Editing IPCC:AR6/WGI/Chapter-2 (section)

== 2.1 Introduction ==

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This chapter assesses the evidence basis for large-scale past changes in selected components of the climate system. As such, it combines much of the assessment performed in Chapters 2 through 5 of the Fifth Assessment Report (AR5) WGI contribution ( [[#IPCC--2013|IPCC, 2013]] ) that, taken together, supported a finding of unequivocal recent warming of the climate system. The Sixth Assessment Report (AR6) WGI Report structure differs substantially from that in AR5 ( [[IPCC:Wg1:Chapter:Chapter-1#1.1.2|Section 1.1.2]] ). This chapter focuses upon observed changes in climate system drivers and changes in key selected large-scale indicators of climate change and in important modes of variability (Cross-Chapter Box 2.2), which allow for an assessment of changes in the global climate system in an integrated manner. This chapter is complemented by Chapters 3 and 4, which respectively consider model assessment/detection and attribution, and future climate projections for subsets of these same indicators and modes. It does not consider changes in observed extremes, which are assessed in Chapter 11. The chapter structure is outlined in the visual abstract (Figure 2.1).

Use is made of paleoclimate, in situ, ground- and satellite-based remote sensing, and reanalysis data products where applicable ( [[IPCC:Wg1:Chapter:Chapter-1#1.5|Section 1.5]] ). All observational products used in the chapter are detailed in Annex I, and information on data sources and processing for each figure and table can be found in the associated chapter Table 2.SM.1 available as an electronic supplement to the chapter. Use of common periods ranging from 56 million years ago through to the recent past is applied to the extent permitted by available data ( [[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] and Cross-Chapter Box 2.1). In all cases, the narrative proceeds from as far in the past as the data permit through to the present. Each sub-section starts by highlighting the key findings from AR5 and any relevant AR6-cycle Special Reports (SROCC, SR1.5, SRCCL), and then outlines the new evidence-basis arising from a combination of: (i) new findings reported in the literature, including new datasets and new versions of existing datasets; and (ii) recently observed changes, before closing with a new summary assessment.

Trends, when calculated as part of this assessment, have wherever possible been calculated using a common approach following that adopted in Box 2.2 of Chapter 2 of AR5 ( [[#Hartmann--2013|Hartmann et al., 2013]] ). In addition to trends, consideration is also made of changes between various time slices/periods in performing the assessment ( [[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] and Cross-Chapter Box 2.1). Statistical significance of trends and changes are assessed at the two-tailed 90% confidence ( ''very likely'' ) level unless otherwise stated. Limited use is also made of published analyses that have employed a range of methodological choices. In each such case the method/metric is stated.

There exist a variety of inevitable and, in some cases, irreducible uncertainties in performing an assessment of the observational evidence for climate change. In some instances, a combination of sources of uncertainty is important. For example, the assessment of global surface temperature over the instrumental record in [[#2.3.1.1.3|Section 2.3.1.1.3]] considers a combination of observational-dataset and trend-estimate uncertainties. Furthermore, estimates of parametric uncertainty are often not comprehensive in their consideration of all possible factors and, when such estimates are constructed in distinct manners, there are often significant limitations to their direct comparability ( [[#Hartmann--2013|Hartmann et al., 2013]] , their Box 2.1).

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'''Cross-Chapter Box 2.1 | Paleoclimate Reference Periods in the Assessment Report'''

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'''Contributing Authors:''' Darrell S. Kaufman (United States of America), Kevin D. Burke (United States of America), Samuel Jaccard (Switzerland), Christopher Jones (United Kingdom), Wolfgang Kiessling (Germany), Daniel J. Lunt (United Kingdom), Olaf Morgenstern (New Zealand/Germany), John W. Williams (United States of America)

Over the long evolution of the Earth’s climate system, several periods have been extensively studied as examples of distinct climate states. This Cross-Chapter Box places multiple paleoclimate reference periods into the unifying context of Earth’s long-term climate history, and points to sections in the report with additional information about each period. Other reference periods, including those of the industrialized era, are described in [[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] .

The reference periods represent times that were both colder and warmer than present, and periods of rapid climate change, many with informative parallels to projected climate (Cross-Chapter Box 2.1, Table 1). They are used to address a wide variety of questions related to natural climate variations in the past (FAQ 1.3). Most of them are used as targets to evaluate the performance of climate models under different climate forcings ( [[IPCC:Wg1:Chapter:Chapter-3#3.8.2|Section 3.8.2]] ), while also providing insight into the ocean-atmospheric circulation changes associated with various radiative forcings and geographical changes.

Global mean surface temperature (GMST) is a key indicator of the changing state of the climate system. Earth’s mean temperature history during the current geological era (Cenozoic, beginning 66 Ma (66 million years ago)) can be broadly characterized as follows (Cross-Chapter Box 2.1, Figure 1): (i) transient warming during the first 15 Myr (15 million years) of the Cenozoic, punctuated by the Paleocene–Eocene Thermal Maximum; (ii) a long-term cooling over tens of millions of years beginning around 50 Ma, driven by (among other factors) the slow drift of tectonic plates, which drove mountain building, erosion and volcanism, and reconfigured ocean passages, all of which ultimately moved carbon from the atmosphere to other reservoirs and led to the development of the Antarctic Ice Sheet (AIS) about 35–30 Ma; (iii) the intensification of cooling by climate feedbacks involving interactions among tectonics, ice albedo, ocean circulation, land cover and greenhouse gases, causing ice sheets to develop in the Northern Hemisphere (NH) by about 3 Ma; (iv) glacial-interglacial fluctuations paced by slow changes in Earth’s astronomical configuration (orbital forcing) and modulated by changes in the global carbon cycle and ice sheets on time scales of tens to hundreds of thousands of years, with particular prominence during the last 1 Myr; (v) a transition with both gradual and abrupt shifts from the Last Glacial Maximum to the present interglacial epoch (Holocene), with sporadic ice-sheet breakup disrupting ocean circulation; (vi) continued warming followed by minor cooling following the mid-Holocene, with superposed centennial- to decadal-scale fluctuations caused by volcanic activity, among other factors; (vii) recent warming related to the build-up of anthropogenic greenhouse gases (Sections 2.2.3 and 3.3.1).

GMST estimated for each of the reference periods based on proxy evidence ( [[#2.3.1.1|Section 2.3.1.1]] ) can be compared with climate projections over coming centuries to place the range of possible futures into a longer-term context (Cross-Chapter Box 2.1, Figure 1). Here, the ''very likely'' range of GMST for the warmer world reference periods are compared with the ''very likely'' range of GSAT projected for the end the 21st century (2080–2100; Table 4.5) and the ''likely'' range for the end of the 23rd century (2300; Table 4.9) under multiple Shared Socio-economic Pathway (SSP) scenarios. From this comparison, there is ''medium confidence'' in the following: GMST estimated for the warmest long-term period of the Last Interglacial about 125 ka (125,000 years ago; 0.5°C–1.5°C relative to 1850–1900) overlaps with the low end of the range of temperatures projected under SSP1-2.6 including its negative emissions extension to the end of the 23rd century (1.0°C to 2.2°C). GMST estimated for a period of prolonged warmth during the mid-Pliocene Warm Period about 3 Ma [2.5°C to 4.0°C] is similar to temperatures projected under SSP2-4.5 for the end of the 23rd century (2.3°C to 4.6°C). GMST estimated for the Miocene Climatic Optimum [5°C to 10°C] and Early Eocene Climatic Optimum [10°C to 18°C], about 15 and 50 Ma, respectively, overlap with the range projected for the end of the 23rd century under SSP5-8.5 (6.6°C to 14.1°C) ''.''

Cross-Chapter Box 2.1

'''Cross-Chapter Box 2.1, Table 1'''

'''|'''

'''Paleo-reference periods, listed from oldest to youngest.''' See ‘AR6 Sections’ (right-hand column) for literature citations related to each ‘Sketch of the climate state.’ See WGII (Chapters 1, 2 and 3) for citations related to paleontological changes. See Interactive [[IPCC:Wg1:Chapter:Atlas|Atlas]] for simulated climate variables for MPWP, LIG, LGM and MH.
{| class="wikitable"
|-
| '''Period'''

| '''Age/Year''' <sup>a</sup>

| '''Sketch of the Climate State (Relative to 1850–1900), and''' Model Experiment Protocols '''(''' italic ''').''' Values for large-scale climate indicators including global temperature, sea level and atmospheric CO <sub>2</sub> are shown in Figure 2.34.

| '''AR6 Sections'''

(partial list)

|-
| Paleocene–Eocene Thermal Maximum (PETM)

| 55.9–55.7 Ma (million years ago)

| A geologically rapid, large-magnitude warming event at the start of the Eocene when a large pulse of carbon was released to the ocean-atmosphere system, decreasing ocean pH and oxygen content. Terrestrial plant and animal communities changed composition, and species distributions shifted poleward. Many deep-sea species went extinct and tropical coral reefs diminished. ''DeepMIP'' ( [[#Lunt--2017|Lunt et al., 2017]] )

| 2.2.3.1

2.3.1.1.1

5.1.2.1

5.3.1.1

7.5.3.4

|-
| Early Eocene Climatic Optimum <sup>b</sup> (EECO)

| 53–49 Ma

| Prolonged ‘hothouse’ period with atmospheric CO <sub>2</sub> concentration &gt;1000 ppm, similar to SSP5-8.5 end-of-century values. Continental positions were somewhat different to present due to tectonic plate movements; polar ice was absent and there was more warming at high latitudes than in the equatorial regions. Near-tropical forests grew at 70°S, despite seasonal polar darkness. ''DeepMIP'' , about 50 Ma ( [[#Lunt--2017|Lunt et al., 2017]] , 2021)

| 2.2.3.1

2.3.1.1.1

7.4.4.1.2

7.5.3.4

7.5.6

|-
| Miocene Climatic Optimum <sup>b</sup> (MCO)

| 16.9–14.7 Ma

| Prolonged warm period with atmospheric CO <sub>2</sub> concentrations 400–600 ppm, similar to SSP2-4.5 end-of-century values. Continental geography was broadly similar to modern. At times, Arctic sea ice may have been absent, and the AIS was much smaller or perhaps absent. Peak in Cenozoic reef development. ''MioMIP1'' , Early and Middle Miocene ( [[#Steinthorsdottir--2021|Steinthorsdottir et al., 2021]] )

| 2.2.3.1

2.3.1.1.1

|-
| Mid-Pliocene Warm Period (MPWP)

| 3.3–3.0 Ma

| Warm period when atmospheric CO <sub>2</sub> concentration was similar to present (Cross-Chapter Box 2.4). The Arctic was much warmer, but tropical temperatures were only slightly warmer. Sea level was higher than present. Treeline extended to the northern coastline of the NH continents. Also called, ‘Piacenzian warm period.’ ''PMIP4'' ''midPliocene-eoi400'' , 3.2 Ma ( [[#Haywood--2016|Haywood et al., 2016]] , 2020)

| CCB2.4

7.4.4.1.2

7.5.3.3

8.2.2.2

9.6.2

|-
| Last Interglacial (LIG)

| 129–116 ka (thousand years ago)

| Most recent interglacial period, similar to mid-Holocene, but with more pronounced seasonal insolation cycle. Northern high latitudes were warmer, with reduced sea ice. Greenland and West Antarctic ice sheets were smaller and sea level was higher. Monsoon was enhanced. Boreal forests extended into Greenland and subtropical animals such as ''Hippopotamus'' occupied Britain. Coral reefs expanded latitudinally and contracted equatorially. ''PMIP4'' ''lig127k'' , 127 ka ( [[#Otto-Bliesner--2017|Otto-Bliesner et al., 2017]] , 2021)

| 2.2.3.2

2.3.1.1.1

2.3.3.3

9.2.2.1

9.6.2

|-
| Last Glacial Maximum (LGM)

| 23–19 ka

| Most recent glaciation when global temperatures were lower, with greater cooling toward the poles. Ice sheets covered much of North America and north-west Eurasia, and sea level was commensurately lower. Atmospheric CO <sub>2</sub> was lower; more carbon was sequestered in the ocean interior. Precipitation was generally lower over most regions; the atmosphere was dustier, and ranges of many plant species contracted into glacial refugia; forest extent and coral reef distribution was reduced worldwide. ''PMIP4lgm'' , 21 ka ( [[#Kageyama--2017|Kageyama et al., 2017]] , 2021a)

| 2.2.3.2

2.3.1.1.1

3.3.1.1

3.8.2.1

5.1.2.2

7.4.4.1.2

7.5.3.1

8.3.2.4

9.6.2

|-
| Last Deglacial Transition (LDT)

| 18–11 ka

| Warming that followed the Last Glacial Maximum, with decreases in the extent of the cryosphere in both polar regions. Sea level, ocean meridional overturning circulation, and atmospheric CO <sub>2</sub> increased during two main steps. Temperate and boreal species ranges expanded northwards. Community turnover was large. Megafauna populations declined or went extinct.

| 2.2.3.2

5.1.2.2

5.3.1.2

8.6.1

9.6.2

|-
| Mid-Holocene (MH)

| 6.5–5.5 ka

| Middle of the present interglacial when the CO <sub>2</sub> concentration was similar to the onset of the industrial era, but the orbital configuration led to warming and shifts in the hydrological cycle, especially NH monsoons. Approximate time during the current interglacial and before the onset of major industrial activities when GMST was highest. Biome-scale loss of North African grasslands caused by weakened monsoons and collapses of temperate tree populations linked to hydroclimate variability. ''PMIP4 mid-Holocene'' , 6 ka ( [[#Otto-Bliesner--2017|Otto-Bliesner et al., 2017]] ; [[#Brierley--2020|Brierley et al., 2020]] )

| 2.3.1.1.2

2.3.2.4

2.3.3.3

3.3.1.1

3.8.2.1

8.3.2.4

8.6.2.2

9.6.2

|-
| Last millennium <sup>c</sup>

| 850–1850 CE

| Climate variability during this period is better documented on annual to centennial scales than during previous reference periods. Climate changes were driven by solar, volcanic, land cover, and anthropogenic forcings, including strong increases in greenhouse gasses since 1750. ''PMIP4 past1000,'' 850–1849 CE ( [[#Jungclaus--2017|Jungclaus et al., 2017]] )

| 2.3.1.1.2

2.3.2.3

8.3.1.6

8.5.2.1

Box 11.3

|}

<sup>a</sup> CE: Common Era; ka: thousands of years ago; Ma: millions of years ago.

<sup>b</sup> The word ‘optimum’ is traditionally used in geosciences to refer to the warmest interval of a geologic period.

<sup>c</sup> The terms ‘Little Ice Age’ and ‘Medieval Warm Period’ (or ‘Medieval Climate Anomaly’) are not used extensively in this report because the timing of these episodes is not well defined and varies regionally. Since AR5, new proxy records have improved climate reconstructions at decadal scale across the last millennium. Therefore, the dates of events within these two roughly defined periods are stated explicitly when possible.

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[[File:0d3ac297d5fc4a0430ecbc376fa3d5fb IPCC_AR6_WGI_CCBox_2_1_Figure_1.png]]

Cross-Chapter Box 2.1, Figure 1 |

'''Global mean surface temperature (GMST) over the past 60 million years (60 Myr) relative to 1850–1900 shown on three time scales.''' Information about each of the nine paleo reference periods (blue font) and sections in AR6 that discuss these periods are listed in Cross-Chapter Box 2.1 Table 1. Grey horizontal bars at the top mark important events. Characteristic uncertainties are based on expert judgement and are representative of the approximate midpoint of their respective time scales; uncertainties decrease forward in time. GMST estimates for most paleo reference periods (Figure 2.34) overlap with this reconstruction, but take into account multiple lines of evidence. Future projections span the range of global surface air temperature best estimates for SSP1–2.6 and SSP5–8.5 scenarios described in [[IPCC:Wg1:Chapter:Chapter-1#1.6|Section 1.6]] . Range shown for 2100 is based on CMIP6 multi-model mean for 2081–2100 from Table 4.5; range for 2300 is based upon an emulator and taken from Table 4.9. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

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