2011, year of forests : Acid rain

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Temps de lecture : 2 minutes  

2011, année des forets : les pluies acides

In the 1970s and 1980s, tens of millions of hectares of forest in Canada and Europe were damaged by a phenomenon about which little was known at the time: acid rain. Even in France, notably in the Vosges Mountains, many trees were affected by this rain as acid as vinegar.

In its natural state, rain is slightly acid, but certain air pollutants such as sulphur dioxide (SO2) and nitrogen oxides (NOx) can considerably increase its acidity. When they enter into contact with the water in the clouds, they are transformed into either sulphuric or nitric acid, and fall to the ground in the form of rain, snow or hail.

By washing out the soils, acid rain deprives trees of mineral salts such as calcium and magnesium that they need to live. These nutritional deficiencies lead to leaf fall and the progressive death of the trees. This is known as forest dieback. Acid rain increases the acidity of lakes and watercourses, endangering the biodiversity to which they are home. It has little effect on man, however, although it is responsible for respiratory diseases and problems, especially among the elderly and children.

The affected zones can be hundreds or even thousands of kilometres from the areas where the gas causing the acid rain originates, for it is carried by the wind. Sulphur emissions have considerably diminished in the West today, largely thanks to deindustrialisation, but also to an international convention signed in 1979 and a series of measures taken to implement the protocol. Nevertheless, the threat of acid rain persists: on the one hand because transport and agriculture still produce high nitrogen emissions, and on the other because the phenomenon has shifted to China, where numerous coal-fired power plants, major sulphur emitters, are operating.

Extrait du livre « Des forêts et des hommes » rédigé par la rédaction de GoodPlanet à l’occasion de l’année internationale des forêts et disponible aux éditions de la Martinière.

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