Friday, November 04, 2011

100 Years: a political construct?

This 100 year timeframe is a policy-determination, not a technical one,” reveals an EcoSecurities report.[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.”[5]

[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


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