Half a degree Additional warming: Prognosis and Projected Impacts on Health (HAPPI-Health)

The most recent Lancet Commissions on climate change and health concluded that "Climate change is the biggest global health threat of the 21st century". Here, we specifically consider the thermal-health component of the future climate-health burden in an attempt to estimate, for the first time, the number of temperature related deaths under future climate change in developing regions of the world. This number is surprisingly hard to calculate even with large error bars. Aside from the uncertainties in climate projections, the relationship between heat stress and human health varies significantly between countries, and even between cities within the same country. Estimates have been made on a regional scale in some developed countries. For instance, in the UK, a ~250% increase in heat related mortality by the year 2050 was estimated from an annual baseline of ~2000 current deaths (Hajat et al, 2014). This proposal aims to provide a comprehensive understanding of extreme temperatures and associated temperature-related mortality in all regions of the globe, including previously avoided regions such as developing nations, by characterising the uncertainties in different methods of climate change projections, mechanisms driving the extremes, and their relationship to the temperature-health burden at the city level.

Specifically, for future climate we consider Paris Agreement climate scenarios. The Paris Agreement aims to limit globally averaged temperatures to well below 2C above pre-industrial levels, and pursue ambitions to limit it to 1.5C. But this aim is currently supported by rather thin scientific evidence (James et al, 2016), in particular with respect to relative risks of high-impact extreme weather events. Sea level rise aside, the impacts of a global warming of 1.5C, and the impacts avoided by stabilising temperatures at 1.5 instead of 2C, will be dominated, in most regions of the world, by changing risks of extreme weather events, hence the relevance of our proposed research. Fischer & Knutti (2015) estimate that, on a global average, the occurrence of heat extremes doubles between 1.5 & 2C warming. For individual regions, large-scale averages do not provide an adequate basis for decisions on risk prevention and resilience. Changes in atmospheric dynamics and factors other than greenhouse gases also affect heat and rainfall extremes, and, locally, may yield changes in risk that are either greater than or even opposed to the global average (Schaller et al, 2016).

The Paris Agreement calls for research into the impacts of a given level of warming, not the impacts of a scenario that is expected, at some probability, to yield a given level of warming. This requires a new approach to estimate future climate which is complementary to the scenario-driven experiments that provide the core of CMIP5 and CMIP6. To address this, we employ the newly developed Half a degree Additional warming; Prognosis and Projected Impacts (HAPPI; Mitchell et al, 2016a) scenario set - a set of targeted experiments specifically designed to address questions related to the Paris Agreement targets of 1.5C and 2C global averaged warming anomalies.

The Paris Agreement is a major step forward for the international climate community, and will play a large role in the next IPCC report (AR6) and well beyond. This proposal brings together experts in climate (Mitchell and Allen) and health (Gasparrini), to provide a comprehensive analysis of one the key impacts of climate change, temperature related mortality. It provides an assessment of what drives extreme temperatures, where the climate change signals are largest, and how these impact on the regional- and city-level health burden around the world.

Policy makers
One of the most important impacts of climate change is the possibility of enhanced extreme weather events. Many aspects of political decision making involve interpreting the scientific evidence presented, this spans the national levels, to the city governance. For instance makes decisions on city preparedness during heat waves. As such, reliable estimates of projected climate are required in order to accurately inform these decisions. Research from this project will directly contribute to the reliability of projected extreme weather risk under 1.5C and 2C scenarios and therefore aid in policy discussions, especially at the city level, where temperature-mortality impacts become more important. Note that it is imperative for our research on 1.5 v. 2C commences immediately to ensure that papers are submitted in time for the IPCC AR6 report, for which the publication deadline is likely to be October 2020. Work beyond that scope will also be highly relevant for other major reports, for instance the Lancet Commission on Climate Change, as well as annual statements made by the World Health Organisation (WHO). Though these reports, and though annual reports from our project partners, the Met Office and Public Health England, our science has the potential to influence policy at a range of levels.

The public
Extreme events are currently at the forefront of public concern according to the World Meteorological Organisation (WMO), while coverage of potential links to climate change and the impacts have increased in the recent years. Examples include the recent long lived Californian drought, widespread flooding in the southern UK, and bleaching of coral off the coast of Australia, all prompting huge media coverage. By increasing our understanding of the link between heat and mortality, and how this might change in the future, this project will clearly be attractive to media outlets and more generally the public community. For instance, a precursor paper to this project (Mitchell et al, 2016, Environmental Research Letters), approaching this question from a climate-attribution point of view, experienced high-levels of coverage both in academic journals (e.g. Brown, 2016, Nature Climate Change), and in the media (e.g. The Guardian, the Daily Mail, Carbon Brief). We will maximise this through inviting new, and previously established contacts from the media, academia and policy circles to an end of project summary meeting, where the project results will be presented and more widely disseminated, including discussions on implications for the different sectors.