Deserts: A Guide to the World’s Arid Lands
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Around 33% of the Earth’s surface is desert land. When most people think of deserts, they imagine a hot, arid region. While this can be the case, deserts are much more diverse than many of us think. With vast, cold, barren areas in the polar regions covering more than 14 million square miles (36 million square kilometers), it’s not hard to see this diversity. Let’s dive a little deeper.
What is a Desert?
The simplest explanation of what a desert is is somewhere that faces extreme conditions. While we are all most familiar with the hot, dry desert, this isn’t the only thing that dictates what a desert is.
Deserts do not experience a lot of precipitation. In fact, in order to qualify as a desert, the region must receive less than 10 inches (25 cm of rain) every year, according to experts. Of course, this is exactly what you’ll find in those dry, hot deserts. The lack of rain means that in order for anything to survive here, it must be well adapted. For this reason, when we look at hot desert flora, we see a wealth of plant adaptations, such as thick skin to conserve water and spine for shade.
Some deserts are much colder and can be found at higher latitudes. While hot deserts usually lie between 15 and 30 degrees, cold deserts, such as the Antarctic are much closer to the poles. Here, you’ll see animal adaptations such as dense, light colored fur and wide feet so the animals don’t sink in the snow. The temperatures in cold deserts are extreme and can reach -110ºF (-80°C) in winter.
Types of Deserts
Deserts are much more than most people first imagine. Their characteristics largely depend on where on earth they are found. In the main, there are four types of desert, and they’re all unique in their own ways.
Subtropical Desert
Subtropical deserts are the ones that are most commonly portrayed in the media and are what people imagine when they think of a desert. These are the hottest type of deserts and some of the hottest places on earth with summer temperatures reaching an average of 122ºF (50°C) during the day. However, at night, owing to low humidity, it’s not uncommon for the mercury to drop to 25ºF (-4°C).
The most well-known subtropical desert is the Sahara; this is also the largest. But there are lots more of these deserts occurring between 15 and 30 degrees north and south, including the Thar desert in India, the Simpson desert in Australia, and the Kalahari desert in Africa.
In these arid regions, there is usually no more than 0.6 inches (1.5 cm) of annual rainfall making it difficult for life to thrive. However, there are some specially adapted plants like succulents, shrubs, and trees like the mesquite tree that are able to survive here. You’ll also find some animals adaptations for these hot conditions like reptiles, insects, and camels.
Subtropical deserts are the result of cool air from the equator coming down and turning warmer and drier. When this happens, it’s difficult for rain clouds to form, which is why these areas are so dry.
Semiarid/Cold Desert
The Gobi desert is perhaps one of the most well-known examples of cold deserts and can be found in China. There’s also the Patagonian desert in Argentina which is well known for being a vast, barren region.
These deserts are categorized by their wet winters and long, dry summers. Temperatures here can reach up to 80ºF (27°C) during the summer but will drop to around freezing in the winter.
Rainfall here usually occurs in winter, and these deserts may experience between 0.7 – 1.6 inches (2 – 4 cm) of precipitation each year. The soil is very rocky and sandy, meaning that it’s not ideal for many types of plants. Therefore you’ll mainly find things like cacti.
The animals here are a little more diverse with things like reptiles, kangaroos, birds, rabbits, and insects.
Coastal Desert
As you can probably guess from their name, coastal deserts occur closer to the ocean and are a result of cold sea currents condensing as they reach the warmer parts of the continent. One of the most famous coastal deserts is the Namib desert in Namibia as well as Chile’s Atacama desert.
These regions tend to experience cold winters with temperatures dipping as low as 41ºF (5°C). However, during the long summers, the climate is quite warm with an average temperature of 75ºF (24°C). Rainfall here is slightly greater than other types of desert and you’d usually expect between 3 – 5 inches (8 – 13 cm) annually.
These conditions mean that it’s much easier for life to thrive here. Plants keep their roots closer to the surface to make the most of what little rainfall occurs, and you’ll find animals like toads, owls, coyotes, eagles, and several species of reptiles.
Polar Desert
The polar deserts can be found at the most northerly and southerly points on earth, and there are only two; the Arctic and the Antarctic. While the latter is pretty much uninhabitable for humans, many people live within the Arctic Circle in countries like Norway, Canada, Greenland, and Russia.
These are some of the coldest places on earth, with winter temperatures reaching a staggering -22°F (-30ºC) on average. Although, they can be warmer or colder in places, with the lowest ever recorded temperature in the Antarctic being -128°F (-89°C)!
Along with these cold conditions are very salty soil conditions, and while there are millions of liters of water, much of it is in the form of glaciers, so it’s difficult for life to thrive. In more habitable parts of polar deserts, you may find some trees like willows, but in terms of plant life, it’s mainly lichen and algae that form.
There is some animal diversity here, with things like jackrabbits, antelope, and various species of mice.
Surprisingly, the summer temperatures in polar deserts can get quite high. While the average is around 50ºF (10°C), there have been records of temperatures near research stations reaching up to 86ºF (30°C).
How are Deserts Formed?
Earth looks very different now to the way it did when it was first formed. Various landscapes litter the surface of the earth such as forests, mountains, and of course, deserts. But where did deserts come from and how did they form?
High Pressure Zones (Hadley Cells)
Subtropical deserts are located between 15 and 30 degrees north and south of the equator, and there’s a scientific reason for this. It’s all to do with how the air is heated at the equator and what happens to it after this.
Air at the equator is heated by the sun and is always very humid. It rises and meets with cooler inrushing air, which causes it to cool. As this air comes back down, it cannot hold as much moisture, so it releases some of this, which is why we see tropical areas.
However, some of this air remains high up in the atmosphere; this air is cold and dry but as it moves further from the equator, it starts to descend and becomes warmer. The warm, dry air is what causes these hot, arid deserts.
Rain Shadow Caused by Mountains
The presence of mountains can vastly alter the conditions of the surrounding land. When mountain ranges sit parallel to coastal areas, rainshadow deserts are often formed.
This occurs because of the prevailing winds that move inland from the ocean and this cool air has nowhere else to go but over the top of the mountains. Any moisture within this air falls over the face of the mountains as it passes over.
However, as these winds go over the peak and start moving down the other side, the air is much drier, having already lost most of its moisture. In addition to this, the presence of this descending air means rain clouds have a much harder time forming so, on the other side of the mountain, the conditions will be more desert-like.
Cold Ocean Currents
Deserts such as the Namib are formed as a result of cold ocean currents, and these deserts always occur on the western side of land masses.
As cold currents from the polar regions move further toward the equator, they are often interrupted by continents. Add to this the cold water from the ocean and what you end up with is very cold air with little moisture that is blown inwards.
But as this air begins moving over the land, it starts to warm up. As the air moves further inland, it’ll gather moisture which will eventually fall as precipitation. However, any moisture already present in this air will fall before it reaches land and that’s why we have coastal deserts.
Interior Areas far Away from the Sea
Some of the world’s continents are huge and therefore some areas are so far away from the sea that it’s almost impossible for them to receive rainfall. For example, the furthest point on land from the ocean is called the pole of inaccessibility which is located near the Kazakhstan border in China. From here, the nearest sea is more than 1644 miles away.
The reason that areas like this become deserts is that when air rises from the sea, most of the moisture is released from this closer to the coast. By the time this air reaches deep inland regions, the moisture is all but depleted.
Why do Hot Deserts Get Cold at Night?
If you’ve ever watched any survival documentaries, the presenters will always tell you that the desert can trick you. One of the reasons for this is that while hot deserts have extremely high temperatures at night, things can drop below freezing once the sun sets.
It’s hard to imagine but it’s true and the reason for this is mainly to do with all that sand. Sand covers most of the ground in hot deserts but this natural material does not have the ability to retain heat. While it might get pretty hot during the day, as soon as the sun stops shining on it, the sand releases all that heat back into the air.
Another reason that things get colder in the desert after dark is the lack of humidity. In tropical regions where the air is humid, it stays warmer even at night. That’s because water is much better at retaining heat than air. So, in arid regions where the air is very dry, it won’t hold heat if the sun isn’t shining on it.
That said, dry air is a lot easier to heat up compared to humid air, which is why it doesn’t take long for the temperatures to soar once the sun rises over the hot desert.
What are the Causes of Desertification?
Desertification is a reduction of the productivity of dryland which can be caused by natural influences and human ones. It’s important to remember that this is not the same as drought and the causes of desertification are a little more complex.
While currently one third of the world’s surface is considered to be desert, this figure will likely increase over time. In fact, according to the UN, in the next ten years, as many as 50 million people may be displaced as a result of desertification of the local landscape. But what is causing this, and is there anything we can do about it?
Climate Change
It’s thought that desertification is one of the greatest threats to the planet at this time, and the difference between human survival and extinction lies within the first 7.9 inches (20 cm) of soil. Climate change is a serious problem, and it’s thought that it’s currently responsible for the degradation of more than 1.9 million square miles (4.9 million square kilometers) of dryland.
Because of climate change, desert regions are now receiving far less annual rainfall than they did just five decades ago. This increases the frequency of drought.
Another issue relating to climate change is the alteration of the soil quality, including things like salinity levels and organic matter decomposition, which is responsible for soil nutrients.
It’s thought that, in lower latitudes, the crop yield could decrease significantly with every degree of temperature increase.
Deforestation
Trees create humidity, so with their removal, the air becomes drier and this can lead to the formation of deserts. Deforestation is one of the main contributors to desertification, but if it were to stop, this could go a long way in reducing the effects of climate change.
Not only do fewer trees mean less humidity and less rainfall, but it also means that the quality of the soil will be affected. Tree roots hold the soil together and prevent erosion, and when soil erosion happens, this decreases the soil fertility, particularly in drylands.
Overgrazing
Humanity is growing at an alarming rate and in order to ensure everyone has enough to eat, more and more animals are being grazed. This means that the bare soil is far more susceptible to soil erosion, which as we have already learned, can be devastating where desertification is concerned.
What’s more, overgrazing also reduces the amount of vegetation cover. Again, fewer plants and trees mean less humidity. While you might think that these plants will just grow back, it’s been demonstrated that overgrazing makes it difficult for new roots to take hold, and therefore, fewer plants actually regrow.
In the once fertile and flourishing Mongolian steppe, the conditions were anything but desert. However, in recent years, it seems that more and more of this area is being overtaken by the Gobi Desert, and the main cause is overgrazing.
Unsustainable Agricultural Practices
Desertification is something that many of us associate with very dry areas like those in Africa. However, research has shown that North America and Europe are now at risk, with countries like Spain and Greece being very vulnerable.
According to research, as much as a third of all available land has been degraded because of human agricultural practices. Experts are now willing us to start taking action to prevent further problems.
By avoiding practices like planting unsuitable or low quality crops, heavy tilling, and not protecting soil from wind and rain, humans are making a rod for their own backs. In drylands, as many as 2 billion people rely on local agriculture but it’s essential that practices are reassessed if we want to continue being able to farm in these areas.
Resource Mismanagement
It seems that, as humans, we really take our natural resources for granted. Humans are demanding more natural resources such as water and this is leading to rapid desertification. Just looking at the evidence we’ve seen so far clearly points to this.
In areas where desertification is already a problem, it’s noted that the birth rate is higher. When there are more people being born, humans are beginning to farm in unsuitable areas which is further contributing to the problem.
As well as exploiting natural resources, humans are also to blame because of things like tourism. This increases carbon emissions, but there are also things such as improper waste disposal and sewage, which both put pressure on the already limited natural resources.
Overpopulation
As I have already mentioned, the planet is rapidly becoming overpopulated, and this means humans require more resources. However, gathering these resources, such as wood and water is putting enormous pressure on the land, and the result is desertification.
For example, in the Sahel region of Africa, illegal logging practices, clearing land for housing and agriculture have all put a massive stain on the land, making this one of the most vulnerable regions to this phenomenon.
How Does an Oasis Form?
Have you ever seen those movies where someone trekking through the hot deserts imagines seeing a beautiful oasis before them? Most of the time it’s nothing but a mirage, but oases do form in the desert despite the hostile conditions.
An oasis is an area within a desert where the water table comes to the surface. As such, the area becomes much more fertile than the surrounding desert. But how and why does this happen?
Naturally formed oases are formed when the water pressure under the ground rises, causing water to seep above the surface. This happens quite often when there is a natural spring present.
However, there are some cases where oases are artificially formed in order to sustain life in otherwise harsh desert conditions. This is usually done by digging wells to reach the water table and these are often used for irrigation purposes. Where artificial oases are created, people will usually plant tall trees around the edges to prevent the water from being contaminated by desert sands.
