When we think about the desert, the first thing that comes to mind is likely a vast, ever-sprawling stretch of sand.  Maybe we throw a few dunes in for good measure, but the predominant idea of the desert is very likely sand to most people on the planet.  Another aspect is likely one of desolation, a certain sense of loneliness coupled with the intense temperature fluctuations that accompany a space with nearly no cover from atmospheric elements.

While this certainly captures the essence of the desert, it is really only a snapshot of the range of desert characteristics.  Maybe it's because a majority of people on Earth don't live with the desert on a daily basis, still roughly a sixth of our population does.  That's over 1 billion people!

Being mindful residents of our planet, let's take a closer look at deserts, what defines them, and how they play into the systems of our home!

What is a Desert

Despite our initial take on what a desert looks like, a sprawling searing sandscape as far as the eye can see, the definition is a bit simpler than that. Deserts are characteristically dry regions receiving less than 250mm of precipitation over the course of a year.  A common term to describe deserts is "arid" which applies to both the atmosphere and, importantly, the soil of the region.  Another way people like to consider a region a desert is "an area unfit to sustain a human population," but that is a highly human-oriented view of the planet.

You might notice that there is no sand qualification for the desert, and this is true.  While sands have a habit of forming in deserts, there are plenty of desert regions where any sand gathering doesn't occur, leaving only the exposed plains of stone and bedrock.
To be a desert, a region can't have more than 250mm (10in) of precipitation in a given year 
​That said, deserts that do host beautiful, rolling dunes of sand get this sand from that underlying bedrock.  The dry atmospheric conditions make the rock brittle allowing for wind erosion to peel off fine particles of the rock.  When deserts do get precipitation, this creates a high likelihood of the rock to erode via water as well.  It's these acts over tremendous periods of geological time that have generated the sandy deserts we love to envision today.

You may also notice heat is not a factor, as the Earth's polar regions also meet the aridity definition for a desert!  Though these deserts are often classified specifically as polar or cold deserts, they still do not receive more than 250mm of precipitation due to the dryness of the atmosphere enveloping the ecosystem.

How Deserts Form

There are many ways that a desert can be formed, so much so that they appear on all continents including Antarctica.  With the exception of Europe, each continent holds one of the top 10 largest deserts in the world.  This ecosystem is certainly not selective of its home!
The Sahara Desert in Africa is the Earth's largest non-polar desert coming in at a whopping 9.1 million square kilometers (3.5 million square miles)!

Trade Winds

The Earth's "trade winds" are winds that flow from the tropics of cancer and capricorn to the equator.  As these winds move, they get warmer, causing cloud cover to disperse.  This has a twofold effect of reducing our planet's water transport and creating a hotter, dryer atmosphere.  A perfect mix for the arid conditions needed for a desert!  Africa's Sahara desert is not only the largest non-polar desert, but also an example of a trade winds desert.

Monsoon and Midlatitude

Another vector for desert creation is being far enough inland that precipitation is simply an uncommon occurrence.  Reflecting on the water cycle, we can recall that rainfall originates in our large bodies of water and is effectively blown inland by our planetary winds.  Midlatitude deserts like the Sonoran in North America are just too far inland for precipitation to be carried over the desert.  Whereas monsoon deserts like the Thar in Asia are just beyond the circulating winds of the monsoon area, resulting in little rainfall reaching the desert.

Rain Shadow

An interesting phenomenon occurs when precipitous clouds approach a mountain.  As the clouds move over the mountain, the air cools reducing the cloud's ability to hold moisture causing it to precipitate.  As the cloud moves over the top of the mountain and begins to descend, its ability to hold moisture increases making it less likely to precipitate.  This area "behind" the mountain is called the "rain shadow" and gives rise to arid regions on the leeward (opposite of windward) sides of mountains, as is the case for the Taklamakan Desert tucked behind the Himalaya Mountains in Asia.

Coastal

Probably one of the more curious examples (at least for human minds thinking being near water would not define a desert) is the coastal desert.  Generally formed on western coastlines alongside cold currents, these deserts tend to get their precipitation from cold fogs that creep out of the ocean before the clouds can rise into the sky.  Despite being close to a water source, the Atacama coastal desert of South America is the driest desert in the world, bringing in a whopping 1mm of rainfall every 5-20 years!

Polar

And, back to what we mentioned earlier, deserts don't have to be hot to be deserts.  The polar regions of the Earth have yearly high temperatures of 10 degrees Celsius and meet the precipitation requirements for deserts coming in at under 250mm per year.  Additionally, cold deserts are the largest deserts on Earth with the Antarctic Desert ringing in at 14,200,000 square kilometers.  That's the size of the United States 1.5 times over!

Desert Inhabitants

Even though deserts are defined as arid places making it difficult for plants to take hold, our fellow species on Earth are a proven resilient bunch.  While not as biodiverse as a rainforest, the desert biome sports an assortment of plants and animals finely tuned to living in a hostile environment.
The Atacama Desert in South America is the driest desert on Earth, only receiving 1mm of rainfall ever 5-20 years!
From a plant's perspective, they need to both be able to find and retain water effectively.  In more livable areas, plants have a wide assortment of root systems and specializations to fit a highly competitive environment, but in the desert, competing with neighbors isn't a great survivability tactic.  As such, plants in the desert tend to be fairly far apart.  Their root systems also tend to favor either a very wide and shallow approach (collecting water from topsoil) or a very narrow and deep (collecting from fossil water reservoirs).  As protection from creatures looking to capitalize on the plant's stored water, it is common in plants like the cactus to produce spikes to deter herbivorous life.

On the animal side, we have a surprising amount of diversity.  Insects are still abound with a particular abundance of ants.  Spiders and scorpions both thrive in desert areas, usually sporting toxins to help defend them in this hostile environment.  Reptiles also thrive particularly well in the desert thanks to the extreme temperatures complementing their cold-blooded systems.  They too may be carrying toxins for predation and defense, and often have interested color displays to ward of anything that might want to eat them!
Despite old beliefs, camels store fat in their humps, not water
There are also a number of birds that inhabit the desert arena ranging from woodpeckers that nest in cacti to the popularized roadrunner which prefers its feet to its wings.  And despite not being the first thing to think of when it comes to deserts, a fair number of mammals have adapted to the environment.  From small burrowers such as rats to foxes and jackals all the way to the mighty lion, mammals prove that there can be a space for them in even the toughest regions of the world.

We'd also be remiss not to mention the camel, a particularly min-maxed mammal when it comes to desert living.  To prevent itself from overheating, the camel stores all of its fat in its characteristic humps.  This also acts as a nutrient store for the camel when it is unable to eat or drink for long durations of time.  These features of the camel have earned it the title of the "ship of the desert!"

A Benefits Balancing Act

After so much talking about the desert being a hostile environment for species across the spectrum, it is hard to imagine benefits of these ecosystems.  In the past, humans even considered foresting or conversion of deserts to productive land as a more helpful approach to deserts than keeping them around.  We might have actually done that if the desertification of land wasn't so difficult to reverse.  However, over time, humans have found other convenient services from deserts that haven't always applied to us centuries ago.

One of the benefits is the ready access to minerals for building, well, most anything.  One of the downsides to the desert being a livable environment is its arid soils, but this really just means the organic content of the soil is low, leaving the mineral content high.  Minerals such as copper, iron, chromite, gold, silver, uranium, lithium, and clays are all valuable and more easily available in desert regions than other ecosystems.
Definitely a newer consideration is the use of deserts for solar energy capture.  Over the course of this page, we've seen that one of the core reasons deserts are arid is due to lack of precipitation in large part because clouds just don't reach deserts.  Without cloud cover, these areas can take full advantage of the sun's incoming energy like no other region on the planet.

Another particularly interesting benefit is the inherent resilience of species that have grown to survive in these regions.  Throughout history, even into today, humans get their inspiration from studying the world around us, and the species found in deserts are some of the most specialized creatures on the planet.  There is likely a ton we can learn by focusing research efforts on deserts and the species that inhabit them.

​​Of course, this is all a balancing act.  Deserts that have naturally formed over geological time take hold due to the reach of the Earth's functions and systems.  Just because they're here doesn't mean they're the most conducive areas for a majority of the life on our planet.  And wouldn't you know that humanity has a way of teetering the formation of deserts as well...
Desertification is the process by which overcultivation of land leads to arid soils and desert expansion.

Humans must adhere to sustainable land practices to ensure we don't throw off the Earth's balance of existing desert cover, driving many species out of their current habitats.
That's right, our deserts are slowly expanding.  Not only due to fluctuations in the Earth's systems, but because humans have found new ways to encourage aridity in our soils and atmosphere.  Aside from the increasing global temperature due to anthropogenic climate change, we also have encouraged generations of unsustainable agricultural practices that have left our soils bereft of organic nutrients.  They cannot hold water and cannot sustain plant life to hold and heal.  Especially on the borders of existing deserts and drylands, this is forcing humanity to abandon land as the deserts continue to grow.  This is desertification, a very real issue facing our planet today.

Thankfully, humanity is more aware of the factors feeding into desertification now, and we're making small steps to spreading more sustainable practices when working our land.  However, simply knowing what caused the problem does not mean the damage we've done is gone.  While our natural deserts certainly have a place on our home, humanity needs to keep our contributions to the Earth's deserts to a minimum so we don't disrupt the balance that allows us and so many of our neighboring species to live here at all.  Hopefully with the knowledge to do better, we can continue containing our accidental growth of the planet's harshest regions, while still admiring the ones built by our home itself.
~ And, as always, don't forget to keep wondering ~
Sources
Deserts
https://pubs.usgs.gov/gip/deserts/
https://www.nationalgeographic.org/encyclopedia/desert/
Arid Soils
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/arid-soils​
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