natureNaturenatureclimatenatureplanet earth

Why Are The Earth's Deserts... You Know... Like That?

Too hot, too dry, too sandy - could they not just calm down?


Dr. Katie Spalding

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer

Photograph of person sprawled on their back in a sandy desert

Pro tip: don't sunbathe in the Sahara. And if you do, at least take your sweater off.

Image Credit: imagoDens/

When someone challenges you to think of the worst weather possible, your mind probably pictures something like the summit of Mount Washington. You know: rainy, windy, with sub-zero temperatures in both Celsius (cold) and Fahrenheit (deadly). But that’s not the only way the Earth can kill you.

Get stuck somewhere too hot or dry, for example, and you’ll end up just as dead as the unfortunate inhabitants of Rainbow Valley. A surfeit of sand can kill you from half a continent away. If you’re really unlucky, you might simply die of irony: succumbing to dehydration while technically surrounded by water.


Where can all this happen? In the planet’s deserts – the drama queens of our supposedly blue-and-green Eden. 

So why, exactly, do they have to be Like That?

The hottest place on Earth: Death Valley

Even for the healthiest of people, extreme heat can be deadly in a matter of hours. With that in mind, the place with the highest recorded temperature on Earth has a very fitting name: it’s Death Valley.

There, on July 10, 1913, in the equally aptly named Furnace Creek, the temperature reached a whopping 56.7°C (134°F) – hotter than most steam rooms. Even on a normal day, it’s scorching: in summer, temperatures often reach 49°C (120°F) in the shade, with overnight lows dropping into the mid-30s°C (90s°F).


“It’s like stepping into a convection oven every day in July and August,” Brandi Stewart, spokesperson for Death Valley National Park, told the New York Times in 2020. “The feeling of that heat on my face, it can almost take your breath away.”

In fact, a convection oven is a pretty good comparison. Death Valley has a very particular geography: it sits at an extremely low altitude, with parts of the park lying at a depth of 86 meters (282 feet) below sea level – but it’s surrounded by high, steep mountains. That means that the warm air of the Valley is essentially trapped: “Heated air rises, cools before it can rise over the valley's mountain walls, and is recycled back down to the valley floor,” explains the US National Park Service. 

“As they descend, they are compressed and heated even more by the low elevation air pressure. These moving masses of super heated air blow through the valley, creating extreme high temperatures.” 

Of course, another reason for the area’s extreme heat is its famous lack of rainfall – water has a very high heat capacity, meaning it can absorb a lot of heat before it actually gets hot. That’s why rainy places are usually cooler than drier places: a whole chunk of the energy from the sun is spent evaporating the water in the air rather than heating up the environment. (It’s also why we haven’t killed all life on Earth many times over by turning the planet into a super-heated megafurnace yet, but that’s by the by.)


But with a few freak exceptions, Death Valley averages only about 57 millimeters (2.24 inches) of rain over the entire year. To put that in perspective, it’s less rain annually than the residents of Oxford, England, saw on one day in October 2020.

“People say, ‘Oh, but it’s a dry heat!’ I want to do a little bit of an eye roll there,” Stewart told the New York Times. “Humidity has its downsides too, but dry heat is also not fun.”

The driest place on Earth: the Atacama*

Despite Death Valley’s barely-there level of moisture, this morbidly-named corner of the Mojave is positively lush when compared with a certain counterpart below the equator. There are parts of the Atacama Desert, Chile, where rainfall has never been recorded at all.

What makes these two places so particularly hot and dry? Part of the story comes from their locations on the globe: they lie roughly around 30 degrees above and below the equator, putting them directly in the path of a global atmospheric circulation known as the Hadley cell.


In short, the Hadley cell is why our planet looks the way it does, with a belt of green around the equator that’s bordered on both sides by desert. At the edges of the cell, where the Mojave and Atacama lie – as do the Sahara, the Sonoran, and even the Australian Outback – humidity is very low, meaning not much rain and, in turn, little respite from the sun.

But that’s not all the Atacama has going for it. It’s not just in a dry spot latitudinally: it also sits right in between the Andes and the Chilean coastal mountain ranges, both of which act as shelter from any rain that might have finally made it to the edge of the Hadley cell.

“The Atacama Desert exists in a kind of meteorological void,” explained science communicator Maiya May in a video for PBS. “[It’s] a place where the conditions for rainfall are essentially negated.”


Since water is kind of essential to live, the Atacama has the dubious honor of another record: it’s so empty of life that NASA has been using it as an analog for Mars for more than a quarter century.

“Even in the Mojave and remote parts of the US mainland […] you still see shrubs, cacti, microorganisms, scorpions, insects; there is still an ecosystem,” Brian Glass, principal investigator of NASA’s Atacama Rover Astrobiology Drilling Studies project, told CNN in 2021.

But in the Atacama, he explained, “you could literally fall down, cut your arm on a rock and you wouldn’t worry about getting an infection because there are no local pathogens.”

The biggest desert on Earth: the Sahara*

For many of us, the Sahara is the desert: the landscape that comes to mind when somebody says “Imagine a desert.” It’s got it all: sand, oases, camels, Brendan Fraser, and Rachel Weisz – and the reason it can fit all of this in is simple. It’s huge.


We all know maps are full of lies, so here’s some raw data: the Sahara is 9,200,000 square kilometers (3,600,000 square miles) in area, or to put it another way, half a million square miles larger than the contiguous United States. It’s about the same size as China. It’s 13 Texases (Texes?) or 20 Californias.

It already covers roughly one-fifteenth of all the land area on Earth, and here’s the really scary part: it’s getting bigger. 

Since 1920, the desert has increased in size by 10 percent, drying out previously fertile areas to the South. “The entire Chad Basin falls in the region where the Sahara has crept southward, and the lake is drying out,” said Sumant Nigam, an atmospheric and ocean scientist at the University of Maryland, in 2018

“It's a very visible footprint of reduced rainfall not just locally, but across the whole region,” he explained. “It's an indicator of declining water in the Chad Basin.”


Why the expansion? Like pretty much every existential threat these days, at least part of it comes down to human-induced climate change, which scientists believe is helping the Earth’s Hadley cells to increase in size.

But for once, this mostly isn’t on us. “Climate change is likely to widen this Hadley circulation, causing the northward advance of subtropical deserts,” Nigam said – but “the southward creep of the Sahara suggests that additional mechanisms are at work.”

It raises an interesting question: if the Sahara is getting bigger, then presumably it must have once been tiny. And unbelievably, that’s actually kind of true: go back in time a dozen millennia or so, and what is now a vast sand-covered desert would have been lush with vegetation and even forests. So what happened?

While early humans may shoulder some of the blame, the fact is that the Sahara’s transformation was inevitable. It’s a consequence of the orbit of the Earth itself: “the story of North African climate is dominantly this 20,000-year beat, going back and forth between a green and dry Sahara,” said David McGee, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, in 2019.


“[The Sahara] seems like such an impenetrable, inhospitable landscape,” he added. “And yet it’s come and gone many times, and shifted between grasslands and a much wetter environment, and back to dry climates, even over the last quarter million years.”

The champion of them all: Antarctica

Okay, so we may have lied a little when we called the Atacama and the Sahara big and dry. There’s one place on Earth that takes the crown as having the world’s biggest desert, driest place, extremes of temperature seen nowhere else on the planet, and, weirdly, the lowest infant mortality rate anywhere: Antarctica.

Just like little brother Sahara, however, the Antarctic was not always a desert wasteland. Some 90 million years ago, the continent enjoyed “a diverse environment,” polar scientist Johann Klages told Vox in 2021, “with such mild temperatures – temperatures that today you have in northern Italy, for example.”

A decrease in global CO2 levels about 34 million years ago meant a drop in temperatures – and at the Earth’s southern pole, it became cold enough for ice to be present throughout the year. As soon as that threshold had been crossed, the continent was able to grow into the ice desert we know and occasionally get murdered by today.


But maybe not tomorrow. The levels of atmospheric COthat resulted in that Mediterranean Antarctica were about 1,100 parts per million, according to Klages’s climate modeling team. And “we are [now at] 420 parts per million CO2,” he warned.

“We are doing a big experiment right now,” he told Vox. “We take fossil fuels from the Earth’s crust that were deposited over millions of years, and usually would have been released back to the atmosphere over millions of years – but we did it within 150 years. Boom.” 

“That has never happened before. That has a massive impact.”


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