We have a long way to go to protect sensitive ecosystems. The acid rain issue faded from public discussion in the 1990s as policy concerns shifted to climate change and the threat of global warming. Many of the latest data were compiled at the 7,800-acre Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. The living laboratory is one of the most influential centers of environmental research and is where Likens became the first scientist to discover acid rain in the United States in 1972. Acid rain occurs when sulfur dioxide, nitrogen oxides and ammonium combine with water vapor in the atmosphere and create acidic solutions.
Sulfur is produced primarily by power plants that burn fossil fuels.
Nitrogen oxides are primarily emitted by automobiles, and ammonium comes from livestock waste and fertilized soil. Calcium and other minerals in the soil can neutralize some acid when it is redeposited on earth, but that ability has diminished over time. In a 1990 report to Congress, the National Acid Precipitation Assessment Program concluded there was insubstantial evidence that acid rain caused the decline of trees other than red spruce growing at high elevations. But the latest data suggest the impact has been broader and more insidious. Pools of acid have built up in forests, altering soils and depriving tree roots of essential nutrients, and heavily acidified lakes and streams have become toxic to plants, fish and other organisms unable to survive abrupt chemical change, the report found.