Editing IPCC:AR6/SROCC/Chapter-1 (section)

== CCB.1 Scenarios, Pathways and Reference Periods ==

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'''Authors:''' Nerilie Abram (Australia), William Cheung (Canada), Lijing Cheng (China), Thomas Frölicher (Switzerland), Mathias Hauser (Switzerland), Shengping He (Norway/China), Anne Hollowed (USA), Ben Marzeion (Germany), Samuel Morin (France), Anna Pirani (Italy), Didier Swingedouw (France)

'''Introduction'''

Assessing the future risks and opportunities that climate change will bring for the ocean and cryosphere, and for their dependent ecosystems and human communities, is a main objective of this report. However, the future is inherently uncertain. A well-established methodological approach that the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) report uses to assess the future under these uncertainties is through scenario analysis (Kainuma et al., 2018 <sup>[[#fn:r112|112]]</sup> ). The ultimate physical driver of the ocean and cryosphere changes that SROCC assesses are greenhouse gas emissions, while the exposure to hazards and the future risks to natural and human systems are also shaped social, economic and governance factors (Cross-Chapter Box 2 in Chapter 1; Section 1.5). This Cross-Chapter Box introduces the main scenarios that are used in the SROCC assessment. Examples of key climate change indicators in the atmosphere and ocean projected under future greenhouse gas emission scenarios are also provided (Table CB1.1).

'''Scenarios and pathways'''

''Scenarios'' are a plausible description of how the future may develop based on a coherent and internally consistent set of assumptions about key driving forces and relationships. ''Pathways'' refer to the temporal evolution of natural and/or human systems towards a future state. In SROCC, assessments of future change frequently use climate model projections forced by pathways of future radiative forcing changes related to different socioeconomic scenarios.

''Representative Concentration Pathways'' (RCPs) are a set of time series of plausible future concentrations of greenhouse gases, aerosols and chemically active gases, as well as land use changes (Moss et al., 2008 <sup>[[#fn:r113|113]]</sup> ; Moss et al., 2010 <sup>[[#fn:r114|114]]</sup> ; van Vuuren et al., 2011a <sup>[[#fn:r115|115]]</sup> ; Figure SM1.1). The word representative signifies that each RCP provides only one of many possible pathways that would lead to the specific radiative forcing characteristics. The term pathway emphasises the fact that not only the long-term concentration levels, but also the trajectory taken over time to reach that outcome are of interest.

Four RCPs were used for projections of the future climate in the Coupled Model Intercomparison Project Phase 5 ( CMIP5) (Taylor et al., 2012 <sup>[[#fn:r116|116]]</sup> ). They are identified by their approximate anthropogenic radiative forcing (in W m -2 , relative to 1750) by 2100: RCP2.6, RCP4.5, RCP6.0, and RCP8.5 (Figure SM1.1). RCP8.5 is a high greenhouse gas emission scenario without effective climate change mitigation policies, leading to continued and sustained growth in atmospheric greenhouse gas concentrations (Riahi et al., 2011 <sup>[[#fn:r117|117]]</sup> ). RCP2.6 represents a low greenhouse gas emission, high mitigation future that gives a two in three chance of limiting global atmospheric surface warming to below 2 o C by the end of the century (van Vuuren et al., 2011b; Collins et al., 2013 <sup>[[#fn:r119|119]]</sup> ; Allen et al., 2018 <sup>[[#fn:r120|120]]</sup> ). Achieving the RCP2.6 pathway would require implementation of negative emissions technologies at a not-yet-proven scale to remove greenhouse gases from the air, in addition to other mitigation strategies such as energy from sustainable sources and existing nature-based strategies (Gasser et al., 2015 <sup>[[#fn:r121|121]]</sup> ; Sanderson et al., 2016 <sup>[[#fn:r122|122]]</sup> ; Royal Society, 2018 <sup>[[#fn:r123|123]]</sup> ; National Academies of Sciences, 2019 <sup>[[#fn:r124|124]]</sup> ). An even more stringent RCP1.9 pathway is considered most compatible with limiting global warming to below 1.5 o C , called a 1.5°C-consistent pathway in the Special Report on Global Warming of 1.5°C ( SR15; O’Neill et al., 2016; IPCC, 2018 <sup>[[#fn:r125|125]]</sup> ), and will be assessed in the IPCC 6th Assessment Report (AR6) using projections of Phase 6 of the Coupled Model Intercomparison Project (CMIP6). Global fossil CO₂ emissions rose more than 2% in 2018 and 1.6% in 2017, after a temporary slowdown in emissions from 2014 – 2016. Current emissions continue to grow in line with the RCP8.5 trajectory (Peters et al., 2012 <sup>[[#fn:r126|126]]</sup> ; Le Quéré et al., 2018 <sup>[[#fn:r127|127]]</sup> ).

In SROCC, the CMIP5 simulations forced with RCPs are used extensively to assess future ocean and cryosphere changes. In particular, RCP2.6 and RCP8.5 are used to contrast the possible outcomes of low-emission versus high-emission futures, respectively (Table CB1.1). In some cases the SROCC assessments use literature that is based on the earlier Special Report on Emission Scenarios (SRES; IPCC, 2000 <sup>[[#fn:r128|128]]</sup> ), and details of these and their approximate RCP equivalents are provided in Tables SM1.3 and SM1.4.

''Shared Socioeconomic Pathways'' (SSPs) complement the RCPs with varying socioeconomic challenges to adaptation and mitigation (e.g., population, economic growth, education, urbanisation and the rate of technological development; O’Neill et al., 2017 <sup>[[#fn:r129|129]]</sup> ). The SSPs describe five alternative socioeconomic futures comprising: sustainable development (SSP1), middle-of-the-road development (SSP2), regional rivalry (SSP3), inequality (SSP4), and fossil-fuelled development (SSP5; Figure SM1.1; Kriegler et al., 2016 <sup>[[#fn:r130|130]]</sup> ; Riahi et al., 2017 <sup>[[#fn:r131|131]]</sup> ). The RCPs set plausible pathways for greenhouse gas concentrations and the climate changes that could occur, and the SSPs set the stage on which reductions in emissions will or will not be achieved within the context of the underlying socioeconomic characteristics and shared policy assumptions of that world. The combination of SSP-based socioeconomic scenarios and RCP-based climate projections provides an integrative frame for climate impact and policy analysis. The SSPs will be included in the CMIP6 simulations to be assessed in AR6 (O’Neill et al., 2016). In SROCC, the SSPs are used only for contextualising estimates from the literature on varying future populations in regions exposed to ocean and cryosphere changes.

'''Baselines and reference intervals'''

A baseline provides a reference period from which changes can be evaluated. In the context of anthropogenic climate change, the baseline should ideally approximate the ‘pre-industrial’ conditions before significant human influences on the climate began. The IPCC 5th Assessment Report (AR5) and SR15 (Allen et al., 2018 <sup>[[#fn:r132|132]]</sup> ) use 1850–1900 as the ''pre-industrial''  baseline for assessing historical and future climate change. Atmospheric greenhouse gas concentrations and global surface temperatures had already begun to rise in this interval from early industrialisation (Abram et al., 2016 <sup>[[#fn:r133|133]]</sup> ; Hawkins et al., 2017 <sup>[[#fn:r134|134]]</sup> ; Schurer et al., 2017 <sup>[[#fn:r135|135]]</sup> ). However, the scarcity of reliable climate observations represents a major challenge for quantifying earlier pre-industrial states (Hawkins et al., 2017 <sup>[[#fn:r136|136]]</sup> ). To maintain consistency across IPCC reports, the 1850–1900 ''pre-industrial'' baseline is used wherever possible in SROCC, recognising that this is a compromise between data coverage and representativeness of typical pre-industrial conditions.

In SROCC, the 1986–2005 reference interval used in AR5 is referred to as the ''recent past'' , and a 2006–2015 reference is used for ''present day'' , consistent with SR15 (Allen et al., 2018 <sup>[[#fn:r137|137]]</sup> ). The 2006–2015 reference interval incorporates near-global upper ocean data coverage and reasonably comprehensive remote-sensing cryosphere data (Section 1.8.1), and aligns this report with a more current reference than the 1986–2005 reference adopted by AR5. This 10-year ''present day'' period is short relative to natural variability. However, at this decadal scale the bias in the ''present day''  interval due to natural variability is generally small compared to differences between ''present day''  conditions and the ''pre-industrial''  baseline. There is also no indication of global average surface temperature in either 1986–2005 or 2006–2015 being substantially biased by short-term variability (Allen et al., 2018 <sup>[[#fn:r138|138]]</sup> ), consistent with the AR5 finding that each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850 (IPCC, 2013 <sup>[[#fn:r139|139]]</sup> ).

SROCC commonly provides future change assessments for two key intervals: A ''near term'' interval of 2031–2050 is comparable to a single generation time scale from present day, and incorporates the interval when global warming is ''likely'' to reach 1.5 o C if warming continues at the current rate (IPCC, 2018 <sup>[[#fn:r140|140]]</sup> ). An ''end-of-century'' interval of 2081–2100 represents the average climate conditions reached at the end of the standard CMIP5 future climate simulations and is relevant to long-term infrastructure planning and climate-resilient development pathways (CRDPs) (Cross-Chapter Box 2 in Chapter 1). In some cases where committed changes exist over multi-century time scales, such as the assessment of future sea level rise (Section 4.3.2) or deep ocean oxygen changes (Section 5.2.4.2, Table 5.5), SROCC also considers model evidence for ''long-term'' changes beyond the end of the current century.

'''Key indicators of future ocean and cryosphere change'''

Table CB1.1 compiles information on key indicators of climate change in the atmosphere and ocean. This information is given for different RCPs and for changes in the ''near term'' and ''end-of-century'' assessment intervals, relative to the ''recent past'' , noting that this does not capture changes that have already taken place since the ''pre-industrial'' baseline. SR15 assessed that global mean surface warming from the pre-industrial (1850 – 1900) to the ''recent past'' (1986 – 2005) reference period was 0.63 o C ( ''likely'' range of 0.57 o C – 0.69 o C), and during the ''present day'' interval (2006 – 2015) was 0.87 o C ( ''likely'' range of 0.75 o C – 0.99 o C) higher than the average over the 1850 – 1900 ''pre-industrial perio'' d ''(very high confidence'' ; IPCC, 2018 <sup>[[#fn:r141|141]]</sup> ).

These key climate and ocean change indicators allow for some harmonisation of the risk assessments in the chapters of SROCC. Projections of future change across a wider range of ocean and cryosphere components is also provided in Figure 1.5. Ocean and cryosphere changes and risks by the end-of-century (2081 – 2100) are expected to be larger under high greenhouse gas emission scenarios, compared with low greenhouse gas emission scenarios ( ''very high confidence'' ) (Table CB1.1, Figure 1.5).

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'''Table CB1.1.''' Projected change in global mean surface air temperature and key ocean variables for the ''near term'' (2031 – 2050) and ''end-of-century'' (2081 – 2100) relative to the ''recent past'' (1986 – 2005) reference period from CMIP5. See Table SM1.2 for the list of CMIP5 models and ensemble member used for calculating these projections. Small differences in the projections given here compared with AR5 (e.g., Table 12.2 in Collins et al., 2013) reflect differences in the number of models available now compared to at the time of the AR5 assessment (Table SM1.2).

<!-- TABLE -->
{| class="wikitable"
|-
|
| colspan="2"| '''Near term: 2031''' – '''2050'''
| colspan="2"| '''End-of-century: 2081''' – '''2100'''
|-
|
| '''Scenario'''
| '''Mean'''
| '''5''' – '''95% range'''
| '''Mean'''
| '''5''' – '''95% range'''
|-
| rowspan="4"| Global Mean Surface Air temperature (°C) a
| RCP2.6
| 0.9
| 0.5–1.4
| 1.0
| 0.3–1.7
|-
| RCP4.5
| 1.1
| 0.7–1.5
| 1.8
| 1.0–2.6
|-
| RCP6.0
| 1.0
| 0.5–1.4
| 2.3
| 1.4–3.2
|-
| RCP8.5
| 1.4
| 0.9–1.8
| 3.7
| 2.6–4.8
|-
| rowspan="2"| Global Mean Sea Surface Temperature (°C) b
(Section 5.2.5)

| RCP2.6
| 0.64
| 0.33–0.96
| 0.73
| 0.20–1.27
|-
| RCP8.5
| 0.95
| 0.60–1.29
| 2.58
| 1.64–3.51
|-
| rowspan="2"| Surface pH (units) b
(Section 5.2.2.3)

| RCP2.6
| -0.072
| -0.072 to -0.072
| -0.065
| -0.065 to -0.066
|-
| RCP8.5
| -0.108
| -0.106 to -0.110
| -0.315
| -0.313 to -0.317
|-
| rowspan="2"| Dissolved Oxygen (100–600 m) (% change)
(Section 5.2.2.4)b

| RCP2.6
| -0.9
| -0.3 to -1.5
| -0.6
| 0.0 to -1.2
|-
| RCP8.5
| -1.4
| -1.0 to -1.8
| -3.9
| -2.9 to -5.0
|}
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Notes:

a Calculated following the same procedure as the IPCC 5th Assessment Report (AR5) (Table 12.2 in Collins et al., 2013). The 5 – 95% model range of global mean surface air temperature across CMIP5 projections was assessed in AR5 as the ''likely'' range, after accounting for additional uncertainties or different levels of confidence in models.

b The 5 – 95% model range for global mean sea surface temperature, surface pH and dissolved oxygen (100 – 600 m) as referred to in the SROCC assessment as the ''very likely'' range (Section 1.9.2, Figure 1.4).

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