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

==== 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).
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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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