Fact box 4. Ocean carbon in the global cycle?
Several studies suggest that the oceans have taken up around mented (including slowing deforestation and wide afforestation
2,000–2,200 Tg C yr
–1
over the past two decades (Gurney et programmes), would by 2050 according to the IPCC amount to ap-
al. 2002, Plattner et al. 2002, Sabine et al. 2004, Bender et al. proximately 12–15% of the required emission reductions. Preventing
2005, Miller et al. 2005, Manning and Keeling 2006). The up- the loss of the oceans blue carbon sinks would mean a significant
take increased slightly from around an estimated 1800 in the contribution to reducing climate change, even compared to slowing
1980s, to 2,200 Tg C yr
–1
in the 1990s and the first half decade deforestation of tropical rainforests. Afforestation programmes of
of the twenty-first century (McNeil et al. 2003, Canadell et al. mangroves could enhance this even further. The upper estimate of
2007). However, only a portion of this carbon is actually stored storage in oceans is approximately 450 Tg C yr
–1
– equivalent near
permanently in the oceans, as much is recycled and released 10% of the required emission reductions. Hence, “Blue” and “Green”
back within a few decades. Coastal ecosystems are currently carbon combined could bind at least 25% of the projected required
storing an amount of carbon equivalent to around 25% of the emission reductions.
estimated annual increase of approximately 2,000 Tg C yr
-1
in
1980s 1990s 2000–2005
the atmosphere.
(Tg C yr
–1
) (Tg C yr
–1
) (Tg C yr
–1
)
Currently, fossil fuel emissions are estimated at 7,200 Tg C yr
–1
, Fossil fuel emissions 5200 ± 300 6400 ± 300 7200 ± 300
which results inapproximately 2,000 Tg C yr
-1
increase in the Atmospheric increase –2900 ± 100 –3200 ± 200 –4200 ± 100
atmosphere per year. Losses of seagrass communities, man- Oceanic uptake –1900 ± 600 –2200 ± 700 –2200 ± 400
groves, and salt marshes have accelerated from around 0.9% Net terrestrial flux –400 ± 700 –100 ± 800 –800 ± 800
per year in the first three quarters of a century to up to 7% per Land-use change 1500 ± 800 1600 ± 800 1500 ± 800
year in the more recent decades. Under current scenarios, most Residual terrestrial flux –1900 ± 1100 –2600 ± 1100 –2300 ± 1100
blue carbon sinks will be lost in the next two decades leading
to a loss of annual carbon binding capacity equivalent to 4–8% Table 2. The Global carbon budget Tg C yr
–1
– around 2,200 Tg C
of the total anthropogenic input. Hence, total emissions would are captured per year in oceans, but only a portion of it is stored,
therefore have to be reduced by an additional 4–8% by 2030 to mainly in sediments in oceans blue carbon sinks, such as man-
retain the status quo, or 10% by 2050. In comparison, the total groves, marshes and seagrass communities (Canadell et al., 2007;
gain estimated from the UN REDD programme if fully imple- Houghton, 2007).
40
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