Heavy fuel oil—the molasses-like sludge left after the oil refining process—is among the dirtiest fuels on the planet, and yet its use by ships is widespread in the Arctic, a pristine environment where it could do significant harm.

Burning the fuel contributes to climate change, and a spill in Arctic waters would be a nightmare for emergency response coordinators. But it's cheap, and attractive for ships making long hauls, the kind of traffic on the rise as climate change makes Arctic shipping easier.

Concerns about the safety and use of the fuel in a delicate and remote environment led to it being banned in the Antarctic in 2011, but efforts to include the Arctic in that ban fell short.

Now—as the United Nations agency that oversees shipping and its associated pollution prepares to meet next week—it's coming to international attention again.

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The week of April 9, the International Maritime Organization's Marine Environment Protection Committee will be grappling with multiple climate change-related issues, including possibly finalizing a first-ever agreement to limit greenhouse gas emissions from shipping. The IMO has wrestled with a major agreement on emissions for decades, but this meeting will be the first time that it also officially discusses regulating heavy fuel oil in the Arctic.

The committee will be presented with several papers addressing heavy fuel oil, including one that proposes banning its use in the Arctic. Another paper summarizes the work that has been done by the Arctic Council's Protection of the Arctic Marine Environment working group. All of the papers—including those that are expected to propose mitigation measures that fall short of a ban—will be considered by IMO member states before a subcommittee makes recommendations.

"A single HFO (heavy fuel oil) spill could have devastating and lasting effects on fragile Arctic marine and coastal environments," the proposal calling for a ban says. "In addition, Arctic shipping is projected to continue to rise, thus increasing the risk of a spill."

The proposal calls for a ban no later than the end of 2021, which is perhaps the earliest it might happen because the IMO is a slow-moving body. Such proposals must wind through committees before a vote for approval, making it a long road from agenda item to action. But for those who have been working on the issue, this is progress.

[Update: On April 13, the committee agreed on plans to move forward with a ban of heavy fuel oil in the Arctic. Countries will submit proposals in October about how best to assess the impact of a ban, and in February 2019, an IMO subcommittee will develop a plan for implementing a ban.]

"Getting the issue on the agenda in July was a huge step," said Liana James, the international policy advisor for the Clean Air Task Force. Now it's a matter of seeing what the IMO decides to do.

As the ice of the polar North dwindles, countries and industries are eyeing new shipping opportunities. Though the number of ships traversing the Arctic has risen recently, the volume remains small. But most industry projections see that changing in the coming years and already there are signs of the Arctic routes' allure. This year—which saw near-record lows in ice levels—a Russian tanker travelled through the northern sea route without an icebreaker for the first time.

With that opportunity comes risk, though. In its 2009 Arctic Marine Shipping Assessment, an Arctic Council working group found that "the most significant threat from ships to the Arctic marine environment is the release of oil through accidental or illegal discharge."

What Is Heavy Fuel Oil?

When oil is refined, once the light and middle distillates (like liquid petroleum gas, kerosene and the gas used in vehicles) are removed, what's left behind are the heavy distillates—wax, lubricating oils, asphalt and heavy fuel oil.

For ships operating across long distances, or in cash-strapped communities across the Arctic, heavy fuel oil is a relatively cheap option. But it also has a lot of impurities. When it's burned, that results in emissions like carbon dioxide, as well as short-lived climate pollutants like nitrogen oxide (a precursor to tropospheric ozone), sulfur oxide and black carbon.

These short-lived climate pollutants don't linger in the atmosphere for as long as carbon dioxide, but they are significantly more potent—meaning that it in their short lifetimes, they can do a lot to accelerate climate change.

In 2015, heavy fuel oil was the most consumed marine fuel in the Arctic, according to the nonprofit International Council on Clean Transportation (ICCT), which provides research and technical and scientific analysis to environmental regulators. This was almost 57 percent of the half-million tons of fuel used by ships in the region.

Who Uses It, and Who Supports a Ban

A recent report by the ICCT found that the biggest user of the fuel in the Arctic is Russia, which used 140,300 tons of it in 2015, followed by Canada, which used 14,612 tons. Denmark used 13,893. When a series of countries banded together to propose a heavy fuel oil ban in the Arctic—an effort led by the United States at the end of the Obama administration—those three countries were notably absent.

Sian Prior, the lead advisor for the Clean Arctic Alliance, a coalition of environmental groups committed to banning heavy fuel oil's use in the Arctic, said each country has different reasons for keeping its name off a proposed ban.

In late 2016, President Barack Obama and Canadian Prime Minister Justin Trudeau jointly called for the phasing down of heavy fuel oil use in the Arctic, but since then, Canada has been less supportive of an all-out ban.

"Canada is in a strange position," Prior said, because it has to balance environmental concerns with the needs of Arctic indigenous communities, which already pay a premium for goods and could see those costs rise further if shipping expenses increase.

In Denmark, she said, there has been support from the capital but no word yet as to whether Greenland is in favor of the ban. Since Greenland's residents are more likely to be impacted by the ban, and since Greenland is a part of Denmark, Denmark hasn't weighed in.

Meanwhile Russia "has never supported a ban," Prior said. "What is encouraging is that they have also not opposed it."

The Black Carbon Factor

The black carbon, or soot, emitted by burning heavy fuel oil is a two-pronged threat. There's climate-warming, when black carbon enters the atmosphere, and then additional warming when the soot falls back to Earth, blanketing the ice or snow below.

One gram of black carbon contributes 100 to 2,000 times more to global warming than one gram of CO2 on a 100-year timescale. A 2015 study found it is responsible for about a half a degree Celsius of warming in the Arctic.

Ice or snow that is white reflects much of the sun's radiation. But when it is darkened by black carbon as it falls from the atmosphere, the black carbon absorbs that radiation instead, leading to further warming. That's why black carbon that is emitted locally in the Arctic is so important. "Black carbon emissions from in-Arctic sources have about a fivefold warming effect over black carbon emitted elsewhere," said James.

In 2015, more than 2,000 ships operated in the Arctic region, as defined by the IMO, emitting more 193 tons of black carbon, according to a report by the ICCT. In total, 68 percent of that black carbon came from the burning of heavy fuel oil.

In that report, the biggest emitter of black carbon from heavy fuel oil was general cargo shipping.

Cleaning Up an Arctic Spill

Any oil spill in the Arctic could be catastrophic. The remoteness, along with the ice cover, periods of dark and vast areas of unmapped seafloor represent challenge after challenge.

But if that spill were heavy fuel oil, the stakes could be even higher. "We know it'd be pretty much impossible to clean up," said James. "When it's spilled, due to its high viscosity, heavy fuel oil essentially emulsifies on the ocean surface. It's mayonnaisey, gross stuff." And if it's cold, it can sink to the ocean floor and then travel on currents to warmer areas, where it can then rise back up to the surface and coat beaches.

A 2002 heavy fuel oil spill in Spain offers a cautionary tale. The Prestige, a Greek-owned ship operating under a Bahamian flag, started leaking after getting caught in a storm off Spain's Galician coast. It was towed out to sea, where it would pose less risk to the fishing and tourism industries, and was broken in two by a second storm, spilling an estimated 63,000 tons of heavy fuel oil.

The spill is known as Spain's worst-ever environmental disaster. Unlike with lighter fuel oils, dispersants couldn't be used and neither could the oil be burned off. It resulted in miles of polluted beaches, damages to wildlife and to the fishing industry—an estimated $4.4 billion in damages.

Momentum at the IMO

Heavy fuel oil earned a seat at the IMO table after a paper was submitted this summer by Canada, Finland, Germany, Iceland, the Netherlands, Norway and the United States, which called for the IMO to develop rules to mitigate its risks.

Though Russia was initially hesitant, it did ultimate support the measure, along with the rest of the Arctic states.

That landed the issue on the agenda for next week's meeting in London.

But a ban is only one possible outcome. Experts expect that other mitigation measures—like alternate sea routes and making certain areas off-limits—could be proposed. Findings will go to a subcommittee, which will make recommendations in a year or two.