How disturbing the deep sea could worsen the climate crisis

How does the seafloor store carbon?

Despite an average depth of over 3,700 meters, the seafloor is closely connected to processes on land and in the ocean's surface waters. Most carbon in the seafloor is generated in the upper sunlit zones of the ocean by phytoplankton, including photosynthetic algae and bacteria. These organisms absorb CO₂ from the water and convert it into organic matter, some of which sinks and is eventually buried in seafloor sediments. A smaller amount of carbon enters the sea via rivers, groundwater, and dust. Incredibly, less than 1% of the carbon fixed by phytoplankton ends up sequestered in the seafloor, as most is degraded or transported away before it reaches the seabed. Though it might seem small, that fraction adds up over time, and once buried, seafloor carbon can remain locked away for millions of years if left undisturbed. 

What are the major threats to seafloor carbon?

With any seafloor disturbance, the mobilization of sediment and stored carbon can result in CO₂ emissions through interconnected physical, biological, and chemical processes. Though much of the sediment resettles, some remains suspended long enough to be degraded, leading to a loss of stored carbon. As microbes break down this newly available carbon, they release CO₂ in the water column, which can eventually make its way to the atmosphere. 

Currently, seafloor degradation is primarily caused by bottom fishing practices, such as trawling and dredging, which involve dragging heavy nets and cages along the seafloor, disturbing seafloor sediment. Other potential disturbances to seafloor carbon include undersea mining, offshore infrastructure, and telecommunications cables.

While we know these disturbances result in impacts to seafloor carbon, we don’t yet know the exact magnitude of CO₂ emissions produced by degrading them. Estimates vary widely, from 0.03 to 0.37 gigatons of carbon dioxide per year, just for bottom trawling. Even fewer studies document changes in seafloor carbon from other impacts. Still, initial research suggests the seafloor of mined areas holds nearly 50% less carbon than adjacent unmined areas. 

Could legal protection minimize seafloor carbon loss?

Marine Protected Areas (MPAs), managed for the long-term conservation of nature, might help minimize human impacts that degrade seafloor carbon stores. Roughly 8% of the ocean is currently in an MPA. When well-funded and effectively managed, MPAs can improve biodiversity and reduce degradation. Expanding ocean protections through MPAs can also generate revenue. Past estimates suggest that increasing protection to 20-30% of the global ocean could cost US$5–19 billion annually, but provide US$4.5–6.7 trillion in ecosystem services. 

Unfortunately, MPA designation alone does not guarantee protection. Fewer than 70% of MPAs are actively managed, and less than 3% of the ocean is at least “highly protected”, a category that minimizes destructive and extractive activities. High-impact activities, such as bottom trawling and dredging, are often allowed in the other 5% of global MPAs. Take Europe's territorial waters, for instance, where MPAs actually experience increased trawling rates. In other words, expanding MPAs have a limited ability to protect seafloor carbon unless they are actively managed, prohibit bottom trawling, and, ideally, function as no-take reserves. 

What about the additional benefits of protection?

Beyond carbon, seafloor protection can also preserve essential ecosystems. While we know surprisingly little about life on the seafloor, recent work has shown that bottom-dwelling organisms can take years to recover after trawling and dredging. Bottom fishing practices are also incredibly inefficient, with a higher carbon footprint than poultry, pork, and lab-cultured meat. Moreover, much of the marine life caught when trawling isn't desired for consumption, so it's thrown back into the ocean. Roughly 7% of the yearly global catch is these 'discards' from trawling. Unfortunately, many bycaught creatures die as a result.

Similarly, critical and globally rare habitats for deep-sea organisms depend on intact seafloor minerals of commercial interest. While some argue that undersea mining has a small footprint compared to other disturbances, mining will irrefutably and irreparably destroy many of these unique habitats since the minerals sought after can take millennia to form. And the ecological impacts aren’t restricted to just the mining sites. For instance, mining waste discharge will likely affect similarly understudied midwater ecosystems that support enough fish to outweigh 1,500 Great Pyramids of Giza.

The bottom line at the bottom of the ocean

Although the full climate impact of destructive activities, such as trawling and mining, remains as mysterious as the deep-sea itself, this much is clear: when the seafloor is adequately protected, its carbon stores stay sequestered and its ecosystems benefit. Considering less than 3% of the ocean is highly protected, MPAs should expand despite current knowledge gaps. The absence of absolute certainty is not an excuse for inaction. It should be incumbent on those who wish to exploit these fragile, carbon-rich environments to prove they will remain intact, not the other way around.

Christina Richardson, Ph.D., is a coastal hydrologist and biogeochemist who studies the impacts of climate change and other disturbances, like wildfire, on aquatic ecosystems. She has worked on a wide range of projects across the terrestrial-marine nexus. At Project Drawdown, her work focuses on climate solutions at the coast and in the ocean.

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