Intergovernmental Panel on Climate Change: Sixth Assessment Report

April 29, 2022 R47082

Intergovernmental Panel on Climate Change’s
April 29, 2022
Sixth Assessment Report
Jonathan D. Haskett
In 2021, the Intergovernmental Panel on Climate Change (IPCC) released Climate Change 2021:
Analyst in Environmental
The Physical Science Basis—Contribution of Working Group I, as part of its Sixth Assessment
Policy
(AR6 WGI). The role of the IPCC as endorsed by the United Nations in plenary session in 1988

is “to provide internationally coordinated scientific assessments of the magnitude, timing and
potential environmental and socio-economic impact of climate change and realistic response

strategies.” This CRS report serves as a primer for the AR6 WGI assessment. It is not
comprehensive, instead presenting key findings pertinent to congressional consideration of risks related to natural and
human-induced climate change and possible legislative responses.
The AR6 WGI presents current evidence of changes in the climate, including, but not limited to, the following: Global
average surface temperature in 2011-2020 increased by approximately 1.0oC above the preindustrial period of 1850-1900;
heatwaves have occurred more often and with greater intensity since the 1950s, while cold extremes have occurred less often
in the same time period; scientists have high confidence that there has been a global increase in co-occurring droughts and
heatwaves since 1950; terrestrial global average precipitation has increased, as has the frequency of heavy precipitation
events; Arctic sea ice has decreased, while Antarctic sea ice has remained largely unchanged.
The AR6 report reinforces earlier IPCC scientific assessments that human activities are the primary driver of many of these
observed changes. The new report concludes, “It is unequivocal that human influence has warmed the atmosphere, ocean, and
land.” While climate change is the result of human and natural climate drivers, greenhouse gases (GHGs) emitted by human
activity have had the greatest effect, including the observed increase in global average surface temperature.
Since the release of the 2014 IPCC Fifth Assessment Report (AR5), information on climatic change has continued to
accumulate, supporting greater scientific certainty regarding some aspects of this change. As the AR6 WGI Summary for
Policy Makers states, “Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and
tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5.”
The AR6 WGI provides five emissions scenarios based on a wide range of assumptions about the future, covering
socioeconomic behavior, technologies and resources, and possible efforts to mitigate GHG emissions, deforestation, air
pollution, and other contributors to climate change. The two highest GHG emissions scenarios are associated with mean
projected temperature increases of greater than 3.5oC (all increases are for the period 2081-2100 relative to 1850-1900).
Mean projected temperature increases for the two lowest GHG emissions scenarios are below 2oC. A middle scenario, which
some associate as closest to current policy trajectories, has a best-estimate increase of 2.7oC. The two lowest emissions
scenarios are both based on net-zero carbon dioxide (CO2) emissions (CO2 emissions entirely offset by CO2 removals) after
2050, and thereafter negative emissions, in which CO2 removals are greater than CO2 emissions and the concentration of CO2
in the atmosphere decreases. Net-zero CO2 emissions and negative CO2 emissions do not occur in the other, higher emissions
scenarios. Only the two lowest GHG emissions scenarios, with substantial GHG mitigation assumed, result in stabilizing the
increase in global mean temperature, while the other emissions scenarios are still on an ascending temperature trajectory at
2100.
The AR6 WGI states the physical science basis of limiting climate change: “From a physical science perspective, limiting
human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2
emissions, along with strong reductions in other greenhouse gas emissions.” Congress may wish to exercise its oversight
authorities in examining national progress toward fulfilling the updated U.S. Nationally Determined Contributions (NDCs) to
mitigating climate change and consider options for shaping and funding the U.S. approach to the Paris Agreement
commitments.

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Introduction
Scientific interest, investigation, and understanding of the effect of carbon dioxide (CO2) and
other greenhouse gases (GHGs) on the climate have been ongoing for over 100 years.1 Since at
least the 1980s, there has been an international focus on potential changes to the earth’s climate
system resulting from increases in the concentrations of CO2 and other GHGs in the atmosphere
due to human activity, as well as on discerning the roles of human and natural drivers of change.
To address this issue, the Intergovernmental Panel on Climate Change (IPCC) was formed by the
United Nations Environment Program and the World Meteorological Organization in 1988.2 In
that year, the United Nations endorsed a role for the IPCC “to provide internationally coordinated
scientific assessments of the magnitude, timing and potential environmental and socio-economic
impact of climate change and realistic response strategies”3 to government policymakers
internationally.4 There are 195 members of the IPCC, including the United States. Since 1990, the
IPCC has provided scientific information to the international effort to address climate change,
including to negotiators of the 1992 United Nations Framework Convention on Climate Change
(UNFCCC); its subsidiary agreements, the Kyoto Protocol (KP) and the Paris Agreement (PA);
and the annual negotiation sessions for these agreements that occur at their respective
Conferences of the Parties (COPs).
The IPCC has three working groups. Working Group I (WGI) is mandated to address the physical
science basis of climate change. Working Group II (WGII) focuses on the impacts of climate
change, vulnerability to these impacts, and the potential for climate change adaptation. Working
Group III (WGIII) has the mitigation of climate change as its topic area. Original research is not
part of the IPCC mandate, but the IPCC, through the various working groups, provides “regular
assessments of the scientific basis of climate change, its impacts and future risks, and options for
adaptation and mitigation.”5
Since the early 1990s, the IPCC has published a series of climate-related reports and assessments.
Assessments by the IPCC are compiled by volunteer experts, organized in working groups who
review a range of materials including peer reviewed, published, scientific studies in an effort to
provide a comprehensive understanding of the current state of knowledge of climate change. The
review process for IPCC assessment reports is extensive and starts with the selection of experts as
authors and reviewers from lists provided by the IPCC member governments and participant
organizations. The assessment reports go through first- and second-order drafts, with expert
review at each stage and with the second-order draft distributed to all IPCC member governments
for review and comment. A final draft is subsequently distributed to all IPCC member
governments. The final report is then subject to the IPCC endorsement process: “all IPCC reports
must be formally endorsed by the responsible Working Group or Working Groups or Task Force
and by the Panel at an IPCC Plenary Session.”6 Climate Change 2021—The Physical Science

1 CRS Report R45086, Evolving Assessments of Human and Natural Contributions to Climate Change, by Jane A.
Leggett.
2 IPCC, About the IPCC, https://www.ipcc.ch/about/.
3 United Nations General Assembly, Resolutions and Decisions Adopted by the General Assembly During Its Forty-
Third Session Volume 1 20 September-22 December 1988, December 31, 1989, pp. 133-134, at https://documents-dds-
ny.un.org/doc/UNDOC/GEN/NR0/761/73/img/NR076173.pdf?OpenElement.
4 IPCC, About the IPCC, https://www.ipcc.ch/about/.
5 IPCC, The Intergovernmental Panel on Climate Change, https://www.ipcc.ch/.
6 IPCC, Preparing Reports, https://www.ipcc.ch/about/preparingreports/.
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Basis is the sixth and most recent physical science assessment (AR6 WGI) of the IPCC on the
physical science basis of climate change.7 The WGII and WGIII reports are scheduled for
publication in 2022.
Congress may find the IPCC assessment useful to understanding the degree to which the earth’s
climate has changed and may change in the future, the contributions of human and natural factors
to observed changes, and some of the projected physical implications of climate change for
human and natural systems. Congress may choose to consider the information in its deliberations
on climate-relevant legislation, oversight of federal climate-related programs, and decisions on
appropriations and oversight with respect to climate change science and federal activities.
For example, as a compendium of the strengths and uncertainties in climate science, AR6 WGI
may provide insights that Congress could consider as part of its oversight and appropriations
decisions for the scientific activities associated with the U.S. Global Change Research Program.8
The AR6 WGI might be a useful resource for Congress in the evaluation of the program’s level of
funding, the allocation of resources among participating agencies, and the setting of research
priorities.
As a concise introduction to the AR6 WGI, this CRS report, while not comprehensive, presents
key findings from the Sixth Assessment Report that Congress may find relevant to the
consideration of possible legislative responses to climate change. This CRS report has four
sections. The first section presents the AR6 WGI findings regarding the current state of climate
science, including the human influence on planetary warming, as well as currently observed
climatic changes. The second section describes the scenario methodology used in AR6 WGI to
provide an analysis of possible climate futures, using greenhouse gas emissions levels associated
with a set of socioeconomic pathways. The third section describes the framework that AR6 WGI
uses to present a scientific basis for limiting future climate change. The fourth section provides an
analysis of the relationship between the scenarios used in AR6 WGI and the temperature
benchmarks of the Paris Agreement, as well as the potential application of AR6 WGI in
congressional evaluation of possible climate mitigation actions.
The Current State of Climate Science: Selected
Findings of AR6 WGI
The IPCC AR6 WGI presents an examination of the current scientific understanding of climate
change. It weighs evidence regarding whether and what changes may have occurred in the
climate and climate-sensitive earth system components. The report’s findings include
 a statement of human influence on warming,
 a detailed analysis of the human and natural drivers of that warming,
 a presentation of climate changes that have already been observed, and
 a section on the unprecedented nature of some of these changes.

7 Intergovernmental Panel on Climate Change (IPCC), Climate Change 2021: The Physical Science Basis—
Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change,
2021 (hereinafter IPCC WGI 2021).
8 The U.S. Global Change Research program was mandated by Congress under the Global Change Research Act of
1990 “to provide for development and coordination of a comprehensive and integrated United States research program
which will assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural
processes of global change” (P.L. 101-606).
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An analysis of each of these findings is presented in the following sections.
Human Influence on Planetary Warming
The question of whether and how human activity, among other potential drivers, has contributed
to climate change has been discussed and debated for many years. Scientists have considered
many potential natural and human-induced drivers of climate change and have examined these
influences for their effect on the current climate using evidence from both historical and
geological records. Each version of the WGI report has included a precisely worded assessment
of the human role in climate change, which has been monitored by many governments, news
media, and civil society organizations around the world. Among the central findings of the AR6
WGI was the following: “It is unequivocal that human influence has warmed the atmosphere,
ocean, and land.”9
The statement is supported by a mechanism of human influence—the increases in well-mixed10
greenhouse gases (GHGs),11 including CO2, methane (CH4), nitrous oxide (N2O), and
halogenated gases (e.g., chlorofluorocarbons [CFCs], hydrochlorofluorocarbons [HCFCs], and
hydrofluorocarbons [HFCs]) in the atmosphere: “Observed increases in well-mixed greenhouse
gas (GHG) concentrations since around 1750 are unequivocally caused by human activities.”12
The heat-trapping character of GHGs in the atmosphere is one of their known physical
properties.13
The AR6 WGI statement on human influence on warming differs from the earlier assessment of
human influence included in the 2014 IPCC Fifth Assessment Report (AR5),14 which states that

9 IPCC, “Summary for Policymakers,” in Climate Change 2021: The Physical Science Basis—Contribution of Working
Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, 2021, p. 4 (hereinafter
IPCC SPM WGI 2021).
10 The glossary in IPCC WGI 2021 defines well-mixed greenhouse gas as follows:
Well-mixed greenhouse gas.... A greenhouse gas that has an atmospheric lifetime long enough (> several
years) to be homogeneously mixed in the troposphere, and as such the global average mixing ratio can be
determined from a network of surface observations. For many well-mixed greenhouse gases, measurements
made in remote regions differ from the global mean by < 15%.
11 The glossary in IPCC, Climate Change 2014: Mitigation of Climate Change—Contribution of Working Group III to
the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2014, provides the following
definition of greenhouse gases:
Greenhouse gases are those gaseous constituents of the atmosphere, both natural and
anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of
terrestrial radiation emitted by the earth’s surface, the atmosphere itself, and by clouds. This
property causes the greenhouse effect.
12 IPCC SPM WGI 2021, p. 4.
13 CRS Report R45086, Evolving Assessments of Human and Natural Contributions to Climate Change, by Jane A.
Leggett.
14 IPCC, Climate Change 2014: Synthesis Report—Contribution of Working Groups I, II and III to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change, 2014 (hereinafter IPCC AR5 WGI,II,III 2014).
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anthropogenic GHGs (i.e., those emitted through human activity) “are extremely likely15 to have
been the dominant cause of the observed warming since the mid-20th century.”16
Patterns and Drivers of Global Surface Temperature Changes
Among the topics the IPCC has addressed are the patterns of surface temperature changes and the
drivers of those observed changes. The AR6 WGI report addresses the observed increase in global
surface temperature of the earth since the industrial revolution in the following statement:
“Global surface temperature was 1.09 [0.95 to 1.20] °C higher in 2011–2020 than 1850–1900.”17
AR6 WGI provides an analysis of the magnitude and causes of this increase, with statements of
confidence and likelihood.18 The observed increase in global mean temperature is stated as the
result of both human and natural influences, known as drivers of variability and potential
changes,19 which have both heating and cooling effects. The AR6 WGI puts this increase into the
context of changes in the earth’s temperature that have occurred in the past. A presentation of
these AR6 WGI findings appears later in this CRS report.
Well-mixed anthropogenic GHGs20 have the largest reported effect of any driver analyzed, and
this is a warming effect. Human emissions of sulfur dioxide aerosols and nitrogen oxides have a
cooling effect. Halogenated gases have a warming effect, although those that destroy
stratospheric ozone (CFCs, HCFCs, and others) had a cooling effect on the stratosphere from
1979 until their control in the mid-1990s21 under the Montreal Protocol.22 Other human drivers

15 IPCC notes the following in Climate Change 2014: Mitigation of Climate Change—Contribution of Working Group
III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2014:
In the Synthesis Report, the certainty in key assessment findings is communicated as in the
Working Group Reports and Special Reports. It is based on the author teams’ evaluations of
underlying scientific understanding and is expressed as a qualitative level of confidence (from very
low to very high) and, when possible, probabilistically with a quantified likelihood (from
exceptionally unlikely to virtually certain). Where appropriate, findings are also formulated as
statements of fact without using uncertainty qualifiers.
16 IPCC AR5 WGI,II,III 2014, p. 17.
17 IPCC SPM WGI 2021, p. 5.
18 IPCC SPM WGI 2021, p. 4:
Each finding is grounded in an evaluation of underlying evidence and agreement. A level of
confidence is expressed using five qualifiers: very low, low, medium, high and very high, and
typeset in italics, for example, medium confidence. The following terms have been used to indicate
the assessed likelihood of a outcome or result: virtually certain 99–100% probability; very likely
90–100%; likely 66–100%; about as likely as not 33–66%; unlikely 0–33%; very unlikely 0–10%;
and exceptionally unlikely 0–1%. Additional terms (extremely likely 95–100%; more likely than
not >50–100%; and extremely unlikely 0–5%) are also used when appropriate. Assessed likelihood
is typeset in italics, for example, very likely. This is consistent with AR5. In this Report, unless
stated otherwise, square brackets [x to y] are used to provide the assessed very likely range, or 90%
interval.
19 For explanation of climate, climate variability, and climate change, see CRS In Focus IF11446, Weather and Climate
Change: What’s the Difference?, by Jane A. Leggett.
20 The U.S. Geological Survey defines anthropogenic as “referring to environmental change caused or influenced by
people, either directly or indirectly.” See U.S. Geological Survey, Earthword: Anthropogenic, https://www.usgs.gov/
news/science-snippet/earthword-anthropogenic.
21 IPCC SPM WGI 2021, p. 5.
22 The Montreal Protocol is an international treaty for the control of substances that deplete the stratospheric ozone
layer. See U.S. Department of State, The Montreal Protocol on Substances That Deplete the Ozone Layer,
https://www.state.gov/key-topics-office-of-environmental-quality-and-transboundary-issues/the-montreal-protocol-on-
substances-that-deplete-the-ozone-layer/.
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include the cooling effect of irrigation and some land-use changes, as well as the warming effect
of aviation contrails and the inefficient combustion of biomass and fossil fuels that release soot,
also known as black carbon. The natural drivers of global surface temperature change include
changes in solar activity and volcanic eruptions, as well as the natural net internal variability of
the climate system itself (see Figure 1).
Regarding the contribution to warming, the AR6 WGI concludes the following:
It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human
drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers
changed global surface temperature by –0.1°C to +0.1°C, and internal variability changed
it by –0.2°C to +0.2°C. It is very likely that well-mixed GHGs were the main driver of
tropospheric warming since 1979 and extremely likely that human-caused stratospheric
ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and
the mid-1990s.23
The observed global temperature increase is the net result of all drivers, both human and natural,
with both warming and cooling effects, acting simultaneously (Figure 1, panel (a)). Human-
caused drivers of both warming and cooling are the largest of these drivers, with the warming
effect of human-emitted well-mixed GHGs being the largest of all (Figure 1, panels (b) and (c)).
Figure 1. Global Surface Warming: Human and Natural Drivers

Source: Adapted from Figure SPM.2, panels (a), (b), and (c), in IPCC, “Summary for Policymakers,” in Climate
Change 2021: The Physical Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the
Intergovernmental Panel on Climate Change, 2021, p. 7.

23 IPCC SPM WGI 2021, p. 5.
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Notes: Panel (a) presents observed warming, the net result of the influences of all drivers pictured in panels (b)
and (c). Panel (b) disaggregates the contributions to the warming between human drivers (e.g., well-mixed GHGs
[warming = red] and stratospheric ozone depletion [cooling = blue]) and natural drivers (i.e., solar and volcanic
activity) and internal climate variability, derived from attribution studies. Panel (c) presents values of individual
drivers based on the calculations of their radiative forcing, or net warming (red bars) or cooling effect (blue
bars).
The global surface temperature response to the combination of human and natural drivers and to
natural drivers alone has been modeled and compared with observed values. This comparison is
presented and discussed below.
Currently Observed Climate Changes
AR6 WGI states that from about 1750 to the present,24 “Widespread and rapid changes in the
atmosphere, ocean, cryosphere and biosphere have occurred.”25 AR6 WGI presents numerous
changes observable in data obtained from monitoring of the climate and climate-sensitive aspects
of the earth system. The report identifies where climate-related changes are detectable, and with
some detectable changes, the report assesses the likelihoods that a particular change is attributable
to human influence. AR6 WGI states varying levels of confidence in being able to attribute
observed changes to human influence. Further, there are varying levels of certainty about whether
particular changes are influenced by human activity, and these are also reported explicitly.26 The
changes that are observed are not uniform with respect to their extent, the direction of the change,
or the regional character of the change. As illustrated in the following quotation, the AR6 WGI
concludes that changes in extremes of hot and cold are virtually certain to have occurred:
It is virtually certain that hot extremes (including heatwaves) have become more frequent
and more intense across most land regions since the 1950s, while cold extremes (including
cold waves) have become less frequent and less severe, with high confidence that human-
induced climate change is the main driver of these changes. Some recent hot extremes
observed over the past decade would have been extremely unlikely to occur without human
influence on the climate system.27
One manifestation of large-scale regional differences in the results of climate change is the
observed quantity and extent of polar sea ice. Seasonal Arctic sea ice has decreased since 1979, a
trend that is very likely to have been human induced, while seasonal Antarctic sea ice has
remained unchanged in the same period.28
Retreat of glaciers is a consistent global phenomenon across many regions and assessed as very
likely to have been influenced by human activity. The warming of the upper ocean since the
1970s is a globally consistent trend that is virtually certain and for which AR6 WGI states human
influence to be extremely likely:29 “ocean warming is largest near the surface, and the upper 75 m
warmed by 0.11 [0.09 to 0.13] °C per decade over the period 1971 to 2010.”30

24 IPCC SPM WGI 2021, pp. 4-6.
25 IPCC SPM WGI 2021, p. 4.
26 Where statements of levels of certainty appear in this CRS report, they are sourced from AR6 WGI unless stated
otherwise.
27 IPCC SPM WGI 2021, p. 8.
28 IPCC SPM WGI 2021, p. 5.
29 IPCC SPM WGI 2021, p. 5.
30 IPCC WGI 2021, p. 1-168.
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AR6 WGI states that there have likely been changes in the occurrence of major tropical cyclones,
but the findings regarding trends and attribution are less certain:
It is likely that the global proportion of major (Category 3–5) tropical cyclone occurrence
has increased over the last four decades ... these changes cannot be explained by internal
variability alone (medium confidence). There is low confidence in long-term (multi-
decadal to centennial) trends in the frequency of all-category tropical cyclones.31
Unprecedented Changes in the Climate System
AR6 WGI states that some of the changes in the climate, or their observable effects, are
unprecedented on a scale of centuries to millennia. This assessment is based on groups of studies
estimating past CO2, CH4, and N2O atmospheric concentrations, using a variety of analytic
techniques, to cover periods going back 800,000 years for CH4 and N2O, and longer periods for
CO2.
In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million
years (high confidence), and concentrations of CH4 and N2O were higher than at any time
in at least 800,000 years (very high confidence). Since 1750, increases in CO2 (47%) and
CH4 (156%) concentrations far exceed—and increases in N2O (23%) are similar to—the
natural multi-millennial changes between glacial and interglacial periods over at least the
past 800,000 years (very high confidence).32
In AR6 WGI there is medium confidence that global surface temperatures are now higher than
they have been for thousands of years (see Figure 2, panel (a)).
Temperatures during the most recent decade (2011–2020) exceed those of the most recent
multi-century warm period, around 6500 years ago [0.2°C to 1°C relative to 1850–1900]
(medium confidence). Prior to that, the next most recent warm period was about 125,000
years ago, when the multi-century temperature [0.5°C to 1.5°C relative to 1850–1900]
overlaps the observations of the most recent decade (medium confidence).33

31 IPCC SPM WGI 2021, p. 9.
32 IPCC SPM WGI 2021, p. 8.
33 IPCC SPM WGI 2021, p. 8.
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Figure 2. Changes in Global Surface Temperature

Source: Adapted from Figure SPM.1, in IPCC, “Summary for Policymakers,” in Climate Change 2021: The Physical
Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on
Climate Change, 2021, p. 6.
Notes: In panel (a), the solid gray line is reconstructed from paleoclimate archives (years 1-2000), while the
black line is from direct observations (years 1850-2020). The gray bar on the left side of the figure is the very
likely temperature range for the warmest multicentury period from 100,000 years ago to the present. The very
likely range of temperature reconstructions is indicated by gray shading. In panel (b), the black line is the
observed global surface temperature over 1850-2020. The brown line is the Coupled Model Intercomparison
Project 6 (CMIP6) climate model simulation for the same period, showing the modeled temperature response to
human and natural climate drivers. The green line is the temperature response simulation of the same model
with only natural drivers. The shaded areas are the very likely ranges of the simulations with corresponding
colors.
In Figure 2, panel (b), the values for simulations using only natural drivers diverge from
observed values after 1950, with observed values increasing to values above 1.0oC by 2020, while
values for natural drivers alone remain below 0.5oC. The values for simulations of the
combination of human and natural drivers are also above 1.0oC by 2020. In some instances, it is
not only the magnitude of the change but the speed with which the change has occurred that is
unprecedented: “Global surface temperature has increased faster since 1970 than in any other 50-
year period over the last 2000 years.”34 Additionally: “Global mean sea level has risen faster since
1900 than over any preceding century in at least the last 3000 years (high confidence).”35
Possible Climate Futures: Climate Pathways
in AR6 WGI
Emissions Scenarios in AR6 WGI
In AR6 WGI, a set of five illustrative emissions scenarios are used as the primary way of
examining and presenting possible climate futures. As AR6 WGI explains, “A set of five new

34 IPCC SPM WGI 2021, p. 8.
35 IPCC SPM WGI 2021, p. 8.
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illustrative emissions scenarios ... explore the climate response to a broader range of greenhouse
gas (GHG), land-use and air pollutant futures than assessed in AR5. This set of scenarios drives
climate model projections of changes in the climate system.”36 The scenarios are designed to span
a range of plausible futures. The following key aspects and issues pertaining to the scenarios, and
their relationship to possible climate futures, are presented in this section of the report:
 descriptions of the scenarios,
 GHG emissions trajectories of the scenarios, and
 scenarios and temperature trends.
At the end of this report, an example is provided of an analysis using the AR6 WGI scenario
information, with potential relevance to aspects of the Paris Agreement.
Scenario Descriptions
AR6 WGI includes five illustrative Shared Socioeconomic Pathway (SSP) scenarios, used to
explore a set of possible climate futures. The SSP scenarios were developed using varying sets of
assumptions about socioeconomic trajectories, resource and technological change, and alternative
policy frameworks for environmental and other possible objectives.37 A specific example of the
kinds of variation in policy frameworks that form the basis of the SSP scenarios are the
differences in the strength of air pollution controls included in the underlying socioeconomic
narratives.38 Each SSP scenario includes values for GHG emissions, land-use changes, and air
pollution levels for the period from 2015 to 2100.39 These are used to model the climate response,
including changes in global surface temperature, arctic sea ice extent, and global sea-level rise,
among others. The scenarios are used as reference points throughout AR6 WGI to identify these
future climate pathways and to provide a common framework for discussion.
There are two low-emissions scenarios, SSP1-1.9 and SSP1-2.6; one that is mid-range, SSP2-4.5;
and two that are high-emissions scenarios, SSP3-7.0 and SSP5-8.5. In the scenario designation,
SSP refers to the Shared Socioeconomic Pathway assumptions underlying the scenario, while the
second number is the scenarios’ radiative forcing40 at 2100, a measure of warming capacity. In
order to reach specified endpoints of radiative forcing by 2100, the creators of the scenarios
generated alternative storylines of population, economic, geopolitical, technological, and other
assumptions that would be consistent with socioeconomic and emissions pathways reaching those
specified endpoints.
Scenarios and Emissions Trajectories
The five emissions scenarios included for analysis in AR6 WGI provide a wide range of different
emissions trajectories spanning the period 2015 to 2100. There are scenarios in which emissions
continue to increase to 2100 and scenarios with sharp, rapid emissions reductions that eventually
turn to net removals of CO2 after mid-century. Across all of the scenarios, CO2 is the largest

36 IPCC SPM WGI 2021, p. 12.
37 IPCC WGI 2021, p. 1-98.
38 IPCC WGI 2021, p. TS-22.
39 IPCC WGI 2021, p. TS-21.
40 Atmospheric GHGs trap heat, creating an energy imbalance with more energy coming in to the earth system than
going out, resulting in warming. The difference between the energy coming in from the sun and the energy emitted
back into space by the earth is referred to as radiative forcing. See National Oceanic and Atmospheric Administration,
Climate Forcing, https://www.climate.gov/maps-data/climate-data-primer/predicting-climate/climate-forcing.
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contributor to global surface temperature increases that the climate models project at the end of
the period of analysis.
By 2100, CO2 emissions under the SSP3-7.0 are slightly more than double CO2 emissions levels
of 2015, while CO2 emissions under SSP5-8.5 are more than triple those of 2015. The CO2
emissions trajectories of these two high-emissions scenarios differ over the period 2015-2100.
Scenario SSP3-7.0 increases continuously over the time period, while SSP5-8.5 begins to decline
after 2085. SSP2-4.5, the mid-range emissions scenario, increases above 2015 levels at a lower
rate than either SSP3-7.0 or SSP5-8.5 and then begins to decline after 2050 (Figure 3).
Global CO2 emissions under the two lowest emissions scenarios, SSP1-1.9 and SSP1-2.6, never
increase above 2015 emissions levels and decline after 2020 (Figure 3). After 2050, both of these
scenarios reach a point of “net zero” CO2 emissions, where emissions of CO2 are balanced by
permanent removals of CO2.41 After reaching net zero emissions, the lowest scenarios assume
“negative emissions”42 in which, as a result of human activities, removals of CO2 from the
atmosphere exceed emissions, such that CO2 concentrations in the atmosphere decline (Figure 3).
Figure 3. Future Emissions and Temperature Increases for the AR6 WGI Scenarios

Source: Adapted from Figure SPM.4, in IPCC, “Summary for Policymakers,” in Climate Change 2021: The Physical
Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on
Climate Change, 2021, p. 13.
Note: The figure il ustrates the human-caused annual emissions of CO2, non-CO2 GHGs, and SO2 for the
scenarios presented in AR6 WGI during the 2015-2100 time frame.
Negative emissions rely on carbon dioxide removals (CDRs), technologies and methods to
remove carbon dioxide from the atmosphere and permanently sequester it. CDR technologies

41 IPCC SPM WGI 2021, p. 30.
42 The glossary in IPCC AR5 WGI,II,III 2014 provides the following definition of net negative emissions: “A situation
of net negative emissions is achieved when, as result of human activities, more greenhouse gases (GHGs) are
sequestered or stored than are released into the atmosphere.”
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include afforestation, reforestation, soil carbon sequestration,43 bioenergy with carbon capture and
storage44 (BECCS), ocean fertilization, enhanced weathering, and direct air carbon capture and
storage45 (DACCS).46 Of these, BECCS and afforestation/reforestation (AR) are the CDR
methods that are more typically assumed in scenarios that hold global temperature increases
below 2°C, although both of these CDRs face land-use constraints.47 The effects of most CDR
technologies and methods are speculative, as their utility has not been demonstrated at the scales
required to achieve the levels of CO2 removal assumed in the lowest emissions scenarios.
The pattern of non-CO2 GHG emissions for the scenarios differs from those of CO2. Under the
SSP1-2.6, SSP1-1.9, and SSP2-4.5 scenarios, CH4 emissions decline below 2015 levels by 2100,
but never achieve net zero. This projection is also the case for N2O. Emissions of CH4 and N2O
are highest at 2100 for SSP3-7.0 and higher than SSP5-8.5, which has the highest CO2 emissions
at this point in the analysis. Emissions of the air pollutant SO2 decline for all scenarios, but do not
reach net zero. (See Figure 3.)
All the SSPs result in global average temperature increases that exceed 1.5oC around mid-century
or earlier. Of the five emissions scenarios included in AR6 WGI, SSP1-2.6 and SSP1-1.9 are very
likely to keep the global surface temperature increase below 2.0oC by 2100, while SSP1-1.9
returns the increase, following a peak, to 1.4oC relative to preindustrial levels, by 2100. The other,
higher emissions scenarios—SSP2-4.5, SSP3-7.0 and SSP5-8.5—have temperature increases at
2100 that have best estimates ranging from 2.7oC to 4.4oC. (See Table 1.)

43 For more information, see CRS Report R46956, Agriculture and Forestry Offsets in Carbon Markets: Background
and Selected Issues, by Genevieve K. Croft et al.
44 For more information, see CRS Report R44902, Carbon Capture and Sequestration (CCS) in the United States, by
Angela C. Jones and Ashley J. Lawson.
45 For more information, see CRS In Focus IF11501, Carbon Capture Versus Direct Air Capture, by Ashley J. Lawson.
46 The following definitions are contained in the IPCC, “Annex I: Glossary” in Global Warming of 1.5°C: An IPCC
Special Report on the Impacts of Global Warming of 1.5°C Above Pre-industrial Levels and Related Global
Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate
Change, Sustainable Development, and Efforts to Eradicate Poverty, 2018:
Afforestation: Planting of new forests on lands that historically have not contained forests.
Reforestation: Planting of forests on lands that have previously contained forests but that have been
converted to some other use. Soil carbon sequestration (SCS): Land management changes which
increase the soil organic carbon content, resulting in a net removal of CO2 from the atmosphere.
Bioenergy with carbon dioxide capture and storage (BECCS): Carbon dioxide capture and storage
(CCS) technology applied to a bioenergy facility. Ocean fertilization: Deliberate increase of
nutrient supply to the near-surface ocean in order to enhance biological production through which
additional carbon dioxide (CO2) from the atmosphere is sequestered. Enhanced weathering:
Enhancing the removal of carbon dioxide (CO2) from the atmosphere through dissolution of silicate
and carbonate rocks by grinding these minerals to small particles and actively applying them to
soils, coasts or oceans. Direct air carbon dioxide capture and storage (DACCS): Chemical process
by which CO2 is captured directly from the ambient air, with subsequent storage.
47 IPCC, Global Warming of 1.5°C: An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre-
industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global
Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty, 2018, p. 316.
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Table 1. Changes in Global Surface Temperature During the 21st Century for
Emissions Scenarios Included in AR6 WGI

Near term, 2021-2040
Mid-term, 2041-2060
Long term, 2081-2100
Best
Best
Very likely
Best estimate
Very likely
estimate
Very likely
Scenario
estimate (°C)
range (°C)
(°C)
range (°C)
(°C)
range (°C)
SSP1-1.9
1.5
1.2 to 1.7
1.6
1.2 to 2.0
1.4
1.0 to 1.8
SSP1-2.6
1.5
1.2 to 1.8
1.7
1.3 to 2.2
1.8
1.3 to 2.4
SSP2-4.5
1.5
1.2 to 1.8
2.0
1.6 to 2.5
2.7
2.1 to 3.5
SSP3-7.0
1.5
1.2 to 1.8
2.1
1.7 to 2.6
3.6
2.8 to 4.6
SSP5-8.5
1.6
1.3 to 1.9
2.4
1.9 to 3.0
4.4
3.3 to 5.7
Source: Adapted from Table SPM.1 in IPCC, “Summary for Policymakers,” in Climate Change 2021: The Physical
Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on
Climate Change, 2021, p. 14.
Notes: The table presents global surface temperature changes reported for three time periods during the 21st
century, for the five emissions scenarios presented in AR6 WGI. Temperature changes are in relation to average
global surface temperature during the time period 1850-1900.
Not all modelers accept that the SSP1-1.9 scenario could be achieved.48 Even should SSP1-1.9
prove feasible, achieving this temperature is not assured; even with the lowest emissions scenario
considered in AR6 WGI, there is uncertainty about achieving this outcome.
For the high-emission SSP5-8.5 scenario, the best estimate for the temperature increase by 2100
is 4.4oC. Experts have questioned whether SSP5-8.5 is a plausible scenario.49 In AR6 WGI no
likelihood is assigned to any of the scenarios, and all are considered possible.50 One objection to
SSP5-8.5 is that it requires “an unprecedented five-fold increase in coal use by the end of the
century”51 to achieve its designated radiative forcing endpoint. An argument supporting SSP5-8.5
is that the levels of emissions need not come from fossil fuels alone, but that emissions from land-
use changes, such as deforestation, could be large enough to make the scenario’s emissions levels
plausible.52 The scenarios included in AR6 WGI are “one of many possible scenarios that could
be consistent with the concentration pathway [resulting in the range of radiative forcing levels
including that of 8.5 Wm-2].”53 Other scenarios, more closely resembling recent GHG levels,
investment patterns, and policies, suggest that current GHG trajectories more closely resemble
SSP2-4.5, and could fall in a range of the middle scenarios. In an article published in Global

48 A key analysis on the scenarios used by the IPCC states, “1.9 W m−2 scenarios could not be achieved in several
models under SSPs with strong inequalities, high baseline fossil-fuel use, or scattered short-term climate policy.” Joeri
Rogelj et al., “Scenarios towards Limiting Global Mean Temperature Increase below 1.5 °C,” Nature Climate Change
vol. 8, no. 4 (April 2018).
49 References to multiple critiques are in Zeke Hausfather, “Explainer: The High-Emissions ‘RCP8.5’ Global Warming
Scenario,” Carbon Brief, August 21, 2019.
50 IPCC WGI 2021, p. TS-22.
51 Zeke Hausfather and Glen P. Peters, “Emissions—The ‘Business as Usual’ Story Is Misleading,” Nature, vol. 577,
no. 7792 (2020), p. 618.
52 Christopher R. Schwalm, Spencer Glendon, and Philip B. Duffy, “Reply to Hausfather and Peters: RCP8.5 Is Neither
Problematic nor Misleading,” Proceedings of the National Academy of Sciences, vol. 117, no. 45 (2020), p. 27793.
53 Detlef van Vuuren et al., “The Representative Concentration Pathways: An Overview,” Climatic Change, vol. 109,
no. 1 (2011), p. 5.
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Environmental Change, researchers stated, “For many dimensions the SSP2 marker
implementation also reflects an extension of the historical experience, particularly in terms of
carbon and energy intensity improvements in its baseline.”54
Scenarios and Temperature Trends
The difference in temperature trend distinguishes the low-emissions scenarios from the other
scenarios. For the low-emissions scenarios SSP1-1.9 and SSP1-2.6, temperature increases halt,
whereas for emissions scenarios SSP2-4.5, SSP3-7.0, and SSP5-8.5, temperatures continue to
increase during the time period analyzed (Figure 4).
Figure 4. Changes in Global Surface Temperatures for Scenarios Included
in AR6 WGI

Source: Adapted from Figure SPM.8, panel (a), in IPCC, “Summary for Policymakers,” in Climate Change 2021:
The Physical Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental
Panel on Climate Change, 2021, p. 22.
Notes: The figure depicts temperature changes under scenarios included in AR6 WGI in relation to 1850-1900.
The shaded areas indicate uncertainty—the very likely global surface temperature change ranges for SSP3-7.0
(red shading) and for SSP1-2.6 (blue shading).
Limiting Future Climate Changes:
The Scientific Framework
A noteworthy finding of AR6 WGI is the observed and modeled, almost linear, relationship
between cumulative anthropogenic emissions of CO2 and global temperature increases (Figure
5). This finding is the stated scientific basis in AR6 WGI for the ability to limit future climate
changes.
This Report reaffirms with high confidence the AR5 finding that there is a near-linear
relationship between cumulative anthropogenic CO2 emissions and the global warming
they cause. Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a
0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C.55

54 Oliver Fricko et al., “The Marker Quantification of the Shared Socioeconomic Pathway 2: A Middle-of-the-Road
Scenario for the 21st Century,” Global Environmental Change, vol. 42 (2017), p. 251.
55 IPCC SPM WGI 2021, p. 28.
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Figure 5. Global Surface Temperature Increases as Function
of Cumulative CO2 Emissions

Source: Adapted from Figure SPM.10, in IPCC, “Summary for Policymakers,” in Climate Change 2021: The
Physical Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel
on Climate Change, 2021, p. 28.
Notes: The black line depicts historical global surface temperature increases from 1850 to 2019 in relation to
cumulative CO2 emissions since 1850. The colored lines and areas show the very likely range of global surface
temperature increases from 2020 to 2050, for the scenarios SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5
in relation to their additional cumulative CO2 emissions. The projected global warming includes all anthropogenic
climate influences.
The AR6 WGI states that with incremental global temperature increases, there are also
measurable changes in some sensitive components of the earth system that become larger.
With every additional increment of global warming, changes in extremes continue to
become larger. For example, every additional 0.5°C of global warming causes clearly
discernible increases in the intensity and frequency of hot extremes, including heatwaves
(very likely), and heavy precipitation (high confidence), as well as agricultural and
ecological droughts in some regions (high confidence). Discernible changes in intensity
and frequency of meteorological droughts, with more regions showing increases than
decreases, are seen in some regions for every additional 0.5°C of global warming (medium
confidence). Increases in frequency and intensity of hydrological droughts become larger
with increasing global warming in some regions (medium confidence). There will be an
increasing occurrence of some extreme events unprecedented in the observational record
with additional global warming, even at 1.5°C of global warming. Projected percentage
changes in frequency are larger for rarer events (high confidence).56

56 IPCC SPM WGI 2021, p. 15.
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Not all changes in components of the earth system are proportional to global temperature
increases. Some, like Antarctic sea ice, show little sensitivity to the climate changes in the
observational record, and AR6 WGI states, “There is low confidence in the projected decrease of
Antarctic sea ice.”57
The scientific basis for limiting future climate change, stated in AR6 WGI, is the relationship
between cumulative CO2 emissions, emissions of other GHGs, other climate drivers, and
increasing temperatures. Limiting global temperature increases and associated changes in
climate-sensitive earth system components requires limiting cumulative CO2 emissions, limiting
emissions of other GHGs, and limiting other climate drivers.
From a physical science perspective, limiting human-induced global warming to a specific
level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions,
along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained
reductions in CH4 emissions would also limit the warming effect resulting from declining
aerosol pollution and would improve air quality.58
The AR6 WGI Scenarios and the Paris Agreement
Temperature Benchmarks
The emissions scenarios of AR6 WGI may be used as a framework for analyzing options for
implementing policies related to the Paris Agreement (PA).59 AR6 WGI frequently uses
benchmarks of global temperature increases of 1.5oC and 2.0oC above preindustrial levels (global
surface temperatures over 1850-1900) for comparing the estimated global temperature trajectories
of the AR6 WGI emissions scenarios. These temperature increase values are stated in the PA as
follows:
Article 2
1. This Agreement, in enhancing the implementation of the Convention, including its
objective, aims to strengthen the global response to the threat of climate change, in the
context of sustainable development and efforts to eradicate poverty, including by:
(a) Holding the increase in the global average temperature to well below 2°C above pre-
industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-
industrial levels, recognizing that this would significantly reduce the risks and impacts of
climate change.60
Emissions and climate projections from the AR6 WGI scenarios can be used to analyze
projections of global GHG emissions based on nonbinding pledges that each party submits as
their “Nationally Determined Contributions (NDCs)” under the PA.61 The NDCs are to be

57 IPCC SPM WGI 2021, p. 16.
58 IPCC SPM WGI 2021, p. 27.
59 The 2015 Paris Agreement (PA) is the second subsidiary agreement to the UNFCCC; the Kyoto Protocol is the first.
The United States became a party to the PA under the Obama Administration, withdrew from the agreement under the
Trump Administration and, on January 20, 2021, rejoined it under the Biden Administration. The PA is a complex,
multifaceted agreement that includes aspects that are specifically relevant to the scenario analysis presented in AR6
WGI, such as the temperature benchmarks stated in the quoted passage.
60 United Nations Framework Convention on Climate Change, Paris Agreement, 2015 p. 3 (hereinafter UNFCCC PA
2015).
61 UNFCCC PA 2015, p. 3.
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submitted every five years and state how each party intends to abate its GHG emissions.62 The
AR6 WGI scenarios include emissions levels and the associated temperature changes that climate
models project for those emissions levels. The estimated emissions levels of the NDCs can be
compared with the AR6 WGI scenario emissions levels as part of an analysis of temperature
changes that may be associated with the NDCs.
Considerations for Congress
In 2021, the Secretariat of the UNFCCC published a synthesis report on “Nationally determined
contributions under the Paris Agreement.”63 The report includes a section on “Contribution
towards achieving the objective of the Convention as set out in its Article 2, and towards Article
2, paragraph 1(a) ... of the Paris Agreement.”64 The information provided in the report suggests
that the current emissions trajectory through 2030, based on the NDCs, is closely aligned with the
IPCC emissions scenario SSP2-4.5. The projected very likely temperature increase for this
scenario at 2100, included in AR6 WGI, exceeds 2.0oC (Table 1) and does not achieve the
objective of the convention set out in Article 2.
Under the terms of the PA, the timeline for communicating NDCs is every five years,65 and as
stated in Article 4, Section 3 of the PA, “Each Party’s successive nationally determined
contribution will represent a progression beyond the Party’s then current nationally determined
contribution and reflect its highest possible ambition.”66 The PA also sets a goal for “peaking of
greenhouse gas emissions ... so as to achieve a balance between anthropogenic emissions by
sources and removals by sinks of greenhouse gases.”67 Net-zero emissions is a similar term for
achieving this balance. Net-zero emissions, or net-zero, refers to situations where human-caused
GHG emissions are balanced by removal of GHG from the atmosphere.68
As part of a climate change mitigation strategy, some governments are developing GHG
emissions reduction strategies that include domestic legislation to realize net-zero emissions and
revising emissions reduction levels in the progression of NDCs. Some countries, including Japan
and the United Kingdom, have updated their NDCs with increasingly ambitious emissions
reduction goals and passed domestic net-zero emissions legislation. The United States has
increased the ambition of the emissions reductions in an updated NDC, but has not, as of this
writing, enacted domestic net-zero legislation.69
With respect to the United States’ most recent NDCs, Congress may wish to exercise its oversight
authorities in examining national progress toward fulfilling the updated U.S. NDCs. Congress

62 UNFCCC, Nationally Determined Contributions (NDCs), https://unfccc.int/process-and-meetings/the-paris-
agreement/nationally-determined-contributions-ndcs/nationally-determined-contributions-ndcs.
63 UNFCC, Nationally Determined Contributions Under the Paris Agreement (Synthesis Report by the Secretariat),
2021.
64 UNFCC, Nationally Determined Contributions Under the Paris Agreement (Synthesis Report by the Secretariat),
2021, p. 26.
65 UNFCCC PA 2015, p. 5.
66 UNFCCC PA 2015, p. 4.
67 UNFCCC PA 2015, p. 3.
68 CRS Report R46945, Greenhouse Gas Emission Reduction Pledges by Selected Countries: Nationally Determined
Contributions and Net-Zero Legislation, by Kezee Procita.
69 CRS Report R46945, Greenhouse Gas Emission Reduction Pledges by Selected Countries: Nationally Determined
Contributions and Net-Zero Legislation, by Kezee Procita.
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could also consider options for shaping and funding the U.S. approach to the PA commitments.70
For example, there have been a number of legislative proposals introduced during the 117th
Congress that seek to address a range of GHG emissions sources as part of an effort to move the
United States toward net-zero emissions.71 The information provided in AR6 WGI may help to
inform Congress when considering the United States’ participation in the PA and the most recent
U.S. NDCs.

Author Information

Jonathan D. Haskett

Analyst in Environmental Policy

Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
shared staff to congressional committees and Members of Congress. It operates solely at the behest of and
under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other
than public understanding of information that has been provided by CRS to Members of Congress in
connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not
subject to copyright protection in the United States. Any CRS Report may be reproduced and distributed in
its entirety without permission from CRS. However, as a CRS Report may include copyrighted images or
material from a third party, you may need to obtain the permission of the copyright holder if you wish to
copy or otherwise use copyrighted material.

70 See CRS In Focus IF11746, United States Rejoins the Paris Agreement on Climate Change: Options for Congress,
by Jane A. Leggett.
71 For example, see H.R. 1512, which would create requirements to reduce GHG emissions with a goal of achieving
net-zero emissions by 2050.
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