There was a time when hippos and elephants lived in the Sahara. Today it's a barren landscape where few creatures can survive. A small lake fed by groundwater that fell to earth as rain thousands of years ago, serves to remind of times gone by. As the climate in the region changed, it led to the creation of a vast sandscape. Deserts are still forming today — often as a result of human activity.

"Desertification" describes the transistion of fertile land to desert. It is often connected to human behavior. When people deplete natural resources — like water sources — in areas with a dry climate, plant life disappears and the soil becomes infertile. This phenomenon can be seen in 70% of the world's arid regions, such as in India, as pictured.

Every year, about 70,000 square kilometres (27,000 square miles) of desert are created. That’s equivalent to the size of Ireland. According to the German development agency, GIZ, 40% of Africa's population live in areas threatened by desertification. In Asia and South America it is 39% and 30% respectively. But places such as Germany, the US and Spain are also at risk.

One reason for the proliferation of deserts is population growth. In China, for example, the soil is being used to feed more and more people. Farmers crowd their pastures with animals that eat every last plant. The soil is loosened and eventually eroded by wind and rain. This creates some 2,500 square kilometers (965 square miles) of desert in the country every year.

The Aral Sea on the border between Kazakhstan and Uzbekistan illustrates a failed agricultural policy that led to desertification. In the past, the Aral Sea was the fourth largest lake in the world; today there is little left of it. Since Soviet times, the two neighboring countries have tapped the lake to irrigate cotton fields. Fishing boats are now stranded on land.

European countries can also suffer from cracked soil and dried up waterways. In Spain, desertification is accelerating — triggered by the demand to accomodate an influx of tourists from all over the world. Whole forests are often cleared for the construction of hotels. The soil is disrupted and removed, or buried under concrete. The Guadalajara region near Madrid is particularly under threat.

Desertification greatly affects lives. Without reliable sources of water and fertile soil, people struggle to survive and often have to leave places they've lived for generations. The GIZ estimates that desertification affects 485 million people in Africa. The UN predicts that more than 60 million people will have been forced to leave the desert regions of Africa by 2020.

Some countries have declared war on desertification. For decades, China has been trying to counter this trend by reforesting. Its "Great Green Wall" project, which began in 1978, has an ambitious goal — to replenish an area the size of Germany by 2050. On the World Day to Combat Desertification, the United Nations wants to draw attention to this increasingly urgent problem.

Of the 2 million-odd people who visit the Great Barrier Reef annually, a 2016 survey found that 69 percent were coming to see the UNESCO World Heritage site "before it's too late." And no wonder. The IPCC says that even if we manage to limit global warming to 2 degrees Celsius, 99 percent of the world's coral will be wiped out. Tourists can hasten their demise by touching or polluting reefs.

And what's the carbon cost of flying to remote natural wonders under threat? A 2010 study found that the business of polar-bear safaris in Churchill, Canada, had an annual CO2 footprint of 20 megatons. Most visitors arrived by plane, and while 88 percent of them said humans were responsible for climate change, only 69 percent agreed that air travel was a contributing cause.

Along with the polar bear, one of the most iconic images of climate change must be the dramatic curves of an iceberg sculpted by the warming atmosphere. Gliding between the melting giants on a cruise ship is a haunting experience that tourists will pay huge sums for. In the early 1990s just 5,000 people visited Antarctica each year, compared to over 46,000 in 2018.

You don't have to go to the poles to see vanishing ice. Kilimanjaro's snowy peaks are a striking sight above the equatorial savannah of the national park, which generates €44 million ($50 million) from tourism annually. Many visitors climb to the Furtwängler Glacier — where 85 percent of the ice has vanished over the last century. The rest is unlikely to survive much beyond mid-century.

When Montana's Glacier National Park opened in 1910, it boasted over 100 of the ice features from which it took its name. Now, there are fewer than two dozen. So dramatic is their retreat, that the park has become a center of climate science research. Some 3 million hikers and holidaymakers also visit the "crown of the continent" each year, soaking in the dying days of its ice-capped glory.

The Maldives are the archetypal tourist paradise: 1,200 coral islands with white beaches rising just 2.5 meters above the turquoise waters. In 2017, the president decided to build new airports and megaresorts to accommodate seven times as many tourists, and use the revenue to build new islands and relocate communities. He has since been voted out of office and faces corruption charges.

It's not just islands that are going under as sea levels rise. Wetlands like Florida's Everglades are disappearing too. Over the last century, around half the Everglades have been drained and turned over to agriculture. Now, saltwater is seeping into what's left, making it the only critically endangered World Heritage site in the United States.

The Galapagos will be forever associated with Darwin, who realized their unique wildlife had evolved over countless generations in isolation. Today, they are besieged by visitors and environmental changes are happening too fast for species to adapt. Ocean warming has left iconic creatures like the marine iguana starving, while UNESCO lists tourism among the greatest threats to the archipelago.