The benefits of limiting global warming to the lower Paris Agreement target of 1.5 °C are substantial with respect to population exposure to heat, and should impel countries to strive towards greater emissions reductions.
Since the Paris Agreement was reached in December 2015 there has been a drive in the scientific community to understand the impacts of global warming at the target levels of 1.5 °C and 2 °C above pre-industrial levels1,2,3. A Special Report on the pathways to limiting global warming to 1.5 °C, and the associated implications of this target, is being prepared by the IPCC. Research has so far focused on changes in different types of climate extremes globally1,3 or regionally2,4, developing and utilizing model experiments to infer differences between the two warming targets5, or the emissions and warming trajectories associated with meeting or breaching the 1.5 °C target6,7. Here we approach the question of how different a 1.5 °C world and a 2 °C world are through the lens of human population exposure to historically unprecedented heat extremes warmer than those observed since 1950 in Europe. We show that the proportion of the population exposed to hot summers above the current record increases dramatically from 1.5 °C of warming to 2 °C. In the past, record summer heat in Europe has been associated with severe heatwaves resulting in thousands of excess deaths8, albeit with high variability in impacts between events — in part due to non-climatic factors. Nonetheless, global warming must be limited to reduce human exposure to historically unprecedented heat.
People tend to remember record hot summers9, and such extremes are well observed over a long period in Europe especially, providing a useful benchmark for investigating future climate extremes. The warmest observed summers (June–August) in Europe from 1950–2017 are associated with average temperatures below 15 °C in parts of Scandinavia, Scotland and the Alps, rising to temperatures exceeding 25 °C around much of the Mediterranean (Fig. 1a). Since populations and ecosystems are well-acclimatized to temperature variability in their locations, summer temperatures exceeding these observed records could have dire consequences even where they may be relatively low — in northern Europe compared with Spain and Italy, for example10.
For the majority of Europe, the hottest summers on record since 1950 occurred after 2000 (Fig. 1b) with the summers of 2003 and 2006 being the hottest over much of western Europe11,12 while 2010 was the hottest further east. There are exceptions, however; for example, in Southern and Central England the hottest summer remains 1976. All the aforementioned summers were associated with shorter spells of record-breaking extreme temperatures and major impacts, such as excess heat-related deaths in western Europe in 20038, wildfires in Russia in 2010 and severe drought in England in 1976.
In future 1.5 °C and 2 °C worlds, represented in bias-adjusted model projections, we find an increase in the likelihood of historically unprecedented hot summers (hereafter used to refer to summer average temperatures that exceed the observed record summer during 1950–2017 at each location). The probability of a hot summer breaking the current record is higher across Europe in a 2 °C world than in a 1.5 °C world, and at least doubles in parts of southern and eastern Europe (Fig. 2a). This illustrates the benefit of limited global warming through reduced heat extremes4,13.
Increasing exposure to summer heat
In each year within each world (natural/pre-industrial, current, 1.5 °C and 2 °C)2,4 we aggregate the population (based on 2010 estimates; see Supplementary Information Section 4) experiencing unprecedented hot summers. Figure 2b shows the probability distributions of aggregated total population exposed to these hot summers in Europe in each world. In the current climate, most summers see a small proportion of Europe’s overall population exposed to temperatures above the observed record, with a median estimate of 45 million (in recent observations, 2003 was an exceptional year with larger numbers of people experiencing a new record). The population exposed to summer heat increases for the simulated Paris Agreement target worlds. On average, in the simulated 1.5 °C world, 90 million people (or 11% of the estimated 2010 population of the continent) are exposed to hot summers beyond the observed record (that is, half of summers would have more than 90 million people exposed to historically unprecedented summer average temperatures). In the simulated 2 °C world, on average there are 163 million Europeans (or 20% of the continent’s population) experiencing summer temperatures exceeding the observed 1950–2017 record. That is equivalent to more than ten times the metropolitan population of Western Europe’s largest city, London, and is about twice the population of Germany.
The exposure of populations to historically unprecedented summer heat increases dramatically even at the relatively low global warming levels of the Paris Agreement (Fig. 2c). For example, the chance of having a summer with such widespread heat that at least 400 million people (or almost 50% of the continental population) experience a summer temperature exceeding the historical record is negligible in the current climate. In contrast, in the modelled 1.5 °C world such an event would occur on average once in 18 years (Fig. 2c) and in the simulations for the 2 °C world the likelihood rises such that a high-exposure event would occur on average once every seven years (Fig. 2c). We have already raised the odds in favour of hotter summers and increased population exposure to summer heat, and even under low global warming scenarios associated with the Paris Agreement this effect is exacerbated.
An incentive for stronger action
As the Earth warms populations will have to cope with more frequent and more intense heat extremes1,3. We show that for the densely populated regions of Europe that have previously experienced devastating impacts of severe heat, particularly in 20038,14 and 201011, there is a substantial benefit (with respect to reduced heat exposure) to limiting global warming to the 1.5 °C Paris target. This benefit is perceptible even when compared with a 2 °C world, let alone higher levels of global warming. This benefit is also likely to extend to other regions of the world15, although we chose to focus only on the European continent (see Supplementary Information Sections 1 and 10 for further discussion).
Before the Paris Agreement, more focus had been placed on 2 °C of global warming and higher. Only since the end of 2015 has there been a shift in the scientific community towards investigating the implications of lower levels of global warming. Although it is recognized that it will be very difficult to meet the aspirational 1.5 °C Paris target, the benefits of doing so would be very great with respect to limiting the frequency and intensity of hot extremes and the consequences of these events. This may act as additional motivation for the world to aim for the 1.5 °C Paris target and develop an emissions pathway and associated technologies that will increase the likelihood of achieving it.
European countries are among the most ambitious in the world in tackling climate change through significant intended reductions in GHG emissions. Here we illustrate that this need not be a selfless act; the countries and peoples of Europe — especially the Mediterranean region, which has suffered in recent hot summers — would benefit from a future of fewer hot summers with limited global warming.
Regardless of the emissions trajectory that the world takes over the next few years, global warming will continue, and heat extremes and associated population exposure will increase. Alongside efforts to limit global warming, strategies to adapt to hotter summers that lie outside of the observed range will be needed to reduce heat–health impacts.
Several authors received funding from the Australian Research Council: A.D.K. and D.J.K (CE110001028), M.G.D. (DE150100456), S.C.L. (DE160100092) and B.J.H. (LP150100062). P.A.S. was supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. We acknowledge the support of the NCI facility in Australia and the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and sharing their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We acknowledge the E-OBS data set from the EU-FP6 project ENSEMBLES (http://ensembles-eu.metoffice.com) and the data providers in the ECA&D project (www.ecad.eu).