Global warming is an environmental issue of great concern as it increases the frequency, intensity, and duration of extreme weather and climate events (IPCC, 2014; Perkins-Kirkpatrick and Gibson, 2018; Trancoso et al., 2020). Heatwave, a type of extreme weather, can induce heat exhaustion, heat oedema, heat cramps, heat syncope, and heatstroke, thereby causing acute cerebrovascular accidents, chronic pulmonary conditions, cardiac conditions, kidney disorders, and psychiatric illness (Chen et al., 2018; McGregor, 2015). The devastating effects of major heatwaves over the past decades have been well documented (Guo et al., 2018; Ma et al., 2015; Patz et al., 2005; Robine et al., 2008; Yang et al., 2019). For instance, the European heatwave in the summer of 2003 caused more than 70,000 deaths (Robine et al., 2008), and nearly 55,000 people died as a result of the Russian heatwave in 2010 (Patz et al., 2005). Therefore, heatwave is still an important public health challenge.

To fully understand the overall risk of global warming, we need to understand how heatwaves can affect mortality in a changing climate scenario, particularly the extent to which the effect of heatwaves will be modified by heatwave characteristics. There have been a few studies projecting the mortality burden of future heatwaves. For example, a study using the definition of heatwave with daily maximum temperature reported that the city of Chicago could experience between 166 and 2217 excess deaths per year during 2081–2100 without considering adaptation scenarios and population change (Peng et al., 2011). Another study in the USA defined the heatwave using daily average temperature and found that heatwave-related excess deaths will increase from 36% to 365% in 2031–2080 compared to that during 1971–2020 (Guo et al., 2018), based on various population scenarios. However, the results of these studies are difficult to compare because of differences in the definitions of heatwaves, climate change scenarios, population change patterns, and adaptation assumptions. These uncertainties are particularly important in projecting future mortality burdens considering future hot weather or heatwaves (Sanderson et al., 2017). Therefore, a study considering all these uncertainties in quantifying future mortality burden in a changing climate is needed.

In addition, it is important to acknowledge future trends regarding the health effects of heatwave characteristics (such as intensity, duration, and timing), which could facilitate the development of targeted heatwave prevention and control policies. Previous investigations have demonstrated that certain heatwave characteristics will change significantly under the context of climate change (Barnett, 2007; IPCC, 2014; Perkins-Kirkpatrick and Gibson, 2018; Trancoso et al., 2020). For example, a study indicated that some tropical regions would experience up to 120 extra heatwave days if global warming of 5 °C is reached, and heatwave duration is projected to increase by 2–10 days/°C (Perkins-Kirkpatrick and Gibson, 2018). Recent attention has focused on the mortality risks of historical heatwave characteristics or their modifications (Anderson and Bell, 2011; Son et al., 2012; Sun et al., 2021), but the evidence regarding future trends of these mortality risks under climate change scenarios is unavailable.

In the present study, we aimed to project the excess mortality caused by heatwaves using diverse definitions under different climate change, population, and adaptation scenarios in Guangzhou, one of the largest cities in China with hot summers. We further differentiated the trends in excess mortality associated with heatwave characteristics and explored the influence of future population growth and adaptation scenarios. This study will contribute to a better understanding of heatwave-related health burdens in the complex context of global warming.