CH19: Carbon Sinks and Sources

FIGURE 19.1 Global carbon cycle. Terrestrial and marine pools and fluxes (arrows) are shown, indicating both natural sources and sinks (white) and the fate of CO2 from the burning of fossil fuels (red). Source: DOE.

FIGURE 19.2 Biological pump. The biological pump moves CO2 from near-surface waters to depth. Phytoplankton fixes CO2. Phytoplankton and zooplankton that are not consumed or decomposed fall through the water column to the ocean floor, where they may become incorporated into sediments and enter the slow (geologic) carbon cycle. Source: Steinberg et al. (2012).

FIGURE 19.3 Net CO2 flux from oceans. The solubility pump results in CO2 moving from cold waters near the poles where CO2 solubility is high (blue-purple), through the thermohaline circulation to upwelling regions in the tropics. The lower solubility of CO2 in warm water results in a CO2 release in the tropics (yellow-red). Source: NASA.

FIGURE 19.4 Carbon in aboveground and belowground biomass. Carbon is fixed from CO2 and stored in vegetation aboveground (woody material and leaves) and belowground (roots). The aboveground store is quickly released if vegetation is burned. The belowground store can be a large fraction of aboveground carbon and is released much more slowly after a fire or disturbance. Source: IPCC.

FIGURE 19.5 Carbon storage in soils. Carbon stored in soils can have long residence times. Carbon density is particularly high in peat soils at high latitudes and in tropical forests. Source: World Resources Institute.

FIGURE 19.6 Annual CO2 emissions partitioning in the carbon cycle. Annual CO2 emissions from fossil fuel use and land use change (upper half of panel b) are balanced by partitioning into the atmosphere, ocean, and land sinks (bottom half of panel b). Panel a shows breakdown of fossil fuel emissions by source type. Source: IPCC. See also Canadell et al. (2007).

FIGURE 19.7 Trends in terrestrial sources and sinks from 2000 to 2009. Recent trends in NPP show that the Amazon has turned from a carbon sink to a carbon source due to drought and fire. Sources are shown in brown, sinks in green. High-latitude forests and Central Africa remain sinks, while subtropical vegetation and forests in Southeast Asia are now sources. So while warming can promote greater plant growth leading to sinks, it can also promote drought and fires that convert sinks to sources. Source: Zhao and Running (2010).

FIGURE 19.8 Carbon sequestration storage options—capacity and longevity. On log scales, capacity and storage time are charted for each option. For instance, Enhanced Oil Recovery (EOR) injections of CO2 into spent oil wells has a low capacity but moderate and variable storage time. Biomass and soil carbon have a low capacity and short residence times (fast carbon cycle), so are part of short-term solutions, but need to be coupled with long-term solutions with residence times typical of the geologic carbon cycle. Ocean acidic is the deep injection of liquid CO2 into ocean bottom waters. Evolving fossil fuel consumption, total fossil fuel reservoirs (including oil sands and shales), and the total oxygen content of the atmosphere (oxygen limit on fossil fuel burning) are shown for comparison. Source: Lackner (2003). Reproduced with permission from AAAS.

FIGURE 19.9 Geologic sequestration. CO2 captured at the source can be injected into abandoned oil wells or geologic formations. A small amount can be used in industrial processes, offsetting CO2 production for those purposes. Source: DOE.

FIGURE 19.10 Artist’s Conception of a FAST array. FAST removes CO2 from the atmosphere. This allows sequestration of past or diffuse emissions. Technologies to remove and sequester free atmospheric CO2 are still in development. Source: Figure courtesy Columbia University.