45
Vegetated marine coastal habitats, blue carbon sinks, rank amongst the most threat
ened marine ecosystems (Duarte
et al.
, 2008; Duarte 2009). Coastal eutrophication,
reclamation, engineering and urbanisation have lead to the loss of a substantial fraction
of the earth’s blue carbon sinks since the 1940s (Duarte
et al.
, 2008; Duarte 2009)
THE WORLD’S OCEAN CARBON
SINKS IN RAPID DECLINE
A recent assessment indicates that about one-third of the glob-
al seagrass area has been already lost, and that these losses are
accelerating, from less than 0.9% year
–1
in the 1970’s to more
than 7% year
–1
since 2000 (Waycott
et al.
, 2009). About 25%
of the area originally covered by salt-marshes has been globally
lost (Bridgham
et al.
, 2006), with current loss rates at about 1
to 2% year
–1
(Duarte
et al.
, 2008). Valiela
et al.
(2001) estimated
that a total of about 35% of the area once covered by mangroves
had been lost globally since the 1940s, with current loss rates
at about 1 to 3% year
–1
. Hence, about one-third of the area covered
by blue carbon sinks has been lost already and the rest is severely
threatened. Marine vegetated habitats, blue carbon sinks, rank
amongst the most threatened habitats in the Biosphere, with global
loss rates 2 to 15 times faster than that of tropical forests (0.5%year
–1
,
Achard
et al.
, 2002). The loss of blue carbon sinks represents, in
addition to the impacts on biodiversity and coastal protection in-
volved, the loss of a natural carbon sink, eroding the capacity of the
biosphere to remove anthropogenic CO
2
emissions.
The Southern Oceans are recognised as an important carbon
sink currently taking up approximately 15% of anthropogenic CO
2
(CSIRO, 2007). Models predict that as the atmospheric concentra-
tion of CO
2
increases, so should the ocean’s absorbtive capacity.
This seems to be happening in most areas, but not so in the South-
ern Ocean (CSIRO, 2007; Le Quéré
et al.
, 2007; Lenton and Metzl,
2009). Whilst scientists agree on the data, there is some debate as
to why this may be – possibly decreased ozone with increased GHG
leading to stronger winds and therefore greater mixing, but despite
the cause, this trend has potentially serious implications for amt-
mospheric CO
2
concentrations in coming years.
Fact box 6. Is the ability of the Southern Oceans
to bind carbon also weakening?
Figure 18: Declining ability of the Southern ocean’s ability to
absorb CO
2
.
Southern Ocean carbon sink change
Gigatonnes of carbon per year
Source : NASA, 2008.
1970
1980
1990
2000
0.0
-0.5
-1.0
1.0
-1.5
1.5
-2.0
0.5
Expected
Observed
