de Moel, Muis et al., 2020).<sup>13</sup> We use the global STORM tropical cyclone wind speed data with a resolution of 6 arcminutes (approximately 11km) for return periods ranging from 10 to 10,000 years. We define exposed locations if the 10-minute average sustained wind speed exceeds a Category-1 threshold of 29 m/s, a Category-2 threshold of 37.6 m/s and a Category 3 threshold of 43.4 m/s.<sup>14</sup> It is important to note that our measure of cyclone intensity leaves out storm surge and heavy precipitation that generally occur in association with a tropical cyclone.

# 3.5 SETTING THRESHOLDS TO DEFINE EXTREME EVENTS AND ESTIMATING JOINT EXPOSURE

Whilst the number of people and poor people exposed are initially estimated for events defined by several intensity thresholds and return periods, for further analysis we set these to a specific “risk threshold”. Choosing comparable risk thresholds across very different types of shocks is challenging, especially given the events have very different types of impact (e.g., sudden loss of assets for tropical cyclones vs. productivity and health impacts for heatwaves). For each event we have selected an intensity threshold that represents serious damage to either assets, income or morbidity and mortality. These thresholds are as shown in Table 3.

Apart from drought, which is based on observed historical frequency, we use a constant return period of 100 years. This means that the flood, heatwave, and cyclone events exceeding chosen thresholds have at least a 1% probability of occurring in any given year. The 100-year event is commonly used in disaster risk management, in part because it represents an event that an individual is more likely than not to experience in her lifetime (Table 1). These thresholds could be adjusted over time to make risks more comparable across hazards.

**Table 3: Thresholds used to define exposure to extreme events**

|Event|Return period<br>(years)|Intensity threshold|
|---|---|---|
|Flood|100|An inundation depth of at least 50cm|
|Agricultural<br>drought|40*|At least 30 percent of cropland or grassland affected in any season, in rural areas|
|Heatwave|100|A three-day running mean maximum Wet Bulb Globe Temperature (WBGT) of > 33°C|
|Cyclone|100|A wind speed of at least the Category2 threshold(37.6 m/s,10 min sustained)|

In Rentschler, Salhab, and Jafino (2022), inundation depths of at least 0.5 meters indicate a high risk that bring disruptions to livelihoods and economic activity, as well as risk to life for select locations and vulnerable groups. In a different work, Huizinga, De Moel, and Szewczyk (2017) show that for a fluvial and marine flood depth of 0.5 meters, the average share of residential assets lost across regions is 0.38 (a range of 0.22-0.49). Cyclone damage functions also indicate direct economic damage in the range of 0.20.5 for category 2 windspeeds for most regions (Eberenz <mark>, Lüthi, and Bresch</mark> , 2021).

Drought results in a loss in income rather than a loss in assets. A moderate (about a 1 in 10 year) drought is predicted to reduce consumption by 15 percent and 9 percent in Uganda and Ethiopia, respectively (World Bank , 2016; Hill and Porter, 2017), and by 10 percent across most agroecological zones in sub-

> 13 The global cyclone hazard maps are available from <u>https://doi.org/10.4121/12705164.v4</u> and as global scale files compiled by Russell (2022) from <u>https://doi.org/10.5281/zenodo.7438144.</u> 14 Category-classifications are based on the Saffir-Simpson scale (converted from 1-min to 10-min thresholds).

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