CLIMATE

Greenhouse gases

Soils are suffering the consequences of climate change in terms of erosion and dryness. Evolving temperatures and humidity are disrupting soil composition and behaviour. Soils themselves are also contributing to these atmospheric mechanisms. By overexploiting the soil, humans are triggering a time bomb with an uncertain detonation schedule.

Érosion éolienne dans l’Erg Eklewa, en Mauritanie. © BRGM im@gé/François Michel
Wind erosion in Erg Eklewa, Mauritania. © BRGM im@gé/François Michel
La forêt, un facteur d’équilibre et un trésor de matières organiques de plus en plus menacé par l’intensification agricole. © Shutterstock
The forest, a balancing force and treasure trove of organic matter that is increasingly under threat from agricultural intensification. ©Shutterstock
Érosion du sol dans un champ après 100 mm de précipitations, en Seine et Marne (FR). © INRA/Thierry Dore
Soil erosion in a field in the Seine-et-Marne region (FR) following 100 mm of rainfall. © INRA/Thierry Dore

"How many decision-makers spare a thought for the earth they walk on day after day?" exclaims Luca Montanarella, from the Institute for Environment and Sus - tainability at the European Commission's Joint Research Centre. "The asphalt insulates us from the geological issues at stake. We need to remember that the soil is where our roots lie and it is the soil that provides us with our sustenance," he adds. The scientist is a farming enthusiast who regrets that issues crucial to the survival of the human race are sometimes overlooked in favour of the higherprofile issues in the news today, like climate change.

For paleoclimatologists, the soil is a useful barometer of the climate's age-old rhythms and patterns. The isotopic composition of the ice in core samples from Greenland and Antarctica reveals changes in the atmospheric concentration of carbon dioxide (CO2) (and hence in temperature) during the transition between glacial and interglacial periods.

Interglacial periods were warmer and experienced CO2 levels of 300 parts per million by volume (ppmv). Today, the concentration is 28 % greater than the maxima recorded over a period of more than 800 000 years. The most familiar consequences of higher temperatures are rising sea levels and coastal erosion owing to more frequent and intense storms.

Geological impact

Rising temperatures also cause geological changes within the soil itself. While such changes receive less media coverage, they are none theless significant. More than half of the world's cultivable surface area (around 1.964 billion hectares) has already fallen prey to slight or serious degradation from a variety of causes, sometimes in combination.

Water erosion from abrasion and run-off carries away soil particles. The wind has the same effect: wind erosion is particularly acute in pre-desert areas. Another source of soil degradation is changes in the soil's chemical composition (acidification, salination, discharge of industrial effluents and use of fertilisers) and physical properties, especially compacting.

CO2 release

These geological changes are accelerated by the multiplier effect of farming: land reclamation dries out soil and diminishes biological life, leaves and branches, which impairs water penetration. "Switching from woodland to farming or from grazing to arable farming increases carbon dioxide release." (1) Plants are disappearing and are therefore no longer able to continue their photosynthesis role of converting CO2. At the same time, we are raiding a treasure trove of organic matter in the soil that it has taken hundreds and thousands of years to amass. A key contributor to soil fertility, organic matter is crucial to climate change.

Agricultural expansion and intensification modify plant cover (deforestation and major changes in vegetation), which increases surface air temperature in desert regions because the soil retains less moisture. Latent energy, which is required to evaporate the moisture, declines in favour of what is known as "sensitive" energy, which is available to heat the air.

The result is that the soil dries out. The ecosystem no longer provides people with the services they need for their survival, and all of these services diminish over a long period. This leads to a fertile region becoming sterile or hostile to life, where annual precipitation tends to be less than 200 mm. In such regions, desertification is a real risk.

How desertification starts

In many cases, soil degradation stems from the settling of nomad populations, as arid soils are less suitable for growing crops than for grazing. With today's globalised markets, inappropriate and excessive use of resources is also one of the leading factors triggering soil degradation. Overproduction undermines selling prices and the resulting incomes of producers of poor countries.

In arid areas, precarious conditions heighten people's dependence on ecosystem services and encourage the use of nitrogenous fertilisers, triggering a vicious circle of overexploitation and, with it, of warming - as fertilisers partially convert to nitrous oxide (N2O), a potent greenhouse gas (GHG).

Climate change also plays a role in desertification, although it is highly complex. First, an increase in atmospheric CO2 boosts the growth of certain plant species, while warming reduces precipitation in these regions, which experience increasingly severe and frequent droughts, leading to drier and poorer-quality soil.

Worldwide consequences

Desertification affects a total of 3.6 billion hectares, including 70 % of the world's arid land, putting around 1 billion people under threat. Every year, a further 10 million hectares of arable land are desertified. In Europe, desertification, which has already hit southern European countries, is progressing northwards. A variety of impacts ensue, including flooding downstream of land that has been stripped of its plant cover, and dust clouds, the effects of which can be felt thousands of kilometres from their point of origin. In human terms, desertification leads to population movements -a source of ethnic friction.

In 1977, the international community recognised the problem by holding the United Nations Conference on Desertification in Nairobi (Kenya), following a series of droughts in the Sahel (1973-74). Since then, the battle against desertification has ground to a halt. "The stumbling block has been the lack of a clear definition. According to the United Nations Convention to combat Desertification (UNCCD)(2), desertification is caused by human activities. This means that the close relationship between desertification and poverty has to be taken into account. While the problem might be the same in both Italy and Burkina Faso, the two countries will not be able to address it in the same way."

Hence desertification has remained more a political - than a scientific - issue until now. "We need a solid scientific basis and clear criteria and standards. An initiative similar to the Intergovernmental Panel on Climate Change (IPCC) is just what is required."

Upsetting the balance

A vicious circle has been triggered: the danger is that, by binding with oxygen in the air to form CO2, enormous reserves of organic carbon (estimated at 1 500 gigatonnes - i.e. double the amount contained in the atmosphere) could be extracted from the soil. This elixir of life, especially plant life, fixes nutrients in the soil, making them available to plants, bacteria, worms and insects. It helps to maintain the geological structure and aids water seepage, as well as accelerating the decomposition of pollutants.

Changing precipitation patterns and rising temperatures, coupled with radical changes in soil management (mechanisation, production specialisation and management simplification), are speeding up the release of stored CO2 by boosting mineralisation mechanisms and the microbial activity that decomposes organic matter. "By affecting temperature and humidity, climate change helps to set off a vicious circle: the higher the GHGs in the atmosphere raise the temperature, the more GHGs the soils release." This has become a serious problem, especially in the peat bogs of northern circumpolar regions.

And research

Alongside efforts to limit global warming, a research project called ECOSSE (3) is studying GHG fluxes between the soil and the atmo - sphere in areas of Scotland and Wales with very high carbon stocks. In its March 2007 report, the project showed that land use is responsible for approximately 5% of total GHG emissions from these regions.

Changes in land use over the past 25 years are just as much to blame as climate change.Thus a second strand of ECOSSE aims to test farming practices that reduce the amount of tillage required for sowing. At European level also, the debate now centres on which techniques to prioritise, with some subsidies being made conditional upon the use of these techniques. Although we in Europe are able to envisage more ecological alternatives for reducing GHGs, this is not an option in the developing world, where the main priority has to be securing people's livelihoods.

To ascertain the scale of the threat from geological changes, projects are underway to collect data, notably with new satellite or electroconductive technologies (see the box on GloSIS). "Such research rarely focuses directly on soil: there is usually a specific aim, such as to study groundwater. Nonetheless, compiling fragmented data will enable us to bridge the gaps in our knowledge."

Geological change happens slowly. Survey data is often very expensive, requiring a longterm approach, spanning several decades or even centuries. The problem is that the duration of research projects does not correspond with this lengthy time scale. "The good news is that leaders are starting to recognise that the key to securing livelihoods and alleviating poverty is to assess the amount of land available for agriculture, which will soon be in short supply."

Delphine d’Hoop

  1. All quotations are from Luca Montanarella.

  2. www.unccd.int

  3. Estimating Carbon in Organic Soils - Sequestration and Emissions, www.scotland.gov.uk/Publications/2007/03/16170508/0


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Greenhouse effect

GHGs are either naturally occurring gases - water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) or ozone (O3) - or fluorinated industrial gases, such as chlorofluorocarbons (CFC). Although more than half of the greenhouse effect is caused by water vapour, human-induced CO2 emissions appear to be responsible for 55 % of the increase in the greenhouse effect. Other GHGs differ in their impact. To compare GHGs, we use the CO2 equivalent, also called global warming potential (GWP) - a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming compared with the same mass of CO2 (that has a GWP of 1 by definition).

Intensification of human activities also affects non-CO2 greenhouse gases with major warming potential. Methane has a GWP 23 times greater than CO2. Produced by the fermentation of organic matter in an anaerobic environment, most CH4 is released when permafrost melts, confining emissions to certain regions. The GWP of N2O, which is produced by the oxidation of the nitrogenous compounds used in, or produced by, crop cultivation and livestock farming, is even greater (310).


GloSIS

The Global Soil Information System (GloSIS), launched in February 2008 in Boston in the USA, marked a major milestone in international collaboration to gather geological data. The project, also known as Global Soil Map, aims to become the world's largest digital soil archive. The project participants, including Americans, Europeans, Australians and the Food and Agriculture Organization of the United Nations (FAO), are working to gather worldwide soil information in a multiscale system. This will provide a vital link between local systems (with scales of between 1:5 000 and 1:250 000) and global databases like the Soil and TERrain database (SOTER), with a scale of 1:5 000 000.

Europe's contribution, the European Soil Information System (EUSIS), is already compiling this data and plans to create by-products, such as soil erosion risk maps, organic carbon estimates, etc. The Joint Research Centre's Soil Data and Information Systems Action (SOIL) is setting up a European Soil Data Centre (ESDAC) with data and procedures that comply with the principles of the Infrastructure for Spatial Information in the European Community (INSPIRE).GloSIS should make it possible to assess the amount of land available for agriculture.



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