Plants or sand

Updated: Feb 23, 2020

Desertification and water conservation.

There have been warnings for years about potential, and actual global water shortages. In 2018 NASA’s satellite survey of global freshwater identified many places where water demand outstrips supply, often due to overuse and reliance on groundwater for irrigation for intensive agriculture. Cities too can run dry as was seen in Cape Town in 2019. Many other cities are at risk. London where supposedly it rains too much, uses more water than the supply in drier summers, and is considered to be in a water stress region.

Andalucia has had steadily decreasing rainfall since the 1960s with many years having between 100 and 200 mm less rain than the average before this. Average rainfall in eastern and southern Spain decreased 12% from 1951 to 2000 (1). The current annual average is between 550 and 600mm (the figure varies according to the source), which is, perhaps strangely, almost as much as London which has just over 600mm per year. Storms in the UK like the ones in February 2020 may affect these figures but do nothing to guarantee long term water supplies.

As noted by many, climate change means many things, according to where in the world you are. The UK for example could get colder, if the Gulf Stream eventually fails, although like most places it has straightforwardly got warmer in the last few years. Many places will get more storms and other extreme events. The predictions for Andalucia as a whole and indeed much of Spain is for reduced rainfall and desertification, with Andalucia as well as Sicily becoming extensions of the Sahara desert. The Sahara has expanded by 10% in the last 10 years (2). Research looking at ancient vegetation patterns, published in Science (3) suggests that if global heating reaches 2 degrees, current Mediterranean vegetation will be challenged in ways it has never had to cope with before.

There are controversies over desertification, as to whether it is really a thing at all, and over what its causes might be. For example the Sahel desert in north Africa, a semi-arid region, has had a long drought starting in the 1960s, getting worse in the 1980s, causing a major environmental and human crisis. Rainfall has increased a little since but overall it remains lower than the first half of the 20th century. The drought was initially blamed on local farming practices and over grazing with cattle, and the Sahel crisis was a major factor in early ideas about desertification and over grazing. It is likely, however, that global climate effects have a major effect, the Sahel has been called the canary in the mine for global climate change, it is sensitive to African, Atlantic and Pacific weather changes (4). Nevertheless increased population, increased food and water demand, as well as military conflict between groups, often for water and grazing resources, are making the situation much worse. The Sahel, after Syria, is a major source of climate and war refugees trying to get to Europe. More hopefully there are many tree planting and other agricultural projects attempting with some success to offset the losses.

Desertification is caused by a number of factors, clearly climate change is a major one. The Sahara was a savannah for several thousand years up until about 5500 years ago when the monsoon type rains stopped and the desert, which was previously present when the last ice age ended 12,000 years ago, returned (5). So natural climate changes can have a major effect, and we do know that the earth’s climate has changed massively over the 4.5 billion years it has existed. However recent climate heating due to human use of fossil fuels increasing carbon dioxide in the air, now is driving desert expansion globally, as rainfall reduces.

Desertification can also be directly increased by changing the vegetation types and distribution, for example cutting down forests and woodland. Even cultivated trees don’t necessarily help, depending on how they are grown. If trees are grown in a tree desert, with bare soil in between, and irrigation and chemicals to feed them and kill diseases, this will continue to exacerbate climate heating, desertification, and loss of biodiversity. This is seen in the problems with bees and poor pollination in the almond deserts of California (6).

There is a recently discussed other side to understanding atmospheric moisture, climate and desertification. The forest pump effect or biotic pump was first described by Makarieva and Gorskov, (7) who are physicists. Simply put, trees transpire a lot of water into the air. This moisture itself drives weather patterns and winds. The most important source of atmospheric moisture, perhaps 50%, is from transpiration. A single tree, in places like the Amazon, can transpire hundreds of litres of water a day. A second effect is that when it rains a significant amount of water is caught by tree leaves and evaporates back into the air when the rain stops. This is a significant addition to the biotic pump, and also part of the reason forests reduce flooding. The Amazon, which has been recently researched in this regard, generates a lot of it’s own rainfall, but also feeds global weather patterns with this moisture pump.

Another aspect of this is the effects of coastal forests. Rain in many places falls first on coastal areas when moist air is blown off the sea. The moisture then pumped into the air by these coastal forests is blown further inland where it can fall as rain again, as the moist air hits higher ground, especially mountains. So if coastal forests are cut down this will reduce rainfall further inland (8). This appears to have been historically important in Australia, and important currently for example in Borneo where significant deforestation has occurred and has been accompanied by reduction in rainfall. In Spain, where deforestation has happened over several previous centuries, there has been a major effort at reforestation in the 20th century and up to now. The 20thcentury drop in rainfall may be partly related to this, but may have other global climatic causes as well.

Trees pump water out of the soil, and catch a lot of rain which then evaporates without touching the ground. How therefore do trees help combat local desertification? It may seem paradoxical, yet the best way to retain soil water is by having forests or woodland. There are many reasons for this, partly it is because of the complex web of life in forest soil, notably the fungi associating with tree and other plant roots. Leaf mould and decaying vegetation feeds the soil life web, retaining water both physically and through the living processes it supports. The gradual breakdown of organic material generates further moisture. One only needs to walk into a wood to feel the cooler moist air around you. Many studies of reforestation projects have shown that dry streams start running again when woodland or forest returns to hillsides. The forest holds water, slows its flow when there are heavy rainfall events, yet allows water to circulate as well, enabling water flow both within its own location, and spreading it further afield, as streams, and as what have been termed “flying rivers” of airborne moisture, from the biotic pump. Trees also shed airborne bacteria which seed water droplet formation, allowing clouds to form. Clouds reduce the amount of sunlight hitting the land, reflecting it back into space, so contribute to global cooling.

Reforesting or converting to agroforestry across the whole Guadalquivir valley would potentially have a massive effect on the whole region, as well as Spain in general. It would contribute significantly to the global need to plant a trillion trees to absorb carbon dioxide, alongside decarbonising the global economy. Agroforestry would allow continued production of annual crops, although in order to protect the soil this would need no till methods and the use of continuous cover crops, as advocated by the conservation agriculture model ( It would also allow animal husbandry, as there is a strong argument that grazing animals in ways that mimic continuously moving wild herds can help restore damaged landscapes. Semi-feral pigs grown for the famous jamón ibérico and fed on acorns are in keeping with land restoring agroforestry. It certainly reduces the animal waste problem, and the animal manure helps feed the forest system.

Desertification is not inevitable, although a global response is ultimately and urgently needed. Nevertheless a regional response in Andalucia could be a beacon, as the reforestation of large areas of Ethiopia has become. It does need to start immediately.


1. Martin de Luis, José Carlos Gonzales-Hidalgo, Luis Alberto Longares, Petr Stepanek. Seasonal precipitation trends in the Mediterranean Iberian Peninsula in second half twentieth century. International Journal of Climatology, 29, 9, December 2008.

2. Josh Gabbatiss, World’s largest desert has grown even larger due to climate change. Independent newspaper 18.03.2018

3. Joel Guiot and Wolfgang Cramer, Climate change: The 2015 Paris Agreement thresholds and Mediterranean basin ecosystems. Science, 28 Oct, 2016

4. see for example Steff Gaulter “Analysis: Understanding the Sahel drought” A Jezeera, 22 Jun 2012.

5. Bjorn Carey, Sahara Desert was once Lush and Populated. Live Science July 20, 2006.

6. Annette McGivney, Like sending bees to war: the deadly truth behind your almond milk obsession. Guardian Newspaper, 8 January, 2020.

7. Anastassia Makarieva, Victor Gorshkov. The Biotic Pump: Condensation, atmospheric dynamics and climate. International Journal of Water 5(4) 365-385; January, 2010.

8. Peter Wohlleben, The Hidden Life of Trees: What they feel, how they communicate – Discoveries from a secret world. Greystone, 2016.

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