An overheated world is bad news for our bodies

For the first time, a UN climate summit is to allocate a full day to health. A sense of urgency is justified by the experience of recent months, which leaves no doubt that the climate crisis is not an abstract scientific projection but a present – and personal – danger. The UAE Presidency of Cop28 has described this as a “human moment” because we have arrived at the point where climate change affects everyone, everywhere much faster than anticipated. The consequence is accelerated global heating, as evidenced by record-high temperatures during the northern hemisphere summer this year. It was the warmest summer on record, averaging 16.77°C (0.66°C above average). That may not sound like much, but it reflects record-breaking extremes exceeding 50°C in parts of China and the US, as well as 40°C in Africa, or exceeding that across large parts of Europe. The southern hemisphere winter was also warmer while the average global ocean surface temperature broke records at 20.98°C. The cyclical El Nino phenomenon is a contributing factor but the underlying causes are heat-trapping greenhouse gas emissions from unsustainable human activity. By last year, the average global temperature increment had reached 1.15°C above the baseline 1850-1900 period. This year, the planet momentarily breached the Paris Agreement’s threshold of 1.5°C. A repeat of this is almost certain. Projections anticipate a one-in-three chance of overall temperature rise exceeding 1.5°C between now and 2027. That limit could be routinely surpassed by 2029, making progression towards the consequential 2°C rise more likely. Prevention requires a 45 per cent reduction in greenhouse gases by 2030, and reaching net zero by 2050. Greater and faster action from all – especially major greenhouse gas producers – is needed. But that is not evident from current voluntary Nationally Determined Contributions. A hotter environment expands air and water. The huge quantities of trapped thermal energy must go somewhere. And so, we see shrinking glaciers and melting ice sheets. They lead to rising sea levels and coastal inundations, risking immense economic and social disruption because 40 per cent of the world lives within 100 kilometres of coasts. Other consequences are frequent and intense storms, rain, flooding and landslides alongside droughts, heatwaves and wildfires. Reduced freshwater availability and soil erosion with reduced carbon storage and productivity are also included. As plants and animals struggle with habitat and biodiversity changes, human food insecurity follows. The human effects are evident in the increased frequency and intensity of weather-related disasters. The five-fold rise over 50 years has caused two million deaths and $3.64 trillion in losses, according to the World Metereological Organisation. Recent Canadian and European forest fires, drought in Somalia, floods in Pakistan and Australia, and hurricanes in the Caribbean indicate the vulnerability of all continents. Trends project a 40 per cent increase in major disasters, from 400 in 2015 to around 560 by 2030. More than two billion people are affected as the climate crisis collides with conflict, with increasing disasters occurring in fragile, war-torn contexts. This is serious enough but more alarming is the direct impact of elevated temperature on the human body. Thanks to our evolutionary origin in Africa, our thermal comfort zone is at ambient temperatures of 21°C and 27°C while clothed. That allows an optimal skin surface temperature of 33°C while the hypothalamus in our brain regulates our core at about 37°C. But as the planet heats, billions of people risk falling out of our natural temperature zone. How that effects human physiology depends on humidity because that dictates our sweating and cooling abilities. Ambient wind speed and solar radiation also make a difference, that is – if sun-exposed or shaded. Studies of heat-and-humidity combinations measure the "wet bulb temperature" at which the body cannot maintain its core temperature. That critical survival limit for young, healthy, fit people is about six hours at 35°C in 50 per cent humidity. That's why humid heat is less bearable than dry heat. The old, very young, and pregnant women are less robust. As are those with non-communicable diseases, which kill 70 per cent of adults, the commonest cause of global mortality. Such factors are worrying because recent heatwaves have exceeded the maximum safe wet bulb limits in several places such as Iran and India with significant numbers of heat-related deaths that, in Europe , were estimated at over 60,000. People die or fall sick from heat because of direct organ effects. Dehydration causes dizziness or fainting, leading to concentration loss, sleep problems, fatigue and accidents. Our blood vessels dilate, lowering blood pressure, forcing the heart to work harder – leading to potentially fatal heart attacks and stroke, especially amongst the 0.5 billion people with cardiovascular disease. A secondary consequence of disrupted blood and oxygen supply is kidney disease from which 0.85 billion people suffer worldwide, including agricultural and other outdoor workers. Another 0.45 billion have respiratory conditions including asthma and chronic pulmonary obstructive disease, that are aggravated by hot air and high particulate pollution that accompanies climate change and causes multi-organ inflammation. Further consequences include premature and low birth weight as heat affects placental blood and nutrient supply to the foetus. Heat also influences the mood hormone, serotonin, increasing mental illness and suicide risks. Heat stress was once called a "silent killer" but no longer. The World Health Organisation projects 250,000 additional deaths each year from 2030 onwards, from heat stress and other health consequences with cost estimates approaching $4 billion. They include the secondary consequences of environmental disruption affecting over half of human pathogens and bringing animal disease vectors closer to us. Familiar examples include malaria, cholera, dengue and the Zika epidemic, as well as Ebola and expected future pandemics. Foodborne diseases that already cause around 600 million cases and 420,000 deaths risk augmentation, as also waterborne diseases that kill 3.4 million annually. Greater antimicrobial resistance is inevitable as our antibiotics and antivirals become less effective. The malnutrition that accompanies food insecurity has direct health impacts. As global heating becomes an existentialist personal threat, grand macro initiatives for future carbon reduction must be accelerated. But, on their own, they are insufficient. Climate adaptation and mitigation strategies must open a track specifically geared towards the urgent daily survival of the most vulnerable among us. For example, keeping cool and managing over-heating is a basic life skill that should be taught alongside other first aid essentials. Work , school, and commercial activity patterns must change to prevent undue heat damage to human health and well-being. Physical infrastructure that creates heat islands could be tackled through more shade and vegetation. When extreme heat disruptions occur, cool spaces and cooling technologies such as air conditioning, fans and water-soaked ventilation must be universally accessible, even as they increase energy demand and carbon emissions. Over-burdened health systems must adapt as medical research identifies better anti-heat life-preserving approaches. Wider changes across all sectors must be anticipated as an over-heated world derails the Sustainable Development Goals and undermines poverty reduction and equity objectives. Challenges for the organisation, governance, security and stability of society are possible. But they are not inevitable if global heating is recognised – not just as one part of the wider climate crisis – but as a perilous emergency in its own right. That is why the health discussions at Cop28 are vital.