The world's population living on low-lying deltas is increasingly vulnerable to flooding, whether from intense rainfall, rivers or from hurricane-induced storm surges. High-resolution SRTM and MODIS satellite data along with geo-referenced historical map analysis allows quantification of the extent of low-lying delta areas and the role of humans in contributing to their vulnerability. Thirty-three major deltas collectively include ~26,000 km 2 of area below local mean sea level and ~96,000 km 2 of vulnerable area below 2 m a.s.l. The vulnerable areas may increase by 50% under projected 21st Century eustatic sea level rise, a conservative estimate given the current trends in the reduction in sedimentary deposits forming on the surface of these deltas. Analysis of river sediment load and delta topographical data show that these densely populated, intensively farmed landforms, that often host key economic structures, have been destabilized by human-induced accelerated sediment compaction from water, oil and gas mining, by reduction of incoming sediment from upstream dams and reservoirs, and from floodplain engineering. IntroductionClose to 0.5 billion people live on, or near, world deltas, inclusively in many mega-cities (1, 2). Ten countries (China, India, Bangladesh, Vietnam, Indonesia, Japan, Egypt, USA, Thailand, and the Philippines) account for 73% of the people that live in the world's coastal zone, defined as within 10 m a.s.l. (3). 20 th -century catchment developments and population and economic growth within subsiding deltas have placed these environments and their populations under a growing risk of coastal flooding, wetland loss, shoreline retreat, and loss of infrastructure (4, 5). It is estimated that more than 10 million people per year experience flooding due to storm surges, and most of these people are living on Asian deltas (6). Using new, globally-consistent and highresolution topographic data, three hypotheses are tested: 1) deltas are rapidly sinking, often to below local sea level, 2) the lack of sediment getting to delta floodplains is the main reason so many deltas are sinking, and 3) human activities are largely responsible for the present vulnerability of deltas. For a representative suite of deltas, Shuttle Radar Topography Mission (SRTM) data are applied to evaluate delta topography, in relation to mean sea level. Historical maps are geo-referenced against detailed topographic data to map morphodynamic patterns and quantify how rivers once flowed through deltas. Visible and near-infrared Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images are used to assess flooding in modern deltas and investigate whether such flooding is mainly from river runoff or instead from coastal storm surges, and whether present river suspended load is sufficient to maintain delta plain aggradation and stability.

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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 Le risque d'inondation et les perspectives de changement climatique mondial et régionalRésumé Cet article trace une perspective globale de l'évolution des risques d'inondation d'origine pluviale pour la fin du 20ème et le début du 21ème siècle. Les pertes économiques dues aux inondations ont fortement augmenté, principalement en raison de l'exposition croissante des actifs à risque. Il n'a pas été possible d'attribuer les tendances de débits de pointe au changement climatique d'origine anthropique au cours des dernières décennies. Les augmentations prévues de la fréquence et de l'intensité des précipitations extrêmes, basées sur des modèles climatiques, devraient contribuer à une augmentation des inondations locales (par exemple, des crues éclairs et des inondations en milieu urbain) provoquées par les pluies. Nous avons évalué la littérature incluse dans le rapport SREX du GIEC et celle qui a été publiée depuis, afin d'estimer l'évolution des risques d'inondation dans les sept régions considérées dans le rapport récent du SREX du GIEC, à savoir l'Afrique, l'Asie, l'Amérique centrale et du Sud, l'Europe, l'Amérique du Nord, l'Océanie et les régions polaires. Tenant compte des publications les plus récentes, le présent article rejoint la récente évaluation SREX du GIEC selon laquelle les impacts du changement climatique sur les caractéristiques des crues sont très sensibles aux détails de ces changements, et qu'à l'heure actuelle nous ne pouvons avoir qu'une confiance limitée dans les projections numériques de l'évolution de l'amplitude ou de la fréquence des inondations résultant du changement climatique.

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