100 Years: it's political, not scientific

Some believe that 100 years is the time it takes for a tonne of CO2 to cycle through the atmosphere. It is not. This is a common misconception. Eg., "The internationally accepted timeframe for ensuring sequestration is equivalent to emissions is 100 years. This is based on the estimated life of one tonne of carbon pollution in the atmosphere." - Carbon Farming Initiative Handbook. (P.17)

Let's ask a scientist:
john.friend@industry.nsw.gov.au wrote

"Regarding your questions about where did I get the "100 year" figure for carbon dioxide and its relation to the issue of permanence. The figure has its origins in the Kyoto protocol. The IPCC have then used 100 year horizon values to compare the other greenhouse gases to carbon dioxide (the IPCC table is here). Regarding a specific reference for the 100 year value, I can't find one. From what I can gather, the rationale behind using 100 years is from this paper which states an "adjustment time" of 50-200 years". This paper actually states that the decay of excess CO2 in the atmosphere cannot be expressed in a single figure, so the 100 year figure seems to be more politically correct than scientifically correct."

Dr John Friend, Leader, Soil and Salinity, Natural Resources Advisory Services, Department of Primary Industries, NSW Department of Trade and Investment, Regional Infrastructure and Services

“This 100 year timeframe is a policy-determination, not a technical one,” reveals a peer--reviewed report by Pedro Moura Costa and Charlie Wilson.(1) It is a period chosen by the IPCC for calculating the Global Warming Potential of each different Greenhouse Gas compared to CO2. For instance, Nitrous Oxide has a GWP of 298 (ie., one tonne of N2O is equivalent to 298 tonnes of CO2).

Some believe that 100 years is the time it takes for a tonne of CO2 to cycle through the atmosphere. It is not. This takes only 4 years, according to an IPCC Report. “The turnover time of CO2 in the atmosphere, measured as the ratio of the content to the fluxes through it, is about 4 years. This means that on average it takes only a few years before a CO2 molecule in the atmosphere is taken up by plants or dissolved in the ocean.” (2.) However, it can take far longer for the atmosphere to adjust to the new levels of CO2, up to 200 years. (3.)

The EcoSecurities analysts calculate that removing a tonne of CO2 and holding it for 55 years is sufficient to counteract its effect on Global Warming. The IPCC uses 20, 100 and 500 year periods in much of its analysis. “The Kyoto Protocol set the time horizon against which [GWPs] are to be determined at 100 years (addendum to the Protocol, Decision 2/CP.3, para. 3)." (4.)

"To be consistent, it can be implied therefore that the Protocol also requires the benefits of sequestration in counteracting the radiative forcing effects of CO2 emissions to be evaluated over a 100 year time horizon. Any uncertainties derive from both this choice of time horizon, as well as future scenarios of atmospheric CO2 concentrations, are not technically driven but rather are a natural consequence of ‘arbitrary’ policy selections.”

“Functional Permanence”

Clearly, there is no definition of Permanence for Biosequestration that is dictated by Scientific Fact. The periods quoted range from 4 years to ‘forever’, with points of 20, 50, 55, 100, 200 and 500 years in between. The choice of 100 Years appears to have been a function of the need to find a scale on which to compare the Global Warming Potential of various Greenhouse Gases. Its choice as a time horizon took place as part of the negotiations around the Kyoto Protocols and was based on functional considerations. One function – the engagement of farmers in soil carbon sequestration activities – was overlooked.

FOOTNOTES:
(1.) Pedro Moura Costa and Charlie Wilson, An equivalence factor between CO2 avoided emissions and sequestration – description and applications in forestry, Mitigation and Adaptation Strategies for Global Change, Volume 5, Number 1, 51-60
(2.) Watson, R.T., Rodhe, H., Oeschger, H. and Siegenthaler, U. 1990. Greenhouse gases and aerosols. In IPCC Report No 1, World Meteorological Organization and United Nations Environment Programme, Cambridge University Press.
(3.) “This short time scale must not be confused with the time it takes tor the atmospheric CO2 level to adjust to a new equilibrium if sources or sinks change This adjustment time… is of the order of 50 - 200 years, determined mainly by the slow exchange of carbon between surface waters and the deep ocean.” ibid
(4.) "Reaffirms that global warming potentials used by Parties should be those provided by the Intergovernmental Panel on Climate Change in its Second Assessment Report (“1995 IPCC GWP values”) based on the effects of the greenhouse gases over a 100-year time horizon, taking into account the inherent and complicated uncertainties involved in global warming potential estimates. In addition, for information purposes only, Parties may also use another time horizon, as provided in the Second Assessment Report.” IPCC, REPORT OF THE CONFERENCE OF THE PARTIES ON ITS THIRD SESSION, HELD AT KYOTO FROM 1 TO 11 DECEMBER 1997, PART TWO: ACTION TAKEN BY THE CONFERENCE OF THE PARTIES AT ITS THIRD SESSION, 25 March 1998, P. 31, Decision 2/CP.3