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OCEANIC
It is difficult to assess the overall impact of climate change on
oceanic carbon uptake capacity. Warming temperatures will
certainly affect the uptake of inorganic carbon, because carbon
dioxide dissolves less readily in warm water than in cold. In-
creasing temperatures may also lead to increased stratification
of sea waters and a slowing down of turnover between surface
and deep waters, leading to less transfer of dissolved inorganic
carbon to the ocean bottom. One study predicted that the ability
of the oceans to absorb inorganic carbon could peak at around
5 Gt per year, and that this peak could be reached by the end of
the 21st century (Cox
et al.
2000).
Increased presence of dissolved inorganic carbon in sea-wa-
ter can have a fertilising effect so that the biomass of pho-
tosynthetic groups such as brown algae and seagrasses in-
creases when CO
2
does (Guinotte and Fabry 2008). In situ
studies recently undertaken at a natural CO
2
vent area in
Ischia, Italy, have shown that seagrass communities flourish
in increased carbon dioxide environments (Hall-Spencer
et
al.
2008).
Cermeno
et al.
(2009) predict that global warming will lead
to an additional decreased efficiency of the so-called biologi-
cal pump in sequestering carbon due to thermal stratification
and a resulting reduction in nutrient supply to the deeper
ocean layers. Carbon models have shown that the rate of or-
ganic uptake of carbon dioxide by the ocean may be reduced
by 9% as a consequence of climate change impacts (through
reduction of wind-borne iron supply to the ocean, resulting
in a decrease in productivity) (Ridgwell
et al.
2002). For the
Southern Ocean, a weakening of the carbon sink has been
observed during the last two decades and whether this trend
may continue or reverse is uncertain (Le Quéré
et al.
2007; Le
Quéré
et al.
2008).
The ecological consequences of ocean acidification caused by
increased uptake of inorganic carbon are largely unknown.
However, progressive acidification is expected to reduce car-
bonate accretion of the shells, bones and skeletons most ma-
rine organisms possess, having impact on marine food chains
from carbonate based plankton up to higher trophic levels (The
Royal Society 2005; Nellemann
et al.
2008).
Overall, while there is agreement between most climate mod-
els that both the land and ocean carbon cycles will be affected
by future climate change, there is still large uncertainty on
the magnitude of these impacts (Friedlingstein
et al.
2006).
There is major uncertainty about the response of South
American and African tropical rainforests to continuing cli-
mate change, largely depending on the severity of changes
in precipitation (Schaphoff
et al.
2006). Large-scale field ex-
periments, such as FLUXNET, could significantly contribute
to improving existing carbon and climate models (Running
2008; Baldocchi 2008).
“The vulnerability of many carbon cycle processes and pools depends
on the magnitude of future climate change. The magnitude of future
climate change, in turn, depends on the vulnerability of the carbon
cycle.”
(Gruber et al. 2004: 52)