TRUDGING through hot, red sand is hard work, especially in temperatures above 40°C (104°F). After about 40 minutes, I am drenched, dehydrated and drained. I can’t imagine doing this for 40 days, dragging all my gear behind me – including 40 litres of water, enough for five days – on a two-wheeled trolley. But that is exactly what the people I am travelling with have just done.
I am in the Nafud desert, a vast tract of sandy and rocky wilderness in northern Saudi Arabia, to experience levels of heat that I am not built to endure – and to meet 20 people participating in an expedition called , dedicated to understanding how humans respond to extreme conditions. “The idea is to study how human beings can adapt to a new kind of environment,” says , the leader of the expedition and director of the Human Adaptation Institute in France.
As the climate warms, the issue is becoming increasingly pressing. Even under the most optimistic scenarios, the scorching heat seen in southern Europe and across the US over the past couple of months, with temperatures exceeding 40°C, will become the norm in many parts of the world.
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That means the question of what happens to our brains and bodies, and the extent to which human physiology can cope with extreme heat, matters for millions of people. “You’re going to see a great big swathe of very densely populated areas go up to unprecedented temperatures that nobody experienced in the historical climate,” says at the University of Exeter, UK, who recently co-authored a research paper called ““.
“Can we adapt to and accept new climate conditions?” asks Clot. “We don’t know. We need to learn more.” The Deep Climate expedition is the first large-scale experiment to attempt to find out.
The human climate niche
Like every other living thing, humans evolved to have an ideal climate niche – an average temperature and rainfall range in which we can live comfortably. “The climate niche describes where people flourish and have flourished for centuries, if not millennia,” says Lenton. Technologies such as clothing, buildings, heating and air conditioning have extended that niche and allowed people to live in some of the hottest and coldest places on Earth, but they haven’t fundamentally changed our biology. The ideal mean temperature for us over the long run is around 12°C (54°F), says Lenton. Which explains why, historically, population density peaks in locations with that mean temperature and falls off rapidly on either side.
Climate change has already altered the geographical distribution of that niche. At current levels of warming, roughly 1.1°C above pre-industrial average temperatures, some areas of Earth that were once close to the niche have been pushed further away from it. “Already, we estimate that some people have been shifted from what was the kind of climate that supported high-density populations in the past to a climate that historically would support lower density,” says Lenton.
I can tell you from experience that constant heat is extremely testing. The average temperature in Saudi Arabia during the summer, when I visited, is around 30°C (86°F), peaking at 45°C (113°F) in the early afternoon and dropping into the 20s (70s) at night. It is brutal and sapping. Sleep is elusive, exertion is incredibly challenging. I found myself prostrate in my tent, fantasising about cold beer, air conditioning and swimming pools. And that was dry heat, which we know to be easier to cope with than humid heat.
It is already well-established how human bodies respond in the short term to extreme high temperatures. The initial symptoms are heat rash and cramps, caused by lost fluids and salts, which can progress to heatstroke: a fast, strong pulse, headaches, dizziness, nausea and confusion. “This is a true medical emergency,” says at the University of Michigan. And the more humid it is, the lower the temperature required to get there. At around 45°C (113°F) and up to 20 per cent humidity, the human body can cool itself through sweating. But above 40 per cent humidity, 45°C can be deadly.
What we know almost nothing about is whether humans can adapt to such conditions in the long term. Enter Clot. In 2016, he realised that there was very little data on how humans will cope with future climates and came up with the idea for his experiment. To begin with, he volunteered himself as a human guinea pig and undertook three 40-day solo expeditions in the extreme climates he thought were most relevant: humid heat in the Brazilian Amazon, freezing cold in Siberia and dry heat in Iran. Every day, he took scientific readings on himself.
Clot’s solo efforts became the pilot project for the Deep Climate expedition. In 2017, the project advertised for volunteers aged between 25 and 50 who were fit, spoke French and were prepared to tolerate tough conditions. They whittled the applicants down to about 30.
There were to be three legs of the experiment, each lasting 40 days and 40 nights: humid heat, extreme cold (which will also become more common as the climate changes) and dry heat. Each leg features a team of 20 “climatonauts” picked from the original 30: 10 women and 10 men, including Clot. There is no motorised transport and everything they need has to be hauled or carried, up to 200 kilograms per person.
The locations have changed due to political instability – French Guyana in place of Brazil, Lapland for Siberia and Saudi Arabia for Iran – but the goal is the same. “We study all the processes of adaptation, and the mechanisms in the brain and in the body, to understand how we can adapt,” says Clot. By adaptation, he doesn’t mean some kind of rapid evolutionary response, rather, he is talking more broadly about how our bodies and brains react, whether that is to cope or otherwise, and how people behave in such extreme conditions.
The French Guyana expedition was completed in January, while the Lapland leg finished in April. The Saudi Arabian experiment began in early May at a village called Sakaka, 210 kilometres as the crow flies from the finishing line at Jubbah, an oasis town on the edge of the . The climatonauts walked up to 10 kilometres a day, hauling their gear over dunes and rocks and running the gauntlet of scorpions, snakes and sandstorms. “The sand is awful,” says Clot. They set off at 5am each morning and trekked for 5 hours, taking shelter during the hottest part of the day, then walked again in the evening. Every fifth day, they paused to rest.
Throughout, they lent their bodies to science. “We study the individual as much as possible – physiology, epigenetics, microbiota,” says chief scientist at Ecole Normale Supérieure in Paris. “We also [study] cognition and brain plasticity, and then we have another set of studies to investigate how the group evolved and the dynamics and how it changed from the beginning of the expedition until the end.”
Some of the data acquisition was continuous. The climatonauts wore accelerometers to monitor their movement and sleep, and sociometers to record their interactions with other group members – who they are near and whether they are talking, though not the content of the conversation. Every few days, they swallowed a sensor to monitor their core temperature. They also filled in daily questionnaires about their emotional and physical state and recorded what they ate – a monotonous diet of rehydrated meals, flatbreads, processed cheese, dried fruits, nuts and cookies.
On rest days, they all underwent a battery of physical and cognitive tests, including heart rate, blood pressure, posture, muscle tone and oxygenation, memory, attention, metacognition and sensory perception. (I took a smell test and flunked it, misidentifying fish as onion; the oppressive heat had clearly messed with my olfactory bulb). They had blood and stool samples taken, the latter to assess their gut microbiome.
This vast pool of data, once compiled and cleaned up, will be sent to for analysis, along with data from the other two expeditions. They will track the changes over time and compare the data with baselines taken before the treks. It will take a year at least to get the full results.
There is some existing research, says Romand-Monnier, but it is limited to short-term physiological adaptation in military personnel and elite athletes. “It’s really frustrating because there is a lot of publications and everybody thinks, come on, we already know everything, what are you going to study?” she says. “But the thing is, for a normal group in that kind of climate and that duration, we know nothing.”
They do have hypotheses, based on the pilot data from Clot’s solo expeditions. Physiology probably can’t adapt very much, says Clot, but the way we think and operate looks like it can respond positively. “The brain can change a lot, and it can change really fast,” he says, based on his experiences. “After 40 days, a lot of human beings are adapted to new conditions.” It is a case of mind over matter.
For a group of people who have been in the desert for 40 days and 40 nights, the climatonauts all appear remarkably healthy. They were, of course, selected precisely because they had the right stuff. Romand-Monnier acknowledges that this is a weakness of the study, since most people who experience climate extremes in the future won’t be young, healthy, decked out in specialist footwear and clothing, fed and watered. Nor will they know there is a definitive endpoint. But you have to start somewhere, she says, and the plan is to re-run the expeditions with participants who represent different backgrounds and ages.
Frequent heatwaves
Understanding how ordinary people respond to extreme climates could hardly be more urgent. In the era before climate change really kicked in, almost nobody lived under extremely hot conditions year-round. In the mid-1960s, perhaps 10 million people lived outside the niche, says Lenton. Today, around 60 million do, overwhelmingly in lower-income countries such as India, Nigeria and Indonesia. That increase is partly due to population growth, but also climate change.
The current distribution of population density still peaks at a mean annual temperature of 12°C (54°F), but there is now another, smaller peak at 25°C (77°F). That is largely due to people living on the Indian subcontinent, says Lenton, where hotter conditions are becoming the norm. It is within our climate niche, but only just, with frequent heatwaves nudging conditions into extreme heat, which Lenton defines as an annual mean of 29°C (84°F). Even places firmly in the niche will find themselves getting warmer. “A bunch of people might be shifted from what was a really great climate historically, say 12 to 13°C, and are now finding themselves at 17°C or maybe even 20°C,” says Lenton.
The numbers affected will continue to grow as temperatures creep up. If we limit global warming to 1.5°C above pre-industrial levels, says Lenton, extreme heat will descend on areas where 400 million people currently live. If we go to 2.7°C, which is where current climate commitments are taking us, that rises to 2 billion. “In the 2.7°C world, there’s over 600 million people in India and over 300 million people in Nigeria exposed to extreme heat,” he says, plus many more millions in Indonesia, the Philippines and vast swathes of the Sahel region of Africa.
Notably, of course, these aren’t wealthy countries. Saudi Arabia is rich, and most people there can afford to modify their living spaces to stay within the niche. Not so for lower-income countries on the front line of extreme heat.
“The super rich can find ways of getting inside air-conditioned buildings and sourcing desalinated seawater, and find other ways of continuing to thrive in extreme climates,” says Lenton. “But that only works if you have enough resources. There’s an awful lot of people on the planet still living below the poverty line, a huge number of people who don’t have the means to have access to some of those adaptation options.”
For those people, the most likely adaptation is mass migration. “The normal response is to move to the better places, we see that happening for birds, for plants, and it has happened throughout human history,” says Lenton’s colleague at Wageningen University in the Netherlands. “It would be a very natural adaptation to consider migration, and not just migration of tens of millions of people. That might be a billion or so.”
How the world will respond to such mass movement isn’t clear, although the West’s current attitude to much smaller numbers of refugees doesn’t bode well. But the shifting niche will at least create more liveable spaces at higher and lower latitudes. “There is a bright side,” says Scheffer. “Some places are getting worse, but other places are getting better. It’s not like the Earth is becoming unliveable.”
It certainly feels unliveable in the Saudi Arabian desert. My two days there felt like an eternity, and I was relieved to get back to the hotel. Cold beer wasn’t an option – Saudi Arabia is a dry country in more than one sense – but there was air conditioning and a swimming pool. Two out of three ain’t bad.
As the climatonauts had wearily broken camp for the last time, I had asked some what they were looking forward to most once they got home. Most said good food, wine, sleep, showers, family, friends, a bit of solitude – and a respite from the heat. Many more of us will be longing for that in the not-too-distant future.
Graham Lawton is a features writer at 91av
