Detecting severe increase in heat stress risk

In addition to advancing the operationalization of the SSP-RCP framework in climate-related risks assessment – which is a scholarly-focused contribution – this doctoral thesis also contributes to a better understanding of the future impact of extreme heat. By projecting future heat stress risk in three different case studies – namely Europe (Chapter 4), African cities (Chapter 5), and Houston (Chapter 6) – this work sheds light on the forthcoming impacts of extreme heat in a context of climate change and socioeconomic development in these regions. More broadly, together with similar studies that focus on other regions of the globe and that use different approaches, this doctoral thesis strengthen our current understanding and characterization of future heat stress risk worldwide. Such a strong and robust knowledge of the forthcoming impacts of rising temperatures across the globe is of utmost importance to (i) raise awareness about the impacts, (ii) minimize them, and (iii) eventually cope with them.

194 The combined results of the different case studies depicted in this doctoral thesis show a large increase in future heat stress risk in all case studies (Figure 8.3). In Chapter 4, I demonstrated that the number of persons at very high risk of heat stress in Europe will increase from ~2.1 million persons currently (i.e., 0.4% of the current European population) to ~122 million persons in the 2050s (i.e., 20.3% of the future European population) under the median scenario (that is, the SSP-RCP combination leading to the closest-to-median increase in risk). In Chapter 5, I showed that the future exposure to dangerous heat in 173 African cities will increase from ~4.2 billion person-days per year currently (aggregated at the continental scale) to ~23 billion person-days per year in the 2030s, ~60 billion person-days per year in the 2060s, and to ~112 billion person-days per year in the 2090s, under the median scenario. Finally, in Chapter 6 I demonstrated that the number of heat-related non-accidental summer mortalities in Houston will increase from 3’825 persons currently to ~22’035 persons in the 2050s, under the median scenario.

Such increase in future heat stress risk throughout the multiple case studies is consistent with findings of similar studies conducted at different scales and/or in different places (e.g. Dong et al., 2015; Anderson et al., 2018; Harrington and Otto, 2018; Jones et al., 2018; Lee et al., 2018). These projections of future heat stress risk are very instrumental for policy-makers to grasp the magnitude of the forthcoming impacts of extreme heat events – in a context of changing socioeconomic conditions –and to envision the level of mitigation and/or adaptation efforts that are required in order to minimize future heat stress risk.

Fig. 8.3 – Combined projections of future heat stress risk in Europe, African cities, and Houston, using the median scenario for each case-study (SSP5-RCP8.5 for Europe and African cities and SSP2-RCP4.5 for Houston). Heat stress risk is represented differently in each case-study, namely the number of persons (million) at very high risk of heat stress in Europe, the number of person-days per year (billion) of exposure to dangerous heat in African cities, and the number of non-accidental summer mortalities (thousand) in Houston.

The projections of future heat stress risk produced in this doctoral thesis are not only instrumental to determine the magnitude of increase in impacts of extreme heat, but are also very useful to pinpoint and highlight the specific areas where the impacts will be the greatest (Figure 8.4). In Europe, results show that populations located in the Mediterranean region, the Iberian Peninsula, and the Southern part of Eastern Europe

195 will be the most impacted by future heat stress risk by the middle of this century. In Africa, the cities with the largest exposure to dangerous heat (in terms of person-days per year) in the 2060s are located in Western Africa (and especially in Nigeria, Niger, and Ivory Coast) as well as in the Southeastern part of Northern Africa (e.g. in Sudan). Finally, in Houston, the Census tracts showing the highest heat-related mortality (in raw number) are located in the outskirts of Harris County as well as in a few urban Census tracts of the City of Houston, together with an increased in mortality generalized to most Census tracts. Such mapping of future heat stress risk – and of climate-related risks in general – appears very useful to inform policy-makers (in Europe, in African cities, and in Houston) about future hotspots of climate risk and to direct attention and funds to geographic areas where impacts are expected to be greatest (de Sherbinin et al., 2019).

Fig. 8.4 – Combined projections of future heat stress risk for Europe (a), 173 different African cities (b), and Houston, Texas (c). Projections were made using the median scenario for each case-study (SSP5-RCP8.5 for Europe and African cities and SSP2-RCP4.5 for Houston). Heat stress risk is represented differently in each case-study, namely the number of persons (million) at very high risk of heat stress in Europe, the number of person-days per year (billion) of exposure to dangerous heat in African cities, and the number of non-accidental summer mortality (thousand) in Houston.

196 8.1.6. Uncovering a wide range of possible outcomes

As mentioned on numerous occasions throughout this thesis, future socioeconomic and climatic conditions are highly uncertain and will depend on the types of socioeconomic development and on the emissions levels. The use of socioeconomic and climate scenarios (such as the SSPs and RCPs) is therefore crucial to account for uncertainty in future socioeconomic and climatic conditions and to explore the future spread of possible outcomes. The SSP-RCP framework, made up of 5 different SSPs and 4 different RCPs (note that climate projections are largely lacking for RCP6.0 – relative to the other RCPs – hence making it difficult to account for), offers the possibility to account for plentiful plausible futures. Throughout the three case studies, I made good use of the numerous possible scenarios and accounted for no less than 9 different scenarios for the European case-study depicted in Chapter 4, 12 different scenarios for the African cities case-study depicted in Chapter 5, and 7 different scenarios for the Houston case-study depicted in Chapter 6 (Figure 8.5).

Fig. 8.5 – Overview of the SSP-RCP combinations employed in the three different case studies of this doctoral thesis.

Employing such a large number of scenarios allows for exploring the full range of possible outcomes, in terms of future heat stress risk. Throughout the three case-studies, I demonstrated that the range of future outcomes is extremely broad. For instance, the number of persons at very high risk of heat stress in Europe in the 2050s ranges from 13 million under SSP1-RCP2.6 (i.e., a low-emissions scenario and a socioeconomic pathways depicting a European population with very low vulnerability) to 216 million under SSP3-RCP8.5 (i.e., a high-emissions scenario and a socioeconomic pathways depicting a highly vulnerable population with disintegration of the social fabric). In the two other case studies, the range of outcomes are less broad but still very substantial (Figure 8.6). It is also worth mentioning that the SSP-RCP combinations leading to the lowest and highest levels of heat stress risk may differ from one case study to another.

In the case study of African cities, it is SSP4-RCP8.5 that leads to the highest exposure to extreme heat – and not SSP3-RCP8.5 like in the European case study – due to the high demographic growth and fast urbanization depicted across Africa under SSP4. In

SSP1 SSP2 SSP3 SSP4 SSP5

RCP8.5 X X X X X X X X X X

RCP4.5 X X X X X X X X X X X X X

RCP2.6 X X X X X

Shared Socioeconomic Pathways

R e p re se n ta ti ve Co n ce n tr at io n P at h w ay s

Europe African cities Houston

197 Houston, it is SSP1-RCP4.5 that leads to the highest number of mortalities, despite the fact that SSP1 depicts a very socially equitable society with low vulnerability. This surprising result is explained by the crucial role that ageing plays in shaping future heat-related mortality in Houston and the increased ageing depicted under SSP1 in Houston.

This shows that the influence of a given SSP on future heat stress risk is highly context-dependent.

Fig. 8.6 – Synthesis of the current level (blue) and the future range (grey) of heat stress risk for each case study, for the 2050s (Europe and Houston) or the 2090s (African cities). Heat stress risk is represented differently in each case-study, namely the number of persons (million) at very high risk of heat stress in Europe, the number of person-days per year (billion) of exposure to dangerous heat in African cities, and the number of non-accidental summer mortality (thousand) in Houston.

Overall, the wide range of outcomes in future heat stress risk is a crucial indicator of the uncertainty associated with projections of climate-related risks and hints at the enormous potential for policy-makers to minimize the future impacts of climate change by taking actions to reduce vulnerability and mitigate climatic hazards.

8.1.7. Demonstrating the central role of socioeconomic pathways

One of the main advantage of the SSP-RCP framework lies in that it offers the possibility to combine a given SSP with different RCP – and vice-versa –, which allows for disentangling the relative contribution of socioeconomic development and climate change to future climate-related risks (van Vuuren et al., 2013; van Ruijven et al., 2014).

Using this feature of the SSP-RCP framework, one can explore the avoided impacts due to shifts in SSPs (that is, a shift from a SSP depicting high vulnerability and/or high exposure to a SSP depicting low vulnerability and/or low exposure) with that due to shifts in RCPs (that is, a shift from a high-emissions RCP to a low-emission RCP). Shifts in SSPs are proxies for adaptation strategies and social policies, while shifts in RCPs are proxies for emissions mitigation strategies.