Editing IPCC:AR6/WGI/TS (section)

== '''Introduction''' ==

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The Working Group I (WGI) contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) assesses the physical science basis of climate change. As part of that contribution, this Technical Summary (TS) is designed to bridge between the comprehensive assessment of the WGI Chapters and its Summary for Policymakers (SPM). It is primarily built from the Executive Summaries of the individual chapters and [[IPCC:Wg1:Chapter:Atlas|Atlas]] and provides a synthesis of key findings based on multiple lines of evidence (e.g., analyses of observations, models, paleoclimate information and understanding of physical, chemical and biological processes and components of the climate system). All the findings and figures here are supported by and traceable to the underlying chapters, with relevant chapter sections indicated in curly brackets.

Throughout this Technical Summary, key assessment findings are reported using the IPCC calibrated uncertainty language (Chapter 1, Box 1.1). Two calibrated approaches are used to communicate the degree of certainty in key findings, which are based on author teams’ evaluations of underlying scientific understanding:

# Confidence <sup>[[#footnote-020|1]]</sup> is a qualitative measure of the validity of a finding, based on the type, amount, quality and consistency of evidence (e.g., data, mechanistic understanding, theory, models, expert judgment) and the degree of agreement.
# Likelihood <sup>[[#footnote-019|2]]</sup> provides a quantified measure of confidence in a finding expressed probabilistically (e.g., based on statistical analysis of observations or model results, or both, and expert judgement by the author team or from a formal quantitative survey of expert views, or both).

Where there is sufficient scientific confidence, findings can also be formulated as statements of fact without uncertainty qualifiers. Throughout IPCC reports, the calibrated language is clearly identified by being typeset in italics.

The context and progress in climate science (Section TS.1) is followed by a Cross-Section Box TS.1 on global surface temperature change. Section TS.2 provides information about past and future large-scale changes in all components of the climate system. Section TS.3 summarizes knowledge and understanding of climate forcings, feedbacks and responses. Infographic TS.1 uses a storyline approach to integrate findings on possible climate futures. Finally, Section TS.4 provides a synthesis of climate information at regional scales. <sup>[[#footnote-018|3]]</sup> The list of acronyms used in the WGI Report is in Annex VIII.

'''Text at the beginning of a section presented in dark blue with a blue vertical bar at the left, as shown here, provides a summary of the findings discussed in that section.'''
The AR6 WGI Report promotes best practices in traceability and reproducibility, including through adoption of the Findable, Accessible, Interoperable, and Reusable (FAIR) principles for scientific data. Each chapter has a data table (in its Supplementary Material) documenting the input data and code used to generate its figures and tables. In addition, a collection of data and code from the report has been made freely-available online via long-term archives. <sup>[[#footnote-017|4]]</sup>

These FAIR principles are central to the WGI Interactive Atlas <sup>[[#footnote-016|5]]</sup> , an online tool that complements the WGI Report by providing flexible spatial and temporal analyses of past, observed and projected climate change information. It comprises a regional information component that supports many of the chapters of the Report and a regional synthesis component that supports the Technical Summary and Summary for Policymakers.

Regarding the representation of robustness and uncertainty in maps, the method chosen for the AR6 <sup>[[#footnote-015|6]]</sup> differs from the method used in the Fifth Assessment Report (AR5). This choice is based on new research on the visualization of uncertainty and on user surveys.

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'''Box TS.1 | Core Concepts Central to This Report'''

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This box provides short descriptions of key concepts that are relevant to the AR6 WGI assessment, with a focus on their use in the Technical Summary and the Summary for Policymakers. The Glossary (Annex VII) includes more information on these concepts along with definitions of many other important terms and concepts used in this Report.

'''Characteristics of Climate Change Assessment'''
'''Global warming:''' Global warming refers to the change of global surface temperature relative to a baseline depending upon the application. Specific global warming levels, such as 1.5°C, 2°C, 3°C or 4°C, are defined as changes in global surface temperature relative to the years 1850–1900 as the baseline (the earliest period of reliable observations with sufficient geographic coverage). They are used to assess and communicate information about global and regional changes, linking to scenarios and used as a common basis for Working Group II (WGII) and Working Group III (WGIII) assessments. (Section TS.1.3, Cross-Section Box TS.1) Links to chapters 1.4.1, 1.6.2, 4.6.1, Cross-Chapter Boxes 1.5, 2.3, 11.1, and 12.1, Atlas Sections 3–11, Glossary

'''Emergence:''' Emergence refers to the experience or appearance of novel conditions of a particular climate variable in a given region. This concept is often expressed as the ratio of the change in a climate variable relative to the amplitude of natural variations of that variable (often termed a ‘signal-to-noise’ ratio, with emergence occurring at a defined threshold of this ratio). Emergence can be expressed in terms of a time or a global warming level at which the novel conditions appear and can be estimated using observations or model simulations. (Sections TS.1.2.3 and TS.4.2) Links to chapters 1.4.2, FAQ 1.2, 7.5.5, 10.3, 10.4, 12.5.2, Cross-Chapter Box Atlas.1, Glossary

'''Cumulative carbon dioxide (CO''' 2 ''') emissions:''' The total net amount of CO <sub>2</sub> emitted into the atmosphere as a result of human activities. Given the nearly linear relationship between cumulative CO <sub>2</sub> emissions and increases in global surface temperature, cumulative CO <sub>2</sub> emissions are relevant for understanding how past and future CO <sub>2</sub> emissions affect global surface temperature. A related term – remaining carbon budget – is used to describe the total net amount of CO <sub>2</sub> that could be released in the future by human activities while keeping global warming to a specific global warming level, such as 1.5°C, taking into account the warming contribution from non-CO <sub>2</sub> forcers as well. The remaining carbon budget is expressed from a recent specified date, while the total carbon budget is expressed starting from the pre-industrial period. (Sections TS.1.3 and TS.3.3) Links to chapters 1.6.3, 5.5, Glossary

'''Net zero CO''' 2 '''emissions:''' A condition that occurs when the amount of CO <sub>2</sub> emitted into the atmosphere by human activities equals the amount of CO <sub>2</sub> removed from the atmosphere by human activities over a specified period of time. Net negative CO <sub>2</sub> emissions occur when anthropogenic removals exceed anthropogenic emissions. (Section TS.3.3) Links to chapters Box 1.4, Glossary

'''Human Influence on the Climate System'''
'''Earth’s energy imbalance:''' In a stable climate, the amount of energy that Earth receives from the Sun is approximately in balance with the amount of energy that is lost to space in the form of reflected sunlight and thermal radiation. ‘Climate drivers’, such as an increase in greenhouse gases or aerosols, interfere with this balance, causing the system to either gain or lose energy. The strength of a climate driver is quantified by its effective radiative forcing (ERF), measured in W m <sup>–2</sup> . Positive ERF leads to warming, and negative ERF leads to cooling. That warming or cooling in turn can change the energy imbalance through many positive (amplifying) or negative (dampening) climate feedbacks. (Sections TS.2.2, TS.3.1 and TS.3.2) Links to chapters 2.2.8, 7.2, 7.3, 7.4, Box 7.1, Box 7.2, Glossary

'''Attribution:''' Attribution is the process of evaluating the relative contributions of multiple causal factors to an observed change in climate variables (e.g., global surface temperature, global mean sea level), or to the occurrence of extreme weather or climate-related events. Attributed causal factors include human activities (such as increases in greenhouse gas concentration and aerosols, or land-use change) or natural external drivers (solar and volcanic influences), and in some cases internal variability. (Sections TS.1.2.4 and TS.2, Box TS.10) Links to chapters Cross-Working Group Box: Attribution in Chapter 1; 3.5; 3.8; 10.4; 11.2.4; Glossary

'''Committed change, long-term commitment:''' Changes in the climate system, resulting from past, present and future human activities, which will continue long into the future (centuries to millennia) even with strong reductions in greenhouse gas emissions. Some aspects of the climate system, including the terrestrial biosphere, the deep ocean and the cryosphere, respond much more slowly than surface temperatures to changes in greenhouse gas concentrations. As a result, there are already substantial committed changes associated with past greenhouse gas emissions. For example, global mean sea level will continue to rise for thousands of years, even if future CO <sub>2</sub> emissions are reduced to net zero and global warming halted, as excess energy due to past emissions continues to propagate into the deep ocean and as glaciers and ice sheets continue to melt. (Section TS.2.1, Box TS.4, Box TS.9) Links to chapters 1.2.1, 1.3, Box 1.2, Cross-Chapter Box 5.3

'''Climate Information for Regional Climate Change and Risk Assessment'''
'''Distillation:''' The process of synthesizing information about climate change from multiple lines of evidence obtained from a variety of sources, taking into account user context and values. It leads to an increase in the usability, usefulness and relevance of climate information, enhances stakeholder trust, and expands the foundation of evidence used in climate services. It is particularly relevant in the context of co-producing regional-scale climate information to support decision-making. (Section TS.4.1, Box TS.11) Links to chapters 10.1, 10.5, 12.6

'''(Climate change) risk:''' The concept of risk is a key aspect of how the IPCC assesses and communicates to decision-makers about the potential for adverse consequences for human or ecological systems, recognizing the diversity of values and objectives associated with such systems. In the context of climate change, risks can arise from potential impacts of climate change as well as human responses to climate change. WGI contributes to the common IPCC risk framing through the assessment of relevant climate information, including climatic impact-drivers and low-likelihood, high-impact outcomes. (Sections TS.1.4 and TS.4.1, Box TS.4) Links to chapters Cross-Chapter Boxes 1.3 and 12.1, Glossary

'''Climatic impact-drivers:''' Physical climate system conditions (e.g., means, events, extremes) that can be directly connected with having impacts on human or ecological systems are described as ‘climatic impact-drivers’ (CIDs) without anticipating whether their impacts are detrimental (i.e., as for hazards in the context of climate change risks) or provide potential opportunities. A range of indices may capture the sector- or application-relevant characteristics of a climatic impact-driver and can reflect exceedances of identified tolerance thresholds. (Sections TS.1.4 and TS.4.3) Links to chapters 12.1–12.3, FAQ 12.1, Glossary

'''Storylines:''' The term storyline is used both in connection to scenarios (related to a future trajectory of emissions or socio-economic developments) or to describe plausible trajectories of weather and climate conditions or events, especially those related to high levels of risk. Physical climate storylines are introduced in AR6 to explore uncertainties in climate change and natural climate variability, to develop and communicate integrated and context-relevant regional climate information, and to address issues with deep uncertainty <sup>[[#footnote-014|7]]</sup> , including low-likelihood, high-impact outcomes ''.'' (Section TS.1.4, Box TS.3, Infographic TS.1) Links to chapters 1.4.4, Box 10.2, Glossary

'''Low-likelihood, high impact outcomes:''' Outcomes/events whose probability of occurrence is low or not well known (as in the context of deep uncertainty) but whose potential impacts on society and ecosystems could be high. To better inform risk assessment and decision-making, such low-likelihood outcomes are considered if they are associated with very large consequences and may therefore constitute material risks, even though those consequences do not necessarily represent the most likely outcome. (Section TS.1.4, Box TS.3, Figure TS.6) Links to chapters 1.4.4, 4.8, Cross Chapter Box 1.3, Glossary

As part of the AR6 cycle, the IPCC produced three Special Reports in 2018 and 2019: the Special Report on Global Warming of 1.5°C (SR1.5), the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC), and the Special Report on Climate Change and Land (SRCCL).

The AR6 WGI Report provides a full and comprehensive assessment of the physical science basis of climate change that builds on the previous assessments and these Special Reports and considers new information and knowledge from the recent scientific literature <sup>[[#footnote-013|8]]</sup> , including longer observational datasets and new scenarios and model results.

The structure of the AR6 WGI Report is designed to enhance the visibility of knowledge developments and to facilitate the integration of multiple lines of evidence, thereby improving confidence in findings. The Report has been peer-reviewed by the scientific community and governments ( [[IPCC:Wg1:Chapter:Annex-x|Annex X]] provides the Expert Reviewer list). The substantive introduction provided by ( [[IPCC:Wg1:Chapter:Chapter-1|Chapter 1]] is followed by a first set of chapters dedicated to large-scale climate knowledge (Chapters 2–4), which encompasses observations and paleoclimate evidence, causes of observed changes, and projections; these are complemented by ( [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] for large-scale changes in extremes. The second set of chapters (Chapters 5–9) is orientated around the understanding of key climate system components and processes, including the global cycles of carbon, energy and water; short-lived climate forcers and their link to air quality; and the ocean, cryosphere and sea level change. The last set of chapters (Chapters 10–12 and the Atlas) is dedicated to the assessment and distillation of regional climate information from multiple lines of evidence at sub-continental to local scales (including urban climate), with a focus on recent and projected regional changes in mean climate, extremes, and climatic impact-drivers. The new online Interactive [[IPCC:Wg1:Chapter:Atlas|Atlas]] allows users to interact in a flexible manner through maps, time series and summary statistics with climate information for a set of updated WGI reference regions. The Report also includes 34 Frequently Asked Questions and answers for the general public ( [[IPCC:Wg1:Faqs|https://www.ipcc.ch/report/ar6/wg1/faqs]] ).

Together, this Technical Summary and the underlying chapters aim at providing a comprehensive picture of knowledge progress since the WGI contribution to AR5. Multiple lines of scientific evidence confirm that the climate is changing due to human influence. Important advances in the ability to understand past, present and possible future changes should result in better-informed decision-making.

Some of the new results and main updates to key findings in this Report compared to AR5, SR1.5, SRCCL, and SROCC are summarized below. Relevant Technical Summary sections with further details are shown in parentheses after each bullet point.

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=== Selected Updates and/or New Results since AR5 ===

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* '''Human influence''' <sup>[[#footnote-012|9]]</sup> '''on the climate system is now an established fact:''' The Fourth Assessment Report (AR4) stated in 2007 that ‘warming of the climate system is unequivocal’, and AR5 stated in 2013 that ‘human influence on the climate system is clear’. Combined evidence from across the climate system strengthens this finding. It is unequivocal that the increase of CO <sub>2</sub> , methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) in the atmosphere over the industrial era is the result of human activities and that human influence is the main driver <sup>[[#footnote-011|10]]</sup> of many changes observed across the atmosphere, ocean, cryosphere and biosphere. (Sections TS.1.2, TS.2.1 and TS.3.1)
* '''Observed global warming to date:''' A combination of improved observational records and a series of very warm years since AR5 have resulted in a substantial increase in the estimated level of global warming to date. The contribution of changes in observational understanding alone between AR5 and AR6 leads to an increase of about 0.1°C in the estimated warming since 1850–1900. For the decade 2011–2020, the increase in global surface temperature since 1850–1900 is assessed to be 1.09 [0.95 to 1.20] °C. <sup>[[#footnote-010|11]]</sup> Estimates of crossing times of global warming levels and estimates of remaining carbon budgets are updated accordingly. (Section TS.1.2, Cross-Section Box TS.1)
* '''Paleoclimate evidence:''' The AR5 assessed that many of the changes observed since the 1950s are unprecedented over decades to millennia. Updated paleoclimate evidence strengthens this assessment; over the past several decades, key indicators of the climate system are increasingly at levels unseen in centuries to millennia and are changing at rates unprecedented in at least the last 2000 years. (Box TS.2, Section TS.2)
* '''Updated assessment of recent warming:''' The AR5 reported a smaller rate of increase in global mean surface temperature over the period 1998–2012 than the rate calculated since 1951. Based on updated observational datasets showing a larger trend over 1998–2012 than earlier estimates, there is now ''high confidence'' that the observed 1998–2012 global surface temperature trend is consistent with ensembles of climate model simulations, and there is now ''very high confidence'' that the slower rate of global surface temperature increase observed over this period was a temporary event induced by internal and naturally forced variability that partly offset the anthropogenic surface warming trend over this period, while heat uptake continued to increase in the ocean. Since 2012, strong warming has been observed, with the past five years (2016–2020) being the hottest five-year period in the instrumental record since at least 1850 ( ''high confidence'' ). (Section TS.1.2, Cross-Section Box TS.1)
* '''Magnitude of climate system response:''' In this Report, it has been possible to reduce the long-standing uncertainty ranges for metrics that quantify the response of the climate system to radiative forcing, such as the equilibrium climate sensitivity (ECS) and the transient climate response (TCR), due to substantial advances (e.g., a 50% reduction in the uncertainty range of cloud feedbacks) and improved integration of multiple lines of evidence, including paleoclimate information. Improved quantification of ERF, the climate system radiative response, and the observed energy increase in the Earth system over the past five decades demonstrate improved consistency between independent estimates of climate drivers, the combined climate feedbacks, and the observed energy increase relative to AR5. (Section TS.3.2)
* '''Improved constraints on projections of future climate change:''' For the first time in an IPCC report, the assessed future change in global surface temperature is consistently constructed by combining scenario-based projections (which AR5 focused on) with observational constraints based on past simulations of warming as well as the updated assessment of ECS and TCR. In addition, initialized forecasts have been used for the period 2019–2028. The inclusion of these lines of evidence reduces the assessed uncertainty for each scenario. (Section TS.1.3, Cross-Section Box TS.1)
* '''Air quality:''' The AR5 assessed that projections of air quality are driven primarily by precursor emissions, including CH <sub>4</sub> . New scenarios explore a diversity of future options in air pollution management. The AR6 reports rapid recent shifts in the geographical distribution of some of these precursor emissions, confirms the AR5 finding, and shows higher warming effects of short-lived climate forcers in scenarios with the highest air pollution. (Sections TS.1.3 and TS.2.2, Box TS.7)
* '''Effects of short-lived climate forcers on global warming:''' The AR5 assessed the radiative forcing for emitted compounds. The AR6 has extended this by assessing the emissions-based ERFs also accounting for aerosol–cloud interactions. The best estimates of ERF attributed to sulphur dioxide (SO <sub>2</sub> ) and CH <sub>4</sub> emissions are substantially greater than in AR5, while that of black carbon is substantially reduced. The magnitude of uncertainty in the ERF due to black carbon emissions has also been reduced relative to AR5. (Section TS.3.1)
* '''Global water cycle:''' The AR5 assessed that anthropogenic influences have ''likely'' affected the global water cycle since 1960. The dedicated chapter in AR6 (Chapter 8) concludes with ''high confidence'' that human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century, with a better understanding of the response to aerosol and greenhouse gas changes. The AR6 further projects with ''high confidence'' an increase in the variability of the water cycle in most regions of the world and under all emissions scenarios. (Box TS.6)
* '''Extreme events:''' The AR5 assessed that human influence had been detected in changes in some climate extremes. A dedicated chapter in AR6 (Chapter 11) concludes that it is now an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since 1850, in particular for temperature extremes. Evidence of observed changes and attribution to human influence has strengthened for several types of extremes since AR5, in particular for extreme precipitation, droughts, tropical cyclones and compound extremes (including fire weather). (Sections TS.1.2 and TS.2.1, Box TS.10)

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=== Selected Updates and/or New Results Since AR5 and SR1.5 ===

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* '''Timing of crossing 1.5°C global warming:''' Slightly different approaches are used in SR1.5 and in this Report. SR1.5 assessed a ''likely'' range of 2030 to 2052 for reaching a global warming level of 1.5°C (for a 30-year period), assuming a continued, constant rate of warming. In AR6, combining the larger estimate of global warming to date and the assessed climate response to all considered scenarios, the central estimate of crossing 1.5°C of global warming (for a 20-year period) occurs in the early 2030s, in the early part of the ''likely'' range assessed in SR1.5, assuming no major volcanic eruption. (Section TS.1.3, Cross-Section Box TS.1)
* '''Remaining carbon budgets:''' The AR5 had assessed the transient climate response to cumulative emissions of CO <sub>2</sub> to be ''likely'' in the range of 0.8°C to 2.5°C per 1000 GtC (1 Gigatonne of carbon, GtC, = 1 Petagram of carbon, PgC, = 3.664 Gigatonnes of carbon dioxide, GtCO <sub>2</sub> ), and this was also used in SR1.5. The assessment in AR6, based on multiple lines of evidence, leads to a narrower ''likely'' range of 1.0°C–2.3°C per 1000 GtC. This has been incorporated in updated estimates of remaining carbon budgets (see Section TS.3.3.1), together with methodological improvements and recent observations. (Sections TS.1.3 and TS.3.3)
* '''Effect of short-lived climate forcers on global warming in coming decades:''' The SR1.5 stated that reductions in emissions of cooling aerosols partially offset greenhouse gas mitigation effects for two to three decades in pathways limiting global warming to 1.5°C. The AR6 assessment updates the AR5 assessment of the net cooling effect of aerosols and confirms that changes in short-lived climate forcers will ''very likely'' cause further warming in the next two decades across all scenarios. (Section TS.1.3, Box TS.7)
* '''COVID-19:''' Temporary emissions reductions in 2020 associated with COVID-19 containment led to small and positive net radiative effect (warming influence). However, global and regional climate responses to this forcing are undetectable above internal climate variability due to the temporary nature of emissions reductions. (Section TS.3.3)

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=== Selected Updates and/or New Results Since AR5, SRCCL and SROCC ===

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* '''Atmospheric concentration of methane:''' The SRCCL reported a resumption of atmospheric CH <sub>4</sub> concentration growth since 2007. The AR6 reports a faster growth over 2014–2019 and assesses growth since 2007 to be largely driven by emissions from the fossil fuels and agriculture (dominated by livestock) sectors. (Section TS.2.2)
* '''Land and ocean carbon sinks:''' The SRCCL assessed that the persistence of the land carbon sink is uncertain due to climate change. The AR6 finds that land and ocean carbon sinks are projected to continue to grow until 2100 with increasing atmospheric concentrations of CO <sub>2</sub> , but the fraction of emissions taken up by land and ocean is expected to decline as the CO <sub>2</sub> concentration increases, with a much larger uncertainty range for the land sink. The AR5, SR1.5 and SRCCL assessed carbon dioxide removal options and scenarios. The AR6 finds that the carbon cycle response is asymmetric for pulse emissions or removals, which means that CO <sub>2</sub> emissions would be more effective at raising atmospheric CO <sub>2</sub> than CO <sub>2</sub> removals are at lowering atmospheric CO <sub>2</sub> . (Section TS.3.3, Box TS.5)
* '''Ocean stratification increase''' '''[[#footnote-009|12]]''' ''':''' Refined analyses of available observations in the AR6 lead to a reassessment of the rate of increase of the global stratification in the upper 200 m to be double that estimated in SROCC from 1970 to 2018. (Section TS.2.4)
* '''Projected ocean oxygen loss:''' Future subsurface oxygen decline in new projections assessed in WGI AR6 is substantially greater in 2080–2099 than assessed in SROCC. (Section TS.2.4)
* '''Ice loss from glaciers and ice sheets:''' Since SROCC, globally resolved glacier changes have improved estimates of glacier mass loss over the past 20 years, and estimates of the Greenland and Antarctic Ice Sheet loss have been extended to 2020. (Section TS.2.5)
* '''Observed global mean sea level change:''' new observation-based estimates published since SROCC lead to an assessed sea level rise estimate from 1901 to 2018 that is now consistent with the sum of individual components and consistent with closure of the global energy budget. (Box TS.4)
* '''Projected global mean sea level change:''' The AR6 projections of global mean sea level are based on projections from ocean thermal expansion and land ice contribution estimates, which are consistent with the assessed ECS and assessed changes in global surface temperature. They are underpinned by new land ice model intercomparisons and consideration of processes associated with ''l'' ''ow confidence'' to characterize the deep uncertainty in future ice loss from Antarctica. The AR6 projections based on new models and methods are broadly consistent with SROCC findings. (Box TS.4)

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