Over the last few years, there’s been a lot of hope placed in seaweed as a way to tackle climate change.
The excitement was a result of studiesSeaweed could be used to capture large amounts of carbon dioxide. This would take advantage of its rapid growth rate, large areas and long-term storage in deep oceans.
At present it’s thought seaweed stores around 175 Million TonnesEach year, 10% of the global carbon emissions comes from cars. This led many scientists to believe that seaweed could become a crucial tool in the fight against climate change.
While we’re all ready for some good news on climate, there is nearly always a “but” in science. Our Research in progressIt has been identified as a major problem that is often overlooked. Is this significant? Unfortunately, yes. Our calculations show that average seaweed ecosystems are not a carbon sink, but a natural source.
How could this be?
There are good reasons to consider coastal seaweed an important global carbon sink. Some species can grow up to 60 centimetres per hour. Seaweed covers about 3.4 million square kilometres in our oceans. When wind and waves break apart fronds or pieces of seaweed, some may escape being eaten and instead be whisked to the deep ocean where they will be deposited.
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Once seaweed has been submerged in sediments or in deep water, it is safe to store the carbon within for several hundred years. This is the time it takes for ocean circulation to bring bottom waters to the surface.
So what’s the issue?
The seaweed canopy is exposed to the ocean’s waters, which bring in large amounts of plankton and organic material from farther out at sea. This provides additional food for filter feeders such as sea squirts and shellfish that live amongst seaweed. It also helps to cover many seaweed fronds with bryozoan animals.
These creatures inhale carbon dioxide more than the seaweed they eat as they consume this additional food supply. The amount of carbon dioxide they produce is very small. But on an ecosystem scale, their numbers and ability to filter large amounts of water are enough to skew what researchers call the net ecosystem production – the balance between carbon dioxide inflows and outflows. Not just a little, but possibly a lot.
How did this happen? We gathered global studies that directly measured or reported key components of net ecosystem production from polar regions to tropical.
We found that seaweed ecosystems were natural carbon sources and released on average around 20 tonnes per square km every year.
But it could be higher. Seaweed could be a much bigger natural source of carbon, if we add in estimates of how much carbon is returned to the atmosphere after it has been washed out into the deep ocean.
We estimate it could be potentially as high as 150 tonnes emitted to the atmosphere per km² every year, in contrast to previous estimates that seaweed absorbs 50 tonnes per km². We need to stress that this number is uncertain due to the difficulty in estimating the quantities.
Is it time to give up on seaweed carbon storage
No. What would replace seaweed if it was lost? It could be urchin barrens – large rocky outcrops dominated by sea urchins – or smaller seaweed species, or mussel beds. Climate change is already evident in some places. Large kelp is being killed by heatwaves in the ocean and background warming in Tasmania, and being replaced with urchin barrens.
To get a true picture of the carbon storage potential of seaweed, we must consider the potential benefits of a substitute ecosystem.
If a substitute ecosystem is a larger carbon source or smaller than the original seaweed ecosystem, we should maintain or rebuild existing seaweed ecosystems to reduce greenhouse gas emissions. We have not yet been able to determine whether all replacement ecosystems have greater or less carbon sources.
What does this mean in terms of climate change efforts? This does not mean that seaweed can be a panacea.
To be able to quantify seaweed carbon storage, mitigation, or farming of seaweed, one must complete an accounting of carbon inputs as well as output. This will ensure that we aren’t unwittingly making the situation worse.
There are many carbon trading programs. Include seaweedWe must not underestimate the value of seaweed in carbon storage.
If we get this wrong, we could see perverse outcomes where industries offset their emissions by funding the preservation or restoration of seaweeds – but in doing so, actually increase their emissions rather than zero them out.