Carbon sequestration through chemical or mechanical processes seems just to displace the problem and is still highly prohibitive in terms of cost.

Seaweed, on the other hand, is a natural solution and has a development potential that humans have not even begun to explore.

Through photosynthesis, seaweed absorbs carbon to create its biomass. It grows at a very fast rate, more than 50 centimetres per day for Macrocystis, or giant kelp, which can reach up to 60 metres in height. Research published in 2022 by a group of leading researchers shows that wild seaweed is present in a surface area of nearly seven million square kilometres, an area comparable in size and productivity to the entire Amazon rainforest, although distributed along the world’s coastlines. But seaweed is much better at absorbing carbon than any terrestrial biotope.55

The other advantage is that throughout its life, seaweed suffers cell losses, mainly due to swell and currents, which it constantly compensates for, just as we do with our skin cells or hair. The same phenomenon occurs in trees that lose their leaves.

These organic particles lost by the seaweed can represent in total almost 50% of its final biomass. Much of the carbon seaweed absorbs is therefore lost to the oceans.

Around half of this exudate will be used to feed the plankton or the “filter feeders” (shellfish, bivalves, krill, sponges, etc), which form the base of the oceanic food pyramid. This half will therefore greatly contribute to restoring the surrounding flora and fauna, which will in turn absorb carbon.

The other half not consumed by the food chain will fall to the bottom of the oceans and sink into the abyssal sediments. If it reaches depths greater than 300 metres, the carbon will be trapped for about a century; if it reaches 1,000 metres, it will be trapped for millennia.

Seaweed therefore plays an essential role in combatting global warming.

Tim Flannery, a well-known Australian environmentalist, suggests that if 9% of the world’s oceans were properly managed to produce seaweed, they would absorb more greenhouse gas emissions than we emit today. The oceans would then begin to actively draw down carbon from the atmosphere to cool it.

First, it would be necessary to determine precisely the capacities of seaweed to transfer carbon into marine sediments depending on the species, external conditions, age, location, etc.

Ocean 2050, an NGO led by Alexandra Cousteau, the famous oceanographer’s granddaughter, with the help of Carlos Duarte, one of the world’s leading researchers in marine ecology, has launched an ambitious research pro- gramme to calculate the level of carbon sequestered in twenty- three existing seaweed farms around the world.57

Preliminary results released in 2022 indicate a carbon sequestration of 3.5 tonnes per hectare, which is three times more carbon sequestration than one hectare of Amazon rainforest. A figure which could rise, according to the NGO, to 10 tonnes per hectare under optimized conditions. Moreover, this carbon can be sequestered on different time scales depending on the depth of the spaces around the farm. This sequestration could perhaps be improved by cultivating seaweed near major fault lines in the ocean, as long as conditions allow. In this way, these farms would become real, all-natural carbon sinks.

Studies estimate that seaweed could absorb 10 billion tonnes of carbon equivalent per year, or almost a fifth of total annual emissions.

In addition, at the end of 2022, the Max Planck Institute for Marine Microbiology in Bremen released fundamental research on the structure of seaweed, which may well represent a potentially revolutionary development in carbon sequestration. Sulphated polysaccharides naturally present in a number of types of seaweed, “fucans” contain “focose” sugars that appear to be resistant to degradation by bacteria in any environment. Thus, certain compounds in the seaweed would trap carbon and be able to hold it for many years, in any environment. These very recent discoveries will have to be further investigated and refined, but if they are confirmed, the real capacity of sea- weed to sequester carbon could be even higher than expected and become truly significant.

Add to this the potentials for emission reduction, and seaweed would not only be carbon neutral, it would actually be carbon negative. This resource would therefore become the only food capable of reversing the curve of climate change and cooling the atmosphere. Every time we eat seaweed, we are contributing in some way to trapping carbon at the bottom of the oceans!

As a result, some are considering a more radical strategy to cool the atmosphere: growing seaweed for the sole purpose of deliberately sinking it to the bottom of the ocean. This is the Azolla strategy!

In this way, these farms would become real, all-natural carbon sinks. According to these researchers, the impact on the ocean would be relatively low, because the ocean’s mass is 250 times greater than the atmosphere.

So, if we transferred half of the carbon from the atmosphere to the oceans it would considerably cool the climate, but only modify the carbon content of the oceans by 2%.

This is a form of geoengineering that is very popular in some countries where they even advocate “seeding iron” into the ocean to accelerate the growth performance of this seaweed.

The solution has numerous advantages, and the produc- tion cost is minimal, as no effort is required for harvesting, transporting or drying the seaweed.

The sequestration result is optimal because when we use our seaweed to produce a food or other resources, most of the carbon held in the seaweed is released and re-enters the carbon cycle. So only the biomass that is lost during growth and not captured by the surrounding ecosystems – ie a maximum of 10 to 15% – ends up being sequestered in the abyssal zone.

If the seaweed sinks into the deep sea, all of the carbon will be sequestered.

Funding for the implementation of these solutions could be supported by carbon offset credits yet to be created, or by “impact investors” seeking to fund large-scale projects to combat climate change.

Whatever it costs, this solution is exceptionally effective at returning carbon from the atmosphere back to the oceans from which we extracted it to satisfy our thirst for energy.

But it does raise the ethical problem of voluntarily sinking millions of tonnes of nutrient-rich food while almost a billion people on earth are starving.

The other necessary reservation – perhaps even more fundamental – concerns the balance of the ocean ecosystem. A build-up of carbon on the ocean floor could lead to an explosion in bacterial growth and uncontrolled phytoplank- ton blooms, resulting in chain reactions that no study can currently measure.

The ocean has numerous and more or less well-known interactions with the surrounding continental shelves, but also with the ocean floor. No one can predict the conse- quences that this massive engulfment of seaweed in the deep sea would generate.

Moreover, seaweed is not only made up of carbon but also of many other nutrients (nitrogen, phosphorus, minerals, etc.) that are part of a cycle and enable life in the oceans and on land.

Sinking seaweed to the depths of the ocean will result in retaining a large amount of carbon there for thousands of years, but also a large amount of nutrients along with it...

This approach divides scientists: there are those who apply the precautionary principle and those who say that the consequences of climate change are already so advanced that not trying anything would be worse.

It is the eternal debate between geochemists and bio- chemists. Can the immense complexity of the balance of our biosphere on earth be reduced to particle exchange models?

It is not just the carbon cycle that we are modifying, but the life cycle!

The two are intimately linked, and meddling with the oceans seems like a dangerous idea...

Excerpted with permission from The Seaweed Revolution: How Seaweed Has Shaped Our Past and Can Save Our Future, Vincent Doumeizel, translated from the French by Charlotte Coombe, Illustrations by Neige Doumeizel, Legend Press.