Editing IPCC:AR6/WGI/SPM (section)

== Footnotes ==

<div id="footnote-048" class="_idFootnote"></div>

[[#footnote-048-backlink|1]] Decision IPCC/XLVI-2.

<div id="footnote-047" class="_idFootnote"></div>

[[#footnote-047-backlink|2]] The three Special Reports are: Global Warming of 1.5°C: An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty (SR1.5); Climate Change and Land: An IPCC Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (SRCCL); IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC).

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[[#footnote-046-backlink|3]] The assessment covers scientific literature accepted for publication by 31 January 2021.

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[[#footnote-045-backlink|4]] Each finding is grounded in an evaluation of underlying evidence and agreement. A level of confidence is expressed using five qualifiers: very low, low, medium, high and very high, and typeset in italics, for example, ''medium confidence'' . The following terms have been used to indicate the assessed likelihood of an outcome or result: virtually certain 99–100% probability; very likely 90–100%; likely 66–100%; about as likely as not 33–66%; unlikely 0–33%; very unlikely 0–10%; and exceptionally unlikely 0–1%. Additional terms (extremely likely 95–100%; more likely than not &gt;50–100%; and extremely unlikely 0–5%) are also used when appropriate. Assessed likelihood is typeset in italics, for example, ''very likely'' . This is consistent with AR5. In this Report, unless stated otherwise, square brackets [x to y] are used to provide the assessed ''very likely'' range, or 90% interval.

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[[#footnote-044-backlink|5]] The Interactive Atlas is available at [https://interactive-atlas.ipcc.ch/ https://interactive-atlas.ipcc.ch]

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[[#footnote-043-backlink|6]] Other GHG concentrations in 2019 were: perfluorocarbons (PFCs) – 109 parts per trillion (ppt) CF <sub>4</sub> equivalent; sulphur hexafluoride (SF <sub>6</sub>) – 10 ppt; nitrogen trifluoride (NF <sub>3</sub>) <sub></sub> – 2 ppt; hydrofluorocarbons (HFCs) – 237 ppt HFC-134a equivalent; other Montreal Protocol gases (mainly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)) – 1032 ppt CFC-12 equivalent). Increases from 2011 are 19 ppm for CO <sub>2</sub>, 63 ppb for CH <sub>4</sub> and 8 ppb for N <sub>2</sub> O.

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[[#footnote-042-backlink|7]] Land and ocean are not substantial sinks for other GHGs.

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[[#footnote-041-backlink|8]] The term ‘global surface temperature’ is used in reference to both global mean surface temperature and global surface air temperature throughout this SPM. Changes in these quantities are assessed with ''high confidence'' to differ by at most 10% from one another, but conflicting lines of evidence lead to ''low confidence'' in the sign (direction) of any difference in long-term trend. Links to chapters Cross-Section Box TS.1

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[[#footnote-040-backlink|9]] The period 1850–1900 represents the earliest period of sufficiently globally complete observations to estimate global surface temperature and, consistent with AR5 and SR1.5, is used as an approximation for pre-industrial conditions.

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[[#footnote-039-backlink|10]] Since AR5, methodological advances and new datasets have provided a more complete spatial representation of changes in surface temperature, including in the Arctic. These and other improvements have also increased the estimate of global surface temperature change by approximately 0.1°C, but this increase does not represent additional physical warming since AR5.

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[[#footnote-038-backlink|11]] The period distinction with A.1.2 arises because the attribution studies consider this slightly earlier period. The observed warming to 2010–2019 is 1.06 [0.88 to 1.21] °C.

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[[#footnote-037-backlink|12]] Throughout this SPM, ‘main driver’ means responsible for more than 50% of the change.

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[[#footnote-036-backlink|13]] As stated in section B.1, even under the very low emissions scenario SSP1-1.9, temperatures are assessed to remain elevated above those of the most recent decade until at least 2100 and therefore warmer than the century-scale period 6500 years ago.

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[[#footnote-035-backlink|14]] As indicated in footnote 12, throughout this SPM, ‘main driver’ means responsible for more than 50% of the change.

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[[#footnote-034-backlink|15]] Agricultural and ecological drought (depending on the affected biome): a period with abnormal soil moisture deficit, which results from combined shortage of precipitation and excess evapotranspiration, and during the growing season impinges on crop production or ecosystem function in general (see Annex VII: Glossary). Observed changes in meteorological droughts (precipitation deficits) and hydrological droughts (streamflow deficits) are distinct from those in agricultural and ecological droughts and are addressed in the underlying AR6 material (Chapter 11).

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[[#footnote-033-backlink|16]] The combined processes through which water is transferred to the atmosphere from open water and ice surfaces, bare soils and vegetation that make up the Earth’s surface (Glossary).

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[[#footnote-032-backlink|17]] The global monsoon is defined as the area in which the annual range (local summer minus local winter) of precipitation is greater than 2.5 mm day <sup>–1</sup> (Glossary). Global land monsoon precipitation refers to the mean precipitation over land areas within the global monsoon.

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[[#footnote-031-backlink|18]] Compound extreme events are the combination of multiple drivers and/or hazards that contribute to societal or environmental risk (Glossary). Examples are concurrent heatwaves and droughts, compound flooding (e.g., a storm surge in combination with extreme rainfall and/or river flow), compound fire weather conditions (i.e., a combination of hot, dry and windy conditions), or concurrent extremes at different locations.

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[[#footnote-030-backlink|19]] Cumulative energy increase of 282 [177 to 387] ZJ over 1971–2006 (1 ZJ = 10 <sup>21</sup> joules).

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[[#footnote-029-backlink|20]] Cumulative energy increase of 152 [100 to 205] ZJ over 2006–2018.

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[[#footnote-028-backlink|21]] Understanding of climate processes, the instrumental record, paleoclimates and model-based emergent constraints (Glossary).

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[[#footnote-027-backlink|22]] Throughout this Report, the five illustrative scenarios are referred to as SSPx-y, where ‘SSPx’ refers to the Shared Socio-economic Pathway or ‘SSP’ describing the socio-economic trends underlying the scenario, and ‘y’ refers to the approximate level of radiative forcing (in watts per square metre, or W m <sup>–2</sup>) resulting from the scenario in the year 2100. A detailed comparison to scenarios used in earlier IPCC reports is provided in Section TS.1.3, and Sections 1.6 and 4.6. The SSPs that underlie the specific forcing scenarios used to drive climate models are not assessed by WGI. Rather, the SSPx-y labelling ensures traceability to the underlying literature in which specific forcing pathways are used as input to the climate models. IPCC is neutral with regard to the assumptions underlying the SSPs, which do not cover all possible scenarios. Alternative scenarios may be considered or developed.

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[[#footnote-026-backlink|23]] Net negative CO <sub>2</sub> emissions are reached when anthropogenic removals of CO <sub>2</sub> exceed anthropogenic emissions (Glossary).

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[[#footnote-025-backlink|24]] Changes in global surface temperature are reported as running 20-year averages, unless stated otherwise.

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[[#footnote-024-backlink|25]] SSP1-1.9 and SSP1-2.6 are scenarios that start in 2015 and have very low and low GHG emissions, respectively, and CO <sub>2</sub> emissions declining to net zero around or after 2050, followed by varying levels of net negative CO <sub>2</sub> emissions.

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[[#footnote-023-backlink|26]] Crossing is defined here as having the assessed global surface temperature change, averaged over a 20-year period, exceed a particular global warming level.

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[[#footnote-022-backlink|27]] The AR6 assessment of when a given global warming level is first exceeded benefits from the consideration of the illustrative scenarios, the multiple lines of evidence entering the assessment of future global surface temperature response to radiative forcing, and the improved estimate of historical warming. The AR6 assessment is thus not directly comparable to the SR1.5 SPM, which reported ''likely'' reaching 1.5°C global warming between 2030 and 2052, from a simple linear extrapolation of warming rates of the recent past. When considering scenarios similar to SSP1-1.9 instead of linear extrapolation, the SR1.5 estimate of when 1.5°C global warming is first exceeded is close to the best estimate reported here.

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[[#footnote-021-backlink|28]] Natural variability refers to climatic fluctuations that occur without any human influence, that is, internal variability combined with the response to external natural factors such as volcanic eruptions, changes in solar activity and, on longer time scales, orbital effects and plate tectonics (Glossary).

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[[#footnote-020-backlink|29]] The internal variability in any single year is estimated to be about ±0.25°C (5–95% range, ''high confidence'').

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[[#footnote-019-backlink|30]] Projected changes in agricultural and ecological droughts are primarily assessed based on total column soil moisture. See footnote 15 for definition and relation to precipitation and evapotranspiration.

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[[#footnote-018-backlink|31]] Monthly average sea ice area of less than 1 million km <sup>2</sup> , which is about 15% of the average September sea ice area observed in 1979–1988.

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[[#footnote-017-backlink|32]] These projected adjustments of carbon sinks to stabilization or decline of atmospheric CO <sub>2</sub> are accounted for in calculations of remaining carbon budgets.

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[[#footnote-016-backlink|33]] The other sectoral emissions are calculated as the residual of the net land and ocean CO <sub>2</sub> uptake and the prescribed atmospheric CO <sub>2</sub> concentration changes in the CMIP6 simulations. These calculated emissions are net emissions and do not separate gross anthropogenic emissions from removals, which are included implicitly.

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[[#footnote-015-backlink|34]] Low-likelihood, high-impact outcomes are those whose probability of occurrence is low or not well known (as in the context of deep uncertainty) but whose potential impacts on society and ecosystems could be high. A tipping point is a critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly. (Glossary) Links to chapters 1.4, Cross-Chapter Box 1.3, 4.7

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[[#footnote-014-backlink|35]] To compare to the 1986–2005 baseline period used in AR5 and SROCC, add 0.03 m to the global mean sea level rise estimates. To compare to the 1900 baseline period used in Figure SPM.8, add 0.16 m.

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[[#footnote-013-backlink|36]] Climatic impact-drivers (CIDs) are physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral, or a mixture of each across interacting system elements and regions (Glossary). CID types include heat and cold, wet and dry, wind, snow and ice, coastal and open ocean.

<div id="footnote-012" class="_idFootnote"></div>

[[#footnote-012-backlink|37]] The main internal variability phenomena include El Niño–Southern Oscillation, Pacific Decadal Variability and Atlantic Multi-decadal Variability through their regional influence.

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[[#footnote-011-backlink|38]] Based on 2500 year reconstructions, eruptions more negative than –1 W m <sup>–2</sup> occur on average twice per century.

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[[#footnote-010-backlink|39]] Regions here refer to the AR6 WGI reference regions used in this Report to summarize information in sub-continental and oceanic regions. Changes are compared to averages over the last 20–40 years unless otherwise specified. Links to chapters 1.4, 12.4, Atlas.1 .

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[[#footnote-009-backlink|40]] The specific level of confidence or likelihood depends on the region considered. Details can be found in the Technical Summary and the underlying Report.

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[[#footnote-008-backlink|41]] In the literature, units of °C per 1000 PgC (petagrams of carbon) are used, and the AR6 reports the TCRE ''likely'' range as 1.0°C to 2.3°C per 1000 PgC in the underlying report, with a best estimate of 1.65°C.

<div id="footnote-007" class="_idFootnote"></div>

[[#footnote-007-backlink|42]] The condition in which anthropogenic carbon dioxide (CO <sub>2</sub>) emissions are balanced by anthropogenic CO <sub>2</sub> removals over a specified period (Glossary).

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[[#footnote-006-backlink|43]] The term ‘carbon budget’ refers to the maximum amount of cumulative net global anthropogenic CO <sub>2</sub> emissions that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers. This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date (Glossary). Historical cumulative CO <sub>2</sub> emissions determine to a large degree warming to date, while future emissions cause future additional warming. The remaining carbon budget indicates how much CO <sub>2</sub> could still be emitted while keeping warming below a specific temperature level.

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[[#footnote-005-backlink|44]] Compared to AR5, and when taking into account emissions since AR5, estimates in AR6 are about 300–350 GtCO <sub>2</sub> larger for the remaining carbon budget consistent with limiting warming to 1.5°C; for 2°C, the difference is about 400–500 GtCO <sub>2</sub>.

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[[#footnote-004-backlink|45]] Potential negative and positive effects of CDR for biodiversity, water and food production are methods-specific and are often highly dependent on local context, management, prior land use, and scale. IPCC Working Groups II and III assess the CDR potential and ecological and socio-economic effects of CDR methods in their AR6 contributions.

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[[#footnote-003-backlink|46]] A general term for how the climate system responds to a radiative forcing (Glossary).

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[[#footnote-002-backlink|47]] The choice of emissions metric depends on the purposes for which gases or forcing agents are being compared. This Report contains updated emissions metric values and assesses new approaches to aggregating gases.

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[[#footnote-001-backlink|48]] For other GHGs, there was insufficient literature available at the time of the assessment to assess detectable changes in their atmospheric growth rate during 2020.

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[[#footnote-000-backlink|49]] Near term: 2021–2040.