WG1: Chapter 8 Summary IPCC 5 th Assessment Report Working Group 1 Chapter 8 Mindy Nicewonger May 15, 2014 ESS 202 All figures/text are copyright IPCC

Table of Contents/Goal 1.Executive Summary 2.Radiative Forcing 3.Atmospheric Chemistry 4.Present-Day Anthropogenic Radiative Forcing 5.Natural Radiative Forcing Changes: Solar and Volcanic 6.Synthesis of Global Mean Radiative Forcing 7.Geographic Distribution of Radiative Forcing 8.Emission metrics 9.FAQs

Executive Summary

8.1.1 The Radiative Forcing Concept RF from natural and anthropogenic components for 2011 relative to 1750 RF is the net change in energy balance due to imposed perturbation Difficult to observe RF, but allows quantitative basis to compare climate response to different agents

Defining Radiative Forcing Radiative Forcing (RF) “Change in net irradiance at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, while holding surface and tropospheric temperatures and state variables such as water vapor and cloud cover fixed at the unperturbed values” TAR and AR4 Stratospherically adjusted RF = RF in AR5

Defining Effective Radiative Forcing Effective Radiative Forcing (ERF) Considers rapid adjustments that can enhance or reduce flux perturbations that may lead to large differences in the long term forcing Rapid adjustments affect clouds & aerosol interactions Previously termed “fast feedbacks” In AR5 this term not used not the distinction from feedbacks involving surface temp. changes “represents the change in net TOA downward radiative flux after allowing for atmospheric temperatures, water vapor, and clouds to adjust but with the sea surface temperatures (SST) and sea ice cover fixed at climatological values” [Box 8.1] ERF & RF nearly equal Analysis of 11 models from CMIP5 shows the rapid adjustments to CO 2 cause ERF to be 2% less than RF

Figure 8.1| RF and ERF Instantaneous RF Strat. RF Fixed sfc T (ERF) Fixed SST (ERF) Equilibrium response

Box 8.2 Well mixed greenhouse gases (WMGHGs) Concentrations measurements from a remote sites can characterize atmospheric burden May still have local variations near sources & sinks, small hemispheric gradients Sometimes referred to as “long-lived GHGs” CO 2, N 2 O, CH 4, SF 6 – no O 3 Near-term climate forcers (NTCFs) Impact on climate occurs primarily within first decade after emissions Short lifetimes, do not accumulate in atmosphere at decadal to centennial time scales CH 4, O 3, aerosols

8.1.2 Calculation of RF due to Concentration or Emission Changes Concentration based forcing vs. emission based forcing Concentrations depend on: Emission variations Wet/dry removal processes Atmospheric chemistry/removal Transport – CTM (chemical transport models) Emissions depend on: Emissions… ERF from emissions – better informed policy decisions Both provide inside into various drivers of climate change

Skipping… Tropospheric ozone, stratospheric ozone and water vapor, methane, nitrous oxide, halogenated species, aerosols

8.3 Present-Day Anthropogenic RF Human activity Land surface changes  albedo GHGs PI concentrations of WMGHGs from firn or ice core records Chemistry-climate models for heterogeneously mixed agents Ozone, aerosols Studies of RF of GHGs are for clear-sky and aerosol free conditions Clouds reduce magnitude of RF by around 25% [Pg. 676]

8.3.2 WMGHGs AR W/m 2 CO 2, CH 4, CFC-12, N 2 0 Halocarbons contributed W/m 2 to total 10% uncertainties AR W/m 2 (range is 2.54 to 3.12 W/m 2 ) Updated atmospheric concentrations New species included: NF 3 & SO 2 F 2 ERF for CO 2 N 2 0 overtaken CFC-12 as 3 rd largest contributor Figure 8.6: RF from major WMGHGs and halocarbons from 1750 to 2011

Carbon Dioxide CO 2 increased from 278 ppm to 391 ppm (in 2011) RF assessed due to changes in concentration, NOT emissions Fraction of historical emissions still in atmosphere Land use change from in AR4: ppm  0.17 to 0.51 W/m 2 Since AR4: RF of CO 2 increased by 0.16 W/m 2 at rate of 0.3 W/m 2 per decade CO 2 dominates the increase in RF of WMGHGs for last 15 years

Methane CH 4 concentrations rose from 722 ppb to 1803 ppb Rise due to mostly anthropogenic emissions of CH 4 and other compounds that affect CH 4 removal rate (through OH) Natural emissions as well; hard to quantify RF is 0.48 ± 0.05 W/m 2 Increase of 0.01 W/m 2 since AR4

Nitrous Oxide Concentration of N 2 O rose from 270 ppb to 324 ppb 3 rd largest forcing of anthropogenic gases 0.17 ± 0.03 W/m 2 increase in RF  6% since AR4

Other WMGHGs Halocarbons made large contribution to increase of RF Since decreased due to Montreal Protocol – RF still positive RF in 2011: W/m 2 RF in 2005: W/m 2 Growth of HCFCs & halogens (SF 6, SO 2 F 2, NF 3 ) compensates for decline in CFCs

Figure 8.6| RF of WMGHGs and minor gases (a) RF from major WMGHGs and halocarbons (b) Same as above but in log scale (c) RF from minor WMGHGs in log scale (d) Rate of change in forcing from major WMGHGs

Table 8.2| Concentrations and RFs from AR4 to AR5

8.3.3 O 3 & Stratospheric Water Vapor Estimates for ozone are almost entirely model based AR5 total ozone RF: 0.35 W/m 2 (0.15 to 0.55 W/m 2 ) 0.40 W/m 2 for ozone in troposphere (0.2 to 0.6 W/m 2 ) W/m 2 for stratospheric ozone ( to W/m 2 ) AR4 RF estimates: 0.35 W/m 2 for tropospheric ozone ± 0.1 W/m 2

Stratospheric Water Vapor Depends on: Deep convection in tropics Direct injection through volcanic plumes & aircraft In situ production from oxidation of CH 4 & hydrogen AR4 estimates remain unchanged 0.07 ± 0.05 W/m 2 Large uncertainty due to large differences in models & studies

8.3.4 Aerosols and Cloud Effects In AR4: Aerosol-radiation interaction Aerosol-cloud interaction Black carbon (BC) impact on snow and ice surface albedo

RF of Aerosol-Radiation Interaction RF due to scattering and absorption of short & longwave radiation AR5: ± 0.50 W/m 2 Estimate smaller in magnitude than AR4, but larger uncertainty Improvements in observations of aerosols with ground based remote sensing and MODIS satellite Limited aerosol observations back in time Growing constraints from ice & lake core records

RF of Aerosol-Cloud Interactions Previously referred to as the Twomey or cloud albedo effect RF can be calculated, but no estimate is given Does not simply translate to the ERF of aerosol-cloud interactions Difficult to separate A-R and A-C interactions TOTAL ERF due to aerosol-radiation and aerosol-cloud interactions: -0.9 W/m 2 best estimate (range of -1.9 to -0.1 W/m 2 ) This value excludes BC on snow and ice ERF estimate in AR5 lower than AR4 AR4 used GCM that did not take into account secondary processes i.e. aerosol effects on mixed-phase or convective clouds and effects on LW radiation

BC Deposition on Snow and Ice BC on snow & ice greatly decreases the albedo Increases absorption Not easily seen by “naked eye” but climatically is very important 0.04 W/m 2 (range of 0.02 – 0.09 W/m 2 )

Figure 8.8 Max RF in 1980 (BC on snow) Slight increase since % lower RF in 2000

Contrails and Contrail-Induced Cirrus RF due to contrails: 0.01 W/m 2 (range of to 0.03 W/m 2 ) ERF of combined contrail & induced cirrus: 0.05 W/m 2 (range of 0.02 to 0.15 W/m 2 ) Since AR4, evidence increased because of increased observations

8.3.5 Land Surface Changes Direct impact on radiation budget through changes in albedo Impacts climate through modifications of: Surface roughness Latent heat flux River runoff Human activity altering water cycle Deforestation – impacts on WMGHG concentrations

Land Cover Changes Estimates of 42-68% if global land surface impacted by land use activities from [Hurtt et al., 2006] Reconstructions of land use Agriculture technique Historical events: Black Death, war invasions Significant uncertainties in anthropogenic land cover change & the time evolution

Surface Albedo and RF Deforestation tends to increase albedo Cultivation of bright surfaces tends to decrease albedo RF due to surface albedo changes: to W/m 2 Recent advances in satellite monitoring (MODIS) improved surface albedo estimates Deforestation directly impacts atmospheric CO 2 Afforestation is a climate mitigation strategy

Figure 8.9 Land use on climate is more complex than just RF Heterogeneous nature of land use change Hydrological cycle Evapotranspiration, root depth, clouds Forcing is not purely radiative and may vary depending on latitude

8.4 Natural RF Changes: Solar & Volcanic Skipping:

8.4.2 Volcanic RF SO 2 and sulfate aerosols CO 2 from volcanic means are 100X smaller than anthro. emissions Stratospheric sulphate aerosols lifetime ~ 1 year Several ways to measure SO 2 and sulphate aerosols in stratosphere: Balloons, airplanes, ground & satellite based remote sensing (IR & UV) 4 mechanisms forcing influences climate: 1.RF due to aerosol-radiation interaction 2.Differential heating producing gradients & changes in circulation 3.Interactions with other modes of circulation (ENSO) 4.Ozone depletion w/ effects on stratospheric heating (depends on Cl) Stratospheric ozone would increase under low Cl conditions

Figure 8.13 Top: Monthly mean extinction ratio (525 nm) profile evolution in the tropics. Position of each volcanic eruption during this period is displayed with its first two letters on the horizontal axis. Bottom: Mean stratospheric aerosol optical depth (AOD) in the tropics.

8.5 Synthesis of Global Mean RF, Past and Future Table 8.5| Confidence level for the forcing estimate associated with each agent for the period.

Figure 8.14| Development of confidence Figure 8.13| Confidence level of the forcing mechanisms in the last 4 IPCC assessments. Level of scientific understating (LOSU) has been used in previous assessments instead of confidence level.

Table 8.6| RF and ERF Best Estimates RF by WMGHGs increase: 16% since TAR 8% since AR5 Incr. concentrations Other RF agents: Re-evaluations Improved understanding Increased studies and obs. Data Aerosol-radiation & BC RFs decreased in magnitude

Figure 8.15| RF and ERF for Solid bars = ERF Hatched bars = RF

Figure 8.16 | PDF of total ERF Important assumption that forcings can be treated additively Monte Carlo simulation Probability density function (PDF) of ERF due to forcings: Total GHG Total aerosol Total anthropogenic Total ERF over Industrial Era Best estimate = 2.29 W/m 2

Figure 2.20 from AR4 WG1 Figures 8.15 and 8.16 from AR5 WG1

Figure 8.17|RF based on emitted compounds Forcing by emitted compounds View RF by emitted species rather than by change in abundance Number of emitted compounds and changes leading to RF is larger than number of compounds causing RF directly Indirect effects & atmospheric chemistry Ex: CH 4, CO and NMVOC led to excess CO 2

Figure 8.18| Time evolution of forcings CO2 and WMGHG dominate Volcanic eruptions CO2 largest contribution to increased anthropogenic forcing since 1960s Total aerosol ERF (A-R and A-C) strongest negative Minus brief volcanic

Figure 8.19|Linear trends in forcings natural forcing very likely negative Reduced uncertainity in anthropogenic from larger domination of WMGHGs and less contribution from aerosol Anthropogenic forcings dominates

Figure 8.20|RF and ERF from % of total anthroprogenic ERF occurred during this period Close to 1.0 W/m 2 (0.7 to 1.3 W/m 2 ) Major uncertainty in time evolution is aerosols Anthropogenic forcing very likely more positive than the natural RF since 1950

8.5.3 Future RF Thank you.