Could Promising Natural Technologies for CO2 Removal Have Unintended Consequences?

Investments in seaweed cultivation for carbon capture have reached tens of millions, promising to mitigate climate change. Yet, potential side effects may hinder significant reductions in atmospheric CO2 levels and could even worsen the situation.

To meet the Paris Agreement target of limiting global warming to 2°C, effective carbon dioxide removal (CDR) is essential. Some believe that seaweed can provide an economical solution. For instance, the US startup Running Tide secured $70 million to grow seaweed in submerged wooden packs designed to sink and sequester carbon in the deep ocean, but it unfortunately ran out of funding and closed last year.

Dutch firm Kelp Blue has allocated at least $2 million to expand its seaweed farms in Namibia, aiming to produce sustainable agricultural fertilizer. This seaweed can break down into small particles, drifting into the ocean depths. They claim it could potentially sequester up to 500 million tonnes of CO2 annually.

However, large-scale seaweed farming can also deplete vital nutrients for phytoplankton, which absorb carbon as they die and sink. Two studies highlighted this concern.

Dr. Manon Berger of the University of Bern warns, “It could backfire locally. In some areas, we might actually reduce the ocean’s ability to absorb carbon dioxide, which would significantly impact the ecosystem.”

Macroalgae, excluding sargassum, thrive near coastal regions abundant in nutrients. They utilize dissolved carbon during photosynthesis, enhancing oceanic CO2 absorption.

Most seaweed is digested or decomposed by marine organisms, generating estimations that it emits roughly 1/90th of the carbon it captures. To enhance carbon sequestration, seaweed would need to be cultivated or transported farther offshore, necessitating packaging or sinking into deeper waters.

However, nutrient scarcity in open oceans poses a challenge, with most research not addressing iron deficiency that limits seaweed growth. Berger and colleagues developed a model suggesting the cultivation of 20 billion tons of seaweed each year, up to 200 nautical miles from shore.

The findings indicated that seaweed rapidly depletes nitrogen, phosphorus, and iron, leading to a 95% decrease in growth after 25 years and potentially reducing global phytoplankton blooms by 8%.

While seaweed farming could theoretically sequester billions of tons of carbon dioxide, the nutrient consumption related to certain seaweed types could lead to half a ton of carbon being added back into the atmosphere for every ton of seaweed grown.

Models indicate that only about 0.05% of the ocean, off the coasts of Senegal and southern Australia, can support seaweed growth without significantly impacting phytoplankton.

“With so few potential locations, we cannot cultivate enough seaweed to achieve gigaton-scale carbon removal,” states Berger.

In a separate study, Andrew Youghal and researchers at the UK’s National Marine Center discovered that fertilizing seaweed cultivation areas with iron could remove up to 40 billion tonnes of CO2 per year. However, this approach would halve plankton populations, severely impacting fish that rely on them.

“You’re effectively depleting the surface ocean’s nutrients, which harms the natural ecosystem,” warns Yull.

Additionally, cultivating and submerging this seaweed requires extensive frameworks across 14% of the ocean’s surface, primarily in nutrient-rich but stormy Southern Ocean, North Pacific Ocean, and Atlantic Ocean.

If insufficient iron exists, the carbon removal potential of seaweed may not offset the decrease in plankton, potentially adding 700 million tons of CO2 annually to the atmosphere.

“Simply growing macroalgae won’t suffice for CDR without addressing the implications for phytoplankton,” explains Chelsea Baker, another researcher at the UK National Marine Centre.