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A quick guide to paleoclimate in the IPCC AR6 2021 report

Last updated by Basil Davis 26 April 2022

The latest IPCC report "AR6 Climate Change 2021: The physical science basis" provides an excellent overview of our current scientific understanding of the climate system and climate change. In AR1-AR5 paleoclimate had its own dedicated chapter, but in AR6 paleoclimate is distributed throughout the different chapters.

To make it easier to find all of the relevant paleoclimate information, you can find below a full list of quick links to all of the sections that mention paleoclimate. This is preceded by an overview kindly provided by Darrell Kaufmann, IPCC AR6 Lead Author. Note that the links may not work in some browsers working on older operating systems (Safari) and/or with certain security settings.

There is also a short paleo section in chapter 1 of the working group II report which you can directly link to here "AR6 Climate Change 2021: Impacts, Adaptation and Vulnerability. "

 

An overview of paleoclimate in the IPCC AR6

SPM = Summary for Policy Makers

CSB = Cross Section Box

CCB = Cross Chapter Box

TS = Technical Summary

SPM A.2.2 is selected statements from Ch 2 on increasingly UNPRECEDENTED CHANGES across the climate system over centuries to millennia (see Ch 2.2 for forcings, 2.3 & Fig 2.34 for indicators). Fig SPM.1 & CSB TS.1 place instrumental temperature into long-term context.

SPM A.4.4 presents the AR6 assessed value for EQUILIBRIUM CLIMATE SENSITIVITY. Paleoclimate evidence is a key constraint, as presented in Ch 7 (7.5.3, 7.5.6) and summarized in TS.3.2.1.

SPM B.1.1 is on global temperature projections, with comparisons to warm paleo reference periods. Fig TS.1 shows CENOZOIC TEMP AND CO2 HISTORY compared with projections. Fig TS.9 depicts greenhouse gases across time scales. TS.2.2 summarizes changes in CLIMATE DRIVERS.

SPM B.5.1 is on IRREVERSIBILITY OF OCEAN & CRYOSPHERE CHANGES over centuries to millennia, grounded in paleo understanding from Ch 9 (9.2.2.1, 9.2.3.2). B.5.4 is projections of sea level rise, with comparisons to warm paleo reference periods (Box TS.4, Ch 9.6.2, 9.6.3.5).

SPM C.3.4 & C.3.5 are on LOW-LIKELIHOOD, HIGH-IMPACT OUTCOMES, including effects of AMOC collapse and a sequence of large volcanic eruptions, respectively, grounded partly in paleo information in Ch 8 & 9, and CCB 4.1.

TS Box TS.2 brings together PROXY- AND MODEL-BASED estimates of global temperature for multiple paleo reference periods, along with CO2 and sea level. Additional paleo data-model comparisons are in Ch 3 (3.3.1.1, 7.4.4.1.2, 7.5.6 for temp, 3.8.2.1 for multiple variables).

Ch 2 CCB 2.1 describes the PALEOCLIMATE REFERENCE PERIODS used in the report and points to other sections where the periods are discussed. PMIP4 and PMIP3 MODEL OUTPUT for four reference periods (temperature & precipitation) is available through the Interactive Atlas.

TS Box TS.2, along with Ch 1.5.1.1 (paleoclimate), highlight other NEW PALEO FINDINGS from several chapters, including the carbon and water cycles (Ch 5 & 8). The context for paleoclimate science is described in Ch 1.3.2.

FREQUENTLY ASKED QUESTIONS that focus on paleoclimate include, FAQ 1.3 “What can past climate teach us about the future?” and FAQ 2.1 “Earth’s temperature has varied before. How is the current warming any different?”

 

The quickest way to navigate through all of the paleoclimate information is to click on the links below which will take you straight to the relevant section in each chapter. Note that each time you click a link, the relevant chapter (size 6-9mb) will open in a new window, although this may not work in some browsers working on older operating systems (Safari) and/or with certain security settings.

The full report is 130mb (smallest version), but if you want a reduced-size copy of the "AR6 Climate Change 2021: The physical science basis" that contains only the chapters with palaeoclimate information (that is, without chapters 6 and 10), it can be downloaded by clicking on the link here (size 66.4mb).

 



Summary for Policy Makers

 

Figure SPM.1 History of global temperature change and causes of recent warming.

SPM A.2.2 Global surface temperature has increased faster since 1970 than in any other 50-year period in at least the last 2000 years..

SPM A.4.4 The equilibrium climate sensitivity.. AR6 2.5-4°C compared to AR5 1.5-4.5°C

SPM B.1.1 Compared to 1850-1900, global surface temperature averaged over 2081-2100 is very likely to be..

SPM B.5.1 Past GHG emissions since 1750 have committed the global ocean to future warming..

SPM B.5.4 In the longer term, sea level rise is committed to rise for centuries to millenia..

SPM C.3.4 The Atlantic Meridional Overturning Circulation is very likely to weaken over the 21st Century..

SPM C.3.5 Unpredictable and rare natural events not related to human influence on climate may lead to low likelihood, high impact outcomes..



Technical Summary

 

Figure TS.1 Changes in atmospheric CO2 and global surface temperature (relative to 1850-1900) from the deep past to the next 300 years

Box TS.2 Paleoclimate

Box TS.2 Figure 1 Paleoclimate and recent reference periods, with selected key indicators

Box TS.2 Figure 2 Global surface temperature as estimated from proxy records (reconstructed) and climate 50 models (simulated).

Cross Section Box TS.1 Global surface Temperature Change

Cross Section Box TS.1, Figure 1 Earth’s surface temperature history and future with key findings annotated 52 within each panel

TS.2.2 Changes in the Drivers of the Climate System

TS.2.2, Figure TS.9 Changes in well-mixed greenhouse gas (WMGHG) concentrations and Effective Radiative Forcing over the past 3.5 million years

Box TS.4 Sea Level

Box TS.4, Figure 1 Global mean sea level change on different time scales and under different scenarios

TS.3.2.1 Equilibrium Climate Sensitivity, Transient Climate Response, and Transient Climate Response to Cumulative Carbon-dioxide Emissions

TS.3.2.1, Figure TS.16 Evolution of equilibrium climate sensitivity (ECS) assessments from the Charney Report through a succession of IPCC Assessment Reports to AR6



Chapter 1 Framing, context, and methods

 

1.3.2 Lines of evidence: paleoclimate

1.5.1.1 Major expansions of observational capacity (Paleoclimate)

FAQ 1.3 What can past climate teach us about the future?

FAQ 1.3, Figure1 Comparison of past, present and future. Schematic of atmospheric carbon dioxide concentrations, global temperature, and global sea level during previous warm periods compared to the present



Chapter 2 Changing state of the climate system

 

Cross Chapter Box 2.1 Paleoclimate Reference Periods in the Assessment Report

Cross Chapter Box 2.1, Figure 1 Global mean surface temperature (GMST) over the past 60 million years relative to 1850–1900 shown on three time scales

2.2 Changes in Climate Drivers

2.2.1 Solar and Orbital Forcing

Figure 2.2 Time series of solar and volcanic forcing for the past 2.5 kyr and since 1850

2.2.2 Volcanic Aerosol Forcing

2.2.3 Well-mixed Greenhouse Gases (WMGHGs)

2.2.3.1 CO2 during 450 Ma to 800 ka

Figure 2.3 The evolution of atmospheric CO2 through the last 450 million years

2.2.3.2 Glacial-interglacial WMGHG Fluctuations from 800 ka

2.2.3.2.1 CO2

2.2.3.2.2 CH4

2.2.3.2.3 N2O

Figure 2.4 Atmospheric WMGHG concentrations from ice cores

2.2.3.4 Summary of changes in WMGHGs

2.2.6 Aerosols

2.2.7 Land use and land cover

2.3 Changes in large-scale climate

2.3.1 Atmosphere and Earth's surface

2.3.1.1 Surface temperatures

2.3.1.1.1 Temperatures of the deep past (65 Ma to 8 ka)

2.3.1.1.2 Temperatures of the post-glacial period (past 7000 years)

Figure 2.11 Earth’s surface temperature history with key findings

Cross Chapter Box 2.3 New estimates of global warming to date and key implications

2.3.1.3 Global hydrological cycle

2.3.1.3.1 Paleo perspective of the global hydrological cycle

2.3.1.4.1 The Hadley and Walker circulations

2.3.1.4.2 Global monsoon changes

2.3.1.4.3 Extratropical jets, storm tracks, and blocking

2.3.2 Cryosphere

2.3.2.1 Sea ice coverage and thickness

2.3.2.1.1 Arctic Sea Ice

2.3.2.1.2 Antarctic Sea Ice

2.3.2.3 Glacier mass

Figure 2.23 Glacier advance and annual mass change

2.3.2.4 Ice sheet mass and extent

2.3.2.4.1 Greenland Ice Sheet

2.3.2.4.2 Antarctic Ice Sheet

2.3.2.5 Terrestrial permafrost

2.3.3 Ocean

2.3.3.1 Ocean temperature, heat content and thermal expansion

2.3.3.2 Ocean salinity

2.3.3.3 Sea level

Figure 2.28 Changes in global mean sea level

2.3.3.4 Ocean circulation

2.3.3.4.1 Atlantic Meridional Overturning circulation (AMOC)

2.3.3.4.2 Western boundary currents and inter-basin exchanges

2.3.3.5 Ocean pH

Figure 2.29 Low latitude surface ocean pH over the last 65 million years

2.3.3.6 Ocean deoxygenation

2.3.4.3 Terrestrial biosphere

2.3.4.3.2 Terrestrial ecosystems

2.3.5 Synthesis of evidence for past changes

Figure 2.34 Selected large-scale climate indicators during paleoclimate and recent reference periods of the Cenozoic Era

Cross Chapter Box 2.4 The climate of the Pliocene (around 3 million years ago), when CO2 concentrations were last similar to those of present day

Cross Chapter Box 2.4, Figure 1 Climate indicators of the mid-Pliocene Warm Period (3.3–3.0 Ma) from models and proxy data

2.4 Changes in modes of variability

2.4.1.1 Northern Annular Mode (NAM) / North Atlantic Oscillation (NAO)

2.4.1.2 Southern Annular Mode (SAM)

Figure 2.35 Southern Annular Mode (SAM) reconstruction over the last millennium

2.4.2 El Niño-Southern Oscillation (ENSO)

Figure 2.36 Reconstructed and historical variance ratio of El Niño–Southern Oscillation (ENSO).

2.4.3 Indian Ocean Basin and dipole modes

2.4.4 Atlantic Meridional and Zonal Modes

2.4.5 Pacific Decadal Variability

2.4.6 Atlantic Multidecadal Variability

FAQ 2.1 The Earth’s temperature has varied before. How is the current warming any different?

FAQ 2.1, Figure 1 Evidence for the unusualness of recent warming



Chapter 3 Human influence on the climate system

 

3.3 Human Influence on the Atmosphere and Surface

3.3.1 Temperature

3.3.1.1 Surface Temperature

Figure 3.2 Changes in surface temperature for different paleoclimates

3.8.2.1 Integrative Measures of Model Performance

Figure 3.44 Multivariate synopsis of paleoclimate model results compared to observational references

FAQ 3.2 What is Natural Variability and How has it Influenced Recent Climate Changes

FAQ 3.2, Figure 1 Annual, decadal and multi-decadal variations in average global surface temperature



Chapter 4 Future global climate: scenario-based projections and near-term information

 

Cross Chapter Box 4.1 The climate effects of volcanic eruption



Chapter 5 Global Carbon and other Biogeochemical Cycles and Feedbacks

 

5.1.2 Paleo Trends and Feedbacks

5.1.2.1 Cenozoic Proxy CO2 Record

Figure 5.3 Atmospheric CO2 concentrations and growth rates for the past 60 million years and projections to 2100.

5.1.2.2 Glacial-Interglacial Greenhouse Gases Records

Figure 5.4 Atmospheric concentrations of CO2, CH4 and N2O in air bubbles and clathrate crystals in ice cores (800,000 BCE to 1990 CE).

5.1.2.3 Holocene Changes

5.3.1 Paleoclimate Context

5.3.1.1 Paleocene-Eocene Thermal Maximum

5.3.1.2 Last Deglacial Transition

BOX 5.1 Permafrost Carbon and Feedbacks to Climate: What does the paleo record tell us about how much emissions to expect?

5.4.7 Climate Feedbacks from CH4 and N2O



Chapter 7 The Earth’s energy budget, climate feedbacks, and climate sensitivity

 

7.4.4.1.2 Polar amplification from proxies and models during past climates associated with CO2 change

Figure 7.13 Polar amplification in paleo proxies and models of the early Eocene climatic optimum (EECO), the mid-Pliocene warm period (MPWP), and the Last Glacial Maximum (LGM)

7.4.4.2.2 Tropical Pacific temperature gradients in past high-CO2 climates

7.5.3 Estimates of ECS based on paleoclimate data

7.5.3.1 Estimates of ECS from the Last Glacial Maximum

7.5.3.2 Estimates of ECS from glacial-interglacial cycles

7.5.3.3 Estimates of ECS from warm periods of the pre-Quaternary

7.5.3.4 Synthesis of ECS based on paleo radiative forcing and temperature

7.5.6 Considerations on the ECS and TCR in global climate models and their role in the assessment

Figure 7.19 Global mean temperature anomaly in models and observations from 5 time periods



Chapter 8: Water cycle changes

 

8.3.2.4.3 West African Monsoon

8.3.2.4.4 North American Monsoon

8.3.2.4.5 South American Monsoon

8.3.2.4.6 Australian and Maritime Continent Monsoon

8.6.1 Abrupt water cycle responses to a collapse of Atlantic Meridional Overturning Circulation

Figure 8.27 Model simulation of precipitation response to the Younger Dryas event and under doubling of 1990 CO2 levels

8.6.2.2 Greening of the Sahara and the Sahel



Chapter 9: Ocean, cryosphere and sea level change

 

9.2.2.1 Ocean Heat Content and Heat Transport

Figure 9.9 Long-term trends of ocean heat content and surface temperature

9.2.3.2 Southern Ocean

9.3.1.1 Arctic Sea-Ice Coverage

9.3.2.1 Antarctic sea-ice coverage

9.5.1.1 Observed and reconstructed glacier extent and mass changes – Drivers of glacier change

9.6.2 Paleo context of global and regional sea-level change

9.6.3.5 Paleo context of global and regional sea-level change



Chapter 11: Weather and climate extreme events in a changing climate

 

BOX 11.3 Extremes in paleoclimate archives compared to instrumental records