Acid rain
In the 1960s, Swedish scientists documented how acid rain was poisoning lakes, killing fish, damaging soils and forests. Crucially they said it was an international problem, because the acid rain was caused by industrial pollution being carried on the prevailing winds from countries thousands of miles away. Acid rain is primarily created by the burning of fossil fuels, particularly coal, which releases large amounts of sulphur dioxide and nitrogen oxides into the air. These particles then mix with moisture in the atmosphere to create sulphuric and nitric acid, which then falls back to earth as acid rain. The phenomenon of acid rain was noticed in the 19th century but the threat was largely ignored. Alex Last spoke to Prof Henning Rodhe of Stockholm University about the research that alerted the world to the dangers of acid rain.Photo: Forest decline caused by acid rain in the Giant Mountains in Poland - 1998 (Getty Images)