A Planetary Health Check chapter puts annual bottom-trawl disturbance at up to 2% of the seafloor

The carbon argument in the chapter turns on repetition. The release pathway it describes is sediment disturbed over and over, which can send stored carbon back out as carbon dioxide ^[6]. The chapter does not break the 2% down by how often the same ground is worked.

Global modeling suggests trawling alone could free more than 100 million tonnes of carbon from sediments each year ^[7]. Set that against the low reading of the shelf inventory, 200 billion tonnes, and it is 0.05% of the stock per year ^[1]. Fully oxidized, 100 million tonnes of carbon would be roughly 367 million tonnes of CO2 ^[2]. How much of it ends up in the atmosphere as additional CO2 remains considerably uncertain ^[8].

Jacob Geersen, the marine geoscientist at the Leibniz Institute for Baltic Sea Research Warnemuende who led the chapter, said "We need to understand the seafloor as an integral part of the Earth system" ^[4]. Levke Caesar, a climate researcher at the Potsdam Institute for Climate Impact Research and co-head of the Planetary Boundaries Science Lab, said that on natural carbon sinks "we must not limit our focus to forests and soils on land" ^[16], and that "we are only just beginning to understand the consequences of compromising its long-term storage function" ^[17].

Both headline numbers are areas, and area is the unit permits are written in. Oceans cover two-thirds of Earth's surface at an average depth of almost 4 kilometres ^[21], so 200 square kilometres is a small patch of that, while the trawling estimate is a share of the whole. They describe different kinds of loss. Deep-sea mining experiments have shown consequences that persist for decades ^[9], and in the deep sea those consequences are still poorly understood ^[10]. What is lost includes living structure: reefs, sponge beds, mussel beds, burrowing organisms and microbial communities regulate the exchange of nitrogen, phosphorus and carbon between sediment and water ^[18].

Seafloor integrity, as the chapter defines it, is the extent to which the physical structure, biotic communities and chemical functions of the seafloor are preserved ^[12]. It is not yet recognized as an additional planetary boundary ^[11]. Protection rests primarily on international maritime law and the precautionary principle, and the chapter sets out the challenges of implementing that framework ^[13].

The worked example is the Baltic Sea, where decades to centuries of human interference are compounded by global warming, acidification, eutrophication and oxygen depletion ^[15].