How do population dynamics affect impact of heat extremes?

Understanding how continuing increases in global mean temperature will exacerbate societal exposure to extreme weather events is a question of profound importance.

Following the signing of the Paris Agreement in December 2015, a targeted focus has emerged within the scientific community to better understand how changes to the global climate system will evolve in response to specific thresholds of future global mean warming, such as 1.5 ◦C or 2◦C above ‘pre-industrial levels’.

However, determining population exposure to the impacts of heat extremes at 1.5 °C and 2 °C of global warming requires not only a robust understanding of the physical climate system response, but also consideration of projected changes to overall population size, as well as where people will live in the future.

A recent analysis [1] by Dr Luke Harrington and Dr Friederike Otto of climateprediction.net 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).

Their results reveal that, when population is kept at 2015 levels, exposure to heat considered severe in the present decade across these regions 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.

Projected population changes by the end of the century 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. Therefore continued African population expansion could place more people in locations where emergent changes to future heat extremes are exceptionally severe.

While local adaptation planners might be primarily be interested in how the patterns of heat extremes align with changes in population over their immediate community, it is equally important for decision makers to recognise the broader implications of heat exposure increases driven by future changes in where people live.

Acknowledgments

The authors would like to thank colleagues at the Oxford e-Research Centre: Andy Bowery, Sihan Li, Mamun Rashid, Sarah Sparrow and David Wallom for their technical expertise. They also acknowledge the MetOffice HadleyCentre PRECIS team for their technical and scientific support for the development and application of Weather@Home.

Finally, they would like to thank all of the volunteers who have donated their computing time to climateprediction.net andWeather@Home.

[1] Changing population dynamics and uneven temperature emergence combine to exacerbate regional exposure to heat extremes under 1.5°C and 2°C of warming, Environmental Research Letters, February 2018