Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

=== 1.3.5 Projections of Future Climate Change ===

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It was recognized in IPCC AR5 that information about the near term was increasingly relevant for adaptation decisions. In response, AR5 WGI made a specific assessment for how global surface temperature was projected to evolve over the next two decades, concluding that the change for the period 2016–2035 relative to 1986–2005 will ''likely'' be in the range of 0.3°C–0.7°C ( ''medium confidence'' ), assuming no major volcanic eruptions or secular changes in total solar irradiance ( [[#IPCC--2013b|IPCC, 2013b]] ). The AR5 was also the first IPCC assessment report to assess ‘decadal predictions’ of the climate, where the observed state of the climate system was used as a starting point for forecasts several years ahead. The AR6 examines updates to these decadal predictions ( [[IPCC:Wg1:Chapter:Chapter-4#4.4.1|Section 4.4.1]] ).

The assessments and predictions for the near-term evolution of global climate features are largely independent of future CO <sub>2</sub> emissions pathways. However, AR5 WGI assessed that limiting climate change in the long-term future will require substantial and sustained reductions of GHG emissions ( [[#IPCC--2013b|IPCC, 2013b]] ). This assessment results from decades of research on understanding the climate system and its perturbations, and projecting climate change into the future. Each IPCC report has considered a range of emissions scenarios, typically including a scenario in which societies choose to continue on their present course, as well as several others reflecting socio-economic and policy responses that may limit emissions and/or increase the rate of CO <sub>2</sub> removal from the atmosphere. Climate models are used to project the outcomes of each scenario. However, future human climate influence cannot be precisely predicted because GHG and aerosol emissions, land use, energy use and other human activities may change in numerous ways. Common emissions scenarios used in the WGI contribution to AR6 are detailed in [[#1.6|Section 1.6]] .

Based on model results and steadily increasing CO <sub>2</sub> concentrations ( [[#Bolin--1970|Bolin and Bischof, 1970]] ; [[#SMIC--1971|SMIC, 1971]] ; [[#Meadows--1972|Meadows et al., 1972]] ), concerns about future ‘risk of effects on climate’ were addressed in Recommendation 70 of the Stockholm Action Plan, resulting from the 1972 United Nations Conference on the Human Environment ( [[#UN--1973|UN, 1973]] ). Numerous other scientific studies soon amplified these concerns (summarized in [[#Schneider--1975|Schneider (1975)]] and [[#Williams--1978|Williams (1978)]] ; see also Nordhaus (1975, 1977). In 1979, a US National Research Council (NRC) group led by Jule Charney reported on the ‘best present understanding of the carbon dioxide/climate issue for the benefit of policymakers’, initiating an era of regular and repeated large-scale assessments of climate science findings.

The 1979 Charney NRC report estimated ECS at 3°C, stating the range as 2°C–4.5°C, based on ‘consistent and mutually supporting’ model results and expert judgment ( [[#NRC--1979|NRC, 1979]] ). ECS is defined in IPCC assessments as the global surface air temperature (GSAT) response to CO <sub>2</sub> doubling (from pre-industrial levels) after the climate has reached equilibrium (stable energy balance between the atmosphere and ocean). Another quantity, transient climate response (TCR), was later introduced as the change in GSAT, averaged over a 20-year period, at the time of CO <sub>2</sub> doubling in a scenario of concentration increasing at 1% per year. Calculating ECS from historical or paleoclimate temperature records, in combination with energy budget models, has produced estimates both lower and higher than those calculated using GCMs and ESMs; in this Report, these are assessed in Chapter 7, Section 7.5.2.

ECS is typically characterized as most relevant on centennial time scales, while TCR was long seen as a more appropriate measure of the 50–100-year response to gradually increasing CO <sub>2</sub> . However, recent studies have raised new questions about how accurately both quantities are estimated by GCMs and ESMs ( [[#Grose--2018|Grose et al., 2018]] ; [[#Meehl--2020|Meehl et al., 2020]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ). Further, as climate models evolved to include a full-depth ocean, the time scale for reaching full equilibrium became longer and new methods to estimate ECS had to be developed ( [[#Gregory--2004|Gregory et al., 2004]] ; [[#Meehl--2020|Meehl et al., 2020]] ; [[#Meinshausen--2020|Meinshausen et al., 2020]] ). Because of these considerations, as well as new estimates from observation-based, paleoclimate, and emergent-constraints studies ( [[#Sherwood--2020|Sherwood et al., 2020]] ), the AR6 definition of ECS has changed from previous reports; it now includes all feedbacks except those associated with ice sheets. Accordingly, unlike previous reports, the AR6 assessments of ECS and TCR are not based primarily on GCM and ESM model results (see Section 7.5.5 and Box. 7.1 for a full discussion).

Today, other sensitivity terms are sometimes used, such as ‘transient climate response to emissions’ (TCRE, defined as the ratio of warming to cumulative CO <sub>2</sub> emissions in a CO <sub>2</sub> -only simulation) and ‘Earth system sensitivity’ (ESS), which includes multi-century Earth system feedbacks such as changes in ice sheets.

Table 1.2 shows estimates of ECS and TCR for major climate science assessments since 1979. The table shows that despite some variation in the range of GCM and (for the later assessments) ESM results, expert assessment of ECS changed little between 1979 and the present Report. Based on multiple lines of evidence, AR6 has narrowed the ''likely'' range of ECS to 2.5°C–4.0°C (Chapter 7, Section 7.5.5).
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'''Table 1.2 |'''
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'''Estimates of equilibrium climate sensitivity (ECS) and transient climate response (TCR) from successive major scientific assessments since 1979.''' No likelihood statements are available for reports prior to 2001 because those reports did not use the IPCC calibrated uncertainty language. The assessed range of ECS differs from the range derived from general circulation model (GCM) and Earth system model (ESM) results because assessments take into account other evidence, other types of models, and expert judgment. The AR6 definition of ECS differs from previous reports, now including all long-term feedbacks except those associated with ice sheets. AR6 estimates of ECS are derived primarily from process understanding, historical observations and emergent constraints, informed by (but not based on) GCM and ESM model results. CMIP6 is the 6th phase of the Coupled Model Intercomparison Project (Section 7.5.5 and Box 7.1).

[[File:c0dbc0614a0a77c3833f127fc722582e IPCC_AR6_WGI_Chapter_1_Table_1_2.png]]
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The AR5 WGI assessed that there is a close relationship of cumulative total emissions of CO <sub>2</sub> and GMST response that is approximately linear ( [[#IPCC--2013b|IPCC, 2013b]] ). This finding implies that continued emissions of CO <sub>2</sub> will cause further warming and changes in all components of the climate system, independent of any specific scenario or pathway. Scenario-based climate projections using the Representative Concentration Pathways (RCPs) assessed in AR5 WGI result in continued warming over the 21st <sup></sup> century in all scenarios except a strong climate change mitigation scenario (RCP2.6). Similarly, under all RCP scenarios, AR5 assessed that the rate of sea level rise over the 21st century will ''very likely'' exceed that observed during 1971–2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets. Further increases in atmospheric CO <sub>2</sub> will also lead to further uptake of carbon by the ocean, which will increase ocean acidification. By the mid-21st century the magnitudes of the projected changes are substantially affected by the choice of scenario. The set of scenarios used in climate change projections assessed as part of AR6 is discussed in [[#1.6|Section 1.6]] .

From the close link between cumulative emissions and warming it follows that any given level of global warming is associated with a total budget of GHG emissions, especially CO <sub>2</sub> as it is the largest long-lived contributor to radiative forcing ( [[#Allen--2009|Allen et al., 2009]] ; [[#Collins--2013|Collins et al., 2013]] ; [[#Rogelj--2019|Rogelj et al., 2019]] ). Higher emissions in earlier decades imply lower emissions later on to stay within the Earth’s carbon budget. Stabilizing the anthropogenic influence on global surface temperature thus requires that CO <sub>2</sub> emissions and removals reach net zero once the remaining carbon budget is exhausted (Cross-Chapter Box 1.4).

Past, present and future emissions of CO <sub>2</sub> therefore commit the world to substantial multi-century climate change, and many aspects of climate change would persist for centuries even if emissions of CO <sub>2</sub> were stopped immediately ( [[#IPCC--2013b|IPCC, 2013b]] ). According to AR5, a large fraction of this change is essentially irreversible on a multi-century to millennial time scale, barring large net removal (‘negative emissions’) of CO <sub>2</sub> from the atmosphere over a sustained period through as yet unavailable technological means (Chapters 4 and 5l; [[#IPCC--2013a|IPCC, 2013a]] , 2018). However, significant reductions of warming due to short-lived climate forcers (SLCFs) could reduce the level at which temperature stabilizes once CO <sub>2</sub> emissions reach net zero, and also reduce the long-term global warming commitment by reducing radiative forcing from SLCFs (Chapter 5).

In summary, major lines of evidence – observations, paleoclimate, theoretical understanding and natural and human drivers – have been studied and developed for over 150 years. Methods for projecting climate futures have matured since the 1950s and attribution studies since the 1980s. We conclude that understanding of the principal features of the climate system is robust and well established.

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