The Natural Fix?

“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)

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).

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