Declining vulnerability to river floods and the global benefits of adaptation

Abstract

The global impacts of river floods are substantial and rising. Effective adaptation to the increasing risks requires an in-depth understanding of the physical and socioeconomic drivers of risk. Whereas the modeling of flood hazard and exposure has improved greatly, compelling evidence on spatiotemporal patterns in vulnerability of societies around the world is still lacking. Due to this knowledge gap, the effects of vulnerability on global flood risk are not fully understood, and future projections of fatalities and losses available today are based on simplistic assumptions or do not include vulnerability. We show for the first time (to our knowledge) that trends and fluctuations in vulnerability to river floods around the world can be estimated by dynamic high-resolution modeling of flood hazard and exposure. We find that rising per-capita income coincided with a global decline in vulnerability between 1980 and 2010, which is reflected in decreasing mortality and losses as a share of the people and gross domestic product exposed to inundation. The results also demonstrate that vulnerability levels in low- and high-income countries have been converging, due to a relatively strong trend of vulnerability reduction in developing countries. Finally, we present projections of flood losses and fatalities under 100 individual scenario and model combinations, and three possible global vulnerability scenarios. The projections emphasize that materialized flood risk largely results from human behavior and that future risk increases can be largely contained using effective disaster risk reduction strategies.

Keywords: adaptation; climate change; development; flooding; vulnerability.

Fig. 1.

Patterns in observed flood impacts can be reproduced using a hydrological modeling chain. (A) Population density and modeled potential inundation depth for Pakistan in 1980 (regular year with minor floods) and 2010 (widespread floods reported); (B) patterns in modeled exposed population in Pakistan between 1990 and 2010 (compared with baseline 1980–1989) are very similar (r = 0.61) to patterns in reported fatalities (1).

Fig. 2.

Relative changes in global modeled flood volume and annual exposure (baseline = 1980–1989) versus absolute fatalities (A) and losses (B). C and E show that the fatalities in high-income countries are declining, despite a range of years with a high number of exposed people, whereas this is not the case in low-income countries. D and F show that both the exposed GDP and reported losses in the period 1990–2010 are higher than the 1980–1989 average for almost each year. Note that the percentage increase in fatalities and losses compared with the baseline may in some years be higher than the percentage increase in exposed population and GDP, but this does not mean that the absolute values are higher. Losses and fatalities as a percentage of exposure are shown in Fig. 3.

Fig. 3.

Rates of loss and mortality due to flooding are shown to be linked to income levels. A and C show that fatalities and losses, respectively, as a percentage of annual exposure are a negative function of GDP per capita. B and D show that vulnerability of people and GDP, respectively, have declined over the past two decades, leading to a relative convergence of vulnerability levels between low- and high-income countries.

Fig. 4.

The 10 recorded events with the largest total number of fatalities (A) and total losses (B), in descending order. The blue lines in both panels show the absolute magnitude of reported fatalities and losses. The red bars display the mortality and loss rates for the specific year and country, which can then be compared with the average mortality and loss rates for the total income group in that year (green bars). The figure shows that the country-specific mortality rates are higher than the income-group average for 7 out of the 10 highest ranking event years in terms of fatalities, and for 10 out of 10 event years in terms of losses.

Fig. 5.

Test of significance and sensitivity to the specific inundation depth threshold. (A) Results of bootstrapping the average mortality rates per income group (n = 1,000; boxes outline the 25th and 75th percentile; whiskers approximate the 1st and 99th percentile; and crossing marks indicate outliers beyond these values); (B) same as A, but for loss rates; (C) global exposed population at minimum inundation depths 0–5 m, for all years 1990–2010; (D) average global mortality rates at minimum inundation depths 0–5 m, for all years 1990–2010.

Fig. 6.

Projections of fatalities and losses using different climate models, emission scenarios, socioeconomic pathways, and adaptation scenarios. Results were computed for two climatic time slices future time slices (2010–2049, labeled 2030; and 2060–2099, labeled 2080), and interpolated linearly in between. A shows that fatalities are expected to increase strongly over the next decades without additional adaptation (red lines) but could be reduced up to 69% under the high-adaptation scenario. B demonstrates that expected losses are likely to increase at a faster pace than fatalities without additional adaptation (red lines) and would be reduced to the currently observed loss level only in the high-adaptation scenario.