Acid rain is well-studied, unpleasant-sounding phenomenon (in everyday jargon, "acidic" is not a term of endearment). The importance of acid rain when it comes to environmental health has been known for decades; its causes and chemistry are well-known, as are its detrimental effects on vegetation and other aspects of the natural environment.
Because it is largely a consequence of human activities that drive much of human industry, efforts to curtail acid rain depend on larger attempts to curb fossil fuel production and utilization.
In the early part of the 21st century, scientists began to uncover evidence of a "silver lining" of acid rain: Its ability to mitigate global warming by preventing the emission of the greenhouse gas methane from wetlands. While this is hardly a reason to ignore its deleterious effects, it gives researchers a potential tool for controlling methane emissions in a more controlled way.
What Causes Acid Rain?
When fossil fuels are burned, some of the products released into the air are sulfur dioxide, SO2, and various nitrogen oxides, which have the generic formula NOX. These particles collide with water and oxygen molecules in the atmosphere, and as water carries hydrogen atoms, some of these wind up being used in reactions that produce sulfuric acid (H2SO4) and nitric acid (HNO3).
Rainwater in most places is usually somewhat acidic, with a pH in the range of 5.6, but these acids can drive it down to 4.2 to 4.4. Volcanic eruptions can launch significant quantities of the same oxides into the atmosphere, as can the action of bacteria on decomposing vegetation. But fossil fuels are by far the primary cause of acid rain.
Disadvantages of Acid Rain
The effects of acid rain on plants is well-studied, particularly those that thrive in aquatic environments, since rain readily finds its way into streams, rivers, lakes and ponds. A major consequence of more acidic water is its retention of aluminum, which can poison forests secondarily by depriving soil of the essential nutrient calcium and making it harder for plants to absorb water from the soil.
There are more harmful effects of acid rain in points where the flora and fauna are naturally more sensitive to fluctuating conditions of acidity. Certain aquatic animals, such as crayfish, are poisoned by the aluminum in habitats rendered more acidic by acid rainfall.
Advantages of Acid Rain
It has been known for some time that the oxides of sulfur and nitrogen that serve as substrates for acid rain are not themselves greenhouse gases that contribute to global warming, as are carbon dioxide (CO2) and methane (CH4).
In fact, sulfur dioxide can exert a local cooling effect by reflecting sunlight rather than trapping it within the atmosphere like greenhouse gases. As China continued to increase fossil-fuel production and saw more acid rain at the turn of the 21st century, the country actually saw a cooling trend.
Also, in 2008, NASA researchers learned that methane production in wetlands could be slowed by the addition of sulfuric acid because this led to decreased methane production by certain bacteria that were "distracted" by the sulfate. However, as of 2020, the utility of acidifying wetlands or other environments as a means of slowing climate change remained largely unexplored.
Acid Rain Solutions
As noted, acid rain is for the most part a consequence of human activities, and is not a problem that can be addressed in isolation. As world leaders continue to move forward with basing their economies more strongly on alternative sources of energy, carbon emissions may slow or reverse, and changes in acid rain worldwide will follow suit.
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