Raise a hand if you've heard of the Gothenburg Protocol.

No? Well, you're in good company. This treaty has been called an "unsung hero" in the fight against air pollution and climate change. It may be unknown in the United States, but it is a landmark international agreement, setting limits on how much black carbon and other pollutants countries can emit.

Black carbon, or soot, is seen as a unique danger to the climate because its ability to accelerate warming in the atmosphere is many times stronger than carbon dioxide. It also speeds up the melting of sea ice. This double-whammy is responsible for a half a degree Celsius of warming in the Arctic so far.

"The science had evolved to a certain degree that it was possible for countries, governments to get involved," said Svante Bodin, the European director of the International Cryosphere Climate Initiative, a network of policy experts and researchers working to preserve the ice-covered portions of the Earth. "It had become clear that this could have a strong climate impact."

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The Gothenburg Protocol, established in 1999, sets limits on nasty pollutants like sulfur dioxide, nitrogen oxides, ammonia and volatile organic compounds, which are hazardous to human health. (Some also contribute to global warming). The pact was amended in 2012 to include black carbon, as the world became more aware of the threat it posed.

As significant as the protocol has been, it remains unfinished. Some countries, including the EU and the United States, have taken concrete steps to achieve its goals via domestic policy, but the amended version has not been ratified by enough countries to enter into force.

As evidence becomes clearer of the impact of black carbon on the Arctic and global climate change, the agreement faces a test: It still needs eight countries before it becomes official policy, which may happen this year. The United States was an early adopter—just two days before President Trump took office.

The protocol is rooted in the 1979 Convention on Long-range Transboundary Air Pollution, known as LRTAP, which was first organized to tackle the problem of acid rain. The idea is that air pollution isn't a local problem. What's emitted by industries, power plants and vehicles in one country can drastically impact another as pollutants are carried across borders.

So here's a cheat sheet: What you need to know about the Gothenburg Protocol and the LRTAP convention, and why.

A Bit of Background

Scientists in the 1960s wanted to get to the bottom of why acid rain was destroying forests, killing fish in lakes and threatening ecosystems in the Northern Hemisphere. They soon found the culprit: air pollutants, often emitted thousands of miles away from where they were raining havoc.

"It started during the Soviet time, so you can imagine that not much happened in those first 10 years," Bodin said. "But if we cannot cooperate about anything else, we can always cooperate about the environment."

Thirty-two countries across Europe, along with the United States and Canada, signed the original LRTAP convention.

"There were some reluctant countries in the beginning," said Michael Oppenheimer, an atmospheric scientist at Princeton University. "The UK was emitting a lot of pollution that was blowing over Scandinavia, and they weren't that interested in doing something about all this."

Over time, that changed. "They established a scientific process, everyone bought into it, and gradually countries shifted under pressure domestically," he said.

There have been eight follow-up protocols—the most recent of which is the Gothenburg Protocol—which broadened the number of regulated substances to include persistent organic pollutants and heavy metals, as well as ground-level ozone and particulate matter (including black carbon). Ground-level ozone and black carbon are both short-lived climate pollutants.

The Gothenburg Protocol came about at a time when the Arctic Council, an intergovernmental forum for the eight Arctic nations as well as indigenous groups, began to focus on black carbon as an accelerator of climate change and a threat to human health.

Why Does It Matter?

The list of problems solved—or at least improved—by LRTAP is impressive. Besides diminishing acid rain, there's been a decrease between 40 percent and 80 percent in a series of hazardous airborne substances in Europe. But that's just a start.

Exposure to air pollution has been responsible for 1 in 20 deaths in the United States, according to recent data in a 2016 report on LRTAP's progress by the United Nations Economic Commission for Europe. In the U.S., a 33 percent decrease in exposure to fine particulate matter and ozone—which are both covered under LRTAP—could avoid 43,000 premature deaths, tens of thousands of non-fatal heart attacks and respiratory and cardiovascular hospitalizations, according to the report.

The 2017 State of the Air report, an annual report by the American Lung Association, found that 125 million people in the United States were exposed to unhealthy levels of air pollution between 2013 and 2015.

The amended version of the Gothenburg Protocol would reduce emissions of sulfur dioxide, nitrogen oxides and particulate matter by 40 to 45 percent between 2005 and 2020, according to 2011 estimates. So far, 10 countries, including the United States, have ratified the amended version. Eight more are needed, and they could sign on over the next year.

How Does It Work?

The LRTAP convention stands out because of the concept of critical loads. Countries report emissions levels for various substances, and a complex computer model at the International Institute for Applied Systems Analysis in Austria assesses how those substances travel in the atmosphere and what ecosystems they might encounter and then projects the impact.

Scientists look at how much of a substance an individual ecosystem can physically handle before it becomes altered.

"There is a certain resilience among all ecosystems, but it varies very much from point to point," Bodin said. Some places have thin topsoil, for instance, and see a greater impact. "In other places, like in Greece, the buffering capacity is enormous because of the minerals in the soil. You can pour sulfuric acid on the soil and it's absorbed and nothing happens."

The models have been effective at determining critical loads across Europe and Asia, but the United States and Canada are not included in the modeling.

Unlike most other environmental or climate-change conventions, the LRTAP convention puts scientists in the room with policymakers, a mix that many observers say is key to the treaty's success.

"As far as I know, it's actually a fairly unique body in this way," said Martin Forsius, a research professor at the Finnish Environment Institute who has participated in environmental working groups of the LRTAP convention. "That really has been this long-term cooperation, and the policymakers clearly seem to listen."

Pam Pearson, who first became involved with the Gothenburg Protocol while working with the State Department, put it another way: "It may come across as really boring—the people involved are the technicians—but it's really effective. They know what they're talking about, and they simply go out and do it."

Impact of National Laws, Like Clean Air Act

The effectiveness of LRTAP and the Gothenburg Protocol in Europe is undeniable. Since 1990, sulfur emissions in Europe have dropped by about 80 percent, and nitrogen emissions have been cut in half.

The European Union adopted the concept of critical loads to set what's called the National Emissions Ceilings Directive—emissions reductions targets for the EU and member states. Though some of the EU's emissions limits mirror what LRTAP requires, the EU's adoption of the limits has more teeth. Failure to meet the requirements set out there comes with repercussions, as opposed to under LRTAP, which is binding but voluntary.

In the United States, it's a little harder to point to how LRTAP and the Gothenburg Protocol has helped. Many of the agreement's targeted pollutants were addressed by the Clean Air Act, which led to an average reduction of 69 percent in emissions of carbon monoxide, lead, nitrogen oxides, volatile organic compounds, particulate matter and sulfur dioxide.

"Basically, for both the U.S. and Canada there was a decision to, in essence, say, 'our Gothenburg Protocol commitments are our national regulations already'," Pearson said. "They didn't do anything new through Gothenburg that they hadn't done before."

That may explain why so few in the United States have heard of it. Expert after expert said they weren't familiar with it, or knew of it in name only.

Oppenheimer, who teaches a course that compares climate treaties, pointed out that there are literally hundreds of similarly unknown international environmental treaties.

"Even at the highest levels of government, if you went to State now—or even during the Obama administration—the number of people who knew and followed exactly what's going on with this convention would probably be two," he said.

Pat yourself on the back. That number is now one higher.

Editor's note: This story has been corrected to reflect that eight more countries must ratify the amended version of the protocol for it to go into force, rather than six.