Evidence from Mumbai, India, illustrates why inadequate infrastructure in low-income neighborhoods exacerbates flood risks (Patankar 2015). The city is prone to recurrent floods during the monsoon season, with significant impacts on poor people. The authorities have identified 40 chronic flood spots and 200 localized flood spots, where waterlogging stems from inadequate drainage and poor land-use planning. Many low-income slum dwellers report floodwaters entering their homes several times during the monsoon season. High-income neighborhoods on the other hand, tend to be equipped with effective drainage systems that can mitigate frequent small floods, but still be at risk from rare and major events that can affect low- and high-income areas alike.

## **Climate change is expected to aggravate risks.**

There is some evidence that climate change will result in increased flood hazards. Hirabayashi et al. (2013) present regionally disaggregated flood risk estimates for the year 2100, based on the outputs of 11 climate models. By modeling changes in river discharge and inundation areas, they show that a high-concentration climate change scenario could lead to a large increase in flood frequencies in Southeast Asia, India, East and Central Africa, and large parts of Latin America.

Yet, existing models are ill-suited to provide reliable long-term projections of climate change at the local level. The IPCC’s Fifth Assessment report shows that forecasts for average temperature and rainfall changes by mid-century vary significantly depending on which model and which assumptions are considered. However, it is not only the change in the mean that matters – already regions around the world are experiencing changes in the variability of climatic events; in the form of increased frequency of formally rare and extreme events, such as droughts and intense floods. In the meanwhile, the continued urbanization of high-risk areas will mean that more and more people will be exposed to flood risks.

Indeed, large coastal cities are likely to be risk hotspots. Hallegatte et al. (2013) estimate the flood risks in the world’s 136 largest coastal cities. They show that population and asset growth, climate change and subsidence are all likely to contribute to a drastic increase in global average flood losses – from $6 billion per year in 2005 to over $60 billion in 2050, <mark>assuming proactive adaptation actions in all cities.</mark> Kulp and Strauss (2019) conduct a global assessment of the potential impact of sea level rise on the world’s coastal population. The global scale of their study and the use of high-resolution flood and population data makes it the closest comparable analysis of flood risk. Their assessment suggests that today between 190 million and 630 million people (depending on the emissions scenario considered) live in areas that could be inundated by sea level rise by the year 2100. They also estimate that one billion people live on land that is less than 10 meters above current high tide lines.

This present study complements the one by Kulp and Strauss (2019) in several ways: First, it considers not only the one billion people living near current high tide zones and coastal flood risk zones – but expands the analysis to the world’s population of 7.7 billion, including those living inland. This study does so by considering fluvial and pluvial flooding, in addition to coastal flooding. Second, rather than focusing on future risk, this study considers the risk levels that already persist today. And third, rather than focusing on the slow-onset hazard of sea level rise, this study estimates people’s exposure to intense flood disasters – for instance those caused by intense rainfall.

# **3. Data**

Several data sets are used in this study to calculate global population exposure to flood risk. The methods for combining the data sets into a single flood risk map per country are described in the methodology section.

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