A pinch of salt for new carbon storage modelling
Coastal ecosystems like salt marshes remove carbon dioxide from the atmosphere and lock it up in their roots, making them an important part of the carbon cycle. They can also act as an important defence against sea level rise, as the carbon laid down by plants such as seagrasses helps to raise the elevation of the land.
A new study in Nature suggests that beyond cycling carbon and offering protection from some of the impacts of climate change, these salty muddy shores may also be able to reduce the rate at which the earth is warming. Sounds too good to be true? In the long run it could turn out to be.
Previous studies have shown that with a bit more warmth, plenty of carbon dioxide and the occasional flood of seawater, sea grasses living in salt marshes could grow better. Since climate change is likely to make all of these conditions more likely, it’s possible that seagrasses will become more effective at locking up carbon. That’s the good news. At the same time however, the rate at which seagrasses decay could speed up, releasing carbon back into the atmosphere.
The net effects of climate change
To figure out what might actually happen scientists created a computer model of a saltmarsh. They then simulated how saltmarshes might respond under different temperature projections taken from the most recent Intergovernmental Panel on Climate Change (IPCC) report. The way this temperature rise would drive sea level rise was based on earlier modelling by Vermeer & Rahmstorf.
Their results suggest that for the first half of the century, seagrasses could thrive in the new climate conditions Vermeer & Rahmstorf’s model projects. But eventually, the rate at which these plants store carbon peaks and declines.
The details vary slightly for different emissions scenarios. Under moderate sea level rise, the model suggests that the rate at which carbon is locked up could initially increase. After 2050 however, that growth rate begins to slow.
By the 2080’s, under medium (A2) and high (A1F1) emissions scenarios, adapting marshes would be unable to keep pace with climate change, and seagrasses would be drowned by rising sea levels. In a low emissions scenario (B1), this happens soon after 2100.
It is worth noting however that these estimates for sea level rise are at the upper end of projections for sea level rise.
Fig 1. Rate of organic matter accumulation (carbon storage) in response to three IPCC climate scenarios. Source: Kirwan & Mudd, 2012 – Supplementary Information
Saltmarsh feedbacks?
Over the last 20 or so years, global sea levels have been rising by about 3.1 millimeters per year. The authors believe 3mm per year is moderate enough for seagrasses to thrive. These results suggest that, in theory, salt marshes might act as a negative (slowing) feedback on climate change for a few decades.
But later in the century, salt marshes begin to act as a positive feedback on climate change, say the authors. If the vegetation is unable to grow upwards at the same pace as sea level rise, it can no longer store carbon.
If sea levels were to change faster, leading to more than a metre rise by 2100, the model predicts that marshes would be submerged and lose productivity. Lead author, Dr Matthew Kirwan explains:
“At fast levels of sea level rise, no realistic amount of carbon accumulation will help [marshes] survive”
Realistic outcomes?
These model predictions should be taken with a pinch of (sea) salt. Models are forced to simplify the processes going on, so more empirical research might be needed to see how saltmarshes respond in the real world. It’s also worth remembering that what actually happens over the next century is likely to be different to the reference IPCC scenarios.
If the research is right, any increases in the amount of carbon locked up in salt marshes will likely be short lived. And in the long run, any positive effects brought about by climate change will peak and sharply decline, as the ecosystems become overwhelmed.
Updated: 15/10/12 Adjusted to reflect that modelling of sea level rise is by Vermeer & Rahmstorf, based on IPCC projections of temperature rise.
Response of salt-marsh carbon accumulation to climate change
Matthew L. Kirwan & Simon M. Mudd
doi:10.1038/nature11440