Changing population dynamics and uneven temperature emergence combine to exacerbate regional exposure to heat extremes under 1.5 °C and 2 °C of warming

Abstract

Understanding how continuing increases in global mean temperature will exacerbate societal exposure to extreme weather events is a question of profound importance. However, determining population exposure to the impacts of heat extremes at 1.5 • C and 2 • C of global mean warming requires not only (1) a robust understanding of the physical climate system response, but also consideration of (2) projected changes to overall population size, as well as (3) changes to where people will live in the future. This analysis introduces a new framework, adapted from studies of probabilistic event attribution, to disentangle the relative importance of regional climate emergence and changing population dynamics in the exposure to future heat extremes across multiple densely populated regions in Southern Asia and Eastern Africa (SAEA). Our results reveal that, when population is kept at 2015 levels, exposure to heat considered severe in the present decade across SAEA will increase by a factor of 4.1 (2.4-9.6) and 15.8 (5.0-135) under a 1.5 • -and 2.0 • -warmer world, respectively. Furthermore, projected population changes by the end of the century under an SSP1 and SSP2 scenario can further exacerbate these changes by a factor of 1.2 (1.0-1.3) and 1.5 (1.3-1.7), respectively. However, a large fraction of this additional risk increase is not related to absolute increases in population, but instead attributed to changes in which regions exhibit continued population growth into the future. Further, this added impact of population redistribution will be twice as significant after 2.0 • C of warming, relative to stabilisation at 1.5 • C, due to the non-linearity of increases in heat exposure. Irrespective of the population scenario considered, continued African population expansion will place more people in locations where emergent changes to future heat extremes are exceptionally severe.

Projections of Human Exposure to Dangerous Heat in African Cities Under Multiple Socioeconomic and Climate Scenarios

“…The few existing studies that performed similar analysis found that a shift from a high to a low radiative forcing pathway led to greater reduction in exposure than a shift from a high to a low population growth pathway. This holds true at the global scale (Jones et al, ) but also (i) in the United States, where Jones et al () found that a shift to a lower emission pathway or to a lower population growth pathway would reduce exposure by ~56% and ~45%, respectively; (ii) in India, where Mishra et al () showed that a shift from high to low population growth has a lower influence on exposure than a shift from +2 to +1.5 °C; (iii) in Eastern Africa, where Harrington and Otto () found that shifting from +2 to +1.5 °C would reduce exposure by ~81%, whereas shifting from a medium (SSP2) to low (SSP1) population growth pathway would reduce exposure by ~28%; and (iv) in North Africa/Middle East and Sub‐Saharan Africa, where Jones et al () demonstrated that a shift from a high (SSP3) to low (SSP5) population growth pathway would lead to a lesser reduction in exposure than a shift from a high (RCP8.5) to low (RCP4.5) emission pathway (~33–39% vs. ~47–57%). Our results—while not entirely comparable because based on different population projections, HI, and scenarios range—differ slightly from those found in the literature in that a shift from a high to a low urban population growth pathway leads to a slightly greater reduction in exposure than a shift from a high to a low emission pathway (~51% vs ~48%).…”

“…Although some recent studies have considered only maximum temperature to define the heat index ( HI ) without accounting for humidity, for example (Dong et al, ; Harrington & Otto, ; Liu et al, ), evidence suggests that humidity plays an important role in temperature discomfort and dangerous heat and thus must be integrated into the construction of the HI (Coffel et al, ; Davis et al, ; Matthews et al, ; Mora et al, ), particularly in South America, Africa, and South Asia (Russo et al, ). Various heat metrics that include both temperature and humidity have been developed over the past few years, all performing well and rather similarly (Anderson et al, ; Matthews et al, ).…”

“…Driven by both high population growth and significant changes in climatic conditions, future exposure to dangerous heat in Africa is projected to show the highest increase worldwide during the 21st century, with South Asia being close behind (Coffel et al, 2018;Dong et al, 2015;Jones et al, 2018;Liu et al, 2017;Matthews et al, 2017). Recent regional studies conducted in Eastern Africa (Harrington & Otto, 2018) and across the Great African Lakes region (Asefi-Najafabady et al, 2018) have provided a closer look at the effects of changes in socioeconomic and climatic conditions on future exposure to dangerous heat in some parts of Africa. However, the effects of such factors on the urban populations of the many large African cities remain to be explored.…”

Global exposure and vulnerability to multi-sector development and climate change hotspots

“…Risks are not just dependent on the severity of climate change and subsequent hazards but critically depend on the population's spatial distribution (exposure) and their vulnerability and capacity to prepare for and manage changing risks [5]. Increasingly studies are showing that the world's poorest are disproportionately exposed to changes in temperature extremes [6,7] and challenging hydro-climatic complexity [8][9][10]. In the water sector, between 8%-14% of the global population are expected to face severe reductions in available water resources between 1.7 • C-2.7 • C [11] and in the energy sector, more than 70% of a 'business as usual' 2050s population could expect climate sensitive changes in energy demand of +/− 5%, with negative impacts overwhelmingly in low and middle income countries [12].…”

Intensified hydroclimatic regime in Korean basins under 1.5 and 2°C global warming

“…This agreement aims to keep the increase in global average temperature well below 2.0 C above preindustrial (PI) levels and to pursue ambitious targets to keep the increase to 1.5 C above PI levels. A significant body of recent studies has attempted to quantify the benefits of avoiding an additional 0.5 C increase in temperature by comparing the emerging patterns of climate change induced by global temperature increases of 1.5 and 2.0 C. Although simple thresholds based on the global average temperature are limited in their ability to represent significant regional variations (Harrington and Otto, 2018), many studies recognize the significant impact of a 0.5 C warming increase on regional climate in terms of temperature-related extremes or heat stress worldwide (e.g., King and Karoly, 2017 for Europe; Sylla et al, 2018 for West Africa; Harrington and Otto, 2018 for Southern Asia and Eastern Africa; Lee and Min, 2018;Li et al, 2018 for East Asia). These studies show that temperature responses to enhanced emissions forcings are relatively clear and straightforward.…”