Large parts of New Zealand and much of Australia have experienced climatic extremes this summer, including intense and prolonged heat. The effects of heat stress on life, including us, depend partly on actual temperature and humidity, and how much higher than usual these are for the season and place.
When it’s unusually hot, what springs into our minds and our newsfeeds? If you live in a town, chances are it’s a cooling dip, ice creams, fans or air conditioners, and maybe scrub, forest or grass fires. In media reports on heatwaves and the associated fires in hotspots of the United States, Australia, Canada and Southern Europe the typical imagery is of fire-fighting aircraft, houses ablaze or under threat, and roads congested with smoke and evacuees.
While the impact on farmers, vulnerable people and their loved ones is very real and rightly of concern, we should also spare a thought for a much larger reality that remains mostly unseen.
It may not feel like it, but we are the most heat-resilient among animal species, and for several reasons. Our capacity to keep our body temperature stable exceeds that of other species, by an enormous margin. Our relatively hairless skin is almost entirely covered by high-output, efficient sweat glands. We also have a rich network of specialised blood vessels in our skin, and dual mechanisms to dilate those vessels. So, we can carry large quantities of heat within blood to the skin, where evaporation of sweat can offload an impressive amount of heat into the environment.
We’re physiologically privileged, and we don’t even use it.
Even more uniquely, we have unmatched opportunities to behaviourally shield ourselves from heat stress, e.g., by reducing clothing, using shelter, and accessing fans and water.
Maybe our biggest advantage for resilience against heat — at least in this rich country away from the tropics — is less intuitive. It is the luxury to be inactive combined with the relative brevity of environmental heat.
Most of our heat stress even on the hottest days comes from inside us — our metabolism — rather than the environment (especially if shaded). Yet, warm and wet skin make us believe it is hot air, sun or high humidity. The major source of metabolic heat is physical activity, with digestion adding slightly to this. Even just walking increases our heat production at least threefold and generates more heat stress than a hot day typically does, but because it’s gained internally, we’re less aware of its impact.
These realities of heat stress raise several issues. We’ll consider four that are seldom discussed.
First, if it’s hot or forecast to be hot, think first about your activity levels. Minimising activity during the warmest periods, if practical, is logical for self-protection. But second, being active during these periods is a valuable way to boost heat tolerance in people who have good cardiovascular health. Third, for most of us, a little heat discomfort is a long way short of heat risk. Fourth, consider the unfathomable number of lives less fortunate than us; for example, birds and mammals living alongside us or especially in warmer climates. They cannot afford to sit idly and wait out severe or sustained heat. They typically don’t have food or water on tap, they need to use or store — and therefore find — energy for reproductive success, migration or winter, and they can neither hide from ambient heat nor offload their higher metabolic heat loads as easily.
We are the single species that is increasingly and knowingly responsible for rising levels of heat and humidity. Yet, as mentioned above, almost all the focus on its impacts is on the detriment to us. Heatwaves or even isolated hot days also give us the ideal reality check; the opportunity to pause and consider the deep inequities. For example, being transported passively in our vehicle that uses at least 40 times more energy and generates a similar amount more heat to go from A to B than we would if we walked or biked. Many such journeys are discretionary, and on a hot day, our vehicle’s air conditioner amplifies the inequity.
We send fire-fighting teams to other high emitting economies to help restore their status quo but rarely spare a thought for the vastly larger number of innocent species impacted, or human populations living today or in the future under more intense heat and pollution despite lower carbon footprints.
Over longer timeframes, effects of heat become more convoluted and less obvious. Many people will be aware that coffee plantations must move higher up hillsides, tropical fish "stocks" are degraded, wood-eating beetles thrive in forests at higher latitudes and affect lumber industries, and tropical diseases will spread into temperate regions. Again, such effects are framed mostly about us, the single species that is responsible and among the least affected.
Jim Cotter is a professor of exercise and environmental physiology in the University of Otago School of Physical Education, Sport and Exercise Sciences Te Kura Akoraka Whakakori. Nic Daniels is completing a PhD in the school. They are part of He Kaupapa Hononga Otago’s Climate Change Research Network.
