Blue carbon and climate change

Of the billions of tonnes of carbon that we emit annually, a significant proportion ends up, ultimately, in the sea where it contributes to so called “blue carbon”. This is carbon locked away in marine and coastal ecosystems. By soaking up carbon, they help to mitigate against climate change. But they may themselves fall victim to climate change and one of the other consequences of global warming: sea level rise. Speaking with Chris Smith, the University of Wollongong’s Kerrylee Rogers has been looking at the present, past and future of blue carbon…

Kerrylee - Blue carbon is the carbon that's sequestered by marine ecosystems. When we use the term sequestered, we are really talking about carbon being trapped for long periods of time. When we're talking about marine ecosystems, we're really talking about mangrove forest and salt marshes and seagrass meadows. They're the biggest blue carbon stores that we have, and actually, mangrove forests are amongst the most efficient ecosystems at sequestering carbon from the atmosphere.

Chris - When we're talking about things like mangroves, how long can they lock away carbon for? Is it long, long, long times like oil and coal or is it just a temporary holding measure for carbon?

Kerrylee - Probably a bit of all of those things actually. So there is some carbon that gets pulled into the mangrove woody material, and, if a forest drops some leaf litter, that organic material might break down relatively quickly and returned back to the atmosphere. Then you've got carbon that is pulled into the root zone. So this is the plants as they photosynthesise, they grab the carbon and they're adding organic mass to the structure of the tree, and some of that is below ground in the roots, and that material can stay for a lot longer because it's below the ground surface. So it doesn't sort of get distributed away on tides like leaf litter might. And it's locked away from exposure to the atmosphere to some degree. And this exposure is partly because those roots are covered up with sediments, but also because tides come in and out relatively regularly, as those tides come in, they actually make the substrates really waterlogged. And for decomposition to occur aerobically, you need air in the substrate. With tides coming in all the time, it stops the availability or limits the amount of air that's available within substrates, and that then slows down the rate of decomposition of that organic material or that root material that's down below the, the substrate surface.

Chris - So it is a legitimate way of locking up carbon for a reasonable length of time?

Kerrylee - Yes, in the right circumstances, and the right circumstances don't occur everywhere. What it really requires is that carbon getting locked away for a really long period of time. So it means they can't, the sediments can't be eroded. It also means that they need to remain relatively unexposed to the atmosphere and to oxygenation. And that only occurs under sort of special circumstances. So it's where tides are coming in and out all the time, and where that rate of inundation actually limits the exposure to oxygen and also slows down another mechanism of decomposition. So when we have tides coming in, they're often salty water and that salty water slows down another process of decomposition called methanogenesis. This is a process that creates another greenhouse gas, not carbon dioxide, but it creates methane gas. And so that inundation with salty water actually means that those methanogenic processes, those processes that create methane, are also slowed down. So we've got a number of mechanisms that create conditions for decomposition of the organic material from mangroves and salt marshes to be relatively slow.

Chris - On the one hand, then this is a good thing because as we release more man-made carbon dioxide and greenhouse gases into the atmosphere, we've got somewhere for it to go. On the other hand, presumably the changes which are happening to temperatures, to sea levels and so on could be jeopardising this very thing, which is there acting as a sponge to mitigate that?

Kerrylee - Yes. So I guess blue carbon is, is not a silver bullet. It's not the solution to climate change. And it's not the overwhelming solution for multiple reasons. One of them being firstly that the capacity of blue carbon ecosystems to sequester carbon or bring more carbon from the atmosphere is actually limited because of the extent of these systems. We've been cutting down mangrove forests, and urban expansion has occurred across salt marsh flats. And that's slowing down our capacity or limiting the capacity of these ecosystems to pull carbon from the atmosphere. The other issue, as you already touched on, is that these ecosystems are actually at risk from climate change themselves. They occur right near the sea. They're roughly at near mean sea level. And if mean sea level is rising, these ecosystems also have to adapt to that change in mean sea level to survive. We've done some modeling work that actually shows that the capacity of these ecosystems to adjust to sea level rise appears to have an upper threshold limit, and we potentially could be hitting those upper threshold limits before the end of this century.