Small, slender and short-lived, with broad noses, big, dark-adapted eyes, living underground, and in the shadows of a shattered, steamy, chaotic world. Richard Musgrove asks: will this be us in 10,000 years?
Climate change is the greatest challenge in human history – current trends could have us eventually approaching extremes not seen on our planet for 15 million years. Will a destabilised global climate wreak economic havoc, leading to societal collapse, mass mortalities, even extinction? Or will we pull ourselves out of this spectacular self-imposed nose-dive?
Which raises the question – what if we don’t? How will humanity change on a much hotter Earth?
Numbers matter
Uncharted territory approaches as we nudge the Paris Agreement’s 1.5°C above preindustrial levels, on track for a potentially catastrophic 2.7°C by 2100. What about 2200, or 3200?
Globally, days above 50°C have doubled since the 1980s – in Australia, Pakistan, India and the Persian Gulf – with the ‘feels-like’ temperature often higher. Even immediately reduced carbon emissions will still mean lingering planet-wide heating and associated effects for many thousands of years.
Our adaptability has led us this far, but what does evolution have in store for our species if we don’t rise to face our greatest challenge? The answer is unlikely to be in the mirror.
Nothing sweats like us
To understand where we are going, we need to examine how we got here and the drivers that made us what we are: the most successful mammal on the planet.
Let’s step back 7 million years. Imagine the leafy-green chaos of a closed canopy rainforest. Noisy, drippingly-humid, multilayered and hugely biodiverse, and humming with life. Apes, which may have been bipedal and would be hauntingly familiar to us, hang out in trees and forage on the forest floor. There’s plenty of food and water. Life is good.
“We are the most superb tropical animals,” says Emeritus Professor Duncan Mitchell, of the School of Physiology at the University of Witwatersrand, South Africa.
“Nobody copes with heat better than humans do, and that’s because of our immense capacity to sweat.
“Problem was, we evolved that sweating capacity in canopy forest where temperatures were 20°C to 32°C, no wind, no solar radiation penetrating through the forest.”
We could comfortably sweat buckets because there was always more water nearby to drink. And we didn’t have to exercise much to survive, says Mitchell, because food was plentiful.
But climates change. Rainforests dry, gaps appear, and savannah grasslands spread. We were left to manage with the sweaty physiological tools inherited from our rainforest ancestors. Evolution does that – working with what’s at hand. Staying alive now meant more exercise, and more sweating. So, staying close to, or carrying, water was now a matter of survival, says Mitchell. And our developing noses (we are the only ape with such endowment) allowed us to carry the jungle with us, in a sense, warming and moistening each breath.
Fast forward 4.6 million years – the Pleistocene Ice Age is well underway. This 2.6-million-year epoch of extremes spawned continent-spanning ice sheets and massive grinding glaciers. Blistering cold has pushed average global temperatures down to around 8oC.
You might see our earliest ancestor in the genus Homo – H. habilis –then around 2.3 million years ago [mya]. H. ergaster (also known as African H. erectus) would appear around 1.8mya. It was the first of our line with human-like proportions – with long legs and relatively short arms, tall and slender and possibly mostly hairless.
H. heidelbergensis follows, giving rise to Neanderthals (H. neanderthalensis), whose compact and sturdy build kept heat around internal organs, says Michelle Langley, Associate Professor of Archaeology, at Griffith University. Their probable cousins, the Denisovans, were high altitude specialists. Then there’s us – the ‘generalist specialist’, H. sapiens.
Now, we stand alone. No cousin-species remain, although Neanderthal and Denisovan DNA in our genome suggests there was a little genetic mixing along the way.
Cultural and behavioural innovation and adaptability allowed this rainforest-evolved ape to withstand the harshest conditions the planet could throw at us, as we spread across the globe, adapting to most conditions anatomically, behaviourally and culturally.
Arctic cold favoured greater body volume to surface area, allowing those with relatively bigger bodies and rounder heads to survive better and pass on their genes more often. Paler skins made more essential Vitamin D production possible. Contrasting hot climates favoured tall, lanky bodies allowing more efficient heat loss, says Langley, and darker skins provided better protection from damaging UV rays and perhaps tropical diseases.
Be cool or be dead
We are still that naked tropical ape under our clothes and inside our air-conditioned spaces.
Heat balance is the key, says Mitchell. “We have to be able to make sure we can dissipate the heat we generate by exercise into the environment, so that we can achieve heat balance even with heat load.” The alternative is heat exhaustion and eventual death.
And sweat is the key to losing that waste heat. Sweat evaporates, cooling our skin, making use of the “water vapour pressure gradient”, says Mitchell.
Water vapour pressure (WVP) is the amount (the ‘partial pressure’) of water vapour in wet air, Mitchell says. Called ‘partial’ pressure because air has other components which also exert pressure, atmospheric water vapour pressure varies with temperature and relates to how much water vapour is in the air (absolute humidity), not how much moisture the air can hold (relative humidity).
Our skins continually respond to temperature, sun and wind as we move through our individual microclimates. Evaporative cooling – sweating from the skin and loss of moisture from the mouth and upper respiratory tract (i.e. nose and nasal cavity, mouth, throat and voice box) windpipe, cools us down. But this evaporative cooling only happens if the WVP of the air touching our bodies is lower than that of the warm sweat on our skin. This works even at 100% relative humidity, although there, your skin must be at a higher temperature than the air, says Mitchell.
But there are limits. And here’s where global warming comes in. As Mitchell says, “we can evaporate sweat if environmental water vapour pressure is below 60 hectopascals [hPa], though it gets harder and harder to evaporate enough as the WVP approaches 60hPa”.
So, what does that mean in the real world? Look at the Bureau of Meteorology WVP map for the week ending January 25, 2025. Water vapour pressure tops out at about 36hPa, around Australia’s far north-west, for now. But water vapour pressure is rising about 7% for every degree of global warming.
Global temperatures have increased 1.3°C since the Industrial Revolution, and we are nudging 1.5°C, on track for around 2.7°C of warming by 2100, call it somewhere between 2.2°C and 3.4°C. This could mean 3.9°C to 5.6°C of warming by 2200-2300.
Around 3oC of warming means 50°C days in Sydney and Melbourne become common, per The Conversation. Double that warming by 2300, and the consequences are unthinkable.
Rising temperatures might push up the 36hPa WVP to around 41hPa by 2100 (at 7% per degree of warming), and to more than 52hPa by 2300 or sooner. Meanwhile, the WVPs in more temperate regions will also increase in concert.
“Assuming water vapour continues to rise, those WVPs are highly likely,” says climate change specialist, Stephen Turton. The last time CO2 levels were this high (400-600 ppm) was the Miocene Climatic Optimum, 16.9-14.7 million years ago.
There goes Northern Australia – no sweat! And Australia won’t be alone.
We can’t live in such environments if we can’t sweat. “Such ‘deadly heat’ might mean that some areas are just abandoned,” says Dr Michael Grose, Senior Climate Researcher at CSIRO.
Higher WVP also means more extremes – droughts and bushfires, rain, storms, floods, and rising sea levels – a destabilised climate wreaking havoc with our economies.
As Grose told Cosmos: “We’ve already locked in ongoing sea level rise for hundreds of years that’s going to just continue inexorably to rise.”
And projected economic impacts include 12% reduction in GDP per 1°C of warming, meaning (assuming now as ‘0’), a 12% reduction by 2100, and perhaps a 48% reduction by 2300 or sooner. As climate change accelerates, effects will multiply, and disruptions will reverberate through supply chains, making everything less efficient and reliable. Those disruptions will affect the very infrastructure we’re building to fight against further warming, and to mitigate against its effects.
People better start thinking about 2200, says Mitchell
Climate wars
Climate change will become very real when the air conditioning fails.
Will we adapt? Of course we will – to a point. We are the world’s most successful mammal after all. But we have no evolutionary experience of the predicted extremes. Yes, there were Miocene mammals, but not us.
There will be conflict, as Mitchell says, “the most catastrophic and probably the most predictable consequence of climate change is war”. Turton agrees: “There would be depopulation because of climate change and possibly political instability. There will be wars fought around climate resources, particularly water; places where you can grow food, and where you can’t because it’ll be too hot.”
The worst possible scenario may be a slow whittling away of capacity, capability and global co-operation. Playing out like a post-apocalyptic movie. Decades of decline amid accelerating climate chaos.
As Professor Emeritus Michael Gillings, of Macquarie University, puts it: “If we don’t get our emissions under control we’re heading for a future where everything becomes less predictable, food supply, energy supply, housing. Everything degrades, and we’ll go back through an agrarian stage to isolated villages and gradually devolve back to being hunter gatherers again.”
Who are the survivors?
If you want to prepare for the worst, it might get a bit uncomfortable. “The only way that you can prepare yourself physiologically for work in the heat is to work in the heat. As soon as you switch on an air conditioner, you’re blocking your ability to cope with climate change,” says Mitchell. But, what about the elderly, the very young, the pregnant, the poor and poorly nourished, the sick?
It’s hard to not envisage a scenario out of the dystopian movie, Mad-Max. Although some form of the moisture-conserving ‘stillsuit’ featured in Frank Herberts’s book, Dune, could become a thing.
Physical work, including farming, and general activity would become nocturnal or crepuscular (dawn and dusk), says Mitchell. Major sporting events are already starting late, including 2023’s Rugby World Cup in Paris, and this may set the pattern, he adds. Farming in warmer areas will become trickier as plants don’t have a choice to hide in the shade – Gillings suggests a focus on hydroponics.
Nocturnalism, of a sort, is not new for us, unlike the sleep-all-night-routine. Before the Industrial Revolution biphasic sleep (early to bed, up at midnight for a few hours activity followed by a second sleep) was common. And many warm climate cultures already integrate a siesta after lunch.
Living underground may also become more attractive. This is a very ancient practice, but we can look to the modern example of Coober Pedy to see how it might turn out. Food again becomes an issue – it would depend on the food sources that could survive the changing climate, says Langley. Mushroom quiche anyone?
“Surviving populations are going to be those which can engineer their way out of it – smaller populations in the right place with the right solutions,” says Langley. “Success could depend on their sociality and willingness to collaborate and work together for the greater good. Whether we go underground or under the water, life is going to be very, very different.”
Human evolution?
It’s impossible to know what exactly would happen to humanity after many thousands of years living on a vastly warmer world. On that timescale, evolutionary pressures might start to change our very makeup.
Perhaps nocturnalism would favour those of us with particularly good night vision. We could end up with more light-sensitive rod cells and larger eyes. Conversely, genes for colour-blindness and short-sightedness may not survive the slide back to hunting and gathering. What colour were those poisonous berries again?
Smaller size could be an advantage, particularly in droughts. Smaller people have comparatively larger surface areas, so they can lose more heat through skin blood flow rather than through sweating. Although this only works while the air temperature is below the 37oC our bodies maintain.
And lankier heat-shedding builds and perhaps broad noses could be favoured. Dry, cold air appears to favour narrower noses – a thinner air-stream being easier to warm and moisten, whereas warm, humid air presents no such challenges.
Lifespans will change. “Lifespan evolution is inversely proportional to mortality rate,” says Gillings. “A slide back to hunting and gathering would mean selection for shorter lifespans, less height, more aggression.” Because that’s what would keep our species alive.
Climate change also means “rapid evolution of pathogens, including vector-borne diseases”, says Mitchell, and as children are more prone to these diseases, there’ll be selection for better immune systems. Mosquitos carrying Zika, Japanese encephalitis and malaria are likely to spread as it warms.
We will survive – in some form
“Climate change is not an existential risk,” says Grose. “Unlike nuclear war, it will not wipe humans out. We will survive in this in one form or another, but there’s a long way between doing great and being extinct, it just depends – what we do will determine how far [we go] down that road.”
Will our distant descendants recognise us as human? Who is that stranger in the mirror?