Issue 11
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The end of acid rain

23rd May 2023
22 Mins

Ending acid rain was one of humanity’s greatest environmental successes. Here’s how it happened.

The Statue of Liberty has not always been its iconic green. When it was first unveiled in 1886, it was a classic copper. But when copper reacts with oxygen and the right mix of pollutants, it takes on a green-blue hue.

The key pollutant in this mix was sulfur. When the Statue of Liberty was built – and in the decades that followed – New York’s air was thick with sulfur from coal burning, which reacts with copper to form copper sulfide and sulfate, changing its color. 

The government should have expected this change in shade. The patina that forms on copper is ‘verdigris’, a green–blue pigment. Humans have known about verdigris for centuries, if not longer. It was widely used as a green pigment in paintings.

But if they did expect a color change, their reaction suggested otherwise. By 1906, Congress was concerned that this was not a patina but corrosion. It wanted to paint the Statue of Liberty to protect it. When The New York Times revealed the plans in a piece titled ‘To Paint Miss Liberty!’ there was a public outcry.

A week later the Times reporter spoke to artists and architects to get their feedback on the plan and published the findings in a follow-up. Artists couldn’t believe the plan. As one put it: ‘I think my attention has never been called to a greater bit of proposed vandalism than this.’ He went on to explain that the patina wasn’t a threat to the sculpture, but a protective layer for the copper underneath.

What had been created was a landmark that artists could only hope to replicate: ‘You may be surprised to know that for years we have been trying to imitate the color effect of the Statue of Liberty by artificial means in our copper work. By architects and artists generally this color effect is considered the type of perfection for this kind of metal.’

Needless to say, the outside of Miss Liberty was never painted. Her green is now legendary, so we might consider the government’s ignorance to be a fortunate mistake. 

But many other sculptures and buildings have been ruined. Limestone and marble monuments of kings, queens, and the ancient Greeks have had their faces melted off. Not even the cherubs were spared.

By Nino Barbieri. From Wikimedia

And if you think it’s easy to restore them, think again. Take a look at the botched restoration efforts of one statue in Palencia in Spain: once a beautiful sculpture of a woman and her livestock, it now looks like a five year old’s creation from Play-Doh.

Human-caused acid rain is to blame for this damage.

The reason that there are still recognizable (albeit disfigured) statues left is that we managed to stop it. Acid rain – and the pollution that causes it – is still a problem in many middle–income countries, but thankfully we know how to fix it. 

How we figured out that rain had become acidic

Acid rain is rain, sleet, or snow that has high levels of sulfuric or nitric acids in it. These make the water more acidic. Not so acidic that it has any direct impact on human health – the acid is too dilute for you, your skin, or your clothes to notice. (Although the air pollutants that cause acid rain can lead to respiratory and heart problems.)

But as we’ll see, it has an impact on other ecosystems.

People started talking about acid rain in the 1970s, but we’ve known about its impact for centuries. This is not so different from climate change: it became mainstream knowledge in the 1990s but research and newspaper clippings about the problem date as far back as the 1800s.

The first hint of acid rain dates back to the 1600s. The writer John Evelyn’s pamphlet, Fumifugium or The Inconvenience of the Aer and Smoak of London is one of the most well known on the subject. This famous writing detailed the extent of air pollution in London, correctly identified many of its sources, and suggested plans to stop it, including moving industries that burnt coal and lime out of the city to the countryside. He had little understanding of atmospheric chemistry, but the strong scent that came off coal and lime kilns gave him some inclination that sulfur was one of the key pollutants. Later, writing in his diary, he described ‘how exceedingly the corrosive air of London had impaired’ the Arundel marbles – a collection of Roman and ancient Greek sculptures. He knew there was some link between the pollution and the damage, but couldn’t fully understand its nature.

Scientific understanding stayed this way until the late 1800s, when the term ‘acid rain’ was first coined. In 1852, the chemist Robert Angus Smith started taking rainwater samples from across the UK to analyze their chemical composition. Sulfur was one of the key substances he studied. As detailed in his book Air and Rain: The Beginnings of a Chemical Climatology, he came to three key conclusions on sulfur pollution:

1. The sulfates rise very high in large towns, because of the amount of sulphur in the coal used as well as of decomposition.

2. When the sulphuric acid increases more rapidly than the ammonia the rain becomes acid.

3. When the air has so much acid that two to three grains are found in a gallon of the rain-water, or forty parts in a million, there is no hope for vegetation in a climate such as we have in the northern parts of the country.

His conclusions came from his study of the air pollution in Manchester in England, the leading industrial city in the world at the time. Coal was making the rain acidic with devastating consequences for plant life in forests downwind of the city. In 1872, after two decades building evidence of the problem, Smith came up with the name ‘acid rain’ and he is now known as ‘the Father of Acid Rain’.

Although scientists had made some initial chemical measurements of the problem, and understood it conceptually, it was almost a century later before acid rain was investigated seriously. In 1963, researchers at the Hubbard Brook Experimental Forest in the United States set up a network of rigorous tests to measure rain acidity and its impact on bodies of water. In their first samples, they found that the rain had a pH level of 3.7. Normally the pH level is 5.6. Each unit higher on the pH scale means something is ten times as acidic. That means the rain in the forest was almost 100 times more acidic than expected.

Since the Industrial Revolution we’ve been adding large amounts of sulfur dioxide (SO2) into the atmosphere by burning coal. When this is emitted, it reacts with oxygen to form sulfur trioxide, SO3. In water, this quickly reacts to form sulfuric acid (H2SO4) in the following reaction:

H2O + SO3 → H2SO4

This sulfuric acid then dissolves in rainwater, making the rain more acidic.

There are some natural processes, such as lightning and volcanic eruptions, that emit SO2. But the amount these produce pales in comparison to the amount that humans have added to the atmosphere over the last few centuries.

And it’s not just sulfur. Emissions of nitrogen oxides (NOx), mainly from car exhausts, can form nitric acid (NHO3) in a similar way.

Why is acid rain bad?

Acid rain has many impacts, but the most visible is damage to old sculptures. The calcite in limestone and marble dissolves when it reacts with sulfuric acid. That’s why many old statues look weathered: parts of their structure have dissolved away. You might also find black crusts or peeling on these sculptures: That’s gypsum, a mineral that forms when sulfuric acid and water react with calcite.

Acid rain can also strip forests bare of their leaves. Acidic water strips soils of calcium and other nutrients that trees need to grow. The trees’ needles can also be stripped of their nutrients, becoming vulnerable to disease.

Yet some of the most troubling impacts of acid rain are less visible. Soils become acidic and are stripped of their aluminum, which runs into rivers and lakes. This can make the waters toxic to aquatic organisms such as crabs, fish and clams. Acid rain has also depleted many lakes of their fish populations. Trout have starved to death as the acidic waters dissolved the skeletons of the tiny organisms that they fed on. 

Image from Lovecz

To some degree, ecosystems can buffer against these impacts. Soils can often counter the normal slightly acidic pH of rainwater to maintain a steady pH. Many lakes contain microbes that produce alkali solutions that neutralize the acidity of their waters. But there is a limit to how much buffering these systems can do, and how fast they can act. 

With acid rain, we went well beyond this limit. But the fact that soils and lakes could counterbalance their pH is why it was possible for them to recover. If we could reduce our SO2 emissions significantly, these natural processes would kick in and restore these ecosystems to a healthy state.

How to reduce SO2 and stop acid rain

By far the biggest source of SO2 is burning coal. That means there are three potential ways to reduce sulfur emissions and stop acid rain.

First, we can burn coal that contains less sulfur. Coal deposits contain very different amounts of sulfur impurities: Deposits called lignite (or ‘brown coal’) tend to have the most sulfur, while sub-bituminous and bituminous coal have moderate levels, and anthracite is the ‘cleanest’. These deposits vary across the world: Australia, Russia, and Germany are home to most lignite coal while India, China, and the US have much more anthracite. 

Burning anthracite instead of lignite can reduce emissions, but it won’t eliminate them completely. And countries can’t completely replace the types of coals they burn with other types. Even if their domestic supplies contain lots of sulfur, they’re still likely to favor that over buying coal from somewhere else.

Second, we can burn less coal overall by opting for other energy sources instead. This is one path to low SO2 emissions, but it’s not a fast one. Energy transitions are becoming faster, but in the 1980s and 1990s, a quick transition away from coal wasn’t easy. Even today, it’s going to take decades – not years – for countries to make the switch.

Finally, we can install technologies in coal plants that remove sulfur from them. It costs money to install these ‘sulfur-scrubbing’ technologies, but they are incredibly effective. They directly react with the SO2 to remove it from the gas that finally makes it out of the smokestack. For such a devastating environmental problem, this solution is remarkably simple.

This last option is the one that most countries took. It allowed them to reduce SO2 emissions quickly without a radical overhaul of their energy systems.

Europe teams up

Europe quickly discovered that acid rain was far from a local or national problem. It was known as early as 1950 that a large share of the UK’s SO2 emissions were blown out to sea – the scale and impact of this problem were not fully understood.

In 1954, a team of Swedish scientists set up a large network of 49 sampling stations across the UK and Scandinavia. For years they rigorously collected data on the chemical composition, pH, and geographical signature of rain and air samples. They saw the fingerprints of British coal in the air and water across Norway and Sweden. But it wasn’t until the 1960s that the ecological damage caused by this transport became known. 

A Swedish scientist named Svante Odén combined data from earlier sampling studies with chemical and ecological data from lakes and forests. Samples from 600 lakes showed that they were becoming more acidic and that there had been a noticeable decline in catch of some salmon and other key fish species. 

After publication in both a national newspaper and a scientific journal, his research galvanized interest that led to the collection of more and more evidence of the cross-country impacts of acid rain throughout the 1970s.

It became clear that Europe had to treat acid rain as a shared problem and act accordingly. But they didn’t yet have the data they needed to quantify how much each country contributed to acid rain across the worst-hit regions. Without this evidence, a group of countries started the ‘30 percent club’ – in which every country was to reduce their SO2 emissions by 30 percent by 1993. 

However, scientists in the UK were stuck in a domestic battle to get the government to take acid rain seriously. Governmental action was largely blocked from within by the Central Electricity Generating Board (CEGB), Britain’s largest polluter. It repeatedly claimed there was no link between sulfur emissions and acidification, and that taking action would be incredibly expensive. This was too much for a country with a coal industry already in peril, it argued. 

Another reason the UK refused to get on board is that it claimed it had already cut its SO2 emissions significantly. This was true: By the 1980s SO2 emissions had fallen by more than 20 percent from their peak in 1970. This was, however, mostly the result of an economic downturn rather than deliberate policies to tackle air pollution.

Pressure on the UK started to mount during the 1980s, both internationally and domestically. There was growing evidence of acid rain at home: Sculptures were weathered; at least 60 lakes were badly affected, with damage to fish and bird populations; and beech forests showed clear signs of dieback. After intense scrutiny the CEGB also reduced its estimates for the cost of tackling SO2 pollution. Reducing emissions was going to be much cheaper than what was initially suggested.

By the 1990s, the governmental position had turned. It joined its European counterparts in the 1994 Oslo Protocol, which increased the ambition of emissions cuts across the continent.

Europe’s emissions of SO2 plummeted, and the pace of this decline has been almost unmatched compared to other gases or pollutants. Countries put strict limits on the emissions from power plants, and industry was forced to install scrubbing technologies. Europe surpassed its first target, with emissions falling by 39 percent between 1987 and 1993. And by 2021, they had fallen by 84 percent.

Some countries went even further: Emissions in the UK and Sweden have fallen by 98 percent since their peak in the 1970s.

The North American battle of the lakes

Across the pond, things moved a bit more slowly. Canada had woken up early to the problem of acid rain, while the US dragged its heels. Public attention in the US was growing during the 1970s – the problem was being covered in the media, making headlines in The New York Times. But political action was much slower.

In 1980, the US Congress passed the Acid Deposition Act, which set off a ten-year research program to study the impacts of acid rain. It was led by the National Acid Precipitation Assessment Program (NAPAP). This was a well-meaning step forward, but it created a decade-long vacuum in political action. 

Meanwhile, tensions were growing between the US and Canada. At one point, acid rain was the two nations’ biggest bilateral issue. The Great Lakes – on both sides of the border – were becoming increasingly acidic, and Canada’s ecosystems were suffering. Scientists claimed that at least half of this acidification was caused by pollution blowing in from the US.

The US government initially denied it. Much like the UK’s, it claimed that the impacts of acid were exaggerated by the media – if they existed at all – and that fixing the problem would be far too expensive: an unsurprising position for a country with a strong coal lobby.

Until this point, Ronald Reagan had largely brushed the problem of acid rain away. However, he relented slightly in the early 1980s, and commissioned the Manhattan Project physicist William Nierenberg to build a panel of experts and draft a report on acid rain. Their final report was submitted in 1983; it firmly concluded that acid rain was a problem and that the US should seek solutions for it. Despite being submitted in April, the publication of this report was delayed until June. In May, before it was published, the US House of Representatives voted against legislation to reduce sulfur emissions. 

The US did not put legislation in place. And it later refused to join Canada in signing the 1985 Helsinki Protocol. The Canadian government was, unsurprisingly, furious. As the Canadian environment minister Charles Caccia put it: ‘You can’t continue to dump on us the garbage that you are producing on your own property.’

A turning point came in the late 1980s and 1990s. There were a few reasons for this. Ronald Reagan had started to build a good relationship with the new Canadian president, Brian Mulroney. As Mulroney details in an account of the acid rain battle, Reagan began to relent on some bilateral issues, often against the advice of his team. Apparently on one state visit in 1987, Reagan became frustrated at his officials for blocking conversations on several issues, including acid rain. As Mulroney writes:

President Reagan took Carlucci [his national security adviser and secretary of defense] aside and said: ‘I think we should do something for Brian.’ Whereupon Carlucci said: ‘Mr. President, we’re doing well holding our positions on acid rain, the free trade agreement and the Northwest passage.’ ‘Oh, no, no, no,’ said Reagan, ‘we ought to do something.’

Mulroney also played this diplomatic game well, making it clear that acid rain was a shared problem between the two countries, and that Canada was doing its bit to reduce the damage to US ecosystems. In a joint session of the US Congress in 1988, Mulroney told them about the impact of acid rain in Canada: It had ‘killed’ almost 15,000 lakes, and damaged 150,000 more. Many salmon-bearing rivers could no longer support them. And its agricultural lands and forests were being acidified.

In 1990, the results from the decade-long study from NAPAP finally confirmed that acid rain was not just a problem for Canada: It was affecting the US too. Many American lakes were acidic, and acidity was affecting the catch of some of its key fish species.

The final nail in the coffin for the USA’s resistance was the inauguration of George HW Bush in 1989. Bush had served as Reagan’s vice president throughout the 1980s and had also developed a good relationship with Mulroney. Before taking office, Bush had pledged to be the ‘environmental president’. Within six months, US Congress passed amendments to its 1963 Clean Air Act, with a specific focus on cutting SO2 emissions to stop acid rain. The first phase of these amendments put limits on emissions from the largest power plants. The limits were then extended to most of the country’s power plants in 2000.

Just like Europe, SO2 emissions in the US plummeted. Between 1990 and 2019, emissions fell by more than 90 percent.

The Kuznets curve: Middle-income countries still have high sulfur emissions

Acid rain hasn’t been solved everywhere, but we have the knowledge to do so. Entire regions have almost eliminated SO2 emissions. All that’s missing is political will and cash.

Most countries that continue to emit a lot of SO2 are in the middle of the global income distribution. Air pollution follows the classic ‘Environmental Kuznets curve’: as incomes rise, pollution levels get worse until a certain point, when they begin to fall again. Large emerging economies such as India, South Africa, and Indonesia are slowly reaching that tipping point where emissions peak and then start to fall.

The Kuznets curve is an observation. But it’s based on a simple logic. When people have very low incomes, they don’t yet have access to electricity or coal as a fuel source. If they do, they can only afford small amounts of energy. As incomes rise, demand for energy (and often coal) does too. Industrial plants appear too, as countries become manufacturing hubs. At this stage of development, paying for technologies that remove sulfur from smokestacks might not make sense given a country’s priorities. The need for cheap energy trumps everything else.

This balance eventually shifts. Once we’ve satisfied our need for energy, our priorities turn to other things, such as cleaner air.

It’s a problem we can solve quickly

It took decades, or even centuries, for today’s rich countries to get to their current position. That’s concerning for today’s emerging economies: Will they still be emitting vast amounts of SO2 in the 2100s? 

No, for several reasons. 

An obvious explanation for the delayed response was ignorance: It took the world a long time to recognize that acid rain was a serious problem. And until sulfur-scrubbing technologies were invented we didn’t have effective or affordable solutions. Low- and middle-income countries don’t need to overcome these barriers today. We have experience and technological learning that makes this transition easier and cheaper, and we can look at China’s story to prove it.

In just over a decade, China’s SO2 emissions have fallen by two thirds. That’s while its coal use more than doubled. 

The government put ambitious regulations in place: Power plants were given limits on how much SO2 they could produce, which forced them into installing scrubbing technology to get rid of the sulfur.

This urgency was largely driven by public pressure. In the run-up to the 2008 Beijing Olympics, Chinese authorities tried to cut air pollution quickly. It worked to some extent (although it was still one of the most polluted events ever). But once the world packed up and went home, pollution across China continued to get worse. It reached a head in 2013, when public anger boiled over. People demanded rigorous monitoring of air quality data. And even Chinese state media were breaking news on the city’s awful pollution levels. The Chinese government declared a ’war on pollution’. It moved quickly, bringing in tough regulations on industrial plants to install scrubbers. It took old cars off the road, shut down nearby coal stations, and switched from coal to gas boilers after public pressure to reduce air pollution reached fever pitch in 2013.

In just six years, SO2 emissions more than halved.

When action is a public priority, and the cost of mitigation is low, countries can move incredibly fast. That might give us hope for tackling climate change – except it’s not quite the same problem.

Are today’s environmental problems like acid rain?

Why was the pace of expected reductions so fast by common environmental standards? A continent-wide proposal to slash CO2 emissions by 30 percent or more, in just a decade, would never have made its way into any early agreement on climate change.

What, then, made acid rain different?

Air pollution – not just acid rain specifically – has very clear domestic impacts. It affects the health of a country’s people; the quality of its forests and agricultural land; and the industries that it relies on, such as fishing. While acid rain does not affect human health directly, the emissions that cause it do cause respiratory, heart and a range of other health problems. Cut out these emissions and you not only stop acid rain, but improve a nation’s health too.

You might wonder how this nationalistic interest connects to the pan-European or US-Canada agreements that seemed to be successful. Don’t be fooled into thinking that countries cut their emissions altruistically, for the sole benefit of their neighbors. Indeed, one of the key arguments that won the US over was Canada’s message that ‘these impacts are bad on both sides of the border; we are cutting our emissions for you and you should do the same in return’. If there were no impacts of acid rain in the US, it might not have been so successful.

This is not the same for climate change, because the damage will not be felt equally across the world. Richer countries at more temperate latitudes (that is, Europe and North America) will experience fewer negative impacts than poorer countries in the tropics. They are also the biggest contributors to the problem. In other words, the biggest emitters have the fewest incentives to act. If the impacts of climate change would be most severe across Europe and the US, they would be much quicker to find ways to reduce or mitigate them.

Another reason acid rain has been easier to tackle is that the solution was much simpler. It meant sticking a relatively cheap piece of technology on top of an existing coal plant.

Finally, it comes down to timescales. Climate change is a long-term problem. Yes it’s already happening today but the worst impacts (and for some countries, the first major impacts) could be decades away. If the problems of air pollution and acid rain were mostly hidden until 2050, countries would not have tackled it so aggressively and so quickly.

If the Statue of Liberty was still the color of copper, old sculptures were still as beautiful as ever, and forests had not been stripped back to nothing, the air across Europe might still be full of sulfur. 

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