Planet Earth is generally considered to be around 4.5 billion years old. That's a long time — an age that's almost impossible to wrap your head around. Humans are used to thinking in terms of centuries and millennium when they consider "long ago," but in terms of how long the Earth has been around, even 1,000 years is a blink of the eye. (If 4.5 billion years was compressed to be a day long, 1,000 years would compress down to a little less than two seconds).

But a lot can happen in a millennium. In a few thousand years, entire landscapes can change completely. Consider that just 10,000 years ago, there was a mile of ice above where I live in Maine.

So it's understandable how easy it is to look at something as huge as a mighty desert and assume that it has always been there. Some of the world's largest deserts can stretch for thousands of miles and cover millions of square miles of area. It's hard to imagine that they have been anything other than vast stretches of dry sand and rocks (or ice in the case of a few deserts). Yet, for every huge desert you show me, I can show you an area that was covered with life sometime in the not-too distant past (again, relative to the age of the planet). It might not be a bad idea, considering the rate that humanity is dumping greenhouse gasses into the environment, for us to take as many lessons as possible from the study of how once verdant areas turned into deserts. If we know how it happens, we stand a better chance of finding a way to slow down or reverse the process.

Here are seven deserts that used to be verdant fields and forests.

Sahara Desert

Sahara

Photo: Wikimedia Commons

The Sahara Desert covers 3.6 million square miles of Northern Africa (that's bigger than the continental United States) and is the largest hot desert in the world. The Sahara has all of the stereotypical desert features and is covered with towering sand dunes, packed with camels and scorpions, and dotted with the occasional oasis (palm trees and all).

The Sahara is not a forgiving environment — temperatures regularly creep over three digits while strong winds whip up sandstorms that darken the sky and choke the lungs of anything caught out unprepared. But it wasn't always that way. The Sahara was actually a lush place as recently as 6,000 or so years ago. If you extend your range of view out into the hundreds of thousands of years (and farther), you see the area cycle through periods of wet and dry, each brought on by larger changes to the climate. Early humans left behind cave art showing crocodiles and large dinosaur fossils, suggesting an environment lush enough to support 20-foot-long animals.

Great Victoria Desert

Victoria Desert

Photo: Amanda Slater/Flickr

The Great Victoria Desert is found in the southwest quadrant of Australia and is one of the least populated (by humans) areas on the planet. Aboriginal people called its wind blasted dunes and sand prairies home before westerners sailed in and conquered the continent. In the '50s and '60s, the Australian government evicted many of the remaining Aboriginals and used the area to test nuclear weapons.

Australia has been a relatively dry mass of land for the last 100,000 years or so, but if you go back a few million years, you'd find it lush and green, covered in rain forests and large animals straight out of an "Avatar" casting call. Today's Australian rain forests are the distant relatives of these ancient forests, pushed to the continents outer fringes by deserts like the Great Victorian Desert.

Gobi Desert

Gobi desert

Photo: Southland Farm Cottages/Flickr

The Gobi Desert, covering a little more than half a million square miles of China and Mongolia, is a diverse, though generally dry, landscape with high elevation plateaus abutting grassy (at least in the wet season) steppes running into sandy dunes. It is a cold desert where winter temperatures generally run below zero. The bone dry air means there is little snow, though frost is a constant winter companion.

It's not hard to find places in the Gobi Desert that used to be green space — it is estimated that the desert eats up hundreds of square miles of grassland every year thanks to overgrazing, deforestation and climate change. Walk to the current border of the desert and look around — a few years ago you would have been looking at grassy fields instead of dry barren expansions of tan sand and rock.

Kalahari Desert

Kalahari Desert

Photo: Jeppestown

Technically, the Kalahari Desert is a semi-desert because seasonal rains regularly soak it, waking dormant grasses and other plants. Even still, when it's dry, the Kalahari is every bit as much a desert as any place listed here. The name Kalahari is derived from a local word meaning "a waterless place." Temperatures can climb above 110 degrees Fahrenheit, driving away any clouds that find strength enough to form in the arid air.

Tens of thousands of years ago, the Kalahari was covered in a huge (about as large as South Carolina) body of fresh water called Lake Makgadikgdi. As the centuries rolled by, the lake was slowly drained away as the rivers that fed from it pulled out more water than was being fed in. By roughly 10,000 years ago, most of the lake had been bled out and the current-day Kalahari started getting dryer and dryer.

Arabian Desert

Arabian desert

Photo: Pedronet

The Arabian Desert, also known as all of Saudi Arabia and part of Egypt, sprawls across almost 1 million square miles and is home to one of the largest contiguous bodies of sand in the world. It is one of the least biologically diverse places on the planet due to its harsh climate and damages from human activity (hunting, industrial pollution, military action.) But just a few tens of thousands of years ago, it was home to a large number of shallow lakes that supported a diverse community of animals, including hippos and water buffalo.

Mojave Desert

Mojave desert

Photo: Ken Lund/Flickr

The Mojave Desert covers most of southern California and parts of Nevada, Utah, and Arizona and is, at only 47,877 square miles, a small fry compared to the other deserts on this list. The Mojave is both a hot and a cold desert, depending on the time of year, with temperatures running from as a low as zero in the winter to as high as 130 degrees F in the summer.

Around 10,000 years ago, as the last ice age faded and melted away, the area that is today known as the Mojave Desert was a much wetter place. It was marked by lakes and streams fed by retreating glaciers and sustained by wetter weather patterns.

Antarctica

Antarctica

Photo: US Embassy

It's sometimes easy to forget that Antarctica is a desert, receiving less than eight inches of precipitation each year. It's a cold and forbidding desert that is covered in darkness for half of the year but even it too was once a green and biologically dense land. In 1986, researchers from Ohio State University found evidence of a temperate rain forest dating from roughly 3 million years ago. If you go farther back than that, far enough to notice continental drift, you'd find an Antarctica enjoying the benefits of a more northern location, slowly on a march towards its current home hugging the South Pole.

Read about more deserts here on MNN:

10 unforgettable desert adventures

Study: Increased carbon dioxide may lead to greener deserts

10 places on Earth that resemble alien planets

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