Editing IPCC:AR6/WGI/Chapter-7 (section)

== 7.1 Introduction, Conceptual Framework, and Advances Since the Fifth Assessment Report ==

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This chapter assesses the major physical processes that affect the evolution of Earth’s energy budget and the associated changes in surface temperature and the broader climate system, integrating elements that were dealt with separately in previous reports.

The top-of-atmosphere (TOA) energy budget determines the net amount of energy entering or leaving the climate system. Its time variations can be monitored in three ways, using: (i) satellite observations of the radiative fluxes at the TOA; (ii) observations of the accumulation of energy in the climate system; and (iii) observations of surface energy fluxes. When the TOA energy budget is changed by a human or natural cause (a ‘radiative forcing’), the climate system responds by warming or cooling (i.e., the system gains or loses energy). Understanding of changes in the Earth’s energy flows helps understanding of the main physical processes driving climate change. It also provides a fundamental test of climate models and their projections.

This chapter principally builds on the IPCC Fifth Assessment Report (AR5; [[#Boucher--2012|Boucher, 2012]] ; [[#Church--2013|Church et al., 2013]] ; M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; [[#Flato--2013|Flato et al., 2013]] ; [[#Hartmann--2013|Hartmann et al., 2013]] ; [[#Myhre--2013b|Myhre et al., 2013b]] ; [[#Rhein--2013|Rhein et al., 2013]] ). It also builds on the subsequent IPCC Special Report on Global Warming of 1.5°C (SR1.5; [[#IPCC--2018|IPCC, 2018]] ), the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#IPCC--2019a|IPCC, 2019a]] ) and the Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (SRCCL; [[#IPCC--2019b|IPCC, 2019b]] ), as well as community-led assessments (e.g., [[#Bellouin--2020|Bellouin et al. (2020)]] covering aerosol radiative forcing and Sherwood et al. (2020) covering equilibrium climate sensitivity).

Throughout this chapter, global surface air temperature (GSAT) is used to quantify surface temperature change (Cross-Chapter Box 2.3 and ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.4|Section 4.3.4]] ). The total energy accumulation in the Earth system represents a metric of global change that is complementary to GSAT but shows considerably less variability on interannual-to-decadal time scales ( [[#7.2.2|Section 7.2.2]] ). Research and new observations since AR5 have improved scientific confidence in the quantification of changes in the global energy inventory and corresponding estimates of Earth’s energy imbalance ( [[#7.2|Section 7.2]] ). Improved understanding of adjustments to radiative forcing and of aerosol–cloud interactions have led to revisions of forcing estimates ( [[#7.3|Section 7.3]] ). New approaches to the quantification and treatment of feedbacks ( [[#7.4|Section 7.4]] ) have improved the understanding of their nature and time-evolution, leading to a better understanding of how these feedbacks relate to equilibrium climate sensitivity (ECS). This has helped to reconcile disparate estimates of ECS from different lines of evidence ( [[#7.5|Section 7.5]] ). Innovations in the use of emissions metrics have clarified the relationships between metric choice and temperature policy goals ( [[#7.6|Section 7.6]] ), linking this chapter to WGIII which provides further information on metrics, their use, and policy goals beyond temperature. ''Very likely'' (5–95%) ranges are presented unless otherwise indicated. In particular, the addition of ‘(one standard deviation)’ indicates that the range represents one standard deviation.

In Box 7.1 an energy budget framework is introduced, which forms the basis for the discussions and scientific assessment in the remainder of this chapter and across the Report. The framework reflects advances in the understanding of the Earth system response to climate forcing since the publication of AR5. A schematic of this framework and the key changes relative to the science reported in AR5 are provided in Figure 7.1.

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'''Figure 7.1''' '''|''' '''Visual guide to Chapter 7.''' Panel '''(a)''' Overview of the chapter.

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'''Figure 7.1:''' Panel '''(b)''' Visual abstract of the chapter, illustrating why the Earth’s energy budget matters and how it relates to the underlying chapter assessment. The methods used to assess processes and key new findings relative to AR5 are highlighted. Upper schematic adapted from Von Schuckmann et al. (2020).

A simple way to characterize the behaviour of multiple aspects of the climate system at once is to summarize them using global-scale metrics. This Report distinguishes between ‘climate metrics’ (e.g., ECS, TCR) and ‘emissions metrics’ (e.g., global warming potential, GWP, or global temperature-change potential, GTP), but this distinction is not definitive. Climate metrics are generally used to summarize aspects of the surface temperature response (Box 7.1). Emissions metrics are generally used to summarize the relative effects of emissions of different forcing agents, usually greenhouse gases (GHGs; [[#7.6|Section 7.6]] ). The climate metrics used in this report typically evaluate how the Earth system response varies with atmospheric gas concentration or change in radiative forcing. Emissions metrics evaluate how radiative forcing or a key climate variable (such as GSAT) is affected by the emissions of a certain amount of gas. Emissions-related metrics are sometimes used in mitigation policy decisions such as trading GHG reduction measures and life cycle analysis. Climate metrics are useful to gauge the range of future climate impacts for adaptation decisions under a given emissions pathway. Metrics such as the transient climate response to cumulative emissions of carbon dioxide (TCRE) are used in both adaptation and mitigation contexts: for gauging future global surface temperature change under specific emissions scenarios, and to estimate remaining carbon budgets that are used to inform mitigation policies ( [[IPCC:Wg1:Chapter:Chapter-5#5.5|Section 5.5]] ).

Given that TCR and ECS are metrics of GSAT response to a theoretical doubling of atmospheric CO <sub>2</sub> (Box 7.1), they do not directly correspond to the warming that would occur under realistic forcing scenarios that include time-varying CO <sub>2</sub> concentrations and non-CO <sub>2</sub> forcing agents (such as aerosols and land-use changes). It has been argued that TCR, as a metric of transient warming, is more policy-relevant than ECS ( [[#Frame--2006|Frame et al., 2006]] ; [[#Schwartz--2018|Schwartz, 2018]] ). However, as detailed in Chapter 4, both established and recent results ( [[#Forster--2013|Forster et al., 2013]] ; [[#Gregory--2015|Gregory et al., 2015]] ; [[#Marotzke--2015|Marotzke and Forster, 2015]] ; [[#Grose--2018|Grose et al., 2018]] ; [[#Marotzke--2019|Marotzke, 2019]] ) indicate that TCR and ECS help explain variation across climate models both over the historical period and across a range of concentration-driven future scenarios. In emission-driven scenarios the carbon cycle response is also important ( [[#Smith--2019|Smith et al., 2019]] ). The proportion of variation explained by ECS and TCR varies with scenario and the time period considered, but both past and future surface warming depend on these metrics ( [[#7.5.7|Section 7.5.7]] ).

Regional changes in temperature, rainfall, and climate extremes have been found to correlate well with the forced changes in GSAT within Earth System Models (ESMs; [[IPCC:Wg1:Chapter:Chapter-4#4.6.1|Section 4.6.1]] ; [[#Giorgetta--2013|Giorgetta et al., 2013]] ; [[#Tebaldi--2014|Tebaldi and Arblaster, 2014]] ; [[#Seneviratne--2016|Seneviratne et al., 2016]] ). While this so-called ‘pattern scaling’ has important limitations arising from, for instance, localized forcings, land-use changes, or internal climate variability ( [[#Deser--2012|Deser et al., 2012]] ; [[#Luyssaert--2014|Luyssaert et al., 2014]] ), changes in GSAT nonetheless explain a substantial fraction of inter-model differences in projections of regional climate changes over the 21st century ( [[#Tebaldi--2018|Tebaldi and Knutti, 2018]] ). This Chapter’s assessments of TCR and ECS thus provide constraints on future global and regional climate change (Chapters 4 and 11).

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'''Box 7.1 | The Energy Budget Framework: Forcing and Response'''

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The forcing and response energy budget framework provides a methodology to assess the effect of individual drivers of global surface temperature response, and to facilitate the understanding of the key phenomena that set the magnitude of this temperature response. The framework used here is developed from that adopted in previous IPCC reports (see [[#Ramaswamy--2019|Ramaswamy et al., 2019]] for a discussion). '''Effective Radiative Forcing (ERF)''' , introduced in AR5 ( [[#Boucher--2013|Boucher et al., 2013]] ; [[#Myhre--2013b|Myhre et al., 2013b]] ) is more explicitly defined in this Report and is employed as the central definition of radiative forcing ( [[#Sherwood--2015|Sherwood et al., 2015]] , Box 7.1, Figure 1a). The framework has also been extended to allow variations in feedbacks over different time scales and with changing climate state (Sections 7.4.3 and 7.4.4).

The global surface air temperature (GSAT) response to perturbations that give rise to an energy imbalance is traditionally approximated by the following linear energy budget equation, in which Δ ''N'' represents the change in the top-of-atmosphere (TOA) net energy flux, Δ ''F'' is an '''effective radiative forcing''' perturbation to the TOA net energy flux, α is the net '''feedback parameter''' and Δ ''T'' is the change in '''GSAT''' :

Δ ''N ='' Δ ''F +'' α Δ ''T''

ERF is the TOA energy budget change resulting from the perturbation, excluding any radiative response related to a change in GSAT (i.e., Δ ''T'' = 0). Climate feedbacks ( α ) represent those processes that change the TOA energy budget in response to a given Δ ''T'' .

The '''effective radiative forcing, ERF''' (Δ ''F'' ; units: W m <sup>–2</sup> ) quantifies the change in the net TOA energy flux of the Earth system due to an imposed perturbation (e.g., changes in greenhouse gas or aerosol concentrations, in incoming solar radiation, or land-use change). ERF is expressed as a change in net downward radiative flux at the TOA following adjustments in both tropospheric and stratospheric temperatures, water vapour, clouds, and some surface properties, such as surface albedo from vegetation changes, that are uncoupled to any GSAT change ( [[#Smith--2018b|Smith et al., 2018b]] ). These adjustments affect the TOA energy balance and hence the ERF. They are generally assumed to be linear and additive ( [[#7.3.1|Section 7.3.1]] ). Accounting for such processes gives an estimate of ERF that is more representative of the climate change response associated with forcing agents than stratospheric-temperature-adjusted radiative forcing (SARF) or the instantaneous radiative forcing (IRF; [[#7.3.1|Section 7.3.1]] ). Adjustments are processes that are independent of GSAT change, whereas feedbacks refer to processes caused by GSAT change. Although adjustments generally occur on time scales of hours to several months, and feedbacks respond to ocean surface temperature changes on time scales of a year or more, time scale is not used to separate the definitions. ERF has often been approximated as the TOA energy balance change due to an imposed perturbation in climate model simulations with sea surface temperature and sea-ice concentrations set to their pre-industrial climatological values (e.g., [[#Forster--2016|Forster et al., 2016]] ). However, to match the adopted forcing–feedback framework, the small effects of any GSAT change from changes in land surface temperatures need to be removed from the TOA energy balance in such simulations to give an approximate measure of ERF (Box 7.1, Figure 1b and ( [[#7.3.1|Section 7.3.1]] ).

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'''Box 7.1, Figure 1'''

'''|'''

'''Schematics of the forcing–feedback framework adopted within the assessment, following Equation 7.1.''' The figure illustrates how the Earth’s top-of-atmosphere (TOA) net energy flux might evolve for a hypothetical doubling of atmospheric CO <sub>2</sub> concentration above pre-industrial levels, where an initial positive energy imbalance (energy entering the Earth system, shown on the y-axis) is gradually restored towards equilibrium as the surface temperature warms (shown on the x-axis). '''(a)''' illustrates the definitions of effective radiative forcing (ERF) for the special case of a doubling of atmospheric CO <sub>2</sub> concentration, the feedback parameter and the equilibrium climate sensitivity (ECS). '''(b)''' illustrates how approximate estimates of these metrics are made within the chapter and how these approximations might relate to the exact definitions adopted in panel (a).
The '''feedback parameter,''' α (units: W m <sup>–2</sup> °C <sup>–1</sup> ) quantifies the change in net energy flux at the TOA for a given change in GSAT. Many climate variables affect the TOA energy budget, and the feedback parameter can be decomposed, to first order, into a sum of terms

[[File:878eaf1b7b3ef7749dc267f37350868d IPCC_AR6_WGI_Formula_Chapter_7_Box_1_1.jpg]]

where ''x'' represents a variable of the Earth system that has a direct effect on the energy budget at the TOA. The sum of the feedback terms (i.e., α in Equation 7.1) governs Earth’s equilibrium GSAT response to an imposed ERF. In previous assessments, α and the related ECS have been associated with a distinct set of physical processes (Planck response and changes in water vapour, lapse rate, surface albedo, and clouds; [[#Charney--1979|Charney et al., 1979]] ). In this assessment, a more general definition of α and ECS is adopted such that they include additional Earth system processes that act across many time scales (e.g., changes in natural aerosol emissions or vegetation). Because, in our assessment, these additional processes sum to a near-zero value, including these additional processes does not change the assessed central value of ECS but does affect its assessed uncertainty range ( [[#7.4.2|Section 7.4.2]] ). Note that there is no standardized notation or sign convention for the feedback parameter in the literature. Here the convention is used that the sum of all feedback terms (the net feedback parameter, α ) is negative for a stable climate that radiates additional energy to space with a GSAT increase, with a more negative value of α corresponding to a stronger radiative response and thus a smaller GSAT change required to balance a change in ERF (Equation 7.1).

A change in process ''x'' amplifies the temperature response to a forcing when the associated feedback parameter α x is positive (positive feedback) and dampens the temperature response when α x is negative (negative feedback). New research since AR5 emphasizes how feedbacks can vary over different time scales ( [[#7.4.4|Section 7.4.4]] ) and with climate state ( [[#7.4.3|Section 7.4.3]] ), giving rise to the concept of an ‘effective feedback parameter’ that may be different from the equilibrium value of the feedback parameter governing ECS ( [[#7.4.3|Section 7.4.3]] ).

The '''equilibrium climate sensitivity, ECS''' (units: °C), is defined as the equilibrium value of Δ ''T'' in response to a sustained doubling of atmospheric CO <sub>2</sub> concentration from a pre-industrial reference state. The value of ERF for this scenario is denoted by Δ ''F'' 2xCO2 , giving ECS = –Δ ''F'' 2xCO2 / α from Equation 7.1 applied at equilibrium (Box 7.1, Figure 1a and ( [[#7.5|Section 7.5]] ). ‘Equilibrium’ refers to a steady state where Δ ''N'' averages to zero over a multi-century period. ECS is representative of the multi-century to millennial Δ ''T'' response to Δ ''F'' 2xCO2 , and is based on a CO <sub>2</sub> concentration change so any feedbacks that affect the atmospheric concentration of CO <sub>2</sub> do not influence its value. As employed here, ECS also excludes the long-term response of the ice sheets ( [[#7.4.2.6|Section 7.4.2.6]] ) which may take multiple millennia to reach equilibrium, but includes all other feedbacks. Due to a number of factors, studies rarely estimate ECS or α at equilibrium or under CO <sub>2</sub> forcing alone. Rather, they give an ‘effective feedback parameter’ ( [[#7.4.1|Section 7.4.1]] and Box 7.1, Figure 1b) or an ‘effective ECS’ ( [[#7.5.1|Section 7.5.1]] and Box 7.1, Figure 1b), which represent approximations to the true values of α or ECS. The ‘effective ECS’ represents the equilibrium value of Δ ''T'' in response to a sustained doubling of atmospheric CO <sub>2</sub> concentration that would occur assuming the ‘effective feedback parameter’ applied at that equilibrium state. For example, a feedback parameter can be estimated from the linear slope of Δ ''n'' against Δ ''T'' over a set number of years within ESM simulations of an abrupt doubling or quadrupling of atmospheric CO <sub>2</sub> (2×CO <sub>2</sub> or 4×CO <sub>2</sub> , respectively), and the ECS can be estimated from the intersect of this regression line with Δ ''N = 0'' (Box 7.1, Figure 1b). To infer ECS from a given estimate of effective ECS necessitates that assumptions are made for how ERF varies with CO <sub>2</sub> concentration ( [[#7.3.2|Section 7.3.2]] ) and how the slope of Δ ''N'' against Δ ''T'' relates to the slope of the straight line from ERF to ECS ( [[#7.5|Section 7.5]] and Box 7.1, Figure 1b). Care has to be taken when comparing results across different lines of evidence to translate their estimates of the effective ECS into the ECS definition used here ( [[#7.5.5|Section 7.5.5]] ).

The '''transient climate response, TCR''' (units: °C), is defined as the Δ ''T'' for the hypothetical scenario in which CO <sub>2</sub> increases at 1% yr <sup>–1</sup> from a pre-industrial reference state to the time of a doubling of atmospheric CO <sub>2</sub> concentration (year 70; [[#7.5|Section 7.5]] ). TCR is based on a CO <sub>2</sub> concentration change, so any feedbacks that affect the atmospheric concentration of CO <sub>2</sub> do not influence its value. It is a measure of transient warming accounting for the strength of climate feedbacks and ocean heat uptake. The '''transient climate response to cumulative emissions of carbon dioxide (TCRE)''' is defined as the transient Δ ''T'' per 1000 Gt C of cumulative CO <sub>2</sub> emissions increase since the pre-industrial period. TCRE combines information on the airborne fraction of cumulative CO <sub>2</sub> emissions (the fraction of the total CO <sub>2</sub> emitted that remains in the atmosphere at the time of doubling, which is determined by carbon cycle processes) with information on the TCR. TCR is assessed in this chapter, whereas TCRE is assessed in ( [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] ( [[IPCC:Wg1:Chapter:Chapter-5#5.5|Section 5.5]] ).

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