First, these numbers only reflect those exposed to severe weather events, as the thresholds used to estimate these numbers were selected to reflect weather events that cause significant damage. Lower intensity events can still cause substantial impacts on poverty, perhaps cumulatively larger given they occur with higher frequency (Hallegatte et al., 2020). The estimates presented here do not consider events of all intensities nor all types of events, so it is important to note they do not represent the total number of people whose welfare is impacted by weather events.

Second, the people exposed or at high risk from extreme weather events are not the same as those exposed to climate change risks, even though there is a strong overlap, especially over the next decades. The approach based on extreme event proposed here may not capture the impacts of changes in average conditions, for instance when water scarcity makes it impossible to produce certain crops, even during good years, or when labor productivity is reduced by heat even during normal days. Also, this indicator will capture the vulnerability to droughts, even in places where climate change makes drought less likely. Replacing the historical hazard data by climate scenarios is relatively straightforward but would require agreement on the scenarios and models to be used. Our approach is justified by the fact that many climate risks will be experienced first as a change in the frequency and intensity of extreme events (e.g., sea level rise will first be experienced as more frequent coastal floods, even though it will eventually lead to permanent land loss). Capturing the risks linked to changes in average conditions but not extreme event would require a deeper adjustment to the methodology.

Third, the data choices and challenges that this paper sets out also highlights the need to invest in data and methods to increase our ability to identify and count who is vulnerable. Some of the data limitations have already been highlighted, and throughout the paper we highlight additional areas where further data investments are needed. Since reducing the number of people at high risk of climate hazards is important to achieve successful development and poverty reduction, it is important to improve how we monitor and track progress in these dimensions. This paper sets out one framework that can be used to do this, even though there is no unique definition.

Finally, the appropriate definition of the methodology depends on how it will be used. At one extreme, everybody can be considered at risk from climate hazards, at least to some extent. Another extreme is to identify the people in the world who are at extremely high risk. The selection of the right thresholds and dimensions of exposure and vulnerability will need to depend on the use of the indicator and the objective of its measure. A final measure will require consensus around the core risks and the core dimensions of vulnerability and meaningful cutoffs to use. This paper uses thresholds that can be adjusted, depending on how the metric is to be used. We hope the framework presented promotes a data-based discussion, and a discussion that also encourages the investments needed to address data gaps.

Section 2 of this paper sets out how exposure and vulnerability are being defined, and the approach taken in the paper to measure vulnerability. Section 3 of this paper outlines how exposure, poverty and vulnerability are measured in this analysis and the data used. Section 4 presents result. Section 5 sets out an agenda for work on improving measurement of vulnerability and concludes.

# 2 PROPOSED APPROACH TO IDENTIFYING THOSE AT RISK

There are three components that determine the impact or risk of an extreme weather event on people: hazard, exposure, and vulnerability (Figure 1). Hazard in this case is the potential occurrence of an extreme weather event “the occurrence of a value of a weather or climate variable above (or below) a threshold 5