The tools are already being made. Deep-sea crawlers the size of small warehouses are being tested by engineers in Rotterdam. In Kingston, Jamaica, international delegates are making comments on PDFs while writing rules for a place most of them have never seen and will never see. The race to mine the bottom of the deep sea is no longer just an idea. It is an institution. A less important question keeps getting pushed to the back of the room amidst all the institutional movement: what happens to the carbon?
The ocean floor deep below is more than just a place to store minerals. Over millions of years, the seafloor has gathered a huge amount of organic matter, such as dead plankton, fish bones, and other dead or decaying organisms that sank to the bottom and settled in the sediment. A lot of this stuff has been stuck in one place, chemically and biologically stable, holding carbon that would have been floating around in the air otherwise. Moving that sediment around by dragging big crawlers over it, vacuuming up nodules, and kicking up thick clouds of silt doesn’t leave the carbon sitting there nicely.
Scientists who study how mining affects sediments have found that it can bring back into the water column large amounts of organic carbon. Some of that carbon makes its way to the surface, where it can be broken down into carbon dioxide and released back into the air. It’s hard to understand how long these things take. Carbon that has been sitting on the seafloor for hundreds of years or even longer without being touched. What took thousands of years to settle could be moved again by a single mining operation.
Take a moment to think about that because it gets lost in the bigger conversation about rare earth minerals and battery supply chains. It makes sense that the deep-sea mining debate has been centered on the loss of biodiversity, like the sponges, corals, and deep-sea octopuses that sit on their eggs for four years. These are real losses that are very sad on their own. It’s more important now because of the carbon liability, though. It’s not just about the things that die in the sediment. It’s about what comes out when you dig up something that’s been buried quietly.
Mining companies are most interested in the 4.5 million square kilometer Clarion-Clipperton Zone, which is between Mexico and Hawaii. This zone has some of the densest deposits of manganese nodules on Earth. There are also layers of sediment that have been there for tens of millions of years. Marine geologists have carefully and not completely written down how sensitive that area is to the environment. On the other hand, the carbon accounting is still mostly unfinished. One gets the impression that the rules are being made faster than the science. This happens a lot in resource extraction, but it’s especially bad in this case because so much is at stake.
Deep-ocean chemistry is very complicated, which makes this harder to pin down. There are different kinds of carbon stored in seafloor sediments, such as dissolved organic carbon, particulate carbon, and methane that is locked up in hydrate structures in some places. Not all of it will come to the surface and rust. Some will be buried again. Some will be eaten by communities of microbes in the water column. As of now, science isn’t accurate enough to say exactly how much will go back into the atmosphere. That there is some doubt is not a reason to ignore the worry. Right now, when the world’s carbon budget is already very low, this is more of a reason to be careful.
There is something a little strange about how all of this happened at this time. The world is trying to lower its carbon footprint by turning everything electric, including cars, heating, and factories. At the same time, it is looking to the deep ocean for the minerals it needs for electricity. In a troubling way, the logic seems to go in circles. It is not a fair trade-off to get cobalt and nickel from the seafloor in order to make batteries that reduce emissions at the surface, since this could mean releasing greenhouse gases that have been stored. There may be no trade-off at all. We might be telling ourselves that the math works because we aren’t looking at both sides of the equation at the same time. The climate cost could just be moved from one ledger to another.
The International Seabed Authority is going to finish up its Mining Code. Companies will use their crawlers at some point. The nodules are going to be picked. A serious look at the carbon liability that’s hiding in those sediment layers won’t happen by itself unless someone makes it happen. It’s still not clear if that day of reckoning will come before the first commercial mining permits are given out or after the plume clouds have settled somewhere else, making someone else’s problem.
