Wetlands dial up carbon storage as sea levels rise

Wetlands and coastal ecosystems will be a topic of discussion at Global Landscapes Forum Kyoto on 13 May. Here’s how to listen.

A collapsed coal mine in the middle of a swamp in southeastern Australia might seem an unpromising location for cutting-edge climate change science. However, it was here that an international team of scientists had the unique opportunity to glimpse how wetlands’ carbon storage abilities are affected by rapid sea level rise.

The mine, located in tidal marshland beside Lake Macquarie just north of Sydney, fell into disuse in the 1980s. In 1986, the mine’s owners decided to remove and reuse some of the pillars that were holding up the structure. They took out too many by mistake, and the mine collapsed, causing the wetland above it to sink. The water level in the wetland rose by about a meter over a couple of months – a very similar figure to that by which global sea levels are projected to rise over the coming century.

“We thought it could be a neat analogy to see how the wetlands respond,” said Kerrylee Rogers, a researcher at the University of Wollongong. Her team had already crunched the numbers, comparing statistics from parts of the world that have experienced different sea level trajectories over the past 6,000 years, to show that wetlands absorb more carbon under long-term sea level rise.

But sea levels are set to rise much more quickly this century than ever before. What’s more, new research suggests the world’s oceans are warming up to 40 percent faster than earlier Intergovernmental Panel on Climate Change (IPCC) estimates, which could boost projections even further, since water expands as it warms up. 

So the scientists were curious as to whether the carbon storage increases would still occur over these much shorter timescales. That’s where the flooded mine came in, says Rogers. “And we found some really interesting results.”

In the resulting study recently published in Nature, the authors highlight that uninterrupted sediment flows are key. “If there is enough sediment being delivered to a site, it will try to recover,” explains Rogers, “and the vegetation component – that is, the addition of organic material to the substrates – is a really important part of that adjustment.”

The researchers found that “the area that was previously mangrove and then converted to open water area had a huge acceleration in sedimentation,” says Rogers, “and a lot of that was organic material. And then even in the area that was salt marsh and became mangrove, again there was an acceleration and an increase in carbon storage below ground.” 

This led the scientists to extrapolate that “perhaps if we can leave our wetlands able to adjust to sea level rise – if sediment levels are high enough and we’re able to give them enough space – they’ll be able to sequester a lot more carbon from the atmosphere,” says Rogers.

So how can we create and secure the conditions for wetlands to perform these important functions?

There are plenty of landscape management activities that can facilitate wetland adjustment, says Rogers. “One is ensuring that they get sediment delivery. That means thinking about the dams that are in place and perhaps not putting more dams in place.”

It is also crucial to ensure wetlands are not destroyed or degraded: “If you clear them, you not only limit future capacity to store carbon, but you also release a whole lot of carbon just by clearing them,” says Rogers.

Allowing the wetlands to shift horizontally to higher elevations should also be considered, she says, and including ensuring that there are buffer zones around their expansion areas.

This means limiting coastal developments and planning to shift potential new infrastructure away from places where wetlands might move.

Once planners start allowing wetlands to adjust to sea level rise, in many places, there’s the opportunity for serious increases in the ecosystems’ extent, adds Rogers. For example, in northern Australia, where there are huge undeveloped coastal plains, “we’re likely to see big increases in the area of coastal wetlands,” she says.

That means more carbon sequestration will be possible as well as added benefits from all the other ecosystem services that wetlands provide. “You get wildlife habitats, nutrient cycling, buffering from storm impacts and flood water storage,” says Rogers.

Indeed, wetlands’ ability to “hold back the ocean” might prove particularly crucial in highly-populated areas like the California coast, wherenearly USD 100 billion worth of property will be threatened by sea level rise and associated flooding by 2100. There, organizations like The Nature Conservancy are already working to maximize the “natural infrastructure” that healthy wetlands provide and mitigate destruction in years to come.