Taking carbon farming out to sea

But to fully take action, we need an approach that is truly global — and that means including our oceans. Just like land agriculture, ocean-based farming can become a climate mitigation tool. Its potential relies on one key crop: seaweed.

Seaweed accounts for half of all food farmed in the ocean and is used for a range of biomedical and industrial purposes. Many countries, most notably China and Indonesia, already harvest millions of tons of seaweed every year. While seaweed is a staple around the world, scientists are only beginning to evaluate its potential to remove carbon from the atmosphere and help combat climate change.

Seaweed’s carbon storage potential

Recent research has discovered that seaweed plays a huge role in oceanic carbon sequestration. Wild, uncultivated seaweed is responsible for sequestering over a hundred million metric tons of carbon in the deep ocean every year. They live and grow, then die and decompose into bits and pieces that slowly make their way down to the ocean floor, taking their carbon with them. Once locked in the deep ocean, this carbon can be stored for hundreds or thousands of years.

Scientists are exploring if seaweed farming practices could mimic this natural sequestration process to draw down and store more carbon in the deep ocean. Preliminary models have estimated that no-harvest seaweed farms on just 3.8% of US West Coast waters could store 34.4 million metric tons of CO2 — an amount equal to all direct emissions from California’s agriculture industry. Kick-starting pilot projects could help us better demonstrate how much carbon this strategy could remove in practice. Since the US does little seaweed farming now, to reach this scale, policy is needed to support research, development, and deployment.

Harnessing even a portion of the untapped potential sitting just off our shores would be meaningful for the environment, and existing seaweed markets are the perfect place to start developing. By upscaling these markets, we can begin to understand the possible costs and benefits of future no-harvest seaweed farming.

The power of shifting diets

Incorporating more seaweed into our diets could also take some of the pressure off of our soils. As a result of conventional practices like intensive tilling, high fertilizer inputs, and monocropping, traditional agriculture contributes as much as 10–12% of global greenhouse gas emissions. Seaweed’s popularity as a food source is only rising — demand for this fresh, salty superfood has grown exponentially over the past 50 years and is likely to continue to climb. But while farmed seaweed naturally stores carbon as it grows, that carbon quickly returns to the atmosphere when we eat it. The real climate-fighting power is in using seaweed to replace some of our land-based food, which can be more emissions intensive than seaweed cultivation.

Other forms of animal aquaculture can also deliver climate benefits when practiced side by side with seaweed farming. Like permaculture systems on land — where farmers use principles of ecology to design integrated “agroecosystems” — seaweed can be grown in conjunction with shellfish aquaculture. Seaweed absorbs excess nutrients and helps shellfish grow by making the nearby waters less acidic. When these permaculture systems are implemented instead of fish farms, it takes away the need to add fish feed and chemical fertilizers, which are the primary sources of aquaculture pollution and carbon emissions.

Farmers in China and Korea have used these regenerative ocean farming practices for years. Within the US, pioneers like Greenwave are beginning to build new farming models specifically for co-cultivating seaweed with shellfish. By shifting human diets toward sustainable seafoods, we can reduce emissions by over one billion metric tons every year. But it’s not just human diets that can make a difference — integrating seaweed into livestock feeds could dramatically reduce the methane gas produced by cows.

Clean energy, clean seas
Seaweed is a very promising feedstock for biofuel production. It contains a lot of high-energy carbohydrates and is relatively easy to break down, making it an ideal candidate. Seaweed grows faster and is more space efficient than competing terrestrial biofuel feedstocks like soy and corn. Seaweed farms can be located offshore, so it’s possible to scale up seaweed growth for biofuel production without competing with the agriculture or forestry sectors.

There is also growing interest in using seaweed to make products to replace those made using fossil fuels. For example, some single-use plastic products could be replaced by alternatives made from seaweed.

Harnessing the power of natural systems to store carbon is crucial to our fight against climate change and, to date, agricultural soils have gotten the spotlight. There’s no doubt that soils can play a huge role in mitigating climate change — farmer adoption of soil carbon practices can remove as much as 250 million metric tons of carbon dioxide from the atmosphere every year. But as we race against time to make meaningful progress in addressing climate change, it’s clear that we’ll need to deploy all of the carbon removal solutions at our disposal as quickly as possible.