1 Introduction
The world is currently experiencing profound systemic shocks, including but not limited to the Coronavirus pandemic (COVID-19), the ongoing conflict between Russia and Ukraine, and looming crises in food and energy. Against this backdrop, climate change has further intensified the health impacts faced by humanity (
Ebi and Hess, 2020;
Romanello et al., 2022). Climate change and pandemics are deemed as the “black elephants” of the Anthropocene−both rare, yet catastrophic “black swan” events and also pressing, but unaddressed “elephants in the room”. These “black elephants” reveal profound and pervasive inequalities within human society (
Asayama et al., 2021).
In recent years, the impact of climate change on human health has attracted widespread attention from both academia and society. Working Group II of the Sixth Assessment (AR6) of the Intergovernmental Panel on Climate Change (IPCC) has assessed the immense risks that climate change poses to human health and well-being. According to the report (
Cissé et al., 2022), there is an increase in climate-related illnesses, premature death, malnutrition in various forms, and the threat posed to mental health and well-being. The report encompasses a range of effects including heat waves, flooding and resultant damages, scarcity of food and water, and the spread of infectious diseases. Moreover, the report assessed the attribution uncertainty of various regions and hazards to climate change, using confidence levels as a measure.
In the IPCC AR6 report of Working Group I (
Ranasinghe et al., 2021) and the report of Lancet Countdown (
Romanello et al., 2022), there are numerous pathways identified through which climate change affects human health. Based on these foundational reports, our work offers a focused examination, specifically aimed at providing a comprehensive summary of potential hazards and health impacts of climate change related to health burden inequalities. This examination is informed by the “climatic impact-drivers” (CIDs) identified by the IPCC (
Chen et al., 2021), as well as the chains of impact made by climate change on human health summarized in the IPCC Working Group II report (
Cissé et al., 2022) and the Lancet Countdown report (
Romanello et al., 2022). Tab.1 is an overview of the various climatic health hazards, their brief descriptions, and the potential impacts on health.
Climate change affects every person on Earth, but the impacts are not equally distributed, and the transmission mechanism of the long chain from climate change drivers to human health is complex and multifaceted (
Romanello et al., 2022). The most vulnerable and disadvantaged groups, including women, children, ethnic minorities, poor communities, migrants or displaced people, older populations, and individuals with underlying health conditions, are disproportionately impacted by climate-sensitive health risks (
World Health Organization, 2021). These groups face a higher risk of adverse health outcomes due to their multiple vulnerabilities, exposure pathways, and health system capacity and resilience. The IPCC developed an analytical framework applied to examine the relationship between climate change and health outcomes by highlighting the roles of vulnerability, hazard, and exposure (
Cissé et al., 2022). The IPCC framework conceptualizes the context in which climate change affects health outcomes and health systems as a function of risk, which is a product of interactions between hazards, exposure, and vulnerability. The resulting impacts can further enhance vulnerability and/or exposure to risks.
The academic community has gained a comprehensive understanding of the health impacts of climate change (
Watts et al., 2019b;
Romanello et al., 2021;
Watts et al., 2021;
Cissé et al., 2022;
Romanello et al., 2022;
Niu et al., 2023), and there is a growing recognition given to the unequal distribution of these impacts across different populations (
Pérez-Peña et al., 2021;
Grobusch and Grobusch, 2022;
van Daalen et al., 2022;
Liu et al., 2023). A range of systematic reviews have explored the intersection of climate change and health inequalities. Khanal et al. highlight the fragmented nature of policy analysis frameworks and the need for a comprehensive approach to understanding health equity in climate change policies (
Khanal et al., 2023). Graham et al. emphasize the importance of USA health care’s response to climate change, particularly in addressing air pollution, extreme weather events, and infectious diseases (
Graham et al., 2019). Khine and Langkulsen underscore the role of climate change in exacerbating inequalities and increasing the risk of diseases among vulnerable populations in South Africa (
Khine and Langkulsen, 2023). Scheelbeek et al. provide evidence from low- and middle-income countries, highlighting the need for more evidence on the effects of climate change adaptation responses on public health (
Scheelbeek et al., 2021). However, few studies have systematically reviewed the broad range of uneven distribution of climate change-related health impacts on a global level from a comprehensive perspective. To address this gap, it is necessary to identify what evidence exists regarding the unequal impact of climate change on health outcomes, to identify the most significant inequalities, and to advance policies to eliminate these inequalities.
The review was guided by the following questions:
1) To what extent do exposures, sensitivity, adaptive capabilities, and health outcomes related to climate change exhibit uneven global distribution?
2) Which factors, including demographic and socioeconomic conditions, physiologic responses, and geographical elements, contribute to the inequality in health impacts of climate change?
3) How has the literature addressed the equality of health burdens due to climate change, focusing on research trends, geographical emphasis, and methodological approaches?
2 Methods
2.1 Search strategy
In preparation for our systematic literature review, we engaged in a rigorous process of keyword identification, informed by an extensive survey of relevant literature and consultations with experts in the field. This process was pivotal in shaping a search strategy adept at capturing studies intricately addressing the intersection of health disparities and environmental aspects within the broader context of climate change. Our strategy honed in on the titles and abstracts of publications across the Web of Science, Pubmed, and Scopus databases, a decision rooted in the pursuit of precision and relevance. This focus was based on the understanding that titles and abstracts succinctly encapsulate the core themes and findings of research, serving as robust indicators of their relevance to our investigative purview.
Our search encompassed an array of carefully chosen terms related to health (e.g., “health” and “healthy”), disparities (e.g., “disparity”, “inequality”, and “justice”), environmental factors (e.g., “environment”, “environmental”), and the overarching theme of climate change. The search was limited to literature published before January 2023 and was confined to English-language journal articles. This language restriction was a pragmatic decision, aimed at harnessing the extensive body of research predominantly available in English, while maintaining consistency and manageability in our analysis. This approach, while acknowledging certain limitations, was deemed most suitable for fulfilling the objectives of our study within the scope of our resources and time frame. A total of 1813 search results were obtained from Web of Science, 266 from Pubmed, and 361 from Scopus. After removing duplicates, a total of 1766 records were obtained.
2.2 Screening
The screening process is shown in Fig.1. According to several criteria, the titles and abstracts were screened. For inclusion, documents were restricted to the relevant peer-reviewed English-language journal articles on the climatic health hazards identified in Tab.1.
Then, 1649 records were excluded after a review of their titles and abstracts. During the process of eligibility screening, two types of articles were excluded. One included those without primary data, which can be quantitative or qualitative data. These were mainly literature reviews. The other included the methodological or predictive studies that did not analyze existing evidence. Finally, 65 articles were selected out of 2440 search results for subsequent literature review. These articles were fully reviewed to identify their methodology, time and region of study, key findings, and other relevant details. Supplementary Material A presents the basic information about these articles.
2.3 Keywords mapping
With Vosviewer 1.6.19 (
van Eck and Waltman, 2010) software applied to conduct co-occurrence analysis, the keywords of these included articles were visualized, including their frequency of occurrence, correlations, and average publication year. The bibliographic data was counted using the default full counting method, with the strength of each link set to 1. In total, 46 keywords were selected by setting the minimum number of occurrences of a keyword to 3. Represented by the size of the circles in the keyword map, the weights were calculated on the basis of occurrence. Besides, the colors indicated the average year of publication of the literature.
2.4 Analytical framework
Under the frameworks provided by the IPCC (
Cissé et al., 2022) and WHO (
World Health Organization, 2021), an analytical framework consisting of causal chains and influencing factors was developed, as shown in Fig.2.
The framework involves three main components in the causal chain from climate drivers to health outcomes: exposure, sensitivity, and adaptive capability. In the left-side box, the causal factors of inequality are identified, including demographic and socioeconomic conditions, physiological conditions, and geographical factors. On the right, there are definitions for exposure, sensitivity, and adaptive capability. Examples are provided for the intersection between causal factors and climate change-related health burdens. Under this analytical framework, the inequality of climate change-related health impacts for individuals and populations was reviewed. Furthermore, the intersection of various factors in the causal chain of health outcomes was examined. In addition, the existing literature was reviewed to identify the research gaps and future research directions in terms of research institutions, topics, regions, and methodologies.
3 Results
3.1 Research trend
Fig.3 illustrates the distribution of the publication year and the country affiliations of all authors’ institutions for each article. In cases where the authors of a single article are affiliated with institutions in “n” different countries, each country is attributed a count of 1/n. This approach ensures an equitable representation of multinational collaborations, reflecting the global nature of research contributions and providing a comprehensive understanding of the geographical diversity in the field. The number of studies showed a significant increase after the publication of the 5th IPCC report in 2014, accounting for 89.2% (58 out of 65) of the total. The majority of publications were authored by researchers from the USA (n = 36.1), followed by China (n = 5.8), Australia (n = 3.6), Germany (n = 3.5), and Canada (n = 2.7). Developed countries contributed most to the studies (54.9 studies, 84.5% of the total).
The co-occurrences and average publication time of keywords were mapped to visualize the trend of changes in the research focus (Fig.4). Initially, the studies (represented in blue) focused mainly on analyzing the health impacts of high and low temperatures through statistical methods, such as regression model. Subsequently, the research paid increasing attention to distinguishing the health effects on susceptible groups, such as the variations in health outcomes based on age, socioeconomic status, and gender/sex (indicated in orange). In recent years, the scope of climate-related hazards has expanded from the initial focus on temperature to the inclusion of drought, extreme weather events, floods, and other hazards. Regarding health impacts, there has been an expansion from traditional indicators such as morbidity and mortality rates to the inclusion of various dimensions such as mental health, vulnerability, and adaptive capacity. Furthermore, COVID-19 has emerged as a new research focus, as researchers have started to explore the overlapping and interactive effects of various health risks faced by humanity.
3.2 Regions in focus
For alignment with the continental regions of the world, the study area was divided into seven regions: North America, Europe, Oceania, Asia, Africa, South America, and the whole world. In this context, the whole world means that the spatial scale of the study is global. Antarctica was excluded from our analysis due to the absence of permanent inhabitants and the limited availability of literature on the health effects of climate change in that region.
Fig.5 illustrates the distribution of publications, with different climatic health hazards covered across various regions. The number of publications related to heat is found to be the highest, with 44 out of the 65 publications involved. Rising sea levels and extreme rainfalls are the second most commonly studied hazards, involving 14 publications. Cold spell ranks third with 11 publications involved. Droughts and wildfires are less frequently studied, each of which involves seven publications. They are followed by food-related and climate-infectious diseases, with six publications each. As for cyclones and storms, the number of publications is the lowest, which is 3. Additionally, the four articles classified as “Other” are common in discussing air pollution, including particulate matter, NO2, and ozone. Notably, each publication in Fig.5 can be counted multiple times if it covers more than one region or hazard.
By region, the majority of studies were focused on North America (n = 36), followed by Europe (n = 9), Asia (n = 8), Africa (n = 7), Oceania (n = 4), global analysis (represented as Whole World, n = 3), and South America (n = 2).
With reference to the IPCC AR6 report by Working Group II (Intergovernmental Panel on Climate Change, 2022), the urgency of regional risks was illustrated. In our review, these key risk categories were corresponded to the hazards of heat, wet and coastal, food, and disease, with the trends in adverse impacts assessed by the IPCC and the confidence degree of attribution to climate change marked on the graph in red and pink for high and medium, respectively. To some extent, this indicates the urgency of the different risks faced in different regions.
As mentioned in the introductory section, the uneven distribution of climate-related health impacts is not only severe but also widespread. Often, a severe crisis of inequality occurs where climate health risks are most urgent. Therefore, the urgency of regional health risks was further investigated as a reference for comparison to the amount of literature on health inequalities in that location. It was discovered that some regions are faced with serious health impact risks. However, there is limited literature on the inequalities in those regions, implying an area for potential progress in future research. For instance, significant health risks are posed in Oceania by wet and coastal hazards, as well as food insecurity. However, there is an absence of literature focusing on the unequal distribution of health impacts in these areas. Similarly, there are health risks faced by Europe due to wet and coastal hazards, food insecurity, and disease. However, most studies revolve around the unequal distribution of health impacts related to high and low temperatures. In contrast, there are few studies exploring the unequal distribution of health impacts caused by other hazards.
It is worth noting that the articles selected for this review are aimed specifically at addressing inequality. They constitute a subset of the broader literature on the health impacts of climate change. By comparing regional health risks with the amount of literature on inequalities, the possible research gaps in this particular sub-field were identified, which is the only purpose of such comparison.
3.3 Methodologies of existing literature
The selected articles were categorized into three broad types depending on the research methodology applied: qualitative studies, quantitative studies, and studies utilizing mixed methods, where both qualitative and quantitative research tools are employed. The spatial and temporal scales of research are also indicated in Fig.6.
Among the 65 articles included in this study, the majority (55 articles) rely on quantitative analysis, while six articles are based on qualitative assessment, and the remaining four articles adopt mixed methods. In terms of research scale, most studies are conducted on a relatively small temporal and spatial scale, which is typically less than ten years (n = 54) and at the city or sub-national level (n = 46).
As a common method used to conduct research on climate change and health, statistical regression is performed in 33 out of 65 articles. The methods of this class encompass a range of statistical techniques, including but not limited to linear regression (
Grineski et al., 2012;
Wanka et al., 2014;
Smith and Hardeman, 2020;
Rhubart and Sun, 2021;
Ellena et al., 2022;
Navas-Martín et al., 2022), Poisson or quasi-Poisson regression (
O’Neill et al., 2003,
2005;
Anderko et al., 2014;
Green et al., 2015;
Yang et al., 2015;
Riley et al., 2018;
Liu et al., 2020;
Conte Keivabu, 2022;
Ellena et al., 2022;
Khatana et al., 2022;
Nyadanu et al., 2022), negative binomial regression (
Abadi et al., 2022;
Lambourg et al., 2022;
Lee and Brown, 2022), spatial regression (
Grineski et al., 2012;
Dialesandro et al., 2021), logistic regression (
Mutic et al., 2018;
Callender et al., 2022), and others.
Through the regression method, the impact of climate change on people’s health outcomes can be described quantitatively and objectively. Besides, the vulnerable populations subjected to climate change inequalities can be identified, along with the variables linked to health outcomes. However, this method is also faced with limitations. One of the most significant drawbacks is that regression models are restricted to showing associations rather than causality. Additionally, it falls short in capturing the complex relationship between climate change and health outcomes, particularly due to challenges in integrating indirect and non-quantitative factors. Another challenge lies in data accessibility, especially in the less-developed regions where climate health challenges are more pronounced.
The main qualitative research methods applied in the selected articles include case studies, interviews, and focus group discussions (
Berrang-Ford et al., 2012;
Hansen et al., 2014;
Méndez et al., 2020;
Abrams et al., 2021;
Ahmed et al., 2022;
Engelman et al., 2022). By using qualitative methods, researchers can better understand the experiences and perspectives of individuals or vulnerable groups. Providing a more nuanced and comprehensive view of how social identities intersect and influence vulnerability to the health impacts of climate change, they can help identify the hidden factors that may be overlooked in quantitative data. For instance, the vulnerability of undocumented Latino and Native immigrants during the Thomas Fire in the USA was analyzed through a case study to gain insight into various factors such as racial discrimination, exploitation, economic hardship, limited English and Spanish language skills, and the fear of deportation in their daily lives and in the context of climate change hazards (
Méndez et al., 2020).
Undoubtedly, qualitative research is also subject to some limitations worth considering. Firstly, the findings of qualitative research apply only to a particular group of individuals, which means they can not be generalized to a larger population. Secondly, qualitative research shows subjectivity and is susceptible to the influence of the personal bias or interpretation of the researcher. Additionally, qualitative research may fail to fully reveal the complexity of the relationship between climate change and health outcomes as it is focused on individual experiences rather than larger population-level patterns. In spite of this, qualitative research remains crucial to exploring the experiences and perspectives of vulnerable populations as it provides insights into the social and cultural factors affecting their vulnerability to climate change impacts.
In some studies, qualitative and quantitative methods are used in combination to thoroughly examine the inequalities in the health impacts of climate change (
Harlan et al., 2006;
Wilder et al., 2016;
Adepoju et al., 2022;
Papworth et al., 2022). This approach can be adopted to better understand the experiences and perceptions of vulnerable populations. Meanwhile, a more objective analysis of the data are enabled. Given the complexity of combining quantitative and qualitative methods, careful planning, data management and analysis, and interpretation of findings are required. Despite these challenges, the use of mixed methods represents an important trend of future research in this area, considering its potential to reveal previously unknown insights and enhance the effectiveness of policy and practice.
3.4 Inequality of exposure, sensitivity, and adaptive capability
The causal factors contributing to inequality in the literature were analyzed, and the elements involved in the causal chain that caused inequality were identified. Fig.7 shows the results. Involving multiple elements or causal factors, some of the literature may have been counted multiple times.
Among these elements, exposure is the most studied (34 out of 65 studies), followed by adaptive capability (31 studies) and inherent sensitivity (8 studies). There are a number of articles restricted to describing the inequality of climate change-related health burdens, with no clear indication of the causal mechanism. For this reason, they were coded as “not clear” in terms of the element in the causal chain.
In terms of causal factors, demographic and socioeconomic conditions are most discussed, with 57 out of 65 in this category. Specifically, socioeconomic status (SES) is most frequently studied in this category, with 45 articles exploring its role in the health inequalities caused by climate change. Race or ethnicity is the second most studied factor, with 38 articles centered on this aspect.
Since both gender and sex play a role in shaping health outcomes, this review distinguishes between them as well. Sex refers to biological and physiologic differences, while gender encompasses socially constructed identities, social roles, and behaviors. For example, there is one study exploring the social and economic factors that contribute to gender inequalities in health impacts, such as the unequal ownership and control of household assets, the gender division of labor, as well as the prevalence of discriminatory legal systems and social practices (
Eastin, 2018). This study is classed as a discussion of the “gender” factor. In another study, the differences in heat-related illness symptoms between male and female farmworkers were reported. It was suggested that various biological factors, including high body surface to mass ratio, morphology, and adipose distribution among women, are the contributors to this difference (
Mutic et al., 2018). This study falls into the category of a discussion about the “sex” factor.
3.4.1 Exposure
Exposure to climate change-related hazards is widely viewed as a major contributor to climate-related health burdens. The impact of exposure inequality is especially evident among vulnerable groups, which is attributable to various demographic and socioeconomic conditions, physiologic conditions, and geographical factors. There is usually an interrelationship present between these factors, which leads to the complex causal mechanisms.
In the domain of exposure research, a well-established consensus delineates the mechanisms whereby populations, particularly those of lower socio-economic status, are disproportionately situated in locales more vulnerable to environmental hazards. This dynamic is aptly demonstrated in the studies (
Collins et al., 2019;
Qiang, 2019) which illustrate how economically disadvantaged populations are more likely to inhabit flood-prone zones. For example, Qiang’s analysis (
Qiang, 2019) assessed the socio-economic disparities in population exposure to flood hazards across USA counties. The findings revealed that, at a national scale, economically disadvantaged populations are more prone to reside in flood zones than outside. At a local scale, economically disadvantaged populations tend to occupy flood zones in inland areas, while coastal flood zones are predominantly occupied by wealthier and elderly individuals. Another dimension of socio-economic vulnerability emerges in urban settings, where poorer individuals tend to live in areas characterized by high building density and limited urban green and water spaces. Such urban environmental features exacerbate exposure to heatwaves (
Scherber et al., 2013;
Mitchell and Chakraborty, 2014).
These studies, therefore, collectively underscore the intersection of socio-economic status and environmental exposure, revealing a pattern in which less affluent communities are disproportionately subjected to diverse environmental hazards, such as floods and heatwaves. This pattern not only poses serious public health challenges but also underscores the urgent issue of environmental justice.
3.4.2 Inherent sensitivity
There is a higher level of sensitivity and vulnerability to climatic hazards among some populations due to their own health status, biological characteristics, etc. The articles included in this review focused mainly on three causal factors: physiologic sex, age, and pre-existing health conditions or disabilities.
In a study conducted in 26 regions across the south and west of China from 2008 to 2011, the effect of temperature on cardiovascular mortality was estimated to reveal that males tend to have a higher risk at cold temperatures, while females tend to have a higher risk at high temperatures (
Yang et al., 2015). Despite a lack of clarity on the underlying mechanism for this observation, it is hypothesized that the higher body fat content of females makes them more resistant to coldness but less heat-resistant than their male counterparts. Furthermore, another study on temperatures and sleep suggested that females tend to experience a decrease in their body temperature earlier in the evening than males, which may cause them to experience higher environmental temperatures at bedtime. Additionally, females tend to have more subcutaneous fat, which may increase their difficulty in releasing heat at night. According to the data obtained from this literature, night-time temperature increase has a more significant per-degree negative impact on women than on men (
P < 0.01) (
Minor et al., 2022).
A recent study conducted in Cincinnati, Ohio, analyzed the data on emergency medical service (EMS) incidents over a 5-year period (2016–2020) at the census block group level (
Lee and Brown, 2022). In the study, the proportion of the population aged over 65 was identified as a statistically significant predictor of an increased risk of medical incidents. In another article, the environmental injustice implications of three climate change-related hazards were examined in the bi-national context of El Paso, Texas and Ciudad Juárez, Chihuahua, which are extreme heat, peak ozone, and floods. According to this study, the children in El Paso are significantly more burdened by peak ozone (
Grineski et al., 2012). Similar conclusions have been drawn from other studies, indicating that vulnerable age groups such as the elderly and children tend to be more sensitive to heat stress (
Hansen et al., 2014;
Heudorf and Schade, 2014;
Liu et al., 2020), air pollution, extreme precipitation (
Bush et al., 2014), and other hazards, and are therefore more likely to experience negative health effects.
Focusing on the experiences of people with disabilities during climate disasters, a case study highlights their physiologic vulnerability to the adverse effects of climatic hazards, such as heat and smog (
Engelman et al., 2022). For instance, individuals with autoimmune diseases−which affect over 70 million people worldwide−may find it difficult to regulate their body temperature, thus increasing their susceptibility to the harm caused by temperature. As further indicated in the study, it is often the case that disabilities and pre-existing health conditions exacerbate the exposure to climate hazards, reduce adaptive capacity, and increase the overall vulnerability of this population, which is a combined effect.
3.4.3 Adaptive capability
As shown in the selected articles, different populations varied in their adaptive capacity to climate change. In many cases, the poorer adaptive capacity results from a combination of various exogenous factors such as SES, ethnicity, gender and a range of endogenous factors such as age and prior health status.
For example, it is highlighted in the previously mentioned research on disability justice that disability affects whether and how people have access to the information and resources needed to prepare, manage and adapt to disasters and climate health risks promptly. Those with disabilities and low incomes may not be able to afford the costs of adapting to climate risks, such as the use of back-up generators to power ventilators. They may also encounter additional barriers to accessing technological solutions (e.g., mobile phones, computers and internet access). Additionally, they may be daunted by the “digital divide” and illiteracy, which are more prevalent in developing countries. A similar circumstance is found among the immigrant and refugee groups, where various socio-cultural barriers hinder them from adapting to climate hazards, including low socio-economic status, poor housing conditions, language barriers, isolation, health problems, cultural factors, and a lack of adaptation (
Hansen et al., 2014).
In some papers, the important role of facilities, such as air conditioning, in climate change adaptation is referred to. As demonstrated by a study exploring disparities by race in heat-related mortality in four USA cities, the prevalence of central air conditioning in black households was lower than half of that in white households in all four cities. Furthermore, black deaths were more significantly correlated with high temperatures, and the prevalence of central air conditioning contributed to the variation in heat effects by race to some extent (
O’Neill et al., 2005). In some other studies on heat waves, it has also been highlighted that the poorer, rural populations of racial minorities are less likely to have access to air conditioning, which reduces their capacity to adapt to extreme heat events (
Bobb et al., 2014;
Hu et al., 2019).
3.5 Vulnerable populations
The selected articles were reviewed to identify 12 categories of vulnerable populations. Tab.2 lists the number of papers in which these populations and the corresponding climate hazards are examined. These articles were counted multiple times, given the possibility of them addressing multiple vulnerable populations and climate hazards. The hazards related to temperature are the most discussed topic, of which heat is a particular focus of attention. Those with low SES are the most discussed vulnerable group (n = 34), followed by marginalized populations such as people of color, indigenous people and migrants (these populations are counted together, n = 28) and females (n = 14). Other vulnerable groups include elderly people (n = 12), people in the regions with certain geo-climatic features (n = 7), people with disadvantaged housing conditions (n = 7), people with pre-existing health issues or disabilities (n = 5), children (n = 4), vulnerable industries (n = 4), people in urban areas (n = 3), males (n = 2), and people in rural areas (n = 2).
According to the relevant research, the inequality of climate change-related health burdens is often attributed to various causal factors and mechanisms in combination. Due to a confluence of inequality in multiple dimensions, the uneven distribution of health burdens is perpetuated and reinforced, including but not limited to residential segregation, healthcare, employment, and educational discrimination.
By gender, both females and males have been identified as vulnerable groups, which is largely attributed to the different causal mechanisms of inequality. For example, research has found that males tend to have a higher risk at cold temperatures, while females tend to have a higher risk at high temperatures (
Yang et al., 2015). Despite no clear explanation provided by the authors for the different causal mechanisms, this is speculated to result from the gender-based physiologic differences.
However, socially constructed gender differences and corresponding additional vulnerabilities of women in society remain the focus of discussion about gender disparities in the context of climate change vulnerability. It is indicated that the gender inequalities in climate change vulnerability reflect pre-existing gender inequalities while reinforcing them. In the context of climate change, women are faced with inequalities in owning and controlling household assets, increased household burdens due to male outmigration, the reduction in food and water supplies, the extra vulnerability caused by pregnancy and childbirth, and increased disaster risks, among others (
Schmeltz et al., 2016;
Eastin, 2018;
Mutic et al., 2018;
Berndt and Austin, 2021;
Ahmed et al., 2022;
Minor et al., 2022;
Navas-Martín et al., 2022;
Zeng et al., 2022a). These factors make women less able to gain economic independence, improve human capital, and maintain health and well-being.
Vulnerable groups are comprised of individuals residing in both urban and rural areas, which appears paradoxical. However, there are variations in the cause of unequal health burdens. Rural residents are often considered vulnerable due to their exposure and limited adaptive capacity. For example, a study conducted in Zhejiang, China, revealed a higher mortality risk linked to extreme temperatures in rural areas than in urban areas, regardless of age group, gender, and disease type (
Hu et al., 2019). This is possibly attributed to the demographic and socio-economic factors associated with the level of urbanization in China, such as age structure, education, GDP, health services, occupation type, and access to air conditioning (1.21 vs. 1.66 units per household). Similarly, a study carried out in the USA demonstrated that flood risk is higher in rural areas on average, and rural residents are more vulnerable to flood-related events, which exposes them to both public health issues and economic insecurity (
Rhubart and Sun, 2021).
The unequal sharing of health burden by urban residents is closely linked to heat-related events. According to the relevant studies, the specific urban environment presents various challenges for urban residents, including higher building densities, greater anthropogenic heat emissions, and less green and water space. Consequently, there is a rise in heat-related risks across urban areas (
Scherber et al., 2013;
Wu et al., 2021;
Zeng et al., 2022b).
4 Discussion
4.1 Addressing regional bias and neglected climate hazards
As revealed by the literature review, there is a regional bias in the existing research, as most studies are focused on developed regions and conducted by authors affiliated with institutions in developed countries. This causes a limitation on the generalizability of findings and a lack of attention paid to some less-developed regions and some areas with high climate risks, as noted in Section 3.2. Furthermore, the existing research is focused mainly on temperature-related hazards, with a particular emphasis on heat, while less attention is brought to other climate hazards with a significant impact on human health. To close these gaps, it is necessary to identify the vulnerable populations facing critical health challenges in future research, where less-developed regions and understudied hazards should be prioritized. This is of particular importance as vulnerable populations may require the mitigation of health impacts caused by climate change through tailored interventions (
World Health Organization, 2021;
Cissé et al., 2022).
4.2 Advancing interdisciplinary approaches in climate-health research
Allowing for the long and complex causal chain of impacts of climate change on human health (
Romanello et al., 2022), innovative and interdisciplinary approaches must be adopted to find multidimensional factors and elements across the climate-health causal chain. During the review, a lack of solid evidence has been identified through an in-depth analysis of the causal mechanisms to account for how natural, biological, and social factors contribute to health inequality. In addition, there is an absence of thorough examination of the correlation between exposure, sensitivity, and adaptive capability, which represent the three concepts in the causal chain from climate drivers to health outcomes. The unequal distribution of health burdens is caused in a complex way, as there are multiple forms of oppression intersecting and interacting with each other. In this context, the impact of climate change on health inequalities must be analyzed from an intersectional perspective (
Kaijser and Kronsell, 2014;
Versey, 2021;
Adepoju et al., 2022;
Callender et al., 2022). Regarding methodology, it is highlighted in the review that quantitative and qualitative analyses have their respective strengths and limitations in assessing inequality. It is suggested that future research should be conducted using more innovative tools that integrate multiple methods.
However, it is important to note that the research gaps identified are within the scope of our review, which primarily focuses on empirical data-based studies. Some modeling or theoretical research articles, which are not based on primary empirical data, may have discussed the causal chain and interactions in the literature. However, they are beyond the scope of our analysis.
4.3 Assessing climate-related health inequality at large-scale
To a large extent, the existing research on climate-related health impacts is focused on short-term exposure to regional climate hazards, which leads to a gap in our understanding of how these impacts are distributed across different countries and regions. This is of particular importance given the long-term and global nature of climate change, which can make the existing global health disparities more significant over time. Thus, large-scale studies are required to capture the wide range of temporal and spatial scales related to climate change. This is conducive to gaining insight into the social, political, and economic factors contributing to health inequalities.
However, there are several methodological challenges posed by such studies, including data acquisition and management, resource allocation, and coordination across multiple research teams. To address these challenges, it is essential to establish a more coherent and clear research framework and assessment criteria for better comparability of studies and uniform, robust assessments across regions of the world. Through a regular collection of new findings and evidence in the global field, as done by the Lancet Countdown team (
Watts et al., 2019a;
Romanello et al., 2021;
Watts et al., 2021;
Romanello et al., 2022) and the IPCC (Intergovernmental Panel on Climate Change, 2014a, 2014b, 2022), contribution can be made to this large-scale view and the most up-to-date evidence can be gathered for policymakers and the public.
Ultimately, a global perspective that integrates diverse perspectives and disciplines will be key to understanding the complex and multifaceted relationships between climate change and health inequalities. As elucidated in the works of scholars like (
Smith et al., 2022) and (
Gould and Rudolph, 2015), the role of entrenched structural inequalities in exacerbating health disparities underlines the necessity for an analysis that transcends traditional disciplinary confines. (
Morris et al., 2017) and (
Rudolph and Gould, 2015) enrich this discourse by dissecting the proximal and distal dimensions of climate impact on health, highlighting the spectrum of direct and indirect effects. The perspectives of (
Khanal et al., 2023) and (
Pearson et al., 2023) reinforce the need for an integrated framework focusing on health equity to ensure inclusive and equitable responses to climate change. Such a comprehensive, globally-oriented perspective, weaving together insights from environmental science, public health, and social sciences, is quintessential in untangling the complexities at the intersection of climate change and health inequalities, paving the way for informed, holistic, and effective interventions.
4.4 Strengths and limitations
The primary strength of this article lies in its comprehensive review of peer-reviewed literature on health burden inequalities caused by climate change, meticulously examining the exposure, sensitivity, and adaptive capacity dimensions and their interplay with demographic, socio-economic, and geographical factors. Through critical analysis, the paper not only synthesizes key contributing factors to health inequalities induced by climate change but also identifies gaps in current research trends.
However, the study does face notable limitations. The literature review’s exclusive reliance on English-language sources may omit essential non-English research, potentially introducing geographic biases. This limitation, while a byproduct of striving for consistency and manageability within our constraints of time and resources, highlights the need for future research to embrace a broader linguistic and geographic scope.
Additionally, in pursuit of focused thematic exploration, the study employed a strict search strategy and inclusion criteria, limiting articles to those with specific keywords in their title or abstract. While this approach ensured a concentrated examination of the most relevant literature, it might have inadvertently excluded significant studies. In future research, expanding the range of considered studies would allow for a more inclusive and comprehensive understanding of this complex issue, capturing the full spectrum of impacts and contributing to a more nuanced discussion in this critical field.
Furthermore, while our screening process implemented stringent quality control measures, it did not extend to quantitative scoring or an exhaustive quality assessment of the selected articles. This was largely attributed to the varied nature of the included studies, rendering the establishment of a standardized quality assessment framework challenging. Future research could consider developing flexible quality assessment models that can adapt to the diversity of studies in this field, thereby enhancing the methodological rigor of literature reviews.
5 Conclusions
In this review, the pressing issue caused by climate change to the inequality of health burdens sharing is highlighted. The growing concern about the impact of climate change on human health is underscored by the severe inequalities in human society. Desirably, there has been an increase in the number of studies and research covering a wider range of topics recently, which further evidences the fact that climate change affects human health unequally. A thorough review of the existing literature is conducted in this study to synthesize the demographic and socioeconomic conditions, physiologic conditions, and geographical factors that cause inequality, with the most affected and vulnerable populations identified.
Meanwhile, this review also highlights the research biases and methodological challenges with the potential to hinder the comprehensive understanding of this complex and multifaceted issue. Specifically, the existing research pays insufficient attention to less-developed regions, with less emphasis placed on non-temperature-related health hazards. Also, climate change is inadequately attributed, and large-scale studies are absent. In future research, the priority should be to address these gaps for a deeper understanding of the unequal impact of climate change on human health.
The pressing requirement to innovate and implement research tools for the detailed assessment and quantification of health impact inequalities deserves reiterated focus. Additionally, it is necessary to better integrate physical evidence and socio-political-economic factors on climate and health through more interdisciplinary approaches and extensive collaboration between research teams. These efforts facilitate decision-making to support the mitigation of adverse impacts on human health, well-being, and social equity caused by current and future climate change. By overcoming these challenges, the unequal distribution of climate change health burdens can be mitigated, and a more equitable and sustainable future can be created for all.
The Author(s) 2024. This article is published with open access at link.springer.com and journal.hep.com.cn
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