Editing IPCC:AR6/SR15/Chapter-3 (section)

== Box 3.3: Lessons from Past Warm Climate Episodes ==

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Climate projections and associated risk assessments for a future warmer world are based on climate model simulations. However, Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models do not include all existing Earth system feedbacks and may therefore underestimate both rates and extents of changes (Knutti and Sedláček, 2012) <sup>[[#fn:r324|324]]</sup> . Evidence from natural archives of three moderately warmer (1.5°C–2°C) climate episodes in Earth’s past help to assess such long-term feedbacks (Fischer et al., 2018) <sup>[[#fn:r325|325]]</sup> .

While evidence over the last 2000 years and during the Last Glacial Maximum (LGM) was discussed in detail in the IPCC Fifth Assessment Report (Masson-Delmotte et al., 2013) <sup>[[#fn:r326|326]]</sup> , the climate system response during past warm intervals was the focus of a recent review paper (Fischer et al., 2018) <sup>[[#fn:r327|327]]</sup> summarized in this Box. Examples of past warmer conditions with essentially modern physical geography include the Holocene Thermal Maximum (HTM; broadly defined as about 10–5 kyr before present (BP), where present is defined as 1950), the Last Interglacial (LIG; about 129–116 kyr BP) and the Mid Pliocene Warm Period (MPWP; 3.3-3.0 Myr BP).

Changes in insolation forcing during the HTM (Marcott et al., 2013) <sup>[[#fn:r328|328]]</sup> and the LIG (Hoffman et al., 2017) <sup>[[#fn:r329|329]]</sup> led to a global temperature up to 1°C higher than that in the pre-industrial period (1850–1900); high-latitude warming was 2°C-4°C (Capron et al., 2017) <sup>[[#fn:r330|330]]</sup> , while temperature in the tropics changed little (Marcott et al., 2013) <sup>[[#fn:r331|331]]</sup> . Both HTM and LIG experienced atmospheric CO <sub>2</sub> levels similar to pre-industrial conditions (Masson-Delmotte et al. 2013). During the MPWP, the most recent time period when CO <sub>2</sub> concentrations were similar to present-day levels, the global temperature was &gt;1°C and Arctic temperatures about 8°C warmer than pre-industrial (Brigham-Grette et al., 2013) <sup>[[#fn:r332|332]]</sup> .

Although imperfect as analogues for the future, these regional changes can inform risk assessments such as the potential for crossing irreversible thresholds or amplifying anthropogenic changes (Box 3.3, Figure 1). For example, HTM and LIG greenhouse gas (GHG) concentrations show no evidence of runaway greenhouse gas releases under limited global warming. Transient releases of CO <sub>2</sub> and CH <sub>4</sub> may follow permafrost melting, but these occurrences may be compensated by peat growth over longer time scales (Yu et al., 2010) <sup>[[#fn:r333|333]]</sup> . Warming may release CO <sub>2</sub> by enhancing soil respiration, counteracting CO <sub>2</sub> fertilization of plant growth (Frank et al., 2010) <sup>[[#fn:r334|334]]</sup> . Evidence of a collapse of the Atlantic Meridional Overturning Circulation (AMOC) during these past events of limited global warming could not be found (Galaasen et al., 2014) <sup>[[#fn:r335|335]]</sup> .

The distribution of ecosystems and biomes (major ecosystem types) changed significantly during past warming events, both in the ocean and on land. For example, some tropical and temperate forests retreated because of increased aridity, while savannas expanded (Dowsett et al., 2016) <sup>[[#fn:r336|336]]</sup> . Further, poleward shifts of marine and terrestrial ecosystems, upward shifts in alpine regions, and reorganizations of marine productivity during past warming events are recorded in natural archives (Williams et al., 2009; Haywood et al., 2016) <sup>[[#fn:r337|337]]</sup> . Finally, past warming events are associated with partial sea ice loss in the Arctic. The limited amount of data collected so far on Antarctic sea ice precludes firm conclusions about Southern Hemisphere sea ice losses (de Vernal et al., 2013) <sup>[[#fn:r338|338]]</sup> .

Reconstructed global sea level rise of 6–9 m during the LIG and possibly &gt;6 m during the MPWP requires a retreat of either the Greenland or Antarctic ice sheets or both (Dutton et al., 2015) <sup>[[#fn:r339|339]]</sup> . While ice sheet and climate models suggest a substantial retreat of the West Antarctic ice sheet (WAIS) and parts of the East Antarctic ice sheet (DeConto and Pollard, 2016) <sup>[[#fn:r340|340]]</sup> during these periods, direct observational evidence is still lacking. Evidence for ice retreat in Greenland is stronger, although a complete collapse of the Greenland ice sheet during the LIG can be excluded (Dutton et al., 2015) <sup>[[#fn:r341|341]]</sup> . Rates of past sea level rises under modest warming were similar to or up to two times larger than rises observed over the past two decades (Kopp et al., 2013) <sup>[[#fn:r342|342]]</sup> . Given the long time scales required to reach equilibrium in a warmer world, sea level rise will ''likely'' continue for millennia even if warming is limited to 2°C.

Finally, temperature reconstructions from these past warm intervals suggest that current climate models underestimate regional warming at high latitudes (polar amplification) and long-term (multi-millennial) global warming. None of these past warm climate episodes involved the high rate of change in atmospheric CO <sub>2</sub> and temperatures that we are experiencing today (Fischer et al., 2018) <sup>[[#fn:r343|343]]</sup> .

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====== Box 3.3, Figure 1 ======

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==== Impacts and responses of components of the Earth System. ====

[[File:fc4e9d430a967a9155c24261904c9124 box-3.3-figure-1024x746.jpg|thumb|400x300px]]

Summary of typical changes found for warmer periods in the paleorecord, as discussed by Fischer et al. (2018) <sup>[[#fn:r344|344]]</sup> . All statements are relative to pre-industrial conditions. Statements in italics indicate that no conclusions can be drawn for the future. Note that significant spatial variability and uncertainty exists in the assessment of each component, and this figure therefore should not be referred to without reading the publication in detail. HTM: Holocene Thermal Maximum, LIG: Last Interglacial, MPWP: Mid Pliocene Warm Period. (Adapted from Fischer et al., 2018)

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