The US ‘0% by 2020’ target: What are the climate consequences, if others delay comparably? Dr. Malte Meinshausen 8th March 2009 Brief Analysis of Todd Stern’s comments in relation to US’ 2020 target

What did Todd Stern say? “[...] What counts is getting on a viable path between now and Reducing 25-40% below 1990 levels would be a good idea if it were doable, since it would allow a less steep reduction path in the time period. But it is not independently necessary; a somewhat steeper path in the latter period could make up for the slightly slower start.” Keynote Remarks at U.S. Climate Action Symposium, Todd Stern, Special Envoy for Climate Change, U.S. Department of State, Senate Hart Office Building, March 3, 2009

2050 numbers are not questioned by T. Stern: 80% or more below 1990 for US by numbers are questioned by T. Stern: The delay (“slightly slower start”) by 2020 does not matter. Question for this brief analysis: How much would such a delay actually matter? Comparison between ‘0% by 2020’ and ‘25% by 2020’ US targets, with all other countries doing comparable efforts.

 0% US target is roughly comparable to a -10% Annex I target

March 2009 Design of default and delayed global emission pathways Default pathway ‘50by50’ Delayed pathway ‘Delayed 50by50’ Annex I Interim Reductions 30% by % by % by 2030 Non-Annex I Interim Reductions 20% below baseline by 2020 (BAU assumed here SRES A2) 20% below baseline by 2030 (BAU assumed here SRES A2) 2050 EmissionsHalved – with Annex I and Non-Annex I shares roughly according to equal percapita emissions 2100 EmissionsAsymptotic approach to zero fossil CO 2 emissions Note: All reductions refer to Kyoto-GHG baskets below 1990 excluding LULUCF emissions, if not otherwise specified. Gas-to-gas mixing ratios, as well as deforestation related CO2 emissions, are calculated using the EQW method (Meinshausen et al. 2006)

Note: Although only Kyoto-GHG emissions are shown, deforestation CO 2 emissions are included in the climate model calculations. Deforestation related CO2 emissions are derived from SRES scenarios using the EQW method, and are turning negative in the 2020’s.

Effect of delay A delay of 2020 emissions targets implies: 1)Steeper reduction rates Too steep reduction rates might render 2050 targets infeasible. 2)Higher cumulative emissions. Too high cumulative emissions will cause less than likely probabilities to achieve 2°C

The Effect of Delay:  320 GtCO 2 higher cumulative emissions  15% higher probability of exceeding 2°C Note: Climate parameter uncertainties assumed according to a climate sensitivity distribution that roughly reflects the IPCC AR4 estimates (3K best guess, 2.5 to 4.0K likely range), i.e. Frame et al. (2006) with uniform priors on TCR, with other climate response parameters constrained by historical observations; and carbon cycle uncertainties constrained by emulations of individual C4MIP carbon cycle models using MAGICC6.0 (see Meinshausen, Wigley, Raper, ACPD, 2006)

The Effect of Delay:  430 GtCO 2 higher cumulative emissions  17% higher probability of exceeding 2°C

Conclusion A somewhat steeper path in the latter period can not make up for the slightly slower start. Both cumulative emissions and reduction rates increase for a delay of interim targets. Even if 2050 emission levels are assumed the same, a delay in US reductions from -25% by 2020 to 0% by 2020 will increase the probability to exceed 2°C by ~15% (when other countries are assumed to follow the US example).

Methods & Further information Multi-gas pathways: If not otherwise stated, multi-gas emission pathways for prescribed emission targets were calculated using the EQW method. A tool for creating EQW pathways is available at > Downloads. The exceedance probability for staying below 1.5°C or 2°C global mean temperatures (relative to pre-industrial levels) were calculated using a medium climate sensitivity distribution (Frame et al with uniform priors on TCR), and a historical constraining to hemispheric temperatures, ocean heat uptake and IPCC AR4 radiative forcing estimates. The used reduced-complexity climate model is MAGICC6.1. The climate sensitivity distribution roughly reflects the IPCC AR4 best estimate (3K) and provided likely range (2.5 to 4.0K) for climate sensitivity. Note that the exceedance probability can diverge from the ones shown here by 15% or more, depending on which climate sensitivity distributions is assumed. Acknowledgements: Michiel Schaeffer, Bill Hare, Joeri Rogelj, Julia Nabel, Kathleen Markmann More information: References: EU Commission (2009) COMMISSION STAFF WORKING DOCUMENT, PART 2 accompanying the COMMUNICATION: Towards a comprehensive climate change agreement in Copenhagen - Extensive background information and analysis, Brussels, Meinshausen, M., B. Hare, T. M. L. Wigley, D. van Vuuren, M. G. J. den Elzen and R. Swart (2006). "Multi-gas emission pathways to meet climate targets." Climatic Change 75(1): Meinshausen, M., S. C. B. Raper and T. M. L. Wigley (2008). "Emulating IPCC AR4 atmosphere-ocean and carbon cycle models for projecting global-mean, hemispheric and land/ocean temperatures: MAGICC 6.0." Atmospheric Chemistry and Physics Discussions 8: 6153–6272. Frame, D. J., D. A. Stone, P. A. Stott and M. R. Allen (2006). "Alternatives to stabilization scenarios." Geophysical Research Letters 33: L Stern, T. (2009) Keynote Remarks at U.S. Climate Action Symposium, U.S. Department of State, Senate Hart Office Building, March 3, 2009