Burden of non-communicable diseases attributable to high temperature in Australia:current and future projections in a changing climate

Abstract

Problem statement: As one of the greatest challenges of the 21st century, global warming leading to increased intensity, frequency and duration of high temperatures, has been reported to cause harm to human health, in particular to vulnerable populations such as older people and those with chronic, non-communicable diseases (NCDs). Australia, as one of the developed countries most vulnerable to the impacts of a changing climate, is experiencing an aging population with a high prevalence of individuals with NCDs. In this context, the health burden of NCDs attributed to high-temperature exposure is expected to substantially increase across Australia due to a warming climate and a larger proportion of vulnerable populations exposed to high temperature. This underscores the significant public health concerns posed by high temperature associated NCDs both globally and in Australia. Accordingly, there is considerable interest in investigating the burden of NCDs attributed to hot weather and projecting the future burden with increasing temperature caused by climate change. Research gap and opportunity: After conducting systematic literature reviews, it was evident that high ambient temperature increases the risk of NCDs. Specifically, cardiovascular diseases (CVDs), mental and behavioral disorders (MBDs), and kidney diseases (KDs) have been well-documented as being associated with hot weather in Australia. In previous epidemiological research, mortality and morbidity were frequently used as indicators to assess the health impact of temperature exposures. These health metrics, while offering valuable insights, do not encompass information regarding the reduction in years of healthy life due to premature deaths and years lived with disability. Therefore, the use of disability-adjusted life years (DALYs) which comprises both years lived with disability (YLDs) and years of life lost (YLLs) is a more comprehensive measure. This metric considers the severity and duration of diseases, reflecting the underlying impact on quality of life and the reduction in life expectancy. However, studies quantifying the health burden attributable to high temperature exposure are very limited. In Australia, no previous study has estimated the burden of NCDs attributable to high temperature, incorporating both YLLs and YLDs, nor projected the future burden of NCDs in the context of a warming climate, considering different climate change scenarios, changes in population size and age structures, and levels of human adaptation. Purpose statement The goal of this research is to obtain a comprehensive national overview of the Australian burden of NCDs linked to high temperature exposure, and project the future burden of NCDs due to climate change. By doing so, this research contributes to new knowledge that may be useful in the development, prioritization, and implementation of current and future climate change and health mitigation and adaptation strategies, planning, and policies. Research questions: 1. What is the epidemiologic evidence of exposure-response relationships between high temperature and each of the three selected NCDs? 2. Given the diverse climate patterns in Australia, what is the magnitude of the current burden of NCDs attributable to high temperature in all states and territories? 3. What is the future burden of NCDs attributable to high temperature exposure under different climatic, demographic change and adaptation scenarios? Methodology: The method undertaken in this research followed the comparative risk assessment framework and aligned with the Australian Institute of Health and Welfare (AIHW)’s Australian Burden of Disease Study. It consisted of four distinct parts: firstly, a systematic review and quantitative synthesis of available epidemiological evidence were conducted to assess the effects of high temperature on selected NCDs, describing the relative risk (RR) per 1°C increase in high temperature. Secondly, the estimated relative risk for each Statistical level 2 Areas (SA2) in Australia was predicted using meta-regression analysis. This estimation, along with observed and projected mean temperatures, was used to calculate the population attributable fraction (PAFs) due to high temperature exposure. Thirdly, the annual average burden of each selected NCD was calculated for the baseline period of 2003-2018 using burden of disease indicators (YLLs and YLDs) sourced from the AIHW, population data from each SA2 per year from the Australian Burau of Statistics (ABS), and the estimated PAFs. Finally, future temperature projections were predicted using eight downscaled climate models to estimate changes in the high temperature attributable burden of each selected NCD. The projections centered around 2030s and 2050s and considered two Representative Concentration Pathway (RCP) greenhouse gas emissions scenarios (RCP4.5 and RCP8.5), as well as demographic changes (i.e., population size and age structure and three levels (0%, 50% and 100%) of human adaptation to climate change. Main findings: Systematic review and meta-analyses Of 14,659 studies identified, 426 were included in the review, with 389 of these studies meeting the criteria for inclusion in meta-analyses (266 for CVDs, 82 for KDs, and 41 for MBDs). Findings from the systematic review and meta-analysis showed that a 1°C increase in temperature is positively associated with CVD-related mortality across all considered diagnoses (International Classification of Diseases, ICD-10 I00-I99). The overall risk of CVD-related mortality increased by 2.1% (95%CI 1.020-1.023). A significant increase in CVD-morbidity was also found for specific CVDs, including arrhythmias, cardiac arrest and coronary heart disease. For KD, the meta-analysis of study findings showed that for each 1°C increase in temperature, the risk of kidney-related morbidity (ICD-10 N00-N39) increased by 1% (95%CI 1.009-1.011), and for MBD, the risk of mental health-related morbidity (ICD F00-F99) increased by 0.7% (95%CI 1.005-1.009). Findings suggested evidence of increased vulnerability for populations living in tropical and subtropical climate zones, and for people aged more than 65 years. Estimated burden of NCDs attributable to current and future high temperature exposures Over the baseline period from 2003 to 2018, high temperature contributed to 7.3% of the observed burden of CVD, 2.7% of the burden of KD, and 1.8% of the observed burden of MBDs in Australia. This suggests that a proportional reduction in the health burden among the population would occur if exposure to high temperatures were reduced to an alternative ideal level. In addition, the projections indicate that, compared to the baseline period, future increasing temperature will correspond to steady increases in the burden of NCDs in Australia across all scenarios examined, except for full (100%) adaptation with no demographic change. When assuming no human adaptation, findings indicated gradual increase in the high temperature-related burden of NCDs using the RCP4.5 and RCP8.5 climate scenarios. This increase was primarily driven by expected changes in population size and age structure. Furthermore, the results suggested that the jurisdiction with a warmer climate, that is the Northern Territory, is projected to experience the highest percentage increase in the burden of NCDs attributable to high temperature by the 2030s and 2050s, when compared to the baseline. Strengths and limitations: One of the key strengths of the systematic review was that it addressed knowledge gaps concerning the quantitative effects of hot weather on each of the selected NCDs, while considering variations in geographical and climate patterns. The review was undertaken in accordance with best practice guidelines developed specifically for environmental health research, followed by several sensitivity analyses to confirm the robustness of findings. The research on the burden of NCDs attributable to high temperature exposure represents the first international investigation to quantify both fatal and non-fatal attributable burden of the selected NCDs at a national level. A detailed assessment of the baseline and projected burden of NCDs attributable to high temperature is provided for each state and territory across Australia. This research also has limitations. With the lack of health information and future population projections at the SA2 level, this investigation relied on health burden estimates and population projections at the state and national levels. A downscaling method was then applied to estimate the burden of NCDs and future populations at the SA2 level. This may result in overestimation or underestimation in certain regions. In addition, the future projections were based on the current estimates of exposure-response relationships, which do not account for potential unknown patterns and magnitudes of temperature-health associations that may occur in the future. Therefore, the reported estimates should be interpreted as potential impacts within well-defined but hypothetical scenarios. Conclusions: This comprehensive research has shown that exposure to high temperature poses a clear increase in the risk of NCDs. As temperatures continue to rise, heat is expected to contribute to a greater burden of NCDs across Australia. These research findings underscore the urgent need for adaptation and mitigation strategies to minimize the negative health impacts of a warming climate on the burden of NCDs. Novelty: This is the first comprehensive Australian investigation to assess both the observed and projected burden of NCDs, with YLLs and YLDs as health indicators, using high temperature as a risk factor at a national, regional, and state level. Additionally, it introduces novelty by extending this assessment across diverse climate zones and projecting future burdens under various climate change, demographic and adaptation scenarios. Implications of findings: Findings from the systematic review and meta-analysis indicate that underlying regional climate conditions need to be accounted for when assessing the risk of NCDs associated with hot weather exposure, as well as characteristics of the population and disease subgroups. Based on the findings of this research, it is likely that future increasing temperature in conjunction with an increase in the proportion of older people in the population will result in an increased risk of poor NCDs-related health outcomes associated with a warming climate. The comprehensive national assessment of the burden of NCDs attributable to high temperature may help to extend our understanding of the associated health impacts and aid in policymaking to reduce the burden of heat attributable NCDs due to increasing temperature in Australia. The disparate geographical patterns of attributable burden of NCDs found in this study suggest that geographically specific adaptation actions will need to be strengthened to maximize the effectiveness of reducing the future burden of NCDs in the context of a climate change.