Reservoirs, pools, and stocks are designations used in box models. A box model is a simplified version of a complex system, in which various parts of that system are reduced to boxes that are linked by fluxes. A reservoir is any box within that system that can accumulate, store, or release the substance of interest. In box models of the global carbon system, the oceans, terrestrial plants, soil, and rocks are examples of reservoirs. The atmosphere is typically not considered to be a reservoir. "Pool" is a term that is synonymous with "reservoir" but can include the atmosphere.

Stocks are the actual accumulations of the substance of interest. Thus, in a box model of the global carbon cycle, the stock is the mass or amount of carbon in various forms--including carbon dioxide (CO2) gas and dissolved species or solids, such as organic matter or inorganic minerals--in each reservoir (Berner, 2004). The stock in each reservoir is assumed to be well mixed, so that regardless of the size of the reservoir, the concentration of stock throughout the reservoir is uniform. The stock present in each reservoir can change over time, as the rate of input or output from each reservoir varies. In simple box models, reservoirs or pools are held to constant volumes. In more complicated models, the size of a reservoir can change. Numerical methods and computer programs are utilized with box models to solve equations to determine parameters such as the amount of stock in a given reservoir over time or the time needed for the amount of stock in a reservoir to change.

Box models, as simplified conceptual models of complex systems and processes, are of fundamental importance for predicting future climate conditions. Results from such models underlie the predictions that are published by the Intergovernmental Panel on Climate Change and other scientific and policy-making bodies. Such models can be applied to any complex system, regardless of scale, such as the global carbon cycle or the biochemical processes in a single cell. With regard to future global warming scenarios, variations in the global carbon cycle are among the fundamental drivers of climate change. Other topics of interest include the global cycles of water, nitrogen, oxygen, and sulfur.

In the development of a box model, the reservoirs and stocks of the species of interest are fundamental properties that must be defined in order to perform the necessary calculations. One of the largest sources of error in box models involves defining the size of reservoirs and pools and determining the amount of stock in each. For example, within the global carbon cycle, the stock of carbon present as gas hydrates in the seafloor is presently not well known (Berner, 2004). This value must be estimated from a relatively low number of measurements in specific regions and extrapolated to all other places on Earth where these compounds might be present. Gas hydrates have been a source of discussion in the scientific community as a trigger for rapid warming in the recent geologic past (as in the clathrate gun hypothesis of James Kennett).

A key question that is often addressed using box models is how the stock within a given reservoir might change in response to certain events. For example, to estimate the amount of global warming that has been caused by anthropogenic production of CO2, scientists must use box models to determine how the stock of CO2 in various pools (such as the atmosphere and oceans) has changed in response to the increased input of CO2 to the environment. Predictions of the length of time needed for atmospheric CO2 stocks to decline in response to future CO2 emissions are also based upon box models of the global carbon cycle.

References

1. Berner, Robert A. The Phanerozoic Carbon Cycle: CO2 and O2. New York: Oxford University Press, 2004.

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