Geo explainer: How are deserts formed?

Deserts are vast and harsh landscapes that cover around 20% of the Earth’s surface, from the Sahara to the Arctic, we delve into how they are formed –

By Susanne Gough

Deserts all have one thing in common: they are dry. To be defined as a desert, an area must receive less than 12.5 centimetres of rainfall a year. Though many deserts reach scorching temperatures by day, heat is largely irrelevant – the Sahara reaches freezing temperatures by night, and the Arctic and Antarctic ice caps qualify as polar deserts. There are many different types of deserts, and how they were formed dictates their type, whether subtropical, rain shadow, interior, coastal, or polar. 

Beyond their formation processes, deserts have histories. When were they formed is as rich a question as how, delving into the human and climactic origins of our driest biome in deep time. And desertification is still happening today. Alongside the plethora of ways our environment is transforming in this era of the climate crisis, deserts are no exception. But how can we prevent and stem the damage of these changes? And how are indigenous desert communities leading the way? 

Subtropical deserts

Occurring between fifteen and thirty degrees north and south of the equator (areas also known as the Tropic of Cancer and the Tropic of Capricorn, respectively), subtropical deserts are the arid, comparably barren neighbours of their wet, equatorial rainforest counterparts. The Sahara, the largest hot desert on earth at 9.2 million square kilometres and unquestionable A-lister of the desert world, is subtropical. 

These deserts are formed by air currents. Hot air rising from the equator loses its moisture in the process, producing clouds and rain, which give rainforests their name. This now dry air cools and descends, generating an arid, formidable desert environment. These wind currents are known as ‘Hadley cells’, named after eighteenth-century amateur meteorologist George Hadley, who was motivated by a need to understand wind patterns for trade purposes. 

Rain shadow and interior deserts

In keeping with their captivating name, rain shadow deserts have fascinating origins. Mountains are their key determinants. Water evaporates from oceans and lakes into vapour and traverses inland, falling as rain and hovering in clouds. Mountain ranges act as barriers for this precipitation, forcing moisture to cling to their seaboard side. The resulting dry area beyond their alternate face is known as a ‘rain shadow’. The Mojave Desert, home to the infamous Death Valley, sits in the rain shadow of the Sierra Nevada Mountains of California. 

The inland location of many rain shadow deserts is also a causative element of their formation. The vast basins and plateaus of the Gobi Desert in the heart of East Asia are flanked by the Himalayas, China’s northern reaches, Mongolia, and the Eurasian Steppes. By the time sea winds have travelled so many miles inland, they have lost their humidity, leaving an inhospitable interior desert.  

Coastal deserts

At this point, coastal deserts may seem a contradiction in terms. Yet these unique habitats confound the other desert types, due to their specific coastal weather patterns. Cold ocean water, when in contact with complex wind currents, produces a thick fog. Hanging over the coastal desert, this fog almost never precipitates as rain. The Namib Desert lies along the Atlantic Ocean shore; a desert buffer to Angola, Namibia, and South Africa. It is nonetheless one of the driest deserts on earth, with an average of less than 1.2 centimetres of rainfall a year. 

Polar deserts

The extreme polar regions of the Arctic and Antarctic hold vast amounts of water, yet paradoxically can be called deserts because it is locked away in glaciers and reservoirs of ice. Their freezing temperatures make rain impossible, producing a cold, dry climate. These deserts are also known as the ‘ice caps’. Not only do the ice caps act as water reserves, stabilising sea levels, they also, as Wadham et al. demonstrated in 2019, act as crucial carbon stores. Their melting is an often-cited catastrophic effect of and perpetuating factor in the climate crisis. 

When were the world’s deserts formed?

During the most recent ice age, which began around 18,000 years ago and ended as the Holocene dawned some 6,300 years later, polar deserts were widespread. Yet as the earth warmed, deserts were not an immediate phenomenon. Around 11,000 years ago, the area where the Sahara Desert now resides was lush, green, and forested, abundant with bodies of water. Slight changes in the tilt of the earth’s axis affect the trajectory of the sun’s rays into the atmosphere, altering the fine climactic balance which produces the earth’s biomes. 

Despite this explanation, there is a gap in the data that scientists have never been able to conclusively explain. The aridity of the Sahara came at an abnormal acceleration. Archaeologist David Wright’s research into the phenomenon posits that humans had a bearing on the change. With the advent of the domestication of livestock, humans grazed animals in the area, and, as Wright argues, this overgrazing led to a decline of plant life and the formation of a desert habitat. Nevertheless, the origins of our deserts remain somewhat shrouded in mystery. 


Deserts are getting bigger. Desertification is the process of land becoming desert and can happen for a variety of reasons, such as overgrazing of livestock and deforestation. Climate change, however, is a crucial factor. Areas close to deserts, such as the Sahel, located south of the Sahara, are particularly at risk. Global warming has led to desert regions becoming more extreme in their aridity and high temperatures, which makes exposed adjacent areas more susceptible to desertification. Various solutions have been posed and tested, such as the ‘Great Green Wall’ in the Sahel – an eight thousand kilometres long ‘wall’ of acacia trees. The moisture stored in these trees and their roots prevents further desertification and provides a lively habitat for other flora and fauna, encouraging increased biodiversity and ecological richness. In the latest Geographical Podcast, see how communities in the Draa River Valley, Morocco, have defiantly resisted desertification and how indigenous knowledge is being used in ‘natural infrastructure’ to reverse the damaging process.

What can we learn from indigenous desert communities?

Ten inches of rainfall a year is nowhere near enough to sustain most of the flora and fauna on earth, let alone a yearly crop. Nevertheless, deserts are habitats for many specialist plant and animal species, whilst also being home to one billion humans, who have adapted to their hostile surroundings in many ways. Dubai and Las Vegas are famous examples of cities which, through intense exploitation of natural resources and use of elaborate irrigation technology, have been able to defy one of nature’s most extreme and inhospitable climates.

Meanwhile, indigenous communities have been adapting the desert environment to human needs, in more symbiotic ways, for thousands of years. Australia’s aboriginal communities, for example, use long-established methods to coax water out of rock holes and roots, even in the desert. Beyond the displacement, prejudice, and erasure these communities have experienced, they are also affected disproportionately by climate change. Indigenous activists such as Paul Kabai, a Torres Strait Islander taking the Australian government to court over failing to prevent climate change, are at the helm of the fight against the climate crisis. Working together with these generationally knowledgeable communities, desertification can be prevented. 

Murcia’s farmers fear for the future as Spain cuts water supplies from River Tagus

By Euronews Green  with AFP  •  Updated: 10/05/2023 – 10:23

Spain’s government has decided to cut water brought from the Tagus River to irrigate crops in the bone-dry southeast of the country. –

Spain is one of the EU’s biggest fruit and vegetable producers.

Almost half of the country’s exports are grown by farmers like Juan Francisco Abellaneda, co-founder of farming cooperative Delior.

His salads and watermelons fill the shelves of Europe’s supermarkets throughout the year. These crops are irrigated by water brought from the River Tagus, hundreds of kilometres to the north of Abellaneda’s 300 hectares of fields near Murcia.

But, as Spain faces the realities of climate change with three-quarters of the country at risk of desertification, the government has decided to limit the flow of water from the Tagus to the southeastern Levante.

“There are many thousands of hectares that are cultivated here, as soon as you cut that (the water supplies) by half, well, everything that is not cultivated will be desert, in a few decades, in a few years,” Abellaneda explains.

Without water, the land can’t be irrigated leading to uncertainty about the future. It means he may have to cut some of the 700 staff employed by the Delior cooperative.

“We need the water. If they take it from us, it will be nothing but a desert here,” Abellaneda says.

Why has water transfer become controversial?

Water levels in the Tagus, the Iberian peninsula’s longest river, have dropped dangerously low. In some places, the dried-up riverbed can be crossed on foot in summer.

As the Tagus runs dry, the right to pull water from the river – which crosses into Portugal before meeting the Atlantic Ocean – has been the centre of a heated debate.

“The Tagus is suffering,” says Domingo Baeza, professor of river ecology at the Autonomous University of Madrid.

“It is degraded in numerous places… because we have far outstripped its capacity (with) uncontrolled expansion of the land it irrigates.”

Water is channelled to the bone-dry southeast of Spain via the Tagus-Segura Water Transfer project – 300 kilometres of tunnels, canals, aqueducts and reservoirs. It brings billions of litres of water from the Tagus to the Segura basin between Murcia and Andalusia.

It was once heralded as a vital solution to drought – now it is accused of making them worse.

Global warming has changed Spain

Since the Water Transfer was built, Spain’s average temperature has increased by 1.3 degrees Celsius, according to the country’s meteorological service. Extreme temperatures and a lack of rainfall have dried up rivers and reservoirs, leading to water shortages.

“Global warming has changed things,” says Julio Barea, a spokesperson for Greenpeace Spain. He adds that the Transfer “no longer works” for Spain.

“The Tagus needs the water (it is losing to farms in the southeast) to survive.”

Residents of the Castile-La Mancha region, where the Tagus’ waters are syphoned away, say the effects have been visible for years. Artificial lakes created by the damming of the river in the 1950s used to attract tourists who would come to swim, boat and eat at local restaurants.

“Everything stopped when the damned water transfers started,” says Borja Castro, vice president of the Association of Riverside Municipalities of Entrepenas and Buendia, whose water is pumped to the southeast.

“With our water went businesses, jobs and a part of our population. They turned the Levante into the garden of Europe, but with water that came from somewhere else. It’s madness.”

A battle over water in Spain

Farmers in the southeast say cutting the flow of water from the Tagus could mean the end of agriculture in the region.

It could lead to 12,200 hectares of arable land being abandoned, the SCRATS farmers lobby group claims. The economic cost would also be colossal, it argues, up to €137 million a year, with 15,000 jobs lost.

Prime Minister Pedro Sanchez’s left-wing government says it had no choice but to cut the flow of water to keep in line with rulings from Spain’s Supreme Court and EU environmental rules.

Minister for Ecological Transition Teresa Ribera has said that the decision was based on “the best scientific knowledge possible”. She has promised more money to develop other sources of water like desalination – something farmers say is too expensive to be viable.

The decree has created a unique political situation ahead of local elections later this month.

The socialist region of Valencia is backing Murcia to try and stop the cuts. But socialist-held Castile-La Mancha, with the help of local right-wing politicians, backs the government’s decision to stop the flow of water.

Environmentalists argue that Spain’s whole agricultural system needs to be completely rethought.

“More than 80 per cent of freshwater in Spain is used by agriculture… it’s just not tenable,” says Greenpeace’s Barea.

“Spain cannot be the garden of Europe if our water is getting more and more scarce.

Spain’s desertification is pitting regions against each other

The crux of the conflict is the decision of the national government to limit the water pipeline flow to Levante farmers

The disastrous domino effect of climate change and bad planning in the past is already playing out in Spain, where a second consecutive year of drought has forced the authorities to turn off the tap on the Tagus-Segura Water Transfer. This decision, however, has been met by contrarian reactions from residents living in Castilla-La Mancha and Murcia region, leading to what some media have called “water wars”.

The water infrastructure in question was built in the 1970s and represents the biggest hydro-engineering project in the country to date. The aim was to redirect some of the Tagus River water to irrigate the arid landscape of the Murcia region some 300 kilometres away via pipes, canals and aqueducts.

The mission was a success as it managed to boost fruit and vegetable farming in Murcia and Alicante, so much so that they are now considered the “Garden of Europe”. Local farmers supply nearly half of Spain’s plant food exports. But then came rising temperatures and falling water reservoir levels and now there’s a dilemma: save the Tagus or save Murcian farming and economy?  

Will salads disappear from European tables?

The Tagus River is the biggest river on the Iberian Peninsula. It crosses through the heart of Spain and into Portugal before flowing into the Atlantic Ocean. However, its water levels have dropped dangerously low. Since the Transfer project was inaugurated in 1979, the average temperature in Spain has risen by 1.3 degrees Celsius and protracted heatwaves have become more common in the 21st century.

Prime Minister Pedro Sanchez’s government says it had no choice but to cut the flow of water to keep in line with rulings from Spain’s Supreme Court and EU environmental rules.

Minister for Ecological Transition Teresa Ribera has said that the decision was based on “the best scientific knowledge possible”, as quoted by Euronews. She has promised more money to develop other sources of water like desalination – something farmers from the region say is too expensive to be viable.

The Murcian farmers are threatening a massive loss of jobs and arable land if the decision is not reversed. Residents and local politicians of Castilla-La Mancha, the region through which a large stretch of the Tagus flows, are supportive of Madrid claiming that the bleeding of the river has damaged their own environment and economy.

Green activists posit that the overall approach to turning Spain into an agricultural garden is unsustainable in the long term given the scarcity of water resources on the peninsula.

More than 80 per cent of freshwater in Spain is used by agriculture… it’s just not tenable,” Greenpeace’s local branch declared.

Desertification Threatens Mediterranean Forests

Heidelberg Earth scientists study natural climate fluctuations of the past 500,000 years –

With a view towards predicting the consequences of human-made climate change for Mediterranean ecosystems, Earth scientists from Heidelberg University have studied natural climate and vegetation fluctuations of the past 500,000 years. Their primary focus was the effects of these fluctuations on the forests in the Mediterranean region. To this end, researchers led by Dr. Andreas Koutsodendris analyzed fossil pollen preserved in a sediment core from Greece. Their investigations suggest that in long-standing drought conditions—as the latest climate models predict—desertification of the forests in the Mediterranean region is likely in the near future.

Mediterranean forests are not only hotspots of biodiversity, but they also provide critical ecosystem services. They protect against soil erosion, regulate the regional climate, and hydrological conditions, and supply food and timber. “Because they are exceptionally sensitive to climate change, concern for their survival is growing in light of anthropogenic CO2 emissions and associated global warming,” explains Koutsodendris. He is a member of the research group of Dr. Jörg Pross, which investigates the Earth’s environmental and ecosystem dynamics at Heidelberg University’s Institute of Earth Sciences.

To trace how Mediterranean forests reacted to climate changes in the past, the Heidelberg researchers, in cooperation with colleagues from France, Germany, Greece, and the United Kingdom, took drill cores from Tenaghi Philippon—a terrestrial climate archive in the northeast of Greece—that provide a complete record of the past 500,000 years, and in which fossil pollen grains are preserved. The data on vegetation development in this period gained from the pollen grains was correlated with geochemical data on contemporaneous fluctuations in precipitation. The results of the team led by Koutsodendris show that, in the past, the Mediterranean forests transformed into steppes within a few decades as soon as specific precipitation thresholds were crossed.

Progressive climate change: desertification threatens Mediterranean forests

Universität Heidelberg

Scientists from Heidelberg University have conducted a study to understand how past climate and vegetation changes in the Mediterranean region can help predict the effects of human-made climate change. They analyzed fossil pollen from a sediment core in Greece to examine the impact of natural climate fluctuations on Mediterranean forests over the past 500,000 years. The study suggests that if current drought conditions continue, as predicted by climate models, the Mediterranean forests may face desertification in the near future.

The Mediterranean forests are important for their biodiversity and services such as soil erosion protection, climate and hydrological regulation, and food and timber supply. These forests are also sensitive to climate change, which is becoming a concern due to human-caused CO2 emissions and global warming. Dr Andreas Koutsodendris, a member of the research group of Prof. Dr Jörg Pross, is investigating the environmental and ecosystem dynamics of the Earth at Heidelberg University’s Institute of Earth Sciences.

To understand how Mediterranean forests have reacted to climate changes in the past, a group of scientists from Heidelberg University and other countries collected drill cores from Tenaghi Philippon in Greece. These cores provide a record of the past 500,000 years, and contain fossil pollen grains that give information about vegetation development. The scientists correlated this information with data on precipitation fluctuations. They found that in the past, Mediterranean forests turned into steppes within a few decades when certain levels of precipitation were crossed.

The scientists used ecological models to investigate what caused the changes in precipitation patterns. They found that changes in atmospheric CO2 levels affected the amount of precipitation in the Mediterranean region. Dr. Koutsodendris explained that in the past, a decrease in rainfall of 40 to 45 percent was enough to cause a sudden change from forest to steppe biomes. Therefore, if no action is taken to protect them, the forests of the Mediterranean region may undergo a similar change in the near future.

Funding for the research was provided by the German Research Foundation, the State of Hessen in the context of its state initiative for the development of scientific and economic excellence, and the Wilhelm Schuler Foundation. The research results were published in the journal Nature Communications.

Estanterías y contenedores de jardinería

El uso de estanterías para incrementar la producción de alimentos es un verdadero éxito en Filipinas. Jojo ROM, Buklod Tao entre otros demostraron que este método se encuentra entre las mejores prácticas para combatir el hambre y la desnutrición. Este vídeo debería convencer a los habitantes de la mayoría de los países subdesarrollados y de campos de refugiados a seguir este hermoso ejemplo. Es asombroso que con una mínima cantidad de irrigación se pueda producir una enorme cantidad de alimentos frescos. Felicitaciones a todos mis amigos filipinos que contribuyeron a promover hacer un huerto en contenedores.

Kitchen gardens to alleviate hunger and malnutrition


Nutritional deficiencies in the Third World affect the daily life of almost all the poor, most of whom suffer from hunger. If we want to alleviate those deficiencies, recurrent food aid will never be a solution. That’s where kitchen or family gardens come in, not to produce more rather cheap carbohydrates, but to grow vitamin-rich, nutritious vegetables and fruits, which are generally quite expensive on the local market.

Low-tech kitchen gardens, simple and cheap like the successful, very efficient container gardens of the Urban Farmers Club in The Philippines, provide useful supplementary nutrition to poor families and their malnourished children. Moreover, container gardeners are recycling all kinds of discarded containers, composting household waste to enrich their potting soil, and reducing the volume of irrigation water by limiting evapotranspiration in containers.

Kitchen gardens play a vital role in the daily life of poor households. They are not just an expensive hobby. Considering that almost 1 billion poor people on earth suffer from continuous hunger or malnutrition, and that the trillions of dollars spent every year on food aid are not fundamentally changing global hunger problems, it is almost unacceptable to argue against kitchen gardens with “mixed feelings” about their effectiveness, citing problems like the costs of gardening “doodads”, extra workload, lack of irrigation water, lack of extra income, or wrong choice of vegetables etc.

If you ask the thousands of people in The Philippines about the effect of their container garden on their families’ nutrition, you will notice that it is never seen as an expensive hobby, but as a real need to create changes in the structural food deficit issues.

Wall gardens

One of the widely appreciated ideas is the construction of a wall garden. Buckets, pails, bins or other containers are stacked to form a vertical garden against a wall.

Wall gardens are generally build to produce vegetables or kitchen herbs close to the house.

See some examples in the photos below.

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