Effects of Acid Rain
Acid rain can harm plants and animals in a variety of ways. As the acid rain lowers the pH of lakes and streams, species like snails and clams that rely on their shells for protection can be the first to suffer. This is because the acids in the water chemically react with calcium carbonate in the water, dissolving it. Because snails and clams need calcium carbonate to build their shells, the acidity can make their shells grow slower and thinner, leaving them vulnerable to predators. This is the same chemical reaction that makes limestone statues vulnerable to acid rain, since limestone is primarily made of calcium carbonate.

Another way acid rain harms ecosystems is by leaching aluminum out of the soil. Aluminum, which occurs naturally in the soil in many areas, can be toxic to plants and animals in high concentrations. When acid rain falls over an area, it can leach, or pull, the aluminum out of the soil, leaving it to be absorbed by plants or be washed into the nearest water body by the rain. If this process continues for long enough, the aluminum concentration builds up enough to harm local species.

A third reason to be concerned about acid rain is a process called eutrophication. When too much nitrogen enters a body of water, often from sources like fertilizer and wastewater, it can cause algae populations in the water to explode as the algae consumes these excess nutrients. When the algae dies and bacteria eat it, the bacteria also consume oxygen in the water, leaving little oxygen for fish and other organisms. High nutrient levels in water is called eutrophication, and the resulting low oxygen level is known as hypoxia. Acid rain can contribute to this problem because the nitrogen in the nitrogen oxides that make up a part of acid rain can fuel these algal blooms.


International Agreements
Because air pollution, like sulfur dioxide and nitrogen oxides, doesn’t respect national borders, it is important for countries to work together on setting limits on air pollution, rather than just relying on each government to cut pollution as they see fit. The international effort to stop acid rain began with the 1979 Convention on Long-Range Transboundary Air Pollution, which was signed by the United States, Canada, and thirty-two European countries. This agreement led to additional agreements to limit sulfur pollution in 1985 and 1994, and an agreement in 1988 to limit nitrogen oxides. The most recent international agreement focused on stopping acid rain is the 1999 Gothenberg Protocol, which combined and strengthened the prior agreements under one treaty.

While only European and North American countries have agreed to the Gothenberg Protocol, other international agreements focused on different types of air pollution have received broader international agreement. The 1987 Montreal Protocol, for example, is the only United Nations treaty that has been ratified by all 197 countries that are UN members. This agreement focused on chemicals known as chlorofluorocarbons, or CFCs. These chemicals are dangerous because they damage the ozone layer of the atmosphere, which protects us from harmful ultraviolet radiation from the sun.

The Kigali Amendment seeks to build on the success of the Montreal Protocol by also phasing out hydrofluorocarbons, or HFCs, which don’t damage the ozone layer like CFCs do, but are powerful greenhouse gasses. Agreed to in 2016, the Kigali Amendment has already been ratified by 110 countries around the world, although the United States has yet to join.

To learn more about innovative climate actions, visit the Harvard Environmental and Energy Law Program.

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