A pinch of salt for new carbon storage modelling
- 27 Sep 2012, 15:25
- Freya Roberts
Source: M.L.Kirwan
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