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The US ‘0% by 2020’ target:
What are the climate consequences,
if others delay comparably?
Brief Analysis of Todd Stern’s comments in relation to US’ 2020 target
Dr. Malte Meinshausen
[email protected], 8th March 2009
What did Todd Stern say?
“[...] What counts is getting on a viable path between
now and 2050. 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 2020- 2050
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 2050
2020 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
Default pathway
‘50by50’
Delayed pathway
‘Delayed 50by50’
Annex I Interim
Reductions
30% by 2020
10% by 2020
30% 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 Emissions
Halved – with Annex I and Non-Annex I shares
roughly according to equal percapita emissions
2100 Emissions
Asymptotic approach to zero fossil CO2 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)
[email protected], March 2009
Design of default and delayed
global emission pathways
Note: Although only Kyoto-GHG emissions are shown, deforestation CO2 emissions are included in the climate model calculations. Deforestation related
scenarios using the EQW method, and are turning negative in the 2020’s.
CO2
emissions are derived from SRES
Effect of delay
• A delay of 2020 emissions targets implies:
1) Steeper reduction rates 2020-2050.
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
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:
 320 GtCO2 higher
cumulative emissions
 15% higher probability
of exceeding 2°C
The Effect of
Delay:
 430 GtCO2
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 www.primap.org > 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. 2006 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:
www.primap.org, www.climateanalytics.org, www.pik-potsdam.de
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,
http://ec.europa.eu/environment/climat/pdf/future_action/part2.pdf
•
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): 151-194.
•
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: L14707.
•
Stern, T. (2009) Keynote Remarks at U.S. Climate Action Symposium, U.S. Department of State, Senate Hart Office Building, March 3, 2009