In 1963, four scientists set out into the White Mountain National Forest in New Hampshire. The mountains there are rugged and richly forested, or at least, they had been. 100 years of logging had clear-cut much of the land, destabilizing hillsides and affecting a nearby river. So the United States Department of Agriculture set aside some land to study the connection between the forest and watershed health.

But when the scientists arrived and began to take measurements, they quickly realized that something was very wrong. The rain in the area had a pH of 3.7, nearly a hundred times more acidic than they'd been expecting, and about the same as a can of soda. They'd stumbled across an extreme example of what would become a hot topic in the decades to come: acid rain.

If you were alive in the '80s, you probably got tired of hearing about it. The stuff was in the news as much as Madonna. But it seems like no one talks about this sinister-sounding precipitation anymore. So, let's look into it. Whatever happened to acid rain?

While it may seem like a recent phenomenon, acid rain has been a thing for centuries. Like in 1667, the English writer John Evelyn noted in his diary that the "corrosive air of London" had eaten away at a collection of marble sculptures.

Which, sure, is a bit vague but we can fast-forward to the mid-19th century to find actual research into the issue. And in a paper published in 1872, the Scottish chemist Robert Angus Smith coined the term "acid rain".

Technically any precipitation can become acidic. There is acid fog and acid snow, so I should really be saying acid precipitation instead of acid rain, but for simplicity's sake, I'm going to stick with the more common term.

As for what causes it, the water in raindrops chemically reacts with the stuff floating around them in the air. This means the liquid part of rain is not pure 100% water.

For example, most rain is at least a little bit acidic due to the water reacting with carbon dioxide to form carbonic acid. Just about anything that reacts with water can potentially alter the chemistry of rain: dust from soils, organic molecules given off by plants, and so on. These chemical reactions have been happening for as long as rain has existed.

But when humans began to reshape the world, we brought a previously rare ingredient into the mix: pollution from fossil fuels. In particular, sulfur and nitrogen compounds found in fossil fuel exhaust can react with rainwater to form acids.

And as for why fossil fuels contain sulfur or nitrogen in the first place, while the most important element in these fuels is carbon, it's not all carbon. Other elements can be introduced either from the organic remains of the life that first made up the fuel or stuff in the ecosystem that got trapped with that organic matter. They may also be chemicals that leeched from the surrounding rock. So when we burn fossil fuels, be it at a coal power plant or inside a gasoline-powered car, the sulfur or nitrogen is burned too.

New molecules like sulfur dioxide or nitrogen dioxide form, and get carried up in smoke or exhaust up into the atmosphere. Of course, sulfur and nitrogen oxides can also be produced by natural sources like volcanoes. But as the industrial revolution happened, we started polluting in new, alarming amounts. And at least some people noticed the effects fairly early, like John Evelyn.

But once scientists realized what was happening, they needed to know more about acid rain than they could learn from say, its effect on marble statues.

So between 1976 and 1983, researchers conducted a, let's say, unconventional study on a lake in western Ontario. Far from where most people live in Canada, this small lake was surrounded by pines and full of fish and plankton. And each May, two to three times a week, someone went out on a boat for a couple of hours and, wait for it... dumped large jugs of sulfuric acid into the lake. You know, for science!

Over the course of a few years, fish like the fathead minnow began to disappear while others started to boom and bust. The acid, it turns out, was killing off juvenile fish in several species before they could become breeding adults. Even for the fish that survived, this may have caused the food web to buckle, as formerly plump fish started to turn up scrawny and emaciated.

Fortunately, less destructive research was also underway in the US and Europe. And to try to find a baseline pH of true untouched rain, one team of scientists traveled to remote areas like southern Chile and an isolated island in the Indian Ocean.

Meanwhile, other researchers investigated more of acid rain's effects. Today we know that it can dissolve shellfish shells, it can indirectly kill trees, it can deplete soil nutrients and cause a toxic amount of aluminum to get released from soil. The extra nitrogen can even cause algal blooms that end up choking lakes and waterways of oxygen. The effect don't have to stay where the pollution is either. They can travel for hundreds of kilometers.

After all that research in the '60s and '70s, well, people started to pay attention. But before I tell you what happened next, a quick ad:

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In 1967, Swedish soil scientist Svante Odén wrote a newspaper article titled "An Insidious Chemical Warfare Among the Nations of Europe", which is certainly one way to frame acid rain. And of course, vested interests pushed back.

Odén's claims were debated. But the big thing was that nations and governments took notice. And this is important, because while acid rain could be countered by adding buffers like lime to important bodies of water, a far more effective solution turned out to be decreasing the emissions that created it in the first place. I mean, who would have thought, right?

In 1970, the United States passed the Clean Air Act, which gave the government the power to set limits on pollution from factories and vehicles. Then in 1972, the UN held their Conference on the Human Environment, the first time world powers got together to address environmental regulations. And in 1979, the Geneva Convention on Long-range Transboundary Air Pollution in Europe was the first international agreement to reduce air pollution across an entire region.

Each of these helped push for new techniques and new technologies. And one of the biggest things people did was install sulfur scrubbers in fossil-fuel-burning power plants.

Scrubbers work by routing the exhaust you create through a chamber where it meets a spray of water and crushed-up limestone or some other chemical that can react with the sulfur dioxide and remove it from the exhaust.

The catalytic converters in cars do something similar but for nitrogen compounds. The molecules are forced into a chamber where they react with metal catalysts, breaking them down into less harmful compounds. So laws started mandating cars have catalytic converters around this time.

Meanwhile, people also started using fossil fuels that naturally had less of those sulfur or nitrogen compounds in them or artificially took those compounds out through physical or chemical processes.

And over the years, regulations tamped down emissions even further. In 1995, the US started the Acid Rain Program, which is a cap and trade system. With an assist from pollution regulations at the state level, it reduced sulfur dioxide emissions in this country by more than 95%.

And in 2001, a bunch of countries in East Asia banded together to create their own acid rain monitoring network called EANET. This stretched all the way from Russia in the north down through Indonesia in the south. More recently, China began clamping down on acid rain by creating pollution control zones that limited sulfur dioxide emissions. In the '90s, more than 30% of China had acid rain. As of 2018, it was only about 5%, a major decrease.

These interventions all worked! Which might be why, if you're one of our younger audience members, you probably have no memory of acid rain's threat being woven into your daily media consumption.

But this recovery all depends on us not putting new pollution into the atmosphere. And though efforts to eliminate acid rain have been effective regionally, there are also countries where it's become a problem lately.

Take India for example: it's currently the world leader in sulfur dioxide emissions and it has the acid rain to prove it. The government has pushed for industrial operations to install those sulfur scrubbers on their power plants with an initial deadline of 2017, but that's now been pushed back to 2026 at the earliest. Meanwhile there have been reports of acid rain as a rising issue in places like Rio de Janeiro in Brazil and parts of West Africa.

But just because a country has had some success in the fight against acid rain, it doesn't mean there isn't more to do. For example, that paper that noted a lower percentage of China was experiencing acid rain also warned that nitrogen oxide emissions appear to be on the rise. The authors say it's due to factors like increased energy consumption and more cars on the road. And if regulations get rolled back in the US and Europe, we could very easily see widespread acid rain return there.

The bottom line is that the reason you don't hear much about acid rain anymore is that we used science to do something about it! But those victories can be temporary. Unless we want to return to news reports of rain corroding our buildings and poisoning our waterways, we need to continue to limit the things that cause this menace.

And really, who wants to go back to the '80s?