The Overlooked Carbon Storage Potential of Tidal Marshes

It’s all about the mud.

Following our release of a “best ever” map of tidal marshes (call them saltmarshes if you prefer, it’s approximately the same), in a new paper in the journal Nature Communications we’ve now calculated how much carbon is packed away in their soils.

Tidal marshes are globally distributed, but for those of us working in the academic and financial hotspots of North America, Europe and East Asia these are our ecosystems. They peak in abundance and in diversity all around our crowded coastlines, a green carpet flourishing on low level muds that stretch out from our estuaries and sheltered shores.

They may lack the buzz of coral reefs or the grandeur of tall, dense mangrove forests, but tidal marshes are among the hyper-productive ecosystems of our planet. The waterlogged mud, rich nutrients, fresh sediments and tidal flushing mean that the grasses and herbs which grow here grow freely, and fast.

About that Tidal Marsh Mud

Tidal marshes may not build forests, but they do build soil. And in that soil they trap, deposit and secure carbon. Furthermore, because that soil is both perpetually wet and salty, the carbon doesn’t rot and it doesn’t give off methane (a potent greenhouse gas). The carbon just builds up, and up, and up. Mangroves are the same. They are the world’s best carbon scrubbers.

Field scientists the world over have driven cores into these muds and measured the carbon they contain. It’s a lot, but it is also a history lesson. Some of this carbon goes back decades, even centuries.

If only there was enough of them, this could solve our climate change problem very quickly, extracting the CO² from the air, converting it to organic matter, then packing it away, every day, year on year.

Of course, there aren’t nearly enough tidal marshes for that, but those that there are must be treasured. If we destroy them we risk releasing yet more greenhouse gases as their carbon is broken down, and we lose the carbon-scrubbing benefits they provide.

To Help Fight Climate Change, Protect and Restore Tidal Marshes

Huge kudos then to Tania Maxwell and Tom Worthington in Cambridge. Even as Tom was completing the global map, Tania was reaching out to field scientists around the world, convening a brilliant team, gathering data (from 3700 soil cores!) and developing a model that would fit to Tom’s map.

The result—we estimate some 1.44 Pg C in the top meter of tidal marsh soils worldwide. (That’s 1,440,000,000 tons.) It’s hard to visualise but the US EPA estimates that burning that much carbon would be the same as the energy consumption of over 700 million US homes.

When a tidal marsh is converted to agriculture, or aquaculture, or urban space, the soil may be dug up, drained or sweetened with freshwater. Typically, the carbon will start to leak out, like air out of a puncture, breaking down into CO² or methane or both. Decades or centuries of captured carbon are on slow release.

This new map shows both the location and the scale of this risk. We know where the carbon is concentrated. We can use this knowledge to fight further losses—we can make a compelling case to STOP, and we can encourage restoration. Letting the sea and the marshes back in can rapidly block the leaking greenhouse gases and restore the carbon scrubbing.

Next task: let’s calculate the rates of carbon sequestration.

And let’s start to build these values into planning, including targets for climate mitigation and restoration.

I encourage you to get into a saltmarsh for an afternoonon a hike, birdwatching or in a kayak. In addition to their value in carbon storage, they’re often wild and beautiful places.