The Causes and Impact of Acid Rain

Examining the Impact of Acid Rain Forests and Wildlife Worldwide

Demolished and dead trees against blue with white clouds
Marek Mnich / Getty Images

Acid rain is a very real phenomenon worldwide, and it's been documented since the 1800s, as the Industrial Revolution caused the burning of fossil fuels like coal, gas, and oil. When these fuels or any other organic material like wood or paper are burned, they release compounds like sulfur dioxide (SO2) and nitrous oxides (NOx) into the air.

The Causes of Acid Rain

Are SO2 and NOx the causes of acid rain?

Indirectly, yes. When SO2 and NOx enter the atmosphere, they react with water vapor, oxygen, and other compounds to form sulfuric acid and nitric acid. This process may take place locally, or -- when winds blow emissions hundreds of miles away -- across international or state boundaries. These acids lower the pH of water condensation in the atmosphere, and when that condensation falls as rain, fog or snow, the resulting acids can wreak havoc on plant and animal life.

(Note: The more acids found in rain, the lower the pH. The pH scale goes from 0 to 14. Values from 0 to 6 are considered acid, 7 is considered neutral, and values from 8 to 14 are considered alkaline. A pH of 1, for example, is far more acidic than a pH of 6.)

The Effects of Acid Rain on Wildlife

The effects of acid rain can vary depending on where it falls and what the local rock and soil are composed of. An alkaline soil can help buffer the effects of acid rain and reduce its impact on local lakes.

However, when acid rain falls on some soils, the acids can wipe out important microbes and insects that live in soil and leaf litter. When acids from rain and snow enter rivers and lakes, it can kill fish and their eggs -- many fish eggs can't survive at pH lower than 5.

This has caused the disappearance of some fish like brook trout from streams in the eastern U.S., where acid rain is more prevalent than in western states.

Crayfish, clams, amphibians and other aquatic wildlife are also killed off by acid rain.

The Effects of Acid Rain on Forests

Trees are among the most visible victims of acid rain. When acid rain or snow falls on forest floors, it leaches out valuable nutrients that are found in the soil, leaving behind aluminum and other elements that can be toxic to plant life. Thus, the trees slowly die from lack of food and from soil toxins -- eventually, an entire forest can be killed off by acid rain.

Trees are especially vulnerable at higher altitude, since they receive more rain and snow, and are often surrounded by acid fog and clouds. The effects of acid rain and snow have been widely seen throughout the Appalachian Mountains, including the Great Smoky Mountains, the Adirondack Mountains and the Catskills in New York. Many forests in Europe, including Germany's famous Black Forest and the high-altitude forests throughout Scandinavia, are also in peril due to acid rain and snow.

The Effects of Acid Rain on Human Health

The amount of acid in rain is too small to have a serious impact on human health, and agricultural land is now amended with lime and other fertilizers to buffer the effect of acid rain.

However, the acid in rain and snow is strong enough to erode rock -- centuries-old buildings, monuments, and statues made of marble, limestone or other rock are slowly eroding away due to the effects of acid rain.

What Can Be Done About Acid Rain?

Though much has been done to reduce the impact of acid rain, much more needs to be accomplished. Smokestack scrubbers that reduce emissions from coal-generated power plants have helped, but with millions of sources like auto tailpipe emissions, sources of acid rain are difficult to manage.

And though international treaties have been signed and implemented throughout Europe and North America, their benefits have been limited, especially as rapidly developing countries in Asia and South America rely heavily on coal and oil for energy. Since the single largest source of acid rain and snow is coal-powered electrical plants, developing alternative sources of energy becomes more important than ever.

Until that time, however, acid rain will continue to destroy trees, forests, wildlife and historical buildings and monuments.

People who are concerned about acid rain can start by saving electricity in their homes, improving their gas mileage and taking other steps to save energy and reduce our dependence on the fossil fuels that cause acid rain.