Climate change, human physiology and the challenge of integration.

That humans are most comfortable when naked and at rest in 28 °C air, with an average skin temperature around 33 °C¹, helps explain why thousands travel to “warm” places every year². Our ancient ancestors are responsible for this thermal preference; these are the conditions of the African homeland in which we evolved. As we moved to cooler climates around 300,000 years ago, we did not adapt much physiologically or morphologically, but instead used our intellect to recreate a “tropical climate” next to the skin.  The Australopithecus hominid made a windbreak in Tanzania ∼3.25 million years ago, Homo erectus was building huts from stones, branches and furs more than 1 million years ago, and clothing might have been used by humans 120,000 years ago².

This intellectual/technological approach also applied to tools, water and food supply, and has underpinned a planetary population explosion: there were one billion humans on the planet in 1804, two billion in 1927, three billion in 1960 - and we’ve added another billion every 12–14 years since, reaching 8 billion in November 2022³.

The problem isn’t just that our numbers have increased eight-fold in 200 years, and doubled in less than 50, it is that we are consuming the planet’s resources at a rate which takes us beyond the planetary boundaries we depend on for our survival³.  This is reflected in our energy use. In 2020, global energy consumption was greater than 580 terajoules/year, equivalent to slightly less than 60 kWh per person per day (equivalent to 8 billion people each boiling a kettle 575 times daily). More than 80% of this energy comes from fossil fuel combustion, meaning that in the span of just 50 years, atmospheric carbon dioxide (CO₂) emissions have reached their highest concentrations in more than 4 million years. This thickening blanket of greenhouse gases traps the energy equivalent of around five, 15 kilotons nuclear bombs every second. This energy gain raises surface temperatures (by 1.1 °C already) and that of the ocean (93% of the energy gain has been absorbed by the seas), and causes increasingly frequent and severe extreme weather events.

The consequences are with us now (Figure 1): rising sea levels (approaching 1 cm every 2 years); ever more common and severe droughts, heat waves, floods and fires; crop failure and famine³. Things are set to get worse: 20% of the CO₂ we emit today will be warming our planet in 33,000 years’ time, and 7% in 100,000 years. Worse, we have already triggered at least eight positive feedbacks, where warming drives warming, including the release of the powerful greenhouse gas methane from carbonate rocks, melting tundra and fermenting wetlands; CO₂ release from fires; carbon monoxide release from fires, which lengthens the atmospheric half-life of methane; loss of the ‘albedo’ effect (white snow and ice reflecting light back into space) which has doubled the rate of Earth’s energy gain in the last 14 years; and warming of the rainforest which, with their destruction by fire, makes them now net emitters of CO₂. Catastrophic change may no longer be gradual: sudden changes in ocean and air currents are likely, which will suddenly ‘flip’ the Earth’s weather, while accelerated polar warming (already at 3x the global average rate) may suddenly increase sea levels.

Global heating will harm health through changes in zoonotic, vector-borne and foodborne disease, and malnutrition: more than half of the known human diseases may be aggravated by climate change⁴. But loss of habitation, livelihoods and food will drive societal disruption and conflict over remaining resources and through climate-driven mass migration. It is no longer just our health, but our very survival which is threatened: the existential threat that has been warned about for so long is becoming a reality⁵

A recurrent theme of the recent COP27 World Health Organisation “Health Pavilion” presentations was recognition of the interdependence of climate and health-related issues, and the need for collaborative action and coherent policies. Certainly, the health-related consequences of climate change should be considered in the context of other societal issues, and of the causes of climate change itself. For example, life expectancy is rising, and will likely average over 70 years across the globe by 2050. In the absence of climate change impacts on survival, one in three born in the UK in 2016 could live to celebrate their 100^th birthday⁶. In contrast, the “healthspan” (years of good health) has remained stable since 2019, meaning that the number of years lived in poor health is increasing; those born in 2019 are forecast to spend up to a quarter of their lives in need of health service support. Much of this physical, and consequent psychological, ill-health, including asthma, allergies, cardiorespiratory disease, infectious disease, maternal ill-health and depression, is related to fossil fuel use. These costs are not factored into the overall costs of fossil fuel industries, which received a net subsidy of $400bn in 2019, with national allocations often exceeding healthcare budgets.

People in industrialised countries spend around 90% of their time indoors⁷, with attendant energy costs and related pollution. At the same time their physiological homeostatic mechanisms that promote resilience to, for example, fluctuations in climatic temperatures, go unchallenged and undeveloped⁸. Car driving pollutes (even electric vehicles release particulates from road, tyre and brake wear), and lack of exercise is harmful to health; combined with poor diet it fuels our epidemic of obesity which is currently costing the NHS 6 billion pounds per annum, rising to 9.7 billion by 2050⁹. By 2034, 70% of adults in the UK are expected to be overweight or obese, which increases the risk of hypertension, heart disease, stroke, cancers, musculoskeletal disease, dementia, depression, kidney disease and anxiety. Worldwide, it is the major modifiable risk factor for type 2 diabetes, and was responsible for healthcare costs of at least $727 billion in 2017 (12 % of adult healthcare spend¹⁰).  

Thus, lifestyles which drive climate change also harm health directly. But the resulting healthcare requirements drive more climate change. NHS England uses £500 million worth of energy each year, with a carbon footprint equivalent to around 20 million tonnes of CO₂. USA healthcare facilities account for 10.3% of total energy consumption in the commercial sector, and the number of these facilities has increased by 22% since 2003 with an associated 21% increase in energy consumption¹¹. 

An understanding of human physiology can help us address these threats, by for example, establishing the degree to which substitution of plant-based protein for animal protein is viable in reducing greenhouse gas emissions and improving health. It can reduce building energy use: 30% of the primary energy supply is used for building heating, ventilation and air-conditioning, and there has been a gradual increase in the thermostat setting, and reduction in clothes worn, in UK homes¹². Reversing this trend would save energy, money and emissions¹³, but the acceptability of doing so requires an understanding of the physiological basis of thermal comfort i.e. the contributions of solar load, air movement, humidity, clothing and physical activity to it, and of the dominant impact of maintenance of peripheral blood flow and warm, stable extremity temperatures on the overall perception of thermal comfort¹⁴. Physiology can help us understand the extent to which humans can acclimatise to heat, as well as determine the body temperature limits for those having to work in warmer environments, or wear protective clothing.

For these reasons, physiologists have recently joined clinicians in advocating for healthier, more active and happier lives¹⁵. Low-carbon living (e.g. clean air from the absence of fossil fuel combustion, active transport and a local, seasonal, vegetable-based diet) offers one means to maintain and promote individual health while preserving planetary health. However, longer more active lives may drive increased resource consumption - this conundrum has yet to be addressed.

The path to comprehensive solutions requires collaborative, integrated action by politicians, policy makers, epidemiologists, engineers, architects, climate change scientists, behavioural psychologists, healthcare professionals, botanists and, physiologists - to provide a fundamental understanding of what humans can tolerate and what needs to be achieved (Figure 2). But we need far more than words, and the time left for action is short. The rise in Earth’s surface temperature of 1.1 °C above preindustrial levels is already proving dangerous, and this danger will continue to increase, even if emissions cease. A rise of 1.5 °C will be more dangerous still, but to stay beneath this requires that global emissions are nearly halved in 7 years. Yet the recent COP27 meeting removed a target to peak in 2025, and also removed any mention of the need to reduce fossil fuel use. Such inertia may soon be fatal. As the 2022 Intergovernmental Panel on Climate Change warned, lack of immediate action at pace and scale means that we may soon miss our last opportunity to “secure a livable future “.

Mike Tipton is a Professor of Human and Applied Physiology at the School of Sport Health and Exercise Science in the Faculty of Science and Health. 

This article is republished from the Parliamentary & Scientific Committee under a Creative Commons Licence. Read the original article.

References

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