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

=== 1.2.1 The Changing State of the Physical Climate System ===

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The WGI contribution to AR5 (AR5 WGI; [[#IPCC--2013a|IPCC, 2013a]] ) assessed that ‘warming of the climate system is unequivocal’, and that since the 1950s, many of the observed changes are unprecedented over decades to millennia. Changes are evident in all components of the climate system: the atmosphere and the ocean have warmed, amounts of snow and ice have diminished, sea level has risen, the ocean has acidified and its oxygen content has declined, and atmospheric concentrations of greenhouse gases (GHGs) have increased ( [[#IPCC--2013b|IPCC, 2013b]] ). This Report documents that, since the AR5, changes to the state of the physical and biogeochemical climate system have continued, and these are assessed in full in later chapters. Here, we summarize changes to a set of key large-scale climate indicators over the modern era (1850 to present). We also discuss the changes in relation to the longer-term evolution of the climate. These ongoing changes throughout the climate system form a key part of the context of the present Report.

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==== 1.2.1.1 Recent Changes in Multiple Climate Indicators ====

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The physical climate system comprises all processes that combine to form weather and climate. The early chapters of this report broadly organize their assessments according to overarching realms: the atmosphere, the biosphere, the cryosphere (surface areas covered by frozen water, such as glaciers and ice sheets), and the ocean. Elsewhere in the report, and in previous IPCC assessments, the land is also used as an integrating realm that includes parts of the biosphere and the cryosphere. These overarching realms have been studied and measured in increasing detail by scientists, institutions and the general public since the 18th century, throughout the era of instrumental observation ( [[#1.3|Section 1.3]] ). Today, observations include those taken by numerous land surface stations, ocean surface measurements from ships and buoys, underwater instrumentation, satellite and surface-based remote sensing, and in situ atmospheric measurements from aeroplanes and balloons. These instrumental observations are combined with paleoclimate reconstructions and historical documentations to produce a highly detailed picture of the past and present state of the whole climate system, and to allow assessments about rates of change across the different realms ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] and [[#1.5|Section 1.5]] ).

Figure 1.4 documents that the climate system is undergoing a comprehensive set of changes. It shows a selection of key indicators of change through the instrumental era that are assessed and presented in the subsequent chapters of this report. Annual mean values are shown as stripes, with colours indicating their value. The transitions from one colour to another over time illustrate how conditions are shifting in all components of the climate system. For these particular indicators, the observed changes go beyond the yearly and decadal variability of the climate system. In this Report, this is termed an ‘emergence’ of the climate signal ( [[#1.4.2|Section 1.4.2]] and FAQ 1.2).

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'''Figure 1.4 |''' '''Changes are occurring throughout the climate system.''' '''Left:''' Main realms of the climate system: atmosphere, biosphere, cryosphere and ocean. '''Right:''' Six key indicators of ongoing changes since 1850, or the start of the observational or assessed record, through 2018. Each stripe indicates the global (except for precipitation which shows two latitude band means), annual mean anomaly for a single ye ar, relative to a multi-year baseline (except for CO2 concentration and glacier mass loss, which are absolute values). Grey indicates that data are not available. Datasets and baselines used are: (i) CO2: Antarctic ice cores ( [[#Lüthi--2008|Lüthi et al., 2008]] ; [[#Bereiter--2015|Bereiter et al., 2015]] ) and direct air measurements ( [[#Tans--2020|Tans and Keeling, 2020]] ) (see Figure 1.5 for details); (ii) precipitation: Global Precipitation Climatology Centre (GPCC) V8 (updated from Becker et al., 2013), baseline 1961–1990 using land areas only with latitude bands 33°N–66°N and 15°S–30°S; (iii) glacier mass loss: [[#Zemp--2019|Zemp et al. (2019)]] ; (iv) global surface air temperature (GMST): HadCRUT5 ( [[#Morice--2021|Morice et al., 2021]] ), baseline 1961–1990; (v) sea level change: ( [[#Dangendorf--2019|Dangendorf et al., 2019]] ), baseline 1900–1929; (vi) ocean heat content (model–observation hybrid): [[#Zanna--2019|Zanna et al. (2019)]] , baseline 1961–1990. Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).

Warming of the climate system is most commonly presented through the observed increase in global mean surface temperature (GMST). Taking a baseline of 1850–1900, GMST change until present (2011–2020) is 1.09°C [0.95 to 1.20] °C ( [[IPCC:Wg1:Chapter:Chapter-2#2.3|Section 2.3]] and Cross-Chapter Box 2.3). This evolving change has been documented in previous assessment reports, with each reporting a higher total global temperature change ( [[#1.3|Section 1.3]] and Cross-Chapter Box 1.2). The total change in global surface air temperature (GSAT) ( [[#1.4.1|Section 1.4.1]] and Cross-Chapter Box 2.3) attributable to anthropogenic activities is assessed to be consistent with the observed change in GSAT ( [[IPCC:Wg1:Chapter:Chapter-3#3.3|Section 3.3]] ). <sup>[[#footnote-007|1]]</sup>

Similarly, atmospheric concentrations of a range of GHGs are increasing. Carbon dioxide (CO <sub>2</sub> , shown in Figure 1.4 and Figure 1.5a, found in AR5 and earlier reports to be the current strongest driver of anthropogenic climate change), has increased from 285.5 ± 2.1 ppm in 1850 to 409.9 ± 0.4 ppm in 2019; concentrations of methane (CH <sub>4</sub> ), and nitrous oxide (N <sub>2</sub> O) have increased as well (Sections 2.2 and 5.2, and Annex V). These observed changes are assessed to be in line with known anthropogenic and natural emissions, when accounting for observed and inferred uptake by land, ocean and biosphere respectively (Section 5.2), and are a key source of anthropogenic changes to the global energy balance (or radiative forcing; Sections 2.2 and 7.3).

The hydrological (or water) cycle is also changing and is assessed to be intensifying, through a higher exchange of water between the surface and the atmosphere (Sections 2.3 and 8.3). The resulting regional patterns of changes to precipitation are, however, different from surface temperature change, and interannual variability is larger, as illustrated in Figure 1.4. Annual land area mean precipitation in the Northern Hemisphere temperate regions has increased, while the subtropical dry regions have experienced a decrease in precipitation in recent decades ( [[IPCC:Wg1:Chapter:Chapter-2#2.3|Section 2.3]] ).

The cryosphere is undergoing rapid changes, with increased melting and loss of frozen water mass in most regions. This includes all frozen parts of the globe, such as terrestrial snow, permafrost, sea ice, glaciers, freshwater ice, solid precipitation, and the ice sheets covering Greenland and Antarctica (Chapter 9; SROCC, [[#IPCC--2019b|IPCC, 2019b]] ). Figure 1.4 illustrates how, globally, glaciers have been increasingly losing mass for the last fifty years. The total glacier mass in the most recent decade (2010–2019) was the lowest since the beginning of the 20th century (Sections 2.3 and 9.5).

The global ocean has warmed unabatedly since at least 1970 (Sections 1.3, 2.3 and 9.2; SROCC, [[#IPCC--2019b|IPCC, 2019b]] ). Figure 1.4 shows how the averaged ocean heat content is steadily increasing, with a total increase of [0.28 to 0.55] yottajoule (YJ; 10 <sup>24</sup> joule) between 1971 and 2018 (Section 9.2). In response to this ocean warming, as well as to the loss of mass from glaciers and ice sheets, the global mean sea level (GMSL) has risen by 0.20 [0.15 to 0.25] metres between 1900 and 2018. GMSL rise has accelerated since the late 1960s (see Section 9.6).

Overall, the changes in these selected climatic indicators have progressed beyond the range of natural year-to-year variability (Chapters 2, 3, 8 and 9, and Sections [[#1.2.1.2|1.2.1.2]] and [[#1.4.2|1.4.2]] ). The indicators presented in Figure 1.4 document a broad set of concurrent and emerging changes across the physical climate system. All indicators shown here, along with many others, are further presented in the coming chapters, together with a rigorous assessment of the supporting scientific literature. Later chapters (Chapters 10, 11, 12 and Atlas) present similar assessments at the regional level, where observed changes do not always align with the global mean picture shown here.

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==== 1.2.1.2 Long-Term Perspectives on Anthropogenic Climate Change ====

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Paleoclimate archives (e.g., ice cores, corals, marine and lake sediments, speleothems, tree rings, borehole temperatures, soils) permit the reconstruction of climatic conditions before the instrumental era. This establishes an essential long-term context for the climate change of the past 150 years and the projected changes in the 21st century and beyond (Chapter 3; [[#IPCC--2013a|IPCC, 2013a]] ; [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] ). Figure 1.5 shows reconstructions of three key indicators of climate change over the past 800,000 years (800 kyr) <sup>[[#footnote-006|2]]</sup> – atmospheric CO <sub>2</sub> concentrations, global mean surface temperature (GMST) and global mean sea level (GMSL) – comprising at least eight complete glacial–interglacial cycles ( [[#EPICA%20Community%20Members--2004|EPICA Community Members, 2004]] ; [[#Jouzel--2007|Jouzel et al., 2007]] ), which are largely driven by oscillations in the Earth’s orbit and consequent feedbacks on multi-millennial time scales ( [[#Berger--1978|Berger, 1978]] ; [[#Laskar--1993|Laskar et al., 1993]] ). The dominant cycles – recurring approximately every 100 kyr – can be found imprinted in the natural variations of these three key indicators. Before industrialisation, atmospheric CO <sub>2</sub> concentrations varied between 174 ppm and 300 ppm, as measured directly in air trapped in ice at Dome Concordia, Antarctica ( [[#Bereiter--2015|Bereiter et al., 2015]] ; [[#Nehrbass-Ahles--2020|Nehrbass-Ahles et al., 2020]] ). Relative to 1850–1900 CE, the reconstructed GMST changed in the range of –6°C to +1°C across these glacial–interglacial cycles (see Chapter 2, [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] for an assessment of different paleo-reference periods). GMSL varied between about –130 m during the coldest glacial maxima and +5 to +25 m during the warmest interglacial periods (Chapter 2; [[#Spratt--2016|Spratt and Lisiecki, 2016]] ). They represent the amplitudes of natural, global-scale climate variations over the last 800 kyr prior to the influence of human activity. Further climate information from a variety of paleoclimatic archives is assessed in Chapters 2, 5, 7 and 9.

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'''Figure 1.5 |''' '''Long-term context of anthropogenic climate change''' '''based on selected paleoclimatic reconstructions over the past 800,000 years (800 kyr) for three key indicators: atmospheric CO''' <sub>2</sub> '''concentrations, global mean surface temperature (GMST), and global mean sea level (GMSL).'''

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'''(a)''' '''Measurements of CO''' '''<sub>2</sub>''' '''in air enclosed in Antarctic ice cores''' (Lüthi et al. , 2008; Bereiter et al. , 2015 [a compilation]; uncertainty ±1.3 ppm; see Sections 2.2.3 and 5.1.2 for an assessment) '''and direct air measurements''' ( [[#Tans--2020|Tans and Keeling, 2020]] ; uncertainty ±0.12 ppm). Projected CO <sub>2</sub> concentrations for five Shared Socio-economic Pathways (SSP) scenarios are indicated by dots on the right-hand side of each panel (grey background; (Meinshausen et al. , 2020; SSPs are described in [[#1.6|Section 1.6]] ). '''(b)''' Reconstruction of GMST from marine paleoclimate proxies (light-grey line: [[#Snyder--2016|Snyder (2016)]] ; dark grey line: Hansen et al. (2013); see [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] for an assessment). Observed and reconstructed temperature changes since 1850 are the AR6 assessed mean (referenced to 1850–1900; Box TS.3; 2.3.1.1); dots/whiskers on the right-hand panels (grey background) indicate the projected mean and ranges of warming derived from Coupled Model Intercomparison Project Phase 6 (CMIP6) SSP-based (2081–2100) and Model for the Assessment of Greenhouse Gas Induced Climate Change (MAGICC7; 2300) simulations (Tables 4.5 and 4.9). '''(c)''' Sea level changes reconstructed from a stack of oxygen isotope measurements on seven ocean sediment cores ( [[#Spratt--2016|Spratt and Lisiecki, 2016]] ; see Chapter 2, [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Chapter 9, Section 9.6.2 for an assessment). The sea level record from 1850–1900 is from Kopp et al. (2016), while the 20th century record is an updated ensemble estimate of GMSL change (Palmer et al. , 2021; Sections 2.3.3.3 and 9.6.1.1). Dots/whiskers on the right-hand panels of the figure (grey background) indicate the projected median and ranges derived from SSP-based simulations (2081–2100: Table 9.9; 2300: Section 9.6.3.5). Best estimates (dots) and uncertainties (whiskers), as assessed in Chapter 2, are included in the left and middle panels for each of the three indicators and selected paleo-reference periods used in this report (CO <sub>2</sub> : Table 2.1; GMST: [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.1|Section 2.3.1.1]] and Cross-Chapter Box 2.3, Table 1; GMSL: Sections 2.3.3.3 and 9.6.2. See also Cross-Chapter Box 2.1). Selected paleo-reference periods: LIG – Last Interglacial; LGM – Last Glacial Maximum; MH – mid-Holocene (Cross-Chapter Box 2.1, Table 1). The non-labelled best estimate in panel (c) corresponds to the sea level high-stand during Marine Isotope Stage 11, about 410 ka (410,000 years ago; Section 9.6.2). Further details on data sources and processing are available in the chapter data Table (Table 1.SM.1).

Paleoclimatic information also provides a long-term perspective on rates of change of these three key indicators. In high-resolution reconstructions from polor ice cores, the rate of increase in atmospheric CO <sub>2</sub> observed over 1919–2019 CE is one order of magnitude higher than the fastest CO <sub>2</sub> fluctuations documented during the Last Glacial Maximum and the last deglacial transition ( [[#Marcott--2014|Marcott et al., 2014]] , see Chapter 2, [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.2.1|Section 2.2.3.2.1]] ). Current multi-decadal GMST exhibit a higher rate of increase than over the past 2 kyr ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.1.2|Section 2.3.1.1.2]] ; [[#PAGES%202k%20Consortium--2019|PAGES 2k Consortium, 2019]] ), and in the 20th century GMSL rise was faster than during any other century over the past 3 kyr ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ).

Paleoclimate reconstructions also shed light on the causes of these variations, revealing processes that need to be considered when projecting climate change. The paleorecords show that sustained changes in global mean temperature of a few degrees Celsius are associated with increases in sea level of several tens of metres (Figure 1.5). During two extended warm periods (interglacials) of the last 800 kyr, sea level is estimated to have been at least six metres higher than today (Chapter 2; [[#Dutton--2015|Dutton et al., 2015]] ). During the last interglacial, sustained warmer temperatures in Greenland preceded the peak of sea level rise (Figure 5.15 in [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] ). The paleoclimate record therefore provides substantial evidence directly linking warmer GMST to substantially higher GMSL.

GMST will remain above present-day levels for many centuries even if net CO <sub>2</sub> emissions are reduced to zero, as shown in simulations with coupled climate models ( [[IPCC:Wg1:Chapter:Chapter-4#4.7.1|Section 4.7.1]] ; [[#Plattner--2008|Plattner et al., 2008]] ; Section 12.5.3 in [[#Collins--2013|Collins et al., 2013]] ; [[#Zickfeld--2013|Zickfeld et al., 2013]] ; [[#MacDougall--2020|MacDougall et al., 2020]] ). Such persistent warm conditions in the atmosphere represent a multi-century commitment to long-term sea level rise, summer sea ice reduction in the Arctic, substantial ice-sheet melting, potential ice-sheet collapse, and many other consequences in all components of the climate system (Section 9.4 and Figure 1.5; [[#Clark--2016|Clark et al., 2016]] ; [[#Pfister--2016|Pfister and Stocker, 2016]] ; H. [[#Fischer--2018|]] [[#Fischer--2018|Fischer et al., 2018]] ).

Paleoclimate records also show centennial- to millennial-scale variations, particularly during the ice ages, which indicate rapid or abrupt changes of the Atlantic Meridional Overturning Circulation (AMOC; Section 9.2.3.1) and the occurrence of a ‘bipolar seesaw’ (opposite-phase surface temperature changes in both hemispheres; [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4.1|Section 2.3.3.4.1]] ; [[#Stocker--2003|Stocker and Johnsen, 2003]] ; [[#EPICA%20Community%20Members--2006|EPICA Community Members, 2006]] ; WAIS Divide Project Members et al., 2015; [[#Lynch-Stieglitz--2017|Lynch-Stieglitz, 2017]] ; [[#Pedro--2018|Pedro et al., 2018]] ; [[#Weijer--2019|Weijer et al., 2019]] ). This process suggests that instabilities and irreversible changes could be triggered if critical thresholds are passed ( [[#1.4.4.3|Section 1.4.4.3]] ). Several other processes involving instabilities are identified in climate models ( [[#Drijfhout--2015|Drijfhout et al., 2015]] ), some of which may now be close to critical thresholds ( [[#1.4.4.3|Section 1.4.4.3]] ; see also Chapters 5, 8 and 9 regarding tipping points; [[#Joughin--2014|Joughin et al., 2014]] ).

Based on Figure 1.5, the reconstructed, observed and projected ranges of changes in the three key indicators can be compared. By the first decade of the 20th century, atmospheric CO <sub>2</sub> concentrations had already moved outside the reconstructed range of natural variation over the past 800 kyr. On the other hand, GMST and GMSL were higher than today during several interglacials of that period (Sections [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|2.3.1]] and [[IPCC:Wg1:Chapter:Chapter-2#2.3.3|2.3.3]] , and Figure 2.34). Projections for the end of the 21st century, however, show that GMST will have moved outside of its natural range within the next few decades, except for the strong mitigation scenarios ( [[#1.6|Section 1.6]] ). There is a risk that GMSL may potentially leave the reconstructed range of natural variations over the next few millennia (Section 9.6.3.5; [[#Clark--2016|Clark et al., 2016]] ; SROCC, [[#IPCC--2019b|IPCC, 2019b]] ). In addition, abrupt changes can not be excluded ( [[#1.4.4.3|Section 1.4.4.3]] ).

An important time period in the assessment of anthropogenic climate change is the last 2 kyr. Since AR5, new global datasets have been produced that aggregate aggregating local and regional paleorecords ( [[#PAGES%202k%20Consortium--2013|PAGES 2k Consortium, 2013]] , 2017, 2019; [[#McGregor--2015|McGregor et al., 2015]] ; [[#Tierney--2015|Tierney et al., 2015]] ; [[#Abram--2016|Abram et al., 2016]] ; [[#Hakim--2016|Hakim et al., 2016]] ; [[#Steiger--2018|Steiger et al., 2018]] ; [[#Brönnimann--2019b|Brönnimann et al., 2019b]] ). Before the global warming that began around the mid-19th century ( [[#Abram--2016|Abram et al., 2016]] ), a slow cooling in the Northern Hemisphere from roughly 1450–1850 CE is consistently recorded in paleoclimate archives ( [[#PAGES%202k%20Consortium--2013|PAGES 2k Consortium, 2013]] ; [[#McGregor--2015|McGregor et al., 2015]] ). While this cooling, primarily driven by an increased number of volcanic eruptions ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1|Section 3.3.1]] ; [[#PAGES%202k%20Consortium--2013|PAGES 2k Consortium, 2013]] ; [[#Owens--2017|Owens et al., 2017]] ; [[#Brönnimann--2019b|Brönnimann et al., 2019b]] ), shows regional differences, the subsequent warming over the past 150 years exhibits a global coherence that is unprecedented in the last 2 kyr ( [[#Neukom--2019|Neukom et al., 2019]] ).

The rate, scale and magnitude of anthropogenic changes in the climate system since the mid-20th century suggested the definition of a new geological epoch: the Anthropocene ( [[#Crutzen--2000|Crutzen and Stoermer, 2000]] ; [[#Steffen--2007|Steffen et al., 2007]] ), referring to an era in which human activity is altering major components of the Earth system and leaving measurable imprints that will remain in the permanent geological record (Figure 1.5; [[#IPCC--2018|IPCC, 2018]] ). These alterations include not only climate change itself, but also chemical and biological changes in the Earth system such as rapid ocean acidification due to uptake of anthropogenic CO <sub>2</sub> , massive destruction of tropical forests, a worldwide loss of biodiversity and the sixth mass extinction of species ( [[#Hoegh-Guldberg--2010|Hoegh-Guldberg and Bruno, 2010]] ; [[#Ceballos--2017|Ceballos et al., 2017]] ; [[#IPBES--2019|IPBES, 2019]] ). According to the key messages of the last global assessment of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services ( [[#IPBES--2019|IPBES, 2019]] ), climate change is a ‘direct driver that is increasingly exacerbating the impact of other drivers on nature and human well-being’, and ‘the adverse impacts of climate change on biodiversity are projected to increase with increasing warming.’

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