Acid Rain Acid rain is a term which is used to describe a variety of processes which might more accurately be referred to as acidic deposition. Natural rainfall is slightly acidic due to dissolved carbon dioxide, picked up in the atmosphere. Organisms and ecosystems all over the planet have adapted to the slightly acidic nature of normal rain, and thus it poses no environmental problems. It is an increase in the acidity of rain, caused by human activities such as the combustion of fossil fuels, that has turned acid rain into a problem. Highly acidic rain can damage or destroy aquatic life, forests, crops and buildings, as well as posing a threat to human health