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

== Cross-Chapter Box 2.4 | The Climate of the Pliocene (Around 3 Million Years Ago), When CO <sub>2</sub> Concentrations Were Last Similar to Those of the Present Day ==

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'''Contributing Authors:''' Alan M. Haywood (United Kingdom), Darrell S. Kaufman (United States of America), Nicholas R. Golledge (New Zealand/United Kingdom), Dabang Jiang (China), Daniel J. Lunt (United Kingdom), Erin L. McClymont (United Kingdom), Ulrich Salzmann (United Kingdom/Germany, United Kingdom), Jessica Tierney (United States of America)

Throughout this Report, information about past climate states is presented in the context of specific climate variables, processes or regions. This Cross-Chapter Box focuses on a single paleoclimate reference period as an example of how proxy data, models and process understanding come together to form a more complete representation of a warm climate state that occurred during the relatively recent geologic past.

'''Introduction'''

The Pliocene Epoch is one of the best-documented examples of a warmer world during which the slow responding components of the climate system were approximately in balance with concentrations of atmospheric CO <sub>2</sub> , similar to present (e.g., [[#Haywood--2016|Haywood et al., 2016]]). It provides a means to constrain Earth’s equilibrium climate sensitivity (Section 7.5.3) and to assess climate model simulations (Section 7.4.4.1.2). During the Pliocene, continental configurations were similar to present (Cross-Chapter Box 2.4 Figure 1a), and many plant and animal species living then also exist today. These similarities increase reliability of paleo-environmental reconstructions compared with those for older geological periods. Within the well-studied mid-Pliocene Warm Period (MPWP, also called the mid-Piacenzian Warm Period, 3.3–3.0 Ma), the interglacial period KM5c (3.212–3.187 Ma) has become a focus of research because its orbital configuration, and therefore insolation forcing, was similar to present (global mean insolation = –0.022 W m <sup>–2</sup> relative to modern; [[#Haywood--2013|Haywood et al., 2013]]), allowing for the climatic state associated with relatively high atmospheric CO <sub>2</sub> to be assessed with fewer confounding variables.

'''Major global climate indicators'''

During the KM5c interglacial, atmospheric CO <sub>2</sub> concentration was typically between 360 and 420 ppm ([[#2.2.3.1|Section 2.2.3.1]]). New climate simulations of this interval from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show a multi-model mean global surface air temperature of 3.2 [2.1 to 4.8] °C warmer than control simulations (Cross-Chapter Box 2.4, Figure 1a; [[#Haywood--2020|Haywood et al., 2020]]). This is consistent with proxy evidence for the broader MPWP, which indicates that global mean surface temperature was 2.5°C–4.0°C higher than 1850–1900 ([[#2.3.1.1.1|Section 2.3.1.1.1]]). Global mean sea level was between 5 and 25 m higher than present ([[#2.3.3.3|Section 2.3.3.3]]). Geological evidence ([[#2.3.2.4|Section 2.3.2.4]]) and ice-sheet modelling (Section 9.6.2) indicate that both the Antarctic and Greenland Ice Sheets were substantially smaller than present (Cross-Chapter Box 2.4, Figure 1c). Attribution of sea level highstands to particular ice-sheet sources (Section 9.6.2) is challenging ([[#DeConto--2016|DeConto and Pollard, 2016]]; [[#Golledge--2020|Golledge, 2020]]), but improving ([[#Berends--2019|Berends et al., 2019]]; [[#Grant--2019|Grant et al., 2019]]).

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'''Cross-Chapter Box 2.4, Figure 1 |''' '''Climate indicators of the mid-Pliocene Warm Period (MPWP; 3.3–3.0 million years ago, Ma) from models and proxy data. (a)''' Simulated surface air temperature (left) and precipitation rate anomaly (right) anomaly (relative to 1850–1900) from the Pliocene Model Intercomparison Project Phase 2 multi-model mean, including CMIP6 (n = 4) and non-CMIP6 (n = 12) models. Symbols represent site-level proxy-based estimates of sea-surface temperature for KM5c (n = 32), and terrestrial temperature (n = 8) and precipitation rate for the MPWP (n = 8). '''(b)''' Distribution of terrestrial biomes was considerably different during the Piacenzian Stage (3.6–2.6 Ma) (upper) compared with present-day (lower). Biome distributions simulated with a model (BIOME4) in which Pliocene biome classifications are based on 208 locations, with model-predicted biomes filling spatial gaps, and the present day, with the model adjusted for CO <sub>2</sub> concentration of 324 parts per million (ppm). '''(c)''' Ice-sheet extent predicted using modelled climate forcing and showing where multiple models consistently predict the former presence or absence of ice on Greenland (n = 8 total) and Antarctica (n = 10 total). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

'''Northern high latitudes'''

The latitudinal temperature gradient during the MPWP was reduced relative to present-day and the consistency between proxy and modelled temperatures has improved since AR5 (Section 7.4.4.1.2). Northern high latitude (&gt;60°N) SSTs were up to 7°C higher than 1850–1900 ([[#Bachem--2016|Bachem et al., 2016]]; [[#McClymont--2020|McClymont et al., 2020]]; [[#Sánchez-Montes--2020|Sánchez-Montes et al., 2020]]), and terrestrial biomes were displaced poleward (e.g., [[#Dowsett--2019|Dowsett et al., 2019]]) (Cross-Chapter Box 2.4, Figure 1b). Arctic tundra regions currently underlain by permafrost were warm enough to support boreal forests, which shifted northward by approximately 250 km in Siberia, and up to 2000 km in the Canadian Arctic Archipelago ([[#Salzmann--2013|Salzmann et al., 2013]]; [[#Fletcher--2017|Fletcher et al., 2017]]). The shift caused high-latitude surface albedo changes, which further amplified the Pliocene global warming ([[#Zhang--2014|Zhang and Jiang, 2014]]). Vegetation changes in north-east Siberia indicate that MPWP summer temperatures were up to 6°C higher than present day ([[#Brigham-Grette--2013|Brigham-Grette et al., 2013]]). Farther south, modern boreal forest regions in Russia and eastern North America were covered with temperate forests and grasslands, whereas highly diverse, warm-temperate forests with subtropical taxa were widespread in central and eastern Europe (Cross-Chapter Box 2.4, Figure 1). While seasonal sea ice was present in the North Atlantic and Arctic oceans, its winter extent was reduced relative to present ([[#Knies--2014|Knies et al., 2014]]; [[#Clotten--2018|Clotten et al., 2018]]), and some models suggest that the Arctic was sea ice free during the summer ([[#Howell--2016|Howell et al., 2016]]; [[#Feng--2020|Feng et al., 2020]]).

'''Tropical Pacific'''

The average longitudinal temperature gradient in the tropical Pacific was weaker during the Pliocene than during 1850–1900 (Section 7.4.4.2.2). Changes in Pacific SSTs and SST gradients had far-reaching impacts on regional climates through atmospheric teleconnections, affecting rainfall patterns in western North America ([[#Burls--2017|Burls and Fedorov, 2017]]; [[#Ibarra--2018|Ibarra et al., 2018]]). The reduced zonal SST gradient has led to suggestions that the Pliocene Pacific experienced a ‘permanent El Niño’ state ([[#Molnar--2002|Molnar and Cane, 2002]]; [[#Fedorov--2006|Fedorov, 2006]]). However, there is no direct geological evidence, nor support from climate models, that ENSO variability collapsed during the Pliocene. Although not located in the centre-of-action region for ENSO, Pliocene corals show temperature variability over 3–7 year timescales ([[#Watanabe--2011|Watanabe et al., 2011]]). In addition, a multi-model intercomparison indicates that ENSO existed, albeit with reduced variability ([[#Brierley--2015|Brierley, 2015]]). Thus, there is ''high confidence'' that ENSO variability existed during the Pliocene.

'''Hydrological cycle'''

Vegetation reconstructions for the late Pliocene indicate regionally wetter conditions resulting in an expansion of tropical savannas and woodlands in Africa and Australia at the expense of deserts (Cross-Chapter Box 2.4, Figure 1b). PlioMIP2 climate models generally simulate higher rates of mean annual precipitation in the tropics and high latitudes, and a decrease in the subtropics, with a multi-model mean global increase of 0.19 [0.13–0.32] mm day <sup>–1</sup> relative to control simulations (Cross-Chapter Box 2.4, Figure 1a; [[#Haywood--2020|Haywood et al., 2020]]). Both simulations and ''limited'' proxy ''evidence'' indicate stronger monsoons in northern Africa, Asia, and northern Australia relative to present, but trends are uncertain in other monsoon regions (X. [[#Li--2018|]] [[#Li--2018|Li et al., 2018]]; [[#Yang--2018|Yang et al., 2018]]; X. [[#Huang--2019a|]] [[#Huang--2019|Huang et al., 2019]] a ; R. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]]). There is thus ''medium confidence'' that monsoon systems were stronger during the Pliocene. Simulations of MPWP climate show that global tropical cyclone intensity and duration increased during the MPWP ([[#Yan--2016|Yan et al., 2016]]); however, there is ''low confidence'' in this result because inter-model variability is high.

'''Summary'''

During the MPWP (3.3–3.0 Ma) the atmospheric CO <sub>2</sub> concentration was similar to present, and the slow-response, large-scale indicators reflect a world that was warmer than present. With ''very high confidence'' , relative to present, global surface temperature, sea level, and precipitation rate were higher, NH latitudinal temperature gradient was lower, and major terrestrial biomes expanded poleward. With ''medium confidence'' from proxy-based evidence alone ([[#2.3.2|Section 2.3.2]]), combined with numerical modelling, analysis of the sea-level budget, and process understanding (Section 9.6.2), there is ''high confidence'' that cryospheric indicators were diminished. There is ''medium confidence'' that the Pacific longitudinal temperature gradient was weaker and monsoon systems were stronger.
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