Habitat lost
since 1940s
30%
25%
20%
15%
Loss rate per
10%
year in
recent times
5%
Source: UNEP-WCMC, 2009;
Seagrass
Waycott et al., 2009.
0%
Figure 16a–c: Distribution of the world’s blue carbon sinks – seagrasses, mangroves, and salt marsh communities (Source: UNEP-WCMC).
et al., 2007). Isotopic analyses of the organic carbon accumu- models, to be about 2,000 Tg C year
–1
(Sarmiento and Gruber,
lated in sediments of vegetated coastal habitats have shown that 2002). However, this 2,000 Tg C year
–1
is the carbon annually
a significant fraction derives from plankton (Gacia et al., 2002). transferred from the atmosphere to the oceans, where it is largely
On the continental shelf and in estuaries, terrestrial sources of stored as dissolved inorganic carbon. The long-term residence
carbon are also significant (Bouillon et al., 2008), adding to the of anthropogenic CO
2
in the oceans is uncertain, as this carbon
carbon sink capacity of these blue carbon sinks. does not penetrate deep enough to remain in the ocean over
extended time scales. Indeed, half of the anthropogenic carbon
A consequence of the capacity of vegetated coastal habitats to stored in ocean waters is contained within the top 400 metres,
accumulate materials in the seafloor is that they act as efficient where it may equilibrate back to the atmosphere within a few de-
carbon sinks, globally responsible for the burial of 120–329 Tg cades, and the amount present in the deep ocean – where it may
C yr
–1
, which accounts for at least half of the lower estimate remain over much longer time scales – is below the detection
for global carbon burial in marine sediments (Table 1). Blue limit (Sabine et al., 2004). Only a minute amount of the carbon
carbon sinks therefore play a major role in the oceanic carbon taken up by the oceans is preserved in the deep-sea sediments,
cycle (Duarte et al., 2005a). The carbon burial capacity of ma- where it is effectively buried over long periods of time, represent-
rine vegetated habitats is phenomenal, 180 times greater than ing 6 Tg C yr
–1
, with a carbon burial per unit area of seafloor 180
the average burial rate in the open ocean. times lower than the rate for blue carbon sink sediments (Table
1). In addition, there are concerns that the capacity of the water
Carbon burial in the ocean represents slightly over 10% of the oce- column of the oceans to act as a sink for atmospheric carbon
anic carbon sink capacity (up to 25% using maximum estimates, will weaken in the future, and there is evidence that it may have
Table 1, see below), estimated, from observations and inverse started to do so (Doney et al., 2009). Hence, only carbon seques-
35
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