From Jonathan D. Haskett comes this CRS report: 'Intergovernmental Panel on Climate Change’s Sixth Assessment Report'.
Download CRS_Report_ IPCC_Sixth_Asessment_Rpt_29April2022
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Summary
In 2021, the Intergovernmental Panel on Climate Change (IPCC) released Climate Change 2021: The Physical Science Basis—Contribution of Working Group I, as part of its Sixth Assessment (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.
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. 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. 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” to government policymakers internationally. 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.”
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.” Climate Change 2021—The Physical Science Basis is the sixth and most recent physical science assessment (AR6 WGI) of the IPCC on the physical science basis of climate change. 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. 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.
An analysis of each of these findings is presented in the following sections.
Cutting to the chase...
Considerations for Congress
In 2021, the Secretariat of the UNFCCC published a synthesis report on “Nationally determined contributions under the Paris Agreement.” 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.” 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, 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.” 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.” 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.
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.
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 could also consider options for shaping and funding the U.S. approach to the PA commitments. 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. 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.
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