Dangerous Climate Impacts and the Kyoto Protocol Brian C. O'Neill and Michael Oppenheimer

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Methods

Temperature change ranges for CO₂ stabilization scenarios. Ranges given by IPCC assume non-CO₂ gases follow the IPCC A1B scenario to 2100 and remain constant thereafter and are based on a simple climate model tuned to represent seven general circulation models with climate sensitivities ranging from 1.7° to 4.2°C. However, the full range of uncertainty in climate sensitivity is estimated to be 1.5° to 4.5°C (S1), a range that was recently interpreted as a 90% confidence interval (S2). Projected temperature change is less sensitive to the choice of non-CO₂ gas scenario (S3), but including this uncertainty would also widen projected temperature ranges. Thus, the temperature ranges used here may underestimate the plausible range of future warming.

Carbon cycle modeling. We use a simple, global carbon-cycle model that was used by the IPCC in its most recent assessment [(20) in main text; (S4)]. We use standard methodology for defining concentration stabilization scenarios and inverting the carbon cycle to calculate emissions pathways (S5). Specifically, concentration pathways through 2010 (or 2020 in the delay scenario) are defined by running the model in forward mode, driven by emissions in the IPCC A1B scenario, with the exception that industrialized countries are assumed to deviate from A1B in 2000 in order to reach the 2010 Kyoto target and in 2010 in order to reach the 2020 Kyoto target. Beyond 2010 (or 2020), concentration pathways are defined using Padé approximants defined such that the rate of change in concentration is equal to the value derived in the forward run in the jump-off year and to zero in 2100. The shape of the stabilization path is adjusted such that implied emissions do not rise above the reference case scenario and do not change discontinuously. Reference scenarios with more slowly growing emissions in the short term ease the task of stabilization; we use the A1B scenario to be consistent with the precautionary framework of analysis.

To represent the current range of uncertainty in terrestrial sink strength, we represent all terrestrial uptake as CO₂ fertilization; strong, weak, and best-estimate sink strengths are defined by calibrating the model to the atmospheric CO₂ record under high, low, and best-estimate assumptions regarding net emissions from land use change over the 1980s (S6). We exclude uncertainty in the oceanic sink and in the mechanism and functional form of terrestrial uptake, which have smaller effects on inversion results, especially over the next few decades. The range of 1980s net land use emissions is taken to be (0.6) 1.7 (2.5) GtC/yr (S7). Model calibration in each case produces a different implied net land use emissions rate in 2000. We assume net land use emissions make a linear transition from their value in 2000 to the values assumed in the A1B scenario by 2010.

References for the Methods

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S3. D. Schimel et al., Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications, J. T. Houghton et al., Eds. (IPCC Technical Paper 3, 1997).

S4. H. Kheshgi et al., Clim. Change 33, 31 (1996).

S5. I. Enting et al., "Future emissions and concentrations of carbon dioxide: Key ocean/atmosphere/land analyses" (CSIRO Technical Paper 31, Commonwealth Scientific and Industrial Research Organization, Collingwood, Victoria, Australia, 1994).

S6. T. M. L. Wigley, in The Carbon Cycle, T. M. L. Wigley and D. S. Schimel, Eds. (Cambridge Univ. Press, Cambridge, 2000).

S7. I. C. Prentice et al., in Climate Change 2001: The Scientific Basis, J. T. Houghton et al., Eds. (Cambridge Univ. Press, Cambridge, 2001) pp. 183-237.