The carbon cycle on land, in the atmosphere, and in the ocean are tightly linked with each other. After having specified the vulnerabilities within the various subsystems, it is important to investigate the interactions of these feedbacks from a global carbon cycle perspective.
Trade-off between Land and Ocean Uptake through Atmospheric CO2
Since atmospheric CO2 is one of the main drivers controlling the oceanic uptake of anthropogenic CO2 and probably has some control over the sink strength of the terrestrial biosphere, there exists a trade-off between these two sinks. For example, if a given amount of carbon is lost from one of the land carbon pools into the atmosphere, the resulting higher atmospheric CO2 concentrations would in turn foster increased oceanic uptake, thereby reducing the overall radiative forcing associated with the initial loss. The magnitude of the oceanic compensation can be significant. Cox et al. (2000), for example, find a compensation of approximately 50 percent. The magnitude of this compensation approaches nearly 100 percent over millennial timescales and played an important role in buffering the large loss of carbon from the land biosphere during transitions from interglacial periods to glacial periods. In a similar manner, a decrease in ocean uptake by, for example, increased stratification would also be partly compensated by increased terrestrial uptake. The magnitude of this trade-off effect, however, in this case depends on the sensitivity of the terrestrial biosphere to atmospheric CO2.
Potential Vulnerability of the Land-Ocean "Conveyor Belt"
The vulnerability of the land-river-marine transport and fate system ("conveyor belt") depends on the regional functioning of the hydrological cycle and on the way its dynamics are driven by patterns of water use, changes in land use practices affecting mobilization of carbon and nutrients to and through channels, and ultimately how the seas of the continental margins respond to these changes in forcing. Under scenarios of an increased hydrological cycle and changes in land use, river flow could increase, leading to increased ability of rivers to carry materials. Conversely, those areas subject to drier conditions would incur reduced flow and a reduced capacity of rivers to mobilize materials. Regardless of climate changes, an increased demand for water for agricultural and for urban and industrial practices would reduce water available to be routed down channels. Changes in forcing from upstream would affect coastal receiving waters. Decreases in river flow (from either drier climates or increased irrigation or retention in reservoirs) would reduce the cross-shelf water exchange because of a reduced buoyancy effect, resulting in a diminished onshore nutrient supply (Chen, Chapter 18, this volume). Primary production on the shelf would decrease proportionately, and with it the ability of continental shelves to act as a sink for carbon. If, on the other hand, there was a significant increase in nutrients (from increased fertilizers and urban use), higher levels of productivity, and even eutrophication, in estuaries or coastal regions could be maintained. Over a longer term, sea-level rises could have pronounced effects on carbon. We are currently not in a position to estimate the exact magnitudes of these effects, but it seems likely that these changes will have a much larger impact on regional issues than on atmospheric CO2.
A third possible feedback arising out of the interaction of the atmosphere-land-ocean system concerns the increasing evidence that micronutrients such as iron play an important role in regulating the strength of the ocean's biological pump, particularly in the high-nutrient low-chlorophyll (HNLC) regions. As a substantial fraction of the iron input into the surface ocean comes from the atmosphere, changes in this input could lead to changes in ocean productivity, export production, and eventually atmospheric CO2. This mechanism has actually been proposed as a possible reason for the low atmospheric CO2 concentration during the last glacial period (e.g., Martin 1990; see Joos and Prentice, Chapter 7, this volume). This feedback may also operate in the future. If degradation or desertification on land leads to increased dust inputs into oceanic HNLC regions, this dust input would stimulate oceanic productivity and thus increase oceanic CO2 uptake. We estimate, however, that this effect will be relatively minor over the next 20 or 100 years in really reducing atmospheric CO2, but this effect might lead to large changes on a regional scale.
It also has been proposed that atmospheric iron input plays a major role in controlling marine N2-fixation and that changes in the iron input would therefore lead to changes in the total inventory of fixed nitrogen in the ocean, which could fuel higher levels of marine productivity throughout the ocean (Falkowski 1997; Broecker and Henderson 1998). It is unlikely, however, that this mechanism will lead to large changes in the ocean atmosphere distribution of inorganic carbon over the next 20 to 100 years.
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