Malte Meinshausen, 1st March 2013 The University of Melbourne & Potsdam Institute for Climate Impact Research

Session 3: The expected temperature changes

associated with different emissions trajectories and stabilisation targets, and the carbon budgets for limiting future global temperature changes at different levels

Overview

1. Why do studies differ on carbon budgets?

2. Relationship between 450ppm CO2eq and 2°C.

3. The global carbon budget

Part 1: Why do studies differ on temperature projections and carbon budgets?

• a) Studies consider different questions. For example: CO2-only versus all anthropogenic forcings; different time horizons (2050, 2100, up to peak); Maximal warming or 2100 warming; Bayesian inference at different probability levels vs. Frequentist most likely outcome; with or without future natural forcing; different definitions of ‘pre-industrial’.

Clarify the question: all anthropogenic forcings and maximal warming; likely achievement of climate target... ?!

• b) Studies make different simplifications. For example: multi-gas emission scenarios vs. idealized exponential shapes.

Avoiding unnecessary simplifications and accounting for unavoidable ones.

• c) Studies represent multiple uncertainties along the cause-effect chain from emissions to global-mean temperature differently. For example: carbon cycle uncertainties; permafrost; clathrates feedbacks; other gas cycle uncertainties, e.g. methane; radiative forcing efficiencies and efficacies (e.g. black carbon); climate sensitivity uncertainty; ocean heat uptake; transient climate response; multi-gas scenario futures; regional aerosol distributions;

Choosing broad uncertainty representation that reflects breadth of current literature.

The analytical steps

Part 2: Relationship between 450ppm CO2eqTOT and 2°C. Three main options to define 450ppm CO2eq.

1) Peaking and stabilising at 450ppm CO2eqTOT with 50%:50% chance.

2) Stabilizing at or below 450ppm CO2eqTOT with likely chance for 2nd half of 21st century, after initial overshooting.

3) As (2), but 450ppm CO2eqGHG.

Equivalence concentrations versus temperature

Rogelj et al., 2012 See as well: Meinshausen, 2006

Knutti et al. 2005

In the very long-term, 450ppm CO2eqTOT stabilisation would entail 50%:50% chance of 2.2°C warming and likely chance to stay below 2.5°C warming

CO2 and CO2eq concentrations

own calculations with RCP default settings, Meinshausen et al. 2011

450ppm CO2/CO2eqTOT closest aligned with RCP3-PD up to 2060. If stabilisation path pursued, for 50%:50% chance, emissions could then remain at higher levels than RCP3-PD.

Conclusions Part 2

• In the very long-term, chance of staying below 2°C, slightly less than 50% for 450ppm CO2eqTOT stabilisation.

• A 1000 GtC budget path is estimated to have 470 ppm CO2eqTOT concentrations in second half of 21st century (median/best estimate).

• RCP3-PD, low IPCC AR5 scenario, peaks CO2-equivalence concentrations around 460, then returns towards 430 ppm CO2eqTOT by end of century. CO2-only concentrations under RCP3-PD peak slightly below 450ppm.

• Modifying RCP3-PD’s emissions after 2050 could produce a concentration stabilisation pathway.

Part 3: The global carbon budget

• Three different kinds of “carbon budgets” in literature:

(i) Most researched: Cumulative CO2 emissions in CO2-only world.

(ii) Most relevant: Cumulative CO2 emissions in all-forcing world.

(iii) Most “Kyoto-like”: Cumulative GHG emissions in all-forcing world.

Why is CO2 special compared to all other GHGs?

Joos et al. 2013 see as well Archer et al. 2009

CO2 does not have a finite lifetime – unlike other greenhouse gases. It accumulates. Therefore cumulative emissions matter. Decreasing airborne fraction and climate system inertia combine to approximately step-function like temperature response to a unit CO2 emission.

International goal: Keeping warming to below 2°C relative to pre-industrial levels (with likely chance). This target is slightly more stringent than 450ppm

CO2eqTOT. International UNFCCC language on 2C target:

“[…] reducing global greenhouse gas emissions so as to hold the

increase in global average temperature below 2 ºC above preindustrial

levels […]”

Cancún Agreements (2010), Decision 1/CP.16, para I.4

“Noting with grave concern the significant gap between […] pledges

[…] and […] pathways consistent with having a likely chance of

holding the increase in global average temperature below 2 ºC or 1.5

ºC above pre-industrial levels”

Durban Platform (2011), Decision 1/CP.17, preamble

Context: +2°C warming last time maybe 3 to 5 Million

years ago.

Different method; similar projection

A 1000 GtC path would be substantially higher; approx.

0.3 K

The CO2-only world would likely not warm above 2°C

Rogelj et al., 2011; UNEP 2010

Study setup / question / assumptions

Transient Climate Response to Emissions (TCRE) °C/PgC

Theoretical on multi-century timescale

Climate Sensitivity of ~3K & airborne fraction of ~25% (Archer et al. 2009)

~1.5°C best estimate; CO2 only.

Allen et al. 2009 Frequentist; CO2 only 1.3 to 3.9°C (5% to 95%) – best estimate 2°C.

Rogelj et al. 2012 / Meinshausen et al. 2009

IPCC AR4 replication with joint uncertainties from Meinshausen 2009

1.0 to 2.0 (CO2-only); 1.5 to 2.5 (all forcing)

Meinshausen et al. 2009 Illustrative default climate sensitivity

~1.5 best estimate (CO2-only); <0.71 PgC for <25% risk of exceeding 2°C;

Johns et al. 2011 ENSEMBLES 1.0 to 4.0 C

See as well: Matthews et al.; Zickfeld et al.; EMICs ensembles etc.

The transient climate response to emissions (TCRE)

Conclusions

• An emission pathway that has 1000 GtC cumulative emissions might have a 50%:50%, but possibly not a likely chance to stay below 2°C.

• RCP3-PD, the lowest of the IPCC climate model “representative concentration pathways” would imply CO2 and CO2eq concentration closest to 450ppm and a likely chance to stay below 2°C.

• New model scenarios could choose RCP-3PD (800-900 GtC cumulative emissions) as a benchmark of their emission characteristics.

• Towards latter half of 21st century: No way around zero or close to zero carbon emissions.

Backup slides

Raupach, Harman, Canadell, 2011

The transient climate response uncertainty

Do not distribute or share. Thanks.

Simple calculation of CO2-only induced warming over 1000-year timescale

• 25% of CO2 emissions remain in atmosphere on 1000-year timescale (Joos et al. 2013, Archer et al. 2009)

1000 PgC x 25% = 250 PgC remain

• 1 ppm atmospheric CO2 concentrations = 2.123 GtC 250 PgC /2.123 = +118 ppm

• 1000 GtC then increase long-term concentrations to approx. 400 ppm CO2 278 ppm + 118 ppm = 395 ppm

• 400ppm induces radiative forcing 5.35 x ln (395/278) = 1.89 W/m2

• ... which leads to a best guess warming of 1.5K. 1.89/3.71 x 3K = 1.5 K

References Allen, M., D. Frame, K. Frieler, W. Hare, C. Huntingford, C. Jones, R. Knutti, J. Lowe, M. Meinshausen, N. Meinshausen and S. Raper (2009). "The exit

strategy." (0905): 56.

Allen, M. R., D. J. Frame, C. Huntingford, C. D. Jones, J. A. Lowe, M. Meinshausen and N. Meinshausen (2009). "Warming caused by cumulative carbon emissions towards the trillionth tonne." Nature 458(7242): 1163.

Bowerman, N. H. A., D. J. Frame, C. Huntingford, J. A. Lowe and M. R. Allen (2011). "Cumulative carbon emissions, emissions floors and short-term rates of warming: implications for policy." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369(1934): 45-66.

Burke, E. J., C. D. Jones and C. D. Koven (2012). "Estimating the permafrost-carbon-climate response in the CMIP5 climate models using a simplified approach." Journal of Climate: early online release.

Canadell, J. G., C. Le Quere, M. R. Raupach, C. B. Field, E. T. Buitenhuis, P. Ciais, T. J. Conway, N. P. Gillett, R. A. Houghton and G. Marland (2007). "Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks." Proceedings of the National Academy of Sciences of the United States of America 104(47): 18866-18870.

Hansen, J., M. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D. L. Royer and J. C. Zachos (2008). "Target Atmospheric CO2: Where Should Humanity Aim?" The Open Atmospheric Science Journal 2: 217-231.

Joos, F., R. Roth, J. Fuglestvedt, G. Peters, I. Enting, W. von Bloh, V. Brovkin, E. Burke, M. Eby, N. Edwards, T. Friedrich, T. L. Frölicher, P. R. Halloran, P. B. Holden, C. Jones, T. Kleinen, F. Mackenzie, K. Matsumoto, M. Meinshausen, G.-K. Plattner, A. Reisinger, J. Segschneider, G. Shaffer, M. Steinacher, K. Strassmann, K. Tanaka, A. Timmermann and A. J. Weaver (2012). "Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: a multi-model analysis." Atmospheric Chemistry & Physics Discussions 12: 19799-19869.

Knutti, R., F. Joos, S. A. Muller, G. K. Plattner and T. F. Stocker (2005). "Probabilistic climate change projections for CO2 stabilization profiles." Geophysical Research Letters 32(20).

Koven, C. D., W. J. Riley and A. Stern (2012). "Analysis of permafrost thermal dynamics and response to climate change in the CMIP5 Earth System Models." Journal of Climate: early online release.

Matthews, H. D. and K. Caldeira (2008). "Stabilizing climate requires near-zero emissions." Geophysical Research Letters 35(4): -.

Matthews, H. D., N. P. Gillett, P. A. Stott and K. Zickfeld (2009). "The proportionality of global warming to cumulative carbon emissions." Nature 459(7248): 829.

Matthews, H. D., S. Solomon and R. Pierrehumbert (2012). "Cumulative carbon as a policy framework for achieving climate stabilization." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370(1974): 4365-4379.

Meinshausen, M., N. Meinshausen, W. Hare, S. C. B. Raper, K. Frieler, R. Knutti, D. J. Frame and M. R. Allen (2009). "Greenhouse-gas emission targets for limiting global warming to 2°C." Nature 458(7242): 1158.

Meinshausen, M. (2006). What does a 2°C target mean for greenhouse gas concentrations? - A brief analysis based on multi-gas emission pathways and several climate sensitivity uncertainty estimates. Avoiding Dangerous Climate Change. J. S. Schellnhuber, W. Cramer, N. Nakicenovic, T. M. L. Wigley and G. Yohe. Cambridge Cambridge University Press: 265-279.

Meinshausen, M., S. Smith, K. Calvin, J. Daniel, M. Kainuma, J. F. Lamarque, K. Matsumoto, S. Montzka, S. Raper, K. Riahi, A. Thomson, G. Velders and D. P. van Vuuren (2011). "The RCP greenhouse gas concentrations and their extensions from 1765 to 2300." Climatic Change 109(1): 213-241.

Meinshausen, M., S. C. B. Raper and T. M. L. Wigley (2011). "Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6: Part I – Model Description and Calibration." Atmospheric Chemistry and Physics 11: 1417-1456.

References (continued) Plattner, G. K., R. Knutti, F. Joos, T. F. Stocker, W. von Bloh, V. Brovkin, D. Cameron, E. Driesschaert, S. Dutkiewicz, M. Eby, N. R. Edwards, T. Fichefet, J. C.

Hargreaves, C. D. Jones, M. F. Loutre, H. D. Matthews, A. Mouchet, S. A. Muller, S. Nawrath, A. Price, A. Sokolov, K. M. Strassmann and A. J. Weaver (2008). "Long-term climate commitments projected with climate-carbon cycle models." Journal Of Climate 21(12): 2721-2751.

Raupach, M. R., J. G. Canadell, P. Ciais, P. Friedlingstein, P. J. Rayner and C. M. Trudinger (2011). "The relationship between peak warming and cumulative CO2 emissions, and its use to quantify vulnerabilities in the carbon–climate–human system." Tellus B 63(2): 145-164.

Raupach, M., I. N. Harman and J. Canadell (2011). Global climate goals for temperature, concentrations, emissionsand cumulative emissions, The Centre for Australian Weather and Climate Research (CAWCR). 74. http://www.cawcr.gov.au/publications/technicalreports/CTR_042.pdf

Raupach, M. R., G. Marland, P. Ciais, C. Le Quere, J. G. Canadell, G. Klepper and C. B. Field (2007). "Global and regional drivers of accelerating CO2 emissions." Proceedings of the National Academy of Sciences of the United States of America 104(24): 10288-10293.

Rogelj, J., C. Chen, J. Nabel, K. Macey, W. Hare, M. Schaeffer, K. Markmann, N. Hohne, K. K. Andersen and M. Meinshausen (2010). "Analysis of the Copenhagen Accord pledges and its global climatic impacts-a snapshot of dissonant ambitions." Environmental Research Letters 5(3): 034013.

Rogelj, J., W. Hare, J. Lowe, D. P. van Vuuren, K. Riahi, B. Matthews, T. Hanaoka, K. Jiang and M. Meinshausen (2011). "Emission pathways consistent with a 2 degree C global temperature limit." Nature Clim. Change 1(8): 413-418.

Rogelj, J., M. Meinshausen and R. Knutti (2012). "Global warming under old and new scenarios using IPCC climate sensitivity range estimates." Nature Climate Change 2(4): 248-253.

Schaphoff, S., U. Heyder, S. Ostberg, D. Gerten, J. Heinke and W. Lucht (2013). "Contribution of permafrost soils to the global carbon budget." Environmental Research Letters 8(1): 014026.

Schneider von Deimling, T., M. Meinshausen, A. Levermann, V. Huber, K. Frieler, D. M. Lawrence and V. Brovkin (2012). "Estimating the near-surface permafrost-carbon feedback on global warming." Biogeosciences 9(2): 649-665.

Solomon, S., G.-K. Plattner, R. Knutti and P. Friedlingstein (2009). "Irreversible climate change due to carbon dioxide emissions." Proceedings of the National Academy of Sciences 106(6): 1704-1709.

Stocker, T. F. (2013). "The Closing Door of Climate Targets." Science 339(6117): 280-282.

UNEP (2010). The Emissions Gap Report - Are the Copenhagen Pledges sufficient to limit global warming to 2C or 1.5C? . Nairobi, Kenya, United Nations Environment Programme. 55. http://www.unep.org/publications/ebooks/emissionsgapreport/

van Vuuren, D., J. Edmonds, M. Kainuma, K. Riahi, A. Thomson, K. Hibbard, G. Hurtt, T. Kram, V. Krey, J.-F. Lamarque, T. Masui, M. Meinshausen, N. Nakicenovic, S. Smith and S. Rose (2011). "The representative concentration pathways: an overview." Climatic Change 109(1): 5-31.

Zickfeld, K., M. Eby, H. D. Matthews and A. J. Weaver (2009). "Setting cumulative emissions targets to reduce the risk of dangerous climate change." Proceedings of the National Academy of Sciences 106(38): 16129-16134.

Johns, T., J.-F. Royer, I. Höschel, H. Huebener, E. Roeckner, E. Manzini, W. May, J.-L. Dufresne, O. Otterå and D. Van Vuuren (2011). "Climate change under aggressive mitigation: the ENSEMBLES multi-model experiment." Climate Dynamics 37(9): 1975-2003.