Why are the Earth’s deserts…did you know…like this?

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

For example, if you get stuck somewhere too hot or too dry, you’ll end up dead just like the hapless residents of Rainbow Valley. An excess of sand can kill you half a continent away. If you’re really unlucky, you may simply die of irony: succumbing to dehydration while technically being surrounded by water.

Where could all this happen? In the deserts of the planet – drama queens in the blue-green Garden of Eden.

So why exactly do they have to be like this?

The hottest place on Earth: Death Valley

Even for the healthiest people, extreme heat can be fatal within hours. With that in mind, the place with the highest recorded temperature on Earth has a very appropriate name: Death Valley.

There, on July 10, 1913, in Furnace Creek of the same name, the temperature reached 56.7 °C (134 °F) – hotter than most steam chambers. Even on a normal day, it’s hot: in summer, temperatures often reach 49 °C (120 °F) in the shade, with overnight lows falling to the mid-30s °C (90 °F).

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

In fact, a convection oven is a pretty good comparison. Death Valley has a very particular geography: it is at a very low elevation—parts of the park are 86 meters (282 feet) below sea level—but it is surrounded by high, steep mountains. This means the valley’s warm air is essentially trapped: “Hot air rises, cools before it can rise over the valley’s mountain walls, and is recycled back to the valley floor,” the U.S. National Park Service explains.

“When they land, they are compressed and heated even more due to the lower air pressure. These moving masses of superheated air blow through the valley, creating extremely high temperatures.

Of course, another reason for the extreme heat in the region is the lack of rainfall – the water has a very high heat capacity, which means it can absorb a lot of heat before it actually becomes hot. This is why rainy places are usually cooler than drier places: almost all of the energy from the sun is spent evaporating water in the air rather than heating the environment. (This is also why we haven’t killed off all life on Earth several times by turning the planet into a giant, superheated oven yet, but that’s over.)

But with a few odd exceptions, Death Valley’s average rainfall is only about 57 mm (2.24 in) over the area. whole year. To put this into perspective, there is less annual rainfall than what the people of Oxford, England, have experienced one day In October 2020.

“People say, ‘Oh, but it’s dry heat!’ “I want to turn some heads there,” Stewart told the New York Times. “Humidity has its downsides too, but dry heat isn’t fun either.”

The driest place on Earth: Atacama*

Despite a humidity level that barely reaches that of Death Valley, this pathologically named corner of the Mojave is favorably fertile when compared to a certain counterpart below the equator. There are parts of the Atacama Desert in Chile where no rainfall has been recorded at all.

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

In short, the Hadley cell is the reason our planet looks the way it does, with a green belt around the equator and bordered by desert on both sides. And on the fringes of the cell, where the Mojave and Atacama are located — as in the Sahara, Sonoran, and even the Australian Outback — humidity is very low, meaning not much rain, and therefore little respite from the sun.

But that’s not all there is to the Atacama. Not only is it a dry spot in terms of latitude, it is also located between the Andes and the Chilean coastal mountain ranges, both of which serve as a shelter from any rain that might eventually reach the edge of the Hadley Cell.

“The Atacama Desert exists in a kind of atmospheric vacuum,” science spokesperson Maya May explained in a PBS video. “(It’s) a place where rainfall conditions are basically eliminated.”

Since water is essential to life, the Atacama has the dubious honor of another record: It is so devoid of life that NASA has been using it as a counterpart to Mars for more than a quarter century.

“Even in the Mojave and remote areas of the mainland United States (…) you still see shrubs, cacti, microorganisms, scorpions, insects; “There’s still an ecosystem there,” Brian Glass, principal investigator for NASA’s Astrobiology Crater Studies Project, told CNN in 2021.

But in the Atacama, he explained, “You can literally fall, cut your arm on a rock, and you won’t have to worry about getting infected because there are no local pathogens.”

The largest desert on Earth: the Sahara*

For many of us, it is the desert the Desert: The scene that comes to mind when someone says “Imagine a desert.” It’s got it all: sand, oases, beauty, Brendan Fraser and Rachel Weisz – and the reason it suits it all is simple. that it huge.

We all know maps are full of lies, so here’s some raw data: The desert area is 9,200,000 square kilometers (3,600,000 square miles), or in other words, half a million square miles larger than the contiguous United States. It is about the same size as China. It’s 13 Texas (Texas?) or 20 California.

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

Since 1920, the area of ​​the desert has increased by 10%, drying up fertile areas in the south. “The entire Chad Basin is in the region where the Sahara has been creeping southward, and the lake is drying up,” Sumant Nigam, an atmospheric and oceanic scientist at the University of Maryland, said in 2018.

“It’s a very clear signature of reduced rainfall, not just locally, but across the region,” he explained. “It is an indication of declining water in the Chad Basin.”

Why expand? Like all existential threats these days, at least part of it is due to human-caused climate change, which scientists believe is helping Hadley cells to increase the size of the Earth.

But this is one time especially Not on us. “Climate change is likely to expand the Hadley circulation, causing subtropical deserts to move northward,” Nigam said, but “the southward creep of the Sahara suggests that there are additional mechanisms at work.”

It raises an interesting question: If the Sahara is getting bigger, it must have once been small. Incredibly, this is kind of true: go back in time about twelve thousand years or so, and what is now a vast, sand-covered desert would have been full of plants and even forests. so what happened?

While early humans may shoulder some of the blame, the truth is that the transformation of the Sahara was inevitable. It’s a consequence of the Earth’s orbit itself: “The story of North Africa’s climate is one of 20,000 years, going back and forth between green and dry Sahara,” says David McGee, an associate professor in the Department of Earth at MIT. Atmospheric and planetary sciences, 2019.

“(The desert) seems like an impenetrable and inhospitable area,” he added. “However, it has come and gone several times, moving between grasslands and a wetter environment, and then back to dry climates, even over the last quarter million years.”

The hero of them all: Antarctica

Well, maybe we were lying a little when we described the Atacama and the desert as big and dry. There is one place on Earth that ranks as having the largest desert in the world, the driest places, temperature extremes unmatched anywhere else on the planet, and, surprisingly, the lowest infant mortality rate anywhere: Antarctica.

Just like little brother Sahara, Antarctica wasn’t always a barren desert land. About 90 million years ago, the continent had a “diverse environment,” polar scientist Johan Klages told Vox in 2021, “with moderate temperatures — the temperatures we have today in northern Italy, for example.”

Global carbon dioxide decline₂ Ice levels about 34 million years ago meant lower temperatures – and at Earth’s Antarctica, it became cold enough for ice to exist year-round. Once this threshold was crossed, the continent was able to grow into the sometimes glacial desert we know today.

But maybe not tomorrow. Atmospheric carbon dioxide levels₂ This resulted in the Antarctica in the Mediterranean being around 1,100 ppm, according to the climate modeling team at Klages. “We are (now) at 420 parts per million carbon dioxide₂“, to caution.

“We’re doing a big experiment now,” he told Vox. “We’re taking fossil fuels from the Earth’s crust that were deposited over millions of years, and normally they would have been released back into the atmosphere over millions of years — but we did that in a 150-year bubble.”

“This has never happened before. This has a huge impact.”