Chapter 11 IPCC WGI Fifth Assessment Report

Chapter 11 IPCC WGI Fifth Assessment Report

First Order Draft Chapter 11 IPCC WGI Fifth Assessment Report 1 2 Chapter 11: Near-term Climate Change: Projections and Predictability 3 4 Coordinating Lead Authors: Ben Kirtman (USA), Scott Power (Australia) 5 6 Lead Authors: Akintayo John Adedoyin (Botswana), George Boer (Canada), Roxana Bojariu (Romania), 7 Ines Camilloni (Argentina), Francisco Doblas-Reyes (Spain), Arlene Fiore (USA), Masahide Kimoto 8 (Japan), Gerald Meehl (USA), Michael Prather (USA), Abdoulaye Sarr (Senegal), Christoph Schaer 9 (Switzerland), Rowan Sutton (UK), Geert Jan van Oldenborgh (Netherlands), Gabriel Vecchi (USA), Hui- 10 Jun Wang (China) 11 12 Contributing Authors: Nathan Bindoff, Yoshimitsu Chikamoto, Javier García-Serrano, Paul Ginoux, 13 Lesley Gray, Ed Hawkins, Marika Holland, Christopher Holmes, Masayoshi Ishii, Thomas Knutson, David 14 Lawrence, Jian Lu, Vaishali Naik, Lorenzo Polvani, Alan Robock, Luis Rodrigues, Doug Smith, Steve 15 Vavrus, Apostolos Voulgarakis, Oliver Wild, Tim Woollings 16 17 Review Editors: Pascale Delecluse (France), Tim Palmer (UK), Theodore Shepherd (Canada), Francis 18 Zwiers (Canada) 19 20 Date of Draft: 16 December 2011 21 22 Notes: TSU Compiled Version 23 24 25 Table of Contents 26 27 Executive Summary..........................................................................................................................................2 28 11.1 Introduction ..............................................................................................................................................7 29 Box 11.1: Climate Prediction, Projection and Predictability .......................................................................7 30 11.2 Uncertainty and Signal-to-Noise in Near-Term Climate Predictions and Projections......................9 31 11.2.1 Uncertainty in Near-Term Climate Projections..............................................................................9 32 11.2.2 Uncertainty in Near-Term Climate Predictions............................................................................10 33 11.3 Near-Term Predictions...........................................................................................................................11 34 11.3.1 Decadal Climate Prediction..........................................................................................................11 35 11.3.2 Prediction Quality.........................................................................................................................14 36 11.4 Near Term Projections...........................................................................................................................19 37 11.4.1 Introduction...................................................................................................................................19 38 11.4.2 Atmosphere and Land Surface ......................................................................................................20 39 11.4.3 Atmospheric Composition and Air Quality...................................................................................30 40 11.4.4 Near-Term Projections of Oceanic Conditions ............................................................................36 41 11.4.5 Cryosphere....................................................................................................................................39 42 11.4.6 Sensitivity of Near-Term Climate to Anthropogenic Emissions and Land-Use............................41 43 11.4.7 The Potential for Surprises in Near-Term Climate.......................................................................44 44 FAQ 11.1: If You cannot Predict the Weather Next Month, How can You Predict Climate for the Next 45 Decades? ..................................................................................................................................................45 46 FAQ 11.2: How do Volcanic Eruptions Affect Climate and Our Ability to Predict Climate?................46 47 References........................................................................................................................................................49 48 Tables...............................................................................................................................................................66 49 Figures .............................................................................................................................................................67 50 Do Not Cite, Quote or Distribute 11-1 Total pages: 102 First Order Draft Chapter 11 IPCC WGI Fifth Assessment Report 1 Executive Summary 2 3 [A major focus of this chapter will be an assessment of research based on analyses of CMIP5 projections and 4 predictions. Extremely little is currently available. Consequently the plots presented that are based on CMIP5 5 models are preliminary, and in some cases (e.g., runoff) may be unreliable estimates of Second Order Draft 6 plots that will be based on a much larger number of CMIP5 models. In many cases analyses of CMIP5 7 output has not been performed to allow key statistics identified below to be calculated.] 8 9 How will climate evolve over the next few decades? In order to address this important question this chapter 10 assesses the literature describing projected (when the climate is most influenced by changes in external 11 forcing such as increasing greenhouse gases) and predicted (when the climate is most influenced by the time 12 evolution of processes related to the observed initial state) changes in climate during the “near term”. “Near 13 term” refers to future decades up to mid-century, the period for which the climate response to different future 14 emissions scenarios are generally similar. Greatest emphasis is given to the period 2016–2035, though some 15 information on projected changes before and after this period (up to mid-century) is assessed. We consider 16 the mechanisms responsible for near-term changes in climate, the degree of “predictability” (as defined 17 below) evident in the climate system, and the processes underpinning predictability. The size of the 18 externally-forced signal relative to internal variability and the degree to which externally-forced near-term 19 changes will emerge from internally generated natural variability are also discussed. 20 21 The near-term projections discussed in this chapter complement the mid-century and longer-term projections 22 presented in Chapters 12 and 14. Projected changes in sea level are presented in Chapter 13. 23 24 This assessment evaluates new emission scenarios for climate change, the four Representative Concentration 25 Pathways (i.e., RCP2.6, RCP4.5, RCP6.0, and RCP8.5), which were developed for, but independently of, 26 this IPCC assessment. The SRES scenarios, developed as part of the IPCC process and used in the TAR and 27 AR4, were mapped from emissions to greenhouse gas abundances to radiative forcing to climate change 28 based on the IPCC assessments. The RCPs use a single parametric box-model for this mapping. The AR5 29 climate model simulations (CMIP5, ACCMIP) use the abundances of the long-lived greenhouse gases as 30 modelled by the RCPs. But even for the same scenario, the CMIP5 models project RF trajectories differing 31 by ±X% (tbd) because of their individual treatment of atmospheric chemistry and aerosols. The UNFCCC 32 reporting and mitigation strategies are emissions-based, and application of current best knowledge of 33 atmospheric chemistry and biogeochemistry using globally resolved models including natural emissions and 34 feedbacks (as in CMIP5 and ACCMIP) projects greenhouse gases and radiative forcing for the RCP 35 emissions that are systematically larger/smaller than those used in CMIP5 by Y% to Z% (tbd), although the 36 uncertainties in RF, typically ±W% (tbd) encompass the results. In terms of the short-lived species (e.g., 37 NOx, CO, NMVOC, NH3, SOx, BC and OC aerosols) that control tropospheric ozone, aerosols and the loss 38 rate of CH4 and HFCs, the RCP emissions are unusually low and do not fully represent the range of possible 39 futures affecting air quality and CH4 abundance. 40 41 For the near term, the range in anthropogenic RF of the RCPs is similar across the RCPs and close to that of 42 the SRES scenarios, but after 2040 RCP2.6 drops well below the lowest SRES used in AR4. Thus the range 43 of new CMIP5 results are similar to those in AR4 for the near term. Considering that RCP2.6 involves major 44 mitigation of greenhouse gas emissions in attempting to keep global mean surface warming below 2°C, the 45 range of continued near-term warming under this pathway is narrow. 46 47 Variations from these paths will be strongly influenced by natural variability. The magnitude of externally- 48 forced near-term changes relative to the magnitude of internally generated natural variability varies from 49 quantity to quantity and from location to location. 50 51 The majority of the information below comes from AOGCM integrations that were performed as part of a 52 major international activity called the Coupled Model Intercomparison Project phase 5 CMIP5 (Taylor et al., 53 2008; 2011). New results on projections corroborate key results in the Fourth Assessment Report (AR4) for 54 the near term. We are already “committed” to further externally forced near-term warming due to past 55 emissions, this warming will be reinforced by emissions over the near term, and the climate we observe in 56 the near term will also be strongly influenced by

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