2 C
`
`hanges in Atmospheric Constituents
`and in Radiative Forcing
`
`Coordinating Lead Authors:
`Piers Forster (UK), Venkatachalam Ramaswamy (USA)
`
`Lead Authors:
`Paulo Artaxo (Brazil), Terje Berntsen (Norway), Richard Betts (UK), David W. Fahey (USA), James Haywood (UK), Judith Lean (USA),
`David C. Lowe (New Zealand), Gunnar Myhre (Norway), John Nganga (Kenya), Ronald Prinn (USA, New Zealand),
`Graciela Raga (Mexico, Argentina), Michael Schulz (France, Germany), Robert Van Dorland (Netherlands)
`
`Contributing Authors:
`G. Bodeker (New Zealand), O. Boucher (UK, France), W.D. Collins (USA), T.J. Conway (USA), E. Dlugokencky (USA), J.W. Elkins (USA),
`D. Etheridge (Australia), P. Foukal (USA), P. Fraser (Australia), M. Geller (USA), F. Joos (Switzerland), C.D. Keeling (USA), R. Keeling (USA),
`S. Kinne (Germany), K. Lassey (New Zealand), U. Lohmann (Switzerland), A.C. Manning (UK, New Zealand), S. Montzka (USA),
`D. Oram (UK), K. O’Shaughnessy (New Zealand), S. Piper (USA), G.-K. Plattner (Switzerland), M. Ponater (Germany),
`N. Ramankutty (USA, India), G. Reid (USA), D. Rind (USA), K. Rosenlof (USA), R. Sausen (Germany), D. Schwarzkopf (USA),
`S.K. Solanki (Germany, Switzerland), G. Stenchikov (USA), N. Stuber (UK, Germany), T. Takemura (Japan), C. Textor (France, Germany),
`R. Wang (USA), R. Weiss (USA), T. Whorf (USA)
`
`Review Editors:
`Teruyuki Nakajima (Japan), Veerabhadran Ramanathan (USA)
`
`This chapter should be cited as:
`Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn,
`G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007:
`The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
`Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University
`Press, Cambridge, United Kingdom and New York, NY, USA.
`
`Page 1 of 106
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`Arkema Exhibit 1105
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`

`
`Changes in Atmospheric Constituents and in Radiative Forcing
`
`Chapter 2
`
`Table of Contents
`
`Executive Summary .................................................... 131
`2.1 Introduction and Scope ................................... 133
`2.2 Concept of Radiative Forcing ....................... 133
`2.3 Chemically and Radiatively
`
`Important Gases ................................................ 137
`2.3.1 Atmospheric Carbon Dioxide .............................. 137
`
`2.3.2 Atmospheric Methane ......................................... 140
`
`2.3.3 Other Kyoto Protocol Gases................................ 143
`
`2.3.4 Montreal Protocol Gases ..................................... 145
`
`2.3.5 Trends in the Hydroxyl Free Radical .................... 147
`
`2.3.6 Ozone .................................................................. 149
`
`2.3.7 Stratospheric Water Vapour ................................ 152
`
`2.3.8 Observations of Long-Lived Greenhouse
`
`Gas Radiative Effects .......................................... 153
`2.4 Aerosols .................................................................. 153
`2.4.1
`Introduction and Summary of the Third
`
`Assessment Report ............................................. 153
`
`2.4.2 Developments Related to Aerosol
`
`Observations ....................................................... 154
`
`2.4.3 Advances in Modelling the Aerosol
`
`Direct Effect ......................................................... 159
`
`2.4.4 Estimates of Aerosol Direct Radiative Forcing .... 160
`
`2.4.5 Aerosol Infl uence on Clouds
`
`(Cloud Albedo Effect) ........................................... 171
`2.5 Anthropogenic Changes in Surface Albedo
`
`and the Surface Energy Budget .................... 180
`2.5.1
`Introduction ......................................................... 180
`
`2.5.2 Changes in Land Cover Since 1750 .................... 182
`
`2.5.3 Radiative Forcing by Anthropogenic Surface
`
`Albedo Change: Land Use .................................. 182
`
`2.5.4 Radiative Forcing by Anthropogenic Surface
`
`Albedo Change: Black Carbon in Snow
`
`and Ice ................................................................. 184
`
`2.5.5 Other Effects of Anthropogenic Changes
`
`in Land Cover ...................................................... 185
`
`2.5.6 Tropospheric Water Vapour from
`
`Anthropogenic Sources ....................................... 185
`
`2.5.7 Anthropogenic Heat Release ............................... 185
`
`2.5.8 Effects of Carbon Dioxide Changes on Climate
`
`via Plant Physiology: ‘Physiological Forcing’ ...... 185
`2.6 Contrails and Aircraft-Induced
`
`Cloudiness ............................................................. 186
`2.6.1
`Introduction ......................................................... 186
`
`130
`
`2.6.2 Radiative Forcing Estimates for Persistent
`
`Line-Shaped Contrails ......................................... 186
`
`2.6.3 Radiative Forcing Estimates for
`
`Aviation-Induced Cloudiness............................... 187
`
`2.6.4 Aviation Aerosols ................................................. 188
`2.7 Natural Forcings ................................................. 188
`2.7.1 Solar Variability .................................................... 188
`
`2.7.2 Explosive Volcanic Activity .................................. 193
`2.8 Utility of Radiative Forcing ............................ 195
`2.8.1 Vertical Forcing Patterns and Surface
`
`Energy Balance Changes .................................... 196
`
`2.8.2 Spatial Patterns of Radiative Forcing .................. 196
`
`2.8.3 Alternative Methods of Calculating
`
`Radiative Forcing ................................................. 196
`
`2.8.4 Linearity of the Forcing-Response
`
`Relationship ......................................................... 197
`
`2.8.5 Effi cacy and Effective Radiative Forcing ............. 197
`
`2.8.6 Effi cacy and the Forcing-Response
`
`Relationship ......................................................... 199
`2.9 Synthesis ................................................................ 199
`2.9.1 Uncertainties in Radiative Forcing ....................... 199
`
`2.9.2 Global Mean Radiative Forcing ........................... 200
`
`2.9.3 Global Mean Radiative Forcing by
`
`Emission Precursor .............................................. 205
`
`2.9.4 Future Climate Impact of Current Emissions ....... 206
`
`2.9.5 Time Evolution of Radiative Forcing and
`
`Surface Forcing ................................................... 208
`
`2.9.6 Spatial Patterns of Radiative Forcing and
`
`Surface Forcing ................................................... 209
`2.10 Global Warming Potentials and Other
`
`Metrics for Comparing Different
`
`Emissions ............................................................... 210
`2.10.1 Defi nition of an Emission Metric and the
`
`Global Warming Potential .................................. 210
`
`2.10.2 Direct Global Warming Potentials ...................... 211
`
`2.10.3 Indirect GWPs .................................................... 214
`
`2.10.4 New Alternative Metrics for Assessing
`
`Emissions ........................................................... 215
`Frequently Asked Question
`
`FAQ 2.1: How Do Human Activities Contribute to Climate
`
`
`Change and How Do They Compare With
`
`
`Natural Infl uences? .............................................. 135
`
`References ........................................................................ 217
`
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`Chapter 2
`
`Changes in Atmospheric Constituents and in Radiative Forcing
`
`Executive Summary
`
`Radiative forcing (RF)1 is a concept used for quantitative
`comparisons of the strength of different human and natural
`agents in causing climate change. Climate model studies since
`the Working Group I Third Assessment Report (TAR; IPCC,
`2001) give medium confi dence that the equilibrium global mean
`temperature response to a given RF is approximately the same
`(to within 25%) for most drivers of climate change.
`For the fi rst time, the combined RF for all anthropogenic
`agents is derived. Estimates are also made for the fi rst time of
`the separate RF components associated with the emissions of
`each agent.
`The combined anthropogenic RF is estimated to be +1.6
`[–1.0, +0.8]2 W m–2, indicating that, since 1750, it is extremely
`likely3 that humans have exerted a substantial warming
`infl uence on climate. This RF estimate is likely to be at least
`fi ve times greater than that due to solar irradiance changes. For
`the period 1950 to 2005, it is exceptionally unlikely that the
`combined natural RF (solar irradiance plus volcanic aerosol)
`has had a warming infl uence comparable to that of the combined
`anthropogenic RF.
`Increasing concentrations of the long-lived greenhouse
`gases (carbon dioxide (CO2), methane (CH4), nitrous oxide
`(N2O), halocarbons and sulphur hexafl uoride (SF6); hereinafter
`LLGHGs) have led to a combined RF of +2.63 [±0.26] W m–2.
`Their RF has a high level of scientifi c understanding.4 The 9%
`increase in this RF since the TAR is the result of concentration
`changes since 1998.
`
`— The global mean concentration of CO2 in 2005 was 379
`ppm, leading to an RF of +1.66 [±0.17] W m–2. Past emissions
`of fossil fuels and cement production have likely contributed
`about three-quarters of the current RF, with the remainder
`caused by land use changes. For the 1995 to 2005 decade, the
`growth rate of CO2 in the atmosphere was 1.9 ppm yr–1 and the
`CO2 RF increased by 20%: this is the largest change observed
`or inferred for any decade in at least the last 200 years. From
`1999 to 2005, global emissions from fossil fuel and cement
`production increased at a rate of roughly 3% yr–1.
`— The global mean concentration of CH4 in 2005 was 1,774
`ppb, contributing an RF of +0.48 [±0.05] W m–2. Over the past
`two decades, CH4 growth rates in the atmosphere have generally
`decreased. The cause of this is not well understood. However,
`
`this decrease and the negligible long-term change in its main
`sink (the hydroxyl radical OH) imply that total CH4 emissions
`are not increasing.
`— The Montreal Protocol gases (chlorofl uorocarbons (CFCs),
`hydrochlorofl uorocarbons (HCFCs), and chlorocarbons) as a
`group contributed +0.32 [±0.03] W m–2 to the RF in 2005. Their
`RF peaked in 2003 and is now beginning to decline.
`— Nitrous oxide continues to rise approximately linearly
`(0.26% yr–1) and reached a concentration of 319 ppb in 2005,
`contributing an RF of +0.16 [±0.02] W m–2. Recent studies
`reinforce the large role of emissions from tropical regions in
`infl uencing the observed spatial concentration gradients.
`— Concentrations of many of the fl uorine-containing Kyoto
`Protocol gases (hydrofl uorocarbons (HFCs), perfl uorocarbons,
`SF6) have increased by large factors (between 4.3 and 1.3)
`between 1998 and 2005. Their total RF in 2005 was +0.017
`[±0.002] W m–2 and is rapidly increasing by roughly 10% yr–1.
`— The reactive gas, OH, is a key chemical species that
`infl uences the lifetimes and thus RF values of CH4, HFCs,
`HCFCs and ozone; it also plays an important role in the
`formation of sulphate, nitrate and some organic aerosol species.
`Estimates of the global average OH concentration have shown
`no detectable net change between 1979 and 2004.
`
`Based on newer and better chemical transport models
`than were available for the TAR, the RF from increases in
`tropospheric ozone is estimated to be +0.35 [–0.1, +0.3]
`W m–2, with a medium level of scientifi c understanding. There
`are indications of signifi cant upward trends at low latitudes.
`The trend of greater and greater depletion of global
`stratospheric ozone observed during the 1980s and 1990s
`is no longer occurring; however, it is not yet clear whether
`these recent changes are indicative of ozone recovery. The
`RF is largely due to the destruction of stratospheric ozone
`by the Montreal Protocol gases and it is re-evaluated to
`be –0.05 [±0.10] W m–2, with a medium level of scientific
`understanding.
`Based on chemical transport model studies, the RF from
`the increase in stratospheric water vapour due to oxidation of
`CH4 is estimated to be +0.07 [± 0.05] W m–2, with a low level
`of scientifi c understanding. Other potential human causes of
`water vapour increase that could contribute an RF are poorly
`understood.
`The total direct aerosol RF as derived from models and
`observations is estimated to be –0.5 [±0.4] W m–2, with a
`
`1 The RF represents the stratospherically adjusted radiative fl ux change evaluated at the tropopause, as defi ned in the TAR. Positive RFs lead to a global mean surface warming
`and negative RFs to a global mean surface cooling. Radiative forcing, however, is not designed as an indicator of the detailed aspects of climate response. Unless otherwise men-
`tioned, RF here refers to global mean RF. Radiative forcings are calculated in various ways depending on the agent: from changes in emissions and/or changes in concentrations,
`and from observations and other knowledge of climate change drivers. In this report, the RF value for each agent is reported as the difference in RF, unless otherwise mentioned,
`between the present day (approximately 2005) and the beginning of the industrial era (approximately 1750), and is given in units of W m–2.
`2 90% confi dence ranges are given in square brackets. Where the 90% confi dence range is asymmetric about a best estimate, it is given in the form A [–X, +Y] where the lower limit
`of the range is (A – X) and the upper limit is (A + Y).
`3 The use of ‘extremely likely’ is an example of the calibrated language used in this document, it represents a 95% confi dence level or higher; ‘likely’ (66%) is another example (See
`Box TS.1).
`4 Estimates of RF are accompanied by both an uncertainty range (value uncertainty) and a level of scientifi c understanding (structural uncertainty). The value uncertainties represent
`the 5 to 95% (90%) confi dence range, and are based on available published studies; the level of scientifi c understanding is a subjective measure of structural uncertainty and
`represents how well understood the underlying processes are. Climate change agents with a high level of scientifi c understanding are expected to have an RF that falls within
`their respective uncertainty ranges (See Section 2.9.1 and Box TS.1 for more information).
`
`131
`
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`

`
`Changes in Atmospheric Constituents and in Radiative Forcing
`
`Chapter 2
`
`the RF remain large. The total solar irradiance, monitored from
`space for the last three decades, reveals a well-established cycle
`of 0.08% (cycle minimum to maximum) with no signifi cant
`trend at cycle minima.
`— Changes (order of a few percent) in globally averaged
`column ozone forced by the solar ultraviolet irradiance 11-year
`cycle are now better understood, but ozone profi le changes are
`less certain. Empirical associations between solar-modulated
`cosmic ray ionization of the atmosphere and globally averaged
`low-level cloud cover remain ambiguous.
`
`The global stratospheric aerosol concentrations in 2005 were
`at their lowest values since satellite measurements began in
`about 1980. This can be attributed to the absence of signifi cant
`explosive volcanic eruptions since Mt. Pinatubo in 1991.
`Aerosols from such episodic volcanic events exert a transitory
`negative RF; however, there is limited knowledge of the RF
`associated with eruptions prior to Mt. Pinatubo.
`The spatial patterns of RFs for non-LLGHGs (ozone, aerosol
`direct and cloud albedo effects, and land use changes) have
`considerable uncertainties, in contrast to the relatively high
`confi dence in that of the LLGHGs. The Southern Hemisphere
`net positive RF very likely exceeds that in Northern Hemisphere
`because of smaller aerosol contributions in the Southern
`Hemisphere. The RF spatial pattern is not indicative of the
`pattern of climate response.
`The total global mean surface forcing5 is very likely negative.
`By reducing the shortwave radiative fl ux at the surface, increases
`in stratospheric and tropospheric aerosols are principally
`responsible for the negative surface forcing. This is in contrast
`to LLGHG increases, which are the principal contributors to the
`total positive anthropogenic RF.
`
`medium-low level of scientifi c understanding. The RF due to the
`cloud albedo effect (also referred to as fi rst indirect or Twomey
`effect), in the context of liquid water clouds, is estimated
`to be –0.7 [–1.1, +0.4] W m–2, with a low level of scientifi c
`understanding.
`
`— Atmospheric models have improved and many now
`represent all aerosol components of signifi cance. Improved in
`situ, satellite and surface-based measurements have enabled
`verifi cation of global aerosol models. The best estimate and
`uncertainty range of the total direct aerosol RF are based on a
`combination of modelling studies and observations.
`— The direct RF of the individual aerosol species is less
`certain than the total direct aerosol RF. The estimates are:
`sulphate, –0.4 [±0.2] W m–2; fossil fuel organic carbon, –0.05
`[±0.05] W m–2; fossil fuel black carbon, +0.2 [±0.15] W m–2;
`biomass burning, +0.03 [±0.12] W m–2; nitrate, –0.1 [±0.1]
`W m–2; and mineral dust, –0.1 [±0.2] W m–2. For biomass
`burning, the estimate is strongly infl uenced by aerosol overlying
`clouds. For the fi rst time best estimates are given for nitrate and
`mineral dust aerosols.
`and
`species
`aerosol
`of more
`Incorporation
`—
`improved treatment of aerosol-cloud
`interactions allow
`a best estimate of the cloud albedo effect. However, the
`uncertainty remains large. Model studies including more
`aerosol species or constrained by satellite observations
`tend
`to yield a relatively weaker RF. Other aspects
`of aerosol-cloud interactions (e.g., cloud lifetime, semi-direct
`effect) are not considered to be an RF (see Chapter 7).
`
`Land cover changes, largely due to net deforestation, have
`increased the surface albedo giving an RF of –0.2 [±0.2]
`W m–2, with a medium-low level of scientifi c understanding.
`Black carbon aerosol deposited on snow has reduced the surface
`albedo, producing an associated RF of +0.1 [±0.1] W m–2, with
`a low level of scientifi c understanding. Other surface property
`changes can affect climate through processes that cannot be
`quantifi ed by RF; these have a very low level of scientifi c
`understanding.
`Persistent linear contrails from aviation contribute an RF
`of +0.01 [–0.007, +0.02] W m–2, with a low level of scientifi c
`understanding; the best estimate is smaller than in the TAR. No
`best estimates are available for the net forcing from spreading
`contrails and their effects on cirrus cloudiness.
`The direct RF due to increases in solar irradiance since 1750
`is estimated to be +0.12 [–0.06, +0.18] W m–2, with a low level
`of scientifi c understanding. This RF is less than half of the TAR
`estimate.
`
`— The smaller RF is due to a re-evaluation of the long-term
`change in solar irradiance, namely a smaller increase from the
`Maunder Minimum to the present. However, uncertainties in
`
`5 Surface forcing is the instantaneous radiative fl ux change at the surface; it is a useful diagnostic tool for understanding changes in the heat and moisture surface budgets.
` However, unlike RF, it cannot be used for quantitative comparisons of the effects of different agents on the equilibrium global mean surface temperature change.
`
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`Chapter 2
`
`Changes in Atmospheric Constituents and in Radiative Forcing
`
`2.1
`
`Introduction and Scope
`
`This chapter updates information taken from Chapters 3
`to 6 of the IPCC Working Group I Third Assessment Report
`(TAR; IPCC, 2001). It concerns itself with trends in forcing
`agents and their precursors since 1750, and estimates their
`contribution to the radiative forcing (RF) of the climate system.
`Discussion of the understanding of atmospheric composition
`changes is limited to explaining the trends in forcing agents and
`their precursors. Areas where signifi cant developments have
`occurred since the TAR are highlighted. The chapter draws
`on various assessments since the TAR, in particular the 2002
`World Meteorological Organization (WMO)-United Nations
`Environment Programme (UNEP) Scientifi c Assessment of
`Ozone Depletion (WMO, 2003) and the IPCC-Technology
`and Economic Assessment Panel (TEAP) special report on
`Safeguarding the Ozone Layer and the Global Climate System
`(IPCC/TEAP, 2005).
`The chapter assesses anthropogenic greenhouse gas changes,
`aerosol changes and their impact on clouds, aviation-induced
`contrails and cirrus changes, surface albedo changes and
`natural solar and volcanic mechanisms. The chapter reassesses
`the ‘radiative forcing’ concept (Sections 2.2 and 2.8), presents
`spatial and temporal patterns of RF, and examines the radiative
`energy budget changes at the surface.
`For the long-lived greenhouse gases (carbon dioxide
`(CO2), methane (CH4), nitrous oxide (N2O), chlorofl uoro-
`carbons
`(CFCs),
`hydrochlorofl uorocarbons
`(HCFCs),
`hydrofl uorocarbons (HFCs), perfl uorocarbons (PFCs) and
`sulphur hexafl uoride (SF6), hereinafter collectively referred
`to as the LLGHGs; Section 2.3), the chapter makes use of
`new global measurement capabilities and combines long-
`term measurements from various networks to update trends
`through 2005. Compared to other RF agents, these trends are
`considerably better quantifi ed; because of this, the chapter does
`not devote as much space to them as previous assessments
`(although the processes involved and the related budgets
`are further discussed in Sections 7.3 and 7.4). Nevertheless,
`LLGHGs remain the largest and most important driver of
`climate change, and evaluation of their trends is one of the
`fundamental tasks of both this chapter and this assessment.
`The chapter considers only ‘forward calculation’ methods
`of estimating RF. These rely on observations and/or modelling
`of the relevant forcing agent. Since the TAR, several studies
`have attempted to constrain aspects of RF using ‘inverse
`calculation’ methods. In particular, attempts have been made
`to constrain the aerosol RF using knowledge of the temporal
`and/or spatial evolution of several aspects of climate. These
`include temperatures over the last 100 years, other RFs, climate
`response and ocean heat uptake. These methods depend on an
`understanding of – and suffi ciently small uncertainties in – other
`aspects of climate change and are consequently discussed in the
`detection and attribution chapter (see Section 9.2).
`Other discussions of atmospheric composition changes and
`their associated feedbacks are presented in Chapter 7. Radiative
`
`forcing and atmospheric composition changes before 1750 are
`discussed in Chapter 6. Future RF scenarios that were presented
`in Ramaswamy et al. (2001) are not updated in this report;
`however, they are briefl y discussed in Chapter 10.
`
`2.2 Concept of Radiative Forcing
`
`The defi nition of RF from the TAR and earlier IPCC
`assessment reports is retained. Ramaswamy et al. (2001) defi ne
`it as ‘the change in net (down minus up) irradiance (solar
`plus longwave; in W m–2) at the tropopause after allowing for
`stratospheric temperatures to readjust to radiative equilibrium,
`but with surface and tropospheric temperatures and state held
`fi xed at the unperturbed values’. Radiative forcing is used to
`assess and compare the anthropogenic and natural drivers of
`climate change. The concept arose from early studies of the
`climate response to changes in solar insolation and CO2, using
`simple radiative-convective models. However, it has proven
`to be particularly applicable for the assessment of the climate
`impact of LLGHGs (Ramaswamy et al., 2001). Radiative
`forcing can be related through a linear relationship to the
`global mean equilibrium temperature change at the surface
`(ΔTs): ΔTs = λRF, where λ is the climate sensitivity parameter
`(e.g., Ramaswamy et al., 2001). This equation, developed from
`these early climate studies, represents a linear view of global
`mean climate change between two equilibrium climate states.
`Radiative forcing is a simple measure for both quantifying
`and ranking the many different infl uences on climate change;
`it provides a limited measure of climate change as it does not
`attempt to represent the overall climate response. However, as
`climate sensitivity and other aspects of the climate response
`to external forcings remain inadequately quantifi ed, it has the
`advantage of being more readily calculable and comparable
`than estimates of the climate response. Figure 2.1 shows how
`the RF concept fi ts within a general understanding of climate
`change comprised of ‘forcing’ and ‘response’. This chapter
`also uses the term ‘surface forcing’ to refer to the instantaneous
`perturbation of the surface radiative balance by a forcing
`agent. Surface forcing has quite different properties than RF
`and should not be used to compare forcing agents (see Section
`2.8.1). Nevertheless, it is a useful diagnostic, particularly for
`aerosols (see Sections 2.4 and 2.9).
`Since the TAR a number of studies have investigated the
`relationship between RF and climate response, assessing the
`limitations of the RF concept; related to this there has been
`considerable debate whether some climate change drivers are
`better considered as a ‘forcing’ or a ‘response’ (Hansen et al.,
`2005; Jacob et al., 2005; Section 2.8). Emissions of forcing
`agents, such as LLGHGs, aerosols and aerosol precursors,
`ozone precursors and ozone-depleting substances, are the more
`fundamental drivers of climate change and these emissions can
`be used in state-of-the-art climate models to interactively evolve
`forcing agent fi elds along with their associated climate change.
`In such models, some ‘response’ is necessary to evaluate the
`
`133
`
`Page 5 of 106
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`

`
`Changes In Atmospheric Constituents and In Radiative Forclng
`
`Chapter 2
`
`RF. This ‘response‘ is 111ost significant for aerosol-
`related cloud changes. where the tropospheric
`state needs to change significantly ir1 order to
`create a radiative perturbation of the climate
`system (Jacob et al.. 2005).
`Over the palaeoclinrate time scales that are
`discussed ir1 Chapter 6.
`long-ter111 changes ir1
`forcing agents arise due to so-called ‘boundary
`condition‘ changes to the Earth‘s climate system
`(such as changes i11 orbital parameters. ice sheets
`and continents). For the purposes of this chapter.
`these ‘boundary conditions’ are assumed to be
`invariant and forcing agent changes are considered
`to be external to the climate system. The natural
`RFs considered are solar changes and volcanoes:
`the other RF agents are all attributed to l1r1111ans.
`For the LLGHGs it
`is appropriate to assume
`that forcing agent concentrations have not been
`significantly altered by biogeochemical responses
`(see Sections 7.3 a11d 7.4). a11d RF is typically
`calculated in off-line radiative transfer schemes.
`
`using observed changes in concentration (i.e..
`humans are considered solely responsible for their
`increase). For the other climate change drivers. RF
`is often estimated using general circulation model
`(GCM) data employing a variety of methodologies
`(Ramaswamy et al.. 2001: Stuber et al.. 2001b:
`Tett et al.. 2002; Shine et al.. 2003; Hansen et
`
`Components of the Climate Change Process
`
`Direct and indirect changes in
`climate change drivers
`(a.g., greenhouse gases. aerosots.
`cloud mlcroplryslcs. and oolarlrndlance)
`
`weather events)
`
`Radiative forcing
`
`Non-inlt|ai-
`radiative
`efleds
`
`Clmete Perturbation and Response
`(e.g., global and regional temperatures
`Indpredplhfion, vugelufion, oxtnrno
`
`Figure 2.1 . Diagram illustrating how RF is linked to other aspects of climate change assessed
`by the /PCC. Human activities and natural processes cause direct and indirect changes in climate
`change drivers. In general, these changes result in specific RF changes, eitherpositive or negative,
`and cause some non -initia/ radiative effects. such as changes in evaporation. Radiative forcing and
`non-initial radiative effects lead to climate perturbations and responses as discussed in Chapters 6,
`7 and 8. Attribution of climate change to natural and anthropogenic factors is discussed in Chapter
`9. The coupling among biogeochemical processes leads to feedbacks from climate change to its
`drivers (Chapter 7). An example of this is the change in wetland emissions of CH, that may occur in
`a warmer dimate. The potential approaches to mitigating climate change by altering human activi-
`ties (dashed lines) are topiw addressed by IPCC’s Working Group III.
`
`al.. 2005: Section 2.8.3). Often. alternative RF calculation
`methodologies that do not directly follow the TAR definition of
`a stratospheric-adjusted RF are used: the most important ones
`are illustrated ir1 Figure 2.2. For r11ost aerosol constituents (see
`Section 2.4). stratospheric adjustment has little effect on the RF.
`and the instantaneous RF at either the top of the atmosphere
`or the tropopause can be substituted. For the climate change
`drivers discussed ir1 Sections 7.5 ar1d 2.5. that are not initially
`radiative in nature. an RF-like quantity can be evaluated by
`
`this is the zero-
`allowing the tropospheric state to change:
`surface-ternperature-change RF ir1 Figure 2.2 (see Shine et al..
`2003: Hansen et al.. 2005; Section 2.8.3). Other water vapour
`and cloud changes are considered climate feedbacks ar1d are
`evaluated i11 Section 8.6.
`
`in the
`changes
`require
`that
`agents
`change
`Climate
`tropospheric state (temperature and/or water vapour amounts)
`prior to causing a radiative perturbation are aerosol-cloud
`lifetime effects. aerosol serni-direct effects and so111e surface
`
`lnstantaneous RF
`
`Stratospheric-
`adjusted RF
`
`Zero-surface
`
`temperature-change RF
`
`Equilibrium
`climate response
`
`fixed at surface
`
`RF = net flux imbalance
`at tropopause
`
`temperature fixed
`everywhere
`
`Stratospheric tem-
`peratures adjust
`
`temperature fixed in
`troposphere and at
`surface
`
`Atmospheric
`temperatures adjust
`
`tem perature
`
`No flux imbalance
`
`temperalures
`adjust everywhere
`
`Figure 2.2. Schema tic comparing RF calculation methodologies. Radiative forcing. defined as the net fiux imbalance at the tropopause, is shown by an arrow. The horizontal
`lines represent the surface (lo wer line) and tropopause (upper line). The unperturbed temperature profile is shown as the blue line and the perturbed temperature profile as
`the orange line. From left to right: instantaneous RF: atmospheric temperatures are fixed everywhere; stratospher1'c~adjusted RF: allows stratospheric temperatures to adjust;
`zero-surface-temperature -change RF: allows atmospheric temperatures to adjust everywhere with surface temperatures fixed; and equilibrium climate response: allows the
`atmospheric and surface temperatures to adjust to reach equilibrium (no tropopause flux imbalance), giving a surface temperature change (A T5).
`
`134
`
`Page 6 of 106
`
`

`
`Chapter 2
`
`Changes in Atmospheric Constituents and in Radiative Forcing
`
`Frequently Asked Question 2.1
`How do Human Activities Contribute to Climate Change
`and How do They Compare with Natural Influences?
`
`Human activities contribute to climate change by causing
`changes in Earth’s atmosphere in the amounts of greenhouse gas-
`es, aerosols (small particles), and cloudiness. The largest known
`contribution comes from the burning of fossil fuels, which releases
`carbon dioxide gas to the atmosphere. Greenhouse gases and aero-
`sols affect climate by altering incoming solar radiation and out-
`going infrared (thermal) radiation that are part of Earth’s energy
`balance. Changing the atmospheric abundance or properties of
`these gases and particles can lead to a warming or cooling of the
`climate system. Since the start of the industrial era (about 1750),
`the overall effect of human activities on climate has been a warm-
`ing infl uence. The human impact on climate during this era greatly
`exceeds that due to known changes in natural processes, such as
`solar changes and volcanic eruptions.
`
`Greenhouse Gases
`
`Human activities result in emissions of four principal green-
`house gases: carbon dioxide (CO2), methane (CH4), nitrous oxide
`(N2O) and the halocarbons (a group of gases containing fl uorine,
`chlorine and bromine). These gases accumulate in the atmosphere,
`causing concentrations to increase with time. Signifi cant increases
`in all of these gases have occurred in the industrial era (see Figure
`1). All of these increases are attributable to human activities.
`
`• Carbon dioxide has increased from fossil fuel use in transpor-
`tation, building heating and cooling and the manufacture of
`cement and other goods. Deforestation releases CO2 and re-
`duces its uptake by plants. Carbon d