EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmos. Chem. Phys., 13, 2793–2825, 2013 Atmospheric Atmospheric www.atmos-chem-phys.net/13/2793/2013/ doi:10.5194/acp-13-2793-2013 Chemistry Chemistry © Author(s) 2013. CC Attribution 3.0 License. and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Carbon dioxide and climate impulse response functions for the Open Access computation of greenhouse gas metrics: a multi-model analysis Biogeosciences Biogeosciences Discussions F. Joos1,2, R. Roth1,2, J. S. Fuglestvedt3, G. P. Peters3, I. G. Enting4, W. von Bloh5, V. Brovkin6, E. J. Burke7, M. Eby8, N. R. Edwards9, T. Friedrich10, T. L. Frolicher¨ 11,1, P. R. Halloran7, P. B. Holden9, C. Jones7, T. Kleinen6, 12 13 5,14 1 15 6 Open Access F. T. Mackenzie , K. Matsumoto , M. Meinshausen , G.-K. Plattner , A. Reisinger , J. Segschneider , Open Access G. Shaffer16,17, M. Steinacher1,2, K. Strassmann1,2, K. Tanaka18, A. Timmermann10, and A. J. Weaver8 Climate 1 Climate Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland of the Past 2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland of the Past 3Center for International Climate and Environmental Research – Oslo (CICERO), P.O. Box 1129 Blindern, 0318 Oslo, Discussions Norway Open Access 4MASCOS, 139 Barry St, The University of Melbourne, Vic 3010, Australia Open Access 5Potsdam Institute for Climate Impact Research, P.O. Box 601203, 14412, Potsdam, GermanyEarth System Earth System 6Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany Dynamics 7Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, UK Dynamics Discussions 8School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada 9The Open University, Environment, Earth and Ecosystems, Milton Keynes, UK Open Access 10International Pacific Research Center, School of Ocean and Earth Science and Technology,Geoscientific University of Hawaii, 1680 Geoscientific Open Access East-West Rd. Honolulu, HI, USA Instrumentation Instrumentation 11AOS Program, Princeton University, Princeton, NJ, USA 12Department of Oceanography, School of Ocean and Earth Science and Technology, UniversityMethods of Hawaii, and Honolulu, Methods and Hawaii, 96822, USA Data Systems Data Systems 13 Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA Discussions Open Access 14School of Earth Sciences, The University of Melbourne, VIC, Australia Open Access 15 Geoscientific New Zealand Agricultural Greenhouse Gas Research Centre, Palmerston North 4442, NewGeoscientific Zealand 16Department of Geophysics, University of Concepcion, Chile Model Development 17Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Model Development Discussions 18Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland Open Access Correspondence to: F. Joos ([email protected]) Open Access Hydrology and Hydrology and Received: 24 July 2012 – Published in Atmos. Chem. Phys. Discuss.: 9 August 2012 Earth System Earth System Revised: 25 January 2013 – Accepted: 11 February 2013 – Published: 8 March 2013 Sciences Sciences Discussions Open Access Abstract. The responses of carbon dioxide (CO2) and other few decades followed by a millennium-scale tail.Open Access For a 100 climate variables to an emission pulse of CO into the atmo- Gt-C emission pulse added to a constant CO concentration 2 2 Ocean Science sphere are often used to compute the Global Warming Poten- of 389 ppm, 25 ± 9 %Ocean is still found Science in the atmosphere after tial (GWP) and Global Temperature change Potential (GTP), 1000 yr; the ocean has absorbed 59 ± 12 % and the land the Discussions to characterize the response timescales of Earth System mod- remainder (16 ± 14 %). The response in global mean surface els, and to build reduced-form models. In this carbon cycle- air temperature is an increase by 0.20 ± 0.12 ◦C within the Open Access climate model intercomparison project, which spans the full first twenty years; thereafter and until year 1000,Open Access tempera- model hierarchy, we quantify responses to emission pulses ture decreases only slightly, whereas ocean heat content and Solid Earth of different magnitudes injected under different conditions. sea level continue to rise. OurSolid best estimateEarth for the Abso- Discussions The CO2 response shows the known rapid decline in the first lute Global Warming Potential, given by the time-integrated Open Access Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. The Cryosphere The Cryosphere Discussions 2794 F. Joos et al.: A multi-model analysis response in CO2 at year 100 multiplied by its radiative ef- climate models, covering the full model hierarchy, and in- −15 −2 ficiency, is 92.5 × 10 yr W m per kg-CO2. This value cluding two large ensembles of simulations by two of the very likely (5 to 95 % confidence) lies within the range of (68 models constrained with observations as well as an ensem- −15 −2 to 117) × 10 yr W m per kg-CO2. Estimates for time- ble of runs of a box model substituting for a suite of more integrated response in CO2 published in the IPCC First, Sec- complex models. This allows us to address model-related un- ond, and Fourth Assessment and our multi-model best esti- certainties by investigating within-model and between-model mate all agree within 15 % during the first 100 yr. The inte- differences. Uncertainties related to the size of the emission grated CO2 response, normalized by the pulse size, is lower pulse, the atmospheric and climatic background conditions for pre-industrial conditions, compared to present day, and or the choice of the future scenario, and the carbon cycle- lower for smaller pulses than larger pulses. In contrast, the climate feedback are assessed in sensitivity simulations. Re- response in temperature, sea level and ocean heat content sults are also compared to CO2 response functions as pub- is less sensitive to these choices. Although, choices in pulse lished in the IPCC First (FAR) (Shine et al., 1990), Second size, background concentration, and model lead to uncertain- (SAR) (Schimel et al., 1996), and Fourth Assessment Report ties, the most important and subjective choice to determine (AR4) (Forster et al., 2007). AGWP of CO2 and GWP is the time horizon. A reevaluation of the CO2 response appears timely as (i) past GWP calculations applied results from a single model and (ii) the atmospheric and climatic conditions influencing the CO2 response continue to change with time. The GWP 1 Introduction adopted for the first commitment period of the Kyoto pro- tocol (2008–2012) (UNFCCC, 1997, 1998) and used for re- Emissions of different greenhouse gases (GHGs) and other porting under the UNFCCC (UNFCCC, 2002) are given by agents that force the climate to change are often compared by the SAR (Schimel et al., 1996) and based on the CO2 re- simplified metrics in economic frameworks, emission trad- sponse of the Bern model (Bern-SAR), an early generation ing and mitigation schemes, and climate policy assessments. reduced-form carbon cycle model (Joos et al., 1996). Its be- The Global Warming Potential (GWP) introduced by the In- haviour was compared to other carbon cycle models in Ent- tergovernmental Panel on Climate Change (IPCC) in 1990 ing et al. (1994) and it was found to be a middle of the (Shine et al., 1990), is the most widely used emission metric. range model. The GWP provided in the AR4 (Forster et al., GWPs are applied for emission reporting under the United 2007) relies on the CO2 response from the Bern2.5CC (here Nations Framework Convention on Climate Change (UN- Bern2.5D-LPJ) Earth System Model of Intermediate Com- FCCC, 2002) and in the emission basket approach of the plexity (EMIC) (Plattner et al., 2008). More recently, the legally-binding Kyoto Protocol (UNFCCC, 1998) to com- Conference of the Parties serving as the meeting of the Par- pare emissions of different GHGs and to compute the so ties to the Kyoto Protocol decided (UNFCCC, 2011b, a) that called “CO2-equivalent” emissions. The initial Kyoto Pro- the GWP from the AR4 should be used for the second com- tocol covered emissions of carbon dioxide (CO2), methane mitment period of the Kyoto Protocol and the Conference (CH4), nitrous oxide (N2O), sulphur hexafluoride (SF6), hy- also noted in its decision that metrics are still being assessed drofluorocarbons (HCFs) and perfluorocarbons (PFCs) in the by IPCC in the context of its Fifth Assessment Report (AR5). first commitment period (2008–2012). The Doha Amend- A much broader set of models covering the whole model hi- ment to the Kyoto Protocol covers emissions in a second erarchy from reduced-form models, to EMICs, to compre- commitment period of 2013–2020 and nitrogen trifluoride hensive Earth System Models (ESMs) are now available. (NF3) is added to the basket of greenhouse gases. The GWP The redistribution of additional CO2 emissions among the compares the radiative forcing (Forster et al., 2007) inte- major carbon reservoirs in the Earth System depends on pre- grated over a time period caused by the emission of 1 kg of vious emissions and on climate. In addition, radiative forc- an agent relative to the integrated forcing caused by the emis- ing of CO2 depends logarithmically on its own concentra- sions of 1 kg CO2. As CO2 is used as a reference gas in the tion. The response functions are calculated by modelling the GWP definition, any changes in the computation of the ra- response to a pulse emission added to a given concentra- diative influence of CO2 affect the GWP of any other agent. tion and climate state, but these background conditions have The purpose of this study is to compute the response in at- changed and will continue to change.