Climate change has a major impact on the factors governing the uptake and storage of carbon by ecosystems and therefore plays a key role in the future capacity of ecosystems to sequester carbon.
Research results from Amazonian and African tropical forests show that carbon storage per hectare has increased over the past few decades, possibly as a result of higher concentrations of carbon dioxide in the atmosphere (Phillips et al. 2008; Lewis et al. 2009). An increase in vegetation biomass is accompanied by an increase in plant-derived carbon input into soils from leaf and root detritus (Davidson and Janssens 2006). Beyond this, “new” carbon sinks may appear in the arctic and at high altitudes if temperature increases allow vegetation to grow here (Schaphoff et al. 2006).
However, a range of models for future changes in biological carbon sequestration project that terrestrial ecosystems will serve as a carbon sink only until 2050. After that, they may become carbon saturated or in the worst case start to act as carbon sources towards the end of the 21st century (White et al. 2000; Cox et al. 2000; Cramer et al. 2001; Joos et al. 2001; Lenton et al. 2006; Schaphoff et al. 2006). Several factors related to climate change have been found to counteract an overall increase in carbon uptake and storage by ecosystems, especially in coaction with other drivers of ecosystem degradation (e.g. Nepstad et al. 2008): An increase in temperature accelerates soil carbon decomposition leading to carbon being released more quickly back into the atmosphere (respiration) (Heath et al. 2005; Davidson and Janssens 2006). Higher autumn respiration rates and resulting soil carbon loss may turn boreal forest areas into carbon sources (Piao et al. 2008). Fertilization experiments in Alaska showed that while annual aboveground plant growth doubled, the loss of carbon and nitrogen from deep soil layers more than offset this increased storage of carbon in plant biomass (Mack et al. 2004). Other factors associated with climate change may turn carbon sinks to sources, for example the thawing of permafrost in northern ecosystems (Gruber et al. 2004; Johansson et al. 2006; Schuur et al. 2008), an increase in ozone levels inhibiting photosynthesis (Felzer et al. 2005) and changing hydrologic regimes contributing to tropical forest dieback (Fung et al. 2005; Hutyra et al. 2005; Nepstad et al. 2007; Huntingford et al. 2008). The serious drought of the year 2005 that hit the Amazon rainforest, for instance, resulted in considerable losses of carbon from aboveground biomass, estimated as in the range of 1.2 to 1.6 Gt (Phillips et al. 2009). Moreover, the species composition of tropical forests is likely to change with changing climate, and this may have considerable impact on their carbon storage capacity (Bunker et al. 2005).
“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)
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. Increasing 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-water can have a fertilising effect so that the biomass of photosynthetic groups such as brown algae and seagrasses increases when CO2 does (Guinotte and Fabry 2008). In situ studies recently undertaken at a natural CO2 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 biological 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 organic 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 carbonate accretion of the shells, bones and skeletons most marine 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 models 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 climate change, largely depending on the severity of changes in precipitation (Schaphoff et al. 2006). Large-scale field experiments, such as FLUXNET, could significantly contribute to improving existing carbon and climate models (Running 2008; Baldocchi 2008).