Sprawling fields of rocks about the size of your fist coat the Pacific seabed. Below miles of ocean, these nodules burst with copper, nickel, manganese and cobalt, all key to building batteries for electric vehicles.

As the global push for electric transportation grows, these metals have converted a remote underwater plain into a battleground over the hard decisions required to address climate change. A nascent industry of deep sea mining is growing to harvest these rocks. The industry's first commercial mining applications may be filed in as little as two years despite incomplete regulations and unsettled science about mining's effects.

Industry proponents say deep sea mining is more environmentally friendly than land-based mining, making it the best option in the face of looming mineral shortages for electric vehicles and a tight timeline to decarbonize transit. Marine and climate scientists counter that there's scant data on the deep sea to gauge potential consequences for oceanic biodiversity and carbon sequestration, and that it would take decades of study to get a holistic assessment.

Because of such serious uncertainties, conservation groups, hundreds of scientists and some battery-reliant manufacturers are calling for a moratorium on deep sea mining. In March, BMW and Volvo Group, along with Samsung and Google, pledged to abstain from sourcing deep sea minerals.

It's a "sustainability paradox," says Kris Van Nijen, managing director of Global Sea Mineral Resources, a deep sea mining contractor for Belgium and Germany. "On the one hand, we have a whole world demanding we deal with climate change ... [but] there is not one solution that does not impact biodiversity that actually helps to mitigate climate change, because, in the end, we have to do something and we have to make choices."

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The world does not have decades to decide how to handle climate change. And, when it comes to regulating deep sea mining, the international community may have even less time.

In June, the 8-square-mile, Pacific island nation of Nauru took the first step in launching the industry. It announced plans to submit an application for commercial extraction as early as 2023 to the International Seabed Authority, the organization overseeing deep sea mining. Such an application will be judged against whatever the deep sea mining rules are at that time — finalized or otherwise.

It's a "concerning move," says Andrew Friedman, who oversees the Pew Charitable Trusts' deep sea mining project. "We're taking what was intended to be a deliberative, consensus-based process to regulate an untested industry in a part of the world that remains largely unexplored, and we're compressing it somewhat arbitrarily into a two-year window. It's raising a lot of questions about what's going to happen next."

Getting critical metals is complicated

Metals to make clean energy batteries can be extracted from the land, sea or recycling. Advocates for marine mining say harvesting ocean minerals would be safer for workers than traditional mining and would have a lower carbon footprint by avoiding deforestation. A traditional land mining project for lithium has already sparked controversy in Nevada because of alleged environmental degradation. Cobalt mining in the Democratic Republic of Congo has long faced accusations of child labor abuses.

"If your nickel comes from [a traditional terrestrial mine in] Indonesia, you are guaranteed that your atmospheric carbon emissions will be many times higher than if your nickel was coming from nodules," DeepGreen, Nauru's Canadian-based contractor, tells NPR. The company points to a peer-reviewed paper in the Journal of Cleaner Production, which it commissioned, that found "nodules put 94 percent less sequestered carbon at risk and disrupt sequestration by 88 percent less" than terrestrial ores. Just like with land mining, DeepGreen says, permits should be decided on a "project-by-project" basis.

Mineral production on land is also concentrated in certain countries, giving them outsize power over extraction and distribution. More than 75% of lithium and rare-earth elements come from Australia, China and Congo, which also holds more than 70% of the world's cobalt.

By contrast, the international waters of the deep sea are home to trillions of mineral-rich nodules. The nodule fields between Hawaii and Mexico known as the Clarion-Clipperton Fracture Zone — about the size of the United States — are estimated to contain six times more cobalt and three times more nickel than land reserves.

Environmentalists argue the best option is to forgo all mining and gather critical materials instead through electronics recycling. But reports by the World Bank and the International Energy Agency conclude recycling alone will not address the world's clean energy mineral needs. By 2050, global demand for minerals such as cobalt and nickel will shoot up nearly 500%, the World Bank predicts.

"Recycling can have a major role, probably, after the 2030s," says Tae-Yoon Kim, an energy analyst for the International Energy Agency and an author of its recent report, when technology has improved and spent clean energy batteries are ready to be recycled.

Until then, experts say, a steady flow of freshly mined minerals will be key to decarbonizing transportation. Shortages could drive up the price of electric vehicles or hamper production, slowing their adoption.

The deep sea holds more than critical minerals

Among the different kinds of deep sea mineral deposits, those that draw the most commercial interest are metal-rich nodules. Though the nodules were discovered more than a century ago, attempts at commercial extraction started only in the 1970s.

The International Seabed Authority was created in 1994 to oversee mining in the high seas. While continuing to draft the Mining Code, or the rules of international seabed mining, it has already issued at least 29 exploratory permits to allow nations and their contractors to search for nodules, test mining equipment and conduct environmental analyses. Sixteen of the permits are for harvesting polymetallic nodules in the Clarion-Clipperton Fracture Zone. The organization did not respond to repeated interview requests for this story.

Unlike the lifeless barrens most people expect the deep sea to be, the remote ocean floors are a biodiverse and complex ecosystem. The intensive logistics to reach the deep sea thwart most research. It's only in the last several decades that scientists have begun to piece this picture together.

Interest in deep sea mining is now drawing industrial and academic dollars, broadening our understanding of this plain. DeepGreen has spent millions of dollars since 2012 to study its leases' environmental "baseline and [ways to] mitigate impacts of nodule collection on the marine environment," it told NPR via email.

Citing an impending merger in which DeepGreen will become The Metals Co., the corporation declined to comment on whether the science of deep sea mining will be settled by 2023, when the Nauru-sponsored mining application is likely to be submitted to the International Seabed Authority.

Other companies also seek a picture of what the deep sea environment is right now. The mining company GSR, which anticipates starting commercial extraction by 2030, is collaborating with an independent European Union scientific consortium, MiningImpact, to assess its leases. MiningImpact periodically monitors the deep sea environment and GSR's mining tests. In April, MiningImpact collected data during field tests of GSR's second vehicle prototype. But this data will not be fully analyzed for several years.

Could mining affect the ocean's storage of carbon dioxide?

As the world's largest carbon sink, the ocean has absorbed a substantial portion of our greenhouse gas emissions, with rising water temperatures and acidification to show for it.

Ocean carbon moves from the surface down into the seabed. Most of this occurs in the relatively shallow continental shelves where plentiful light and animals push carbon down the water column. A small portion makes its way down to the deep seabed. How exactly mining could affect this deep sea carbon is unclear, scientists say, and depends, in part, on exactly how mining technology functions.

If carbon-packed sediment that's dislodged by mining stays near the seabed, it is unlikely to contribute to atmospheric carbon levels on a meaningful time scale, says Trisha Atwood, an associate professor of watershed sciences at Utah State University. Atwood is co-author of a recent study that discovered bottom trawling releases as much carbon dioxide annually as global aviation.

"Once you get below 2,000 meters, carbon cycling slows down a lot," Atwood says. While deep sea mining "might expose carbon ... there's a pretty big time lag between when microbes would turn that into CO2, and when that CO2 is likely to hit the atmosphere," potentially thousands of years.

Still, Atwood is one of the nearly 500 scientists who called in June for a deep sea mining moratorium. "Even if that carbon isn't coming out into the atmosphere, we are weakening the oceans' capacity to take up more CO2," she says.

But it's uncertain if mining sediment will remain at the bottom of the ocean. Both GSR's and DeepGreen's mining vehicles use seawater to displace several centimeters of sediment and nodules so they can be sucked into the vehicle, which separates muck from rocks. The nodules are then raised to the surface for processing while the sediment is returned to the seafloor. Early tests indicate the sediment plumes generated by their activities disperse several kilometers across the seabed but do not rise significantly. Scientists recently developed a model to estimate how plumes will move around in the water to inform regulations and affect estimates.

Keeping the sediment close to the seabed is relatively expensive, says Matthias Haeckel, who oversees MiningImpact. It requires additional technology to re-cool the sediment so it stays dense and does not rise in the water column. Mining operators looking to save money could decide to release the sediment at or near the surface.

There are currently no provisions in the Mining Code that regulate the height at which sediment can be released in the water column, according to Pew's Friedman.

Hoovering up deep ocean mud may also affect microbes living in the seabed that consume carbon dioxide. These microbes absorb so-called natural carbon dioxide, or carbon that was not emitted by people but from ongoing organic and inorganic processes instead. The bacteria could be responsible for up to 10% of background "natural" carbon absorbed by the ocean annually.

Scientists do not know if disturbing the microbes through mining would alter the ocean's ability to capture and hold carbon long term. What is evident is that this bacteria recover slowly from disturbances. Researchers dragged small plows in the Peru Basin in 1989 to simulate small-scale mining. Decades later, the sites remain scarred, and they house decreased microbial activity.

"The microbial community does take a long time to recover following simulated mining disturbance," says Andrew Sweetman, a deep sea ecologist at Heriot-Watt University, who discovered carbon-consuming bacteria in the Clarion-Clipperton Fracture Zone in 2018. "So if these processes are important and are taking up CO2, then it's possible that mining may impact those processes to a large extent."

Deep sea mining companies say the amount of seabed that could be disturbed is inconsequential to carbon levels. Even if "nodule collection severely disrupted the carbon-cycling" in mining areas, writes sustainability researcher Daina Paulikas in the Cleaner Production paper, harvesting enough rocks to build 1 billion electric vehicles would disturb only 0.2% of the ocean's deep sea bacteria.

Down the road, DeepGreen anticipates pivoting from oceanic mining to recycling minerals in support of a circular economy. Several U.S. port cities are under consideration as potential sites for future recycling plants.

Energy Secretary Jennifer Granholm said in June that the Biden administration wants to see the supply for clean energy minerals met through "responsible" mining "that respects the environment."

A June report from the White House on potential supply chain shortages notes that "significant quantities of strategic and critical materials may be found on the seabed, but the industry to extract these resources remains nascent, given both technical challenges of mining in the marine environment and the potential for significant environmental harm."

In the end, experts say, mining the deep sea will likely come down to a value judgment.

The carbon impact of deep sea mining "is probably not going to be huge," Heriot-Watt's Sweetman says, "but I would say, at what point does a small number become significant?"

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