File:Carbon Dioxide Residence Time.png

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Description

Shows a number of scenarios that would allow atmospheric concentrations of carbon dioxide to stabilize. Note that in every case the actually emissions must ultimately fall towards zero due to the long lifetime of carbon dioxide.

This figure shows the degree to which carbon dioxide (CO2) emissions persist in the atmosphere over time. The lifetime of a gas in the atmosphere is generally known as its "residence time", but unlike other greenhouse gases, carbon dioxide does not undergo a simple decline over a single predictable timescale. Instead, the excess carbon is first diluted by the carbon cycle as it mixes into the oceans and biosphere (e.g. plants) over a period of a few hundred years, and then it is slowly removed over hundreds of thousands of years as it is gradually incorporated into carbonate rocks.

The dilution of carbon is such that only 15-30% is expected to remain in the atmosphere after 200 years, with most of the rest being either incorporated into plants or dissolved into the oceans. This leads to a new equilibrium being established; however, the total amount of carbon in the ocean-atmosphere-biosphere system remains elevated. To restore the system to a normal level, the excess carbon must be incorporated into carbonate rocks through geologic processes that progress exceedingly slowly. As a result, it is estimated that between 3 and 7% of carbon added to the atmosphere today will still be in the atmosphere after 100,000 years (Archer 2005, Lenton & Britton 2006). This is supported by studies of the Paleocene-Eocene Thermal Maximum, a large naturally occurring release of carbon 55 million years ago that apparently took ~200,000 years to fully return to pre-event conditions (Zachos et al. 2001).

In the figure, the colored curves are based on a modified version of the Wigley (1993) carbon cycle model (described below) and range from very small to very large carbon emissions. The smallest perturbation (red curve, 2 gigatonnes carbon [GtC], equivalent to about 1/4 today's annual emissions) is initially removed about twice as fast as a very large perturbation (green curve, 1500 GtC, roughly equivalent to about a tripling atmospheric carbon dioxide concentrations), but otherwise the trajectories are similar. This slowing is caused by predicted, but not yet observed, saturation effects that may reduce the ability for the ocean and biosphere to absorb carbon (Wigley 1993).

The gray band on the figure indicates a range of possible scenarios for carbon absorption based on the results of Archer (2005) and Lenton & Britton (2006). The ultimate trajectory that is realized depends upon the total amount of carbon dioxide released and the action of a number of poorly understood feedback systems. For example, it has been suggested that global warming resulting from the greenhouse effects of carbon dioxide could release additional carbon from methane clathrates and enhance decomposition of soils, both of which effects would lead to a slower uptake of carbon. Alternatively, stronger storms accompanying global warming may accelerate the weathering of rocks leading to faster sequestration (Lenton & Britton 2006). Uncertainties such as these make it more difficult to predict the long-term natural response to elevated carbon dioxide levels.

Wigley Carbon Cycle Model

The Wigley (1993) carbon cycle model used to generate the colored curves is a box model simulating interactions between the ocean, atmosphere and biosphere through five reservoirs and fourteen fluxes between them. This is far less sophisticated than more modern 2-D and 3-D models, but was chosen because it could be easily implemented and run on a desktop computer and its results were found to be consistent with more sophisticated models within the range of existing uncertainties (e.g. Archer 2005, Lenton & Britton 2006). The Wigley model incorporates changes in biological productivity and ocean carbon uptake as a response to elevated carbon dioxide levels, but does not model temperature dependent feedbacks.

Model parameters were adjusted to match the partitioning of carbon uptake into oceanic and terrestrial reservoirs as more recently determined by Bender et al. (2005). Further, following Archer (2005) a 30 kyr lifetime was added for carbon mineralization.

Copyright

This figure was prepared by Robert A. Rohde and is licensed under the Global Warming Art license.


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References

  • [abstract] [full text] [DOI] Archer, David (2005). "Fate of fossil fuel CO2 in geologic time". Journal of Geophysical Research 110: C09S05. 
  • [abstract] [DOI] Bender, Michael L., David T. Ho, Melissa B. Hendricks, Robert Mika, Mark O. Battle, Pieter P. Tans, Thomas J. Conway, Blake Sturtevant, Nicolas Cassar (2005). "Atmospheric O2/N2 changes, 1993-2002: Implications for the partitioning of fossil fuel CO2 sequestration". Global Biogeochemical Cycles 19: GB4017. 
  • [abstract] [DOI] Lenton, Timothy M. and Clare Britton (2006). "Enhanced carbonate and silicate weathering accelerates recovery from fossil fuel CO2 perturbations". Global Biogeochemical cycles 20: GB3009. 
  • [abstract] [full text] [DOI] Wigley, TML (1993). "Balancing the carbon budget. Implications for projections of future carbon dioxide concentration changes". Tellus 45B: 409-425. 
  • [abstract] [full text] [DOI] Zachos, James, Mark Pagani, Lisa Sloan, Ellen Thomas, and Katharina Billups (2001). "Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present". Science 292 (5517): 686–693. 

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