Tens of millions of dollars have been allocated to the cultivation of seaweed for effective carbon dioxide capture, aiming to mitigate climate change. However, this emerging technology may face challenges that hinder its ability to significantly lower atmospheric CO2 levels and could even lead to unintended increases.

Meeting the Paris Agreement target of limiting global warming to 2°C necessitates robust carbon dioxide removal (CDR) strategies. According to reports submitted to the United Nations, many stakeholders are optimistic about utilizing seaweed as a cost-effective solution. For example, the U.S. startup Running Tide raised $70 million intended for seaweed cultivation in wooden structures designed to sink into the deep ocean, facilitating carbon sequestration, but ultimately ran out of funding and closed last year.

Dutch enterprise Kelp Blue has successfully secured over $2 million to scale up its current seaweed production aimed at generating sustainable agricultural fertilizers in Namibia. This initiative claims the potential to “sequester and offset” approximately 500 million tonnes of CO2 annually as the seaweed particles drift into deeper ocean layers. Learn more.

However, extensive seaweed farming initiatives can adversely affect nutrient levels in local phytoplankton populations. Research indicates that phytoplankton play a crucial role in carbon sequestration as they die and descend into ocean depths.

“It could have localized negative impacts,” stated Manon Berger from the University of Bern in Switzerland. “In specific areas, we might actually reduce the ocean’s capacity to absorb carbon dioxide. The overall potential for beneficial outcomes is limited and could have severe ecosystem impacts.”

Most types of macroalgae, excluding sargassum, thrive near nutrient-rich coastal regions, where they absorb dissolved carbon during photosynthesis, aiding the ocean in capturing more CO2 from the atmosphere.

A significant portion of the seaweed is ultimately digested or decomposed by marine organisms and microorganisms, believed to emit just 1/90th of the carbon captured. To enhance carbon sequestration, seaweed cultivation would need to extend further offshore, necessitating specialized packaging or sinking measures in deep waters.

Nevertheless, nutrient availability in the open ocean is limited, and past studies have often overlooked how iron deficiencies can restrict seaweed growth. Berger and colleagues developed a simulation to assess the feasibility of cultivating 20 billion tons of seaweed annually within 200 nautical miles from coastlines.

The findings revealed that seaweed cultivation significantly depleted nitrogen, phosphorus, and iron levels in surrounding waters, leading to a 95% decline in growth after 25 years. Additionally, this could potentially result in an 8% reduction in global phytoplankton blooms.

While some scenarios suggest that seaweed farming could still remove billions of tons of CO2, the specific species cultivated and their nutrient consumption patterns could mean that for every ton of carbon stored in seaweed, an additional half-ton may be released into the atmosphere.

Models indicate that only about 0.05% of ocean territory near Senegal and southern Australia is conducive to seaweed growth without significantly impacting phytoplankton populations.

“If we rely on a limited number of specialized sites, we simply cannot cultivate enough seaweed to achieve gigaton-scale carbon removal,” Berger commented.

In a separate study, Andrew Youghal and his team at the UK’s National Marine Center modeled the effects of iron fertilization on seaweed regions. They found it could potentially eliminate 40 billion tons of CO2 per year, but this would come at the cost of halving plankton populations, with severe repercussions for fish that depend on them for food.

“This process extracts nutrients from the surface ocean and redistributes them to deeper waters,” Yull explained. “Essentially, this action would diminish or slowly suffocate the natural ecosystem.”

Moreover, cultivating and submerging vast quantities of seaweed would necessitate substantial infrastructure, such as cages, spanning 14% of the ocean’s surface, predominantly in nutrient-infused yet tumultuous waters like the Southern Ocean, North Pacific, and Atlantic Oceans.

Ultimately, if significant areas of the ocean lack iron, the potential carbon removal benefits of seaweed cultivation may not fully counterbalance the loss of plankton, which could amount to as much as 700 million tons of CO2 released annually into the atmosphere.

“It’s not enough to simply grow macroalgae; for effective carbon dioxide removal, we must also factor in the effects on phytoplankton growth,” cautioned Chelsea Baker, another researcher at the UK National Marine Centre.