But a robust body of research shows very little evidence to support the claim that the rising costs associated with weather and climate events are associated with changes in the frequency or intensity of events themselves.1 Instead, the research that has sought to explain increasing disaster losses has found that the trend has far more to do with the nature of societal vulnerability to those events. This conclusion is borne out in literature from the natural hazards community (e.g., Mileti 1999; Tierney 2001) and the societal impacts of the climate community (e.g., Glantz 2003; Changnon et al. 2000), and is consistent with the findings of the most recent assessment of the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al. 2001; McCarthy et al. 2001).

On the Flood Risk in the Netherlands

The Netherlands are protected from storm surges and river floods by the Deltaworks: a reinforcement of the primary flood defence system consisting of coastal dunes, dikes and storm-surge barriers. These were implemented in response to the dramatic flooding disaster in 1953. Over the last 50 years, billions of euros have been invested in this scheme creating a feeling of safety in society. However, in this paper we argue that the current sense of safety may be inappropriate. Scientific evidence is growing, which shows that the hydraulic baseline conditions like storm wave properties and maximum river discharges may be different and more severe than recently thought. Climate change and sea-level rise may aggravate this situation. Moreover, the number of people and the value of properties behind the dikes have increased significantly since the coastal protection schemes were designed. In the present situation the flood risk appears to be disproportionately large compared to other daily risks. We conclude that on the short term the existing coastal and river flood protection should be reinforced to accommodate the more extreme hydraulic conditions and protect these valuables. Alternatively, protection levels of different areas should be reconsidered and perhaps reduced. The latter would in fact mean retreat from particular areas