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

== 3.1 Scope and Overview ==

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This chapter assesses the extent to which the climate system has been affected by human influence and to what extent climate models are able to simulate observed mean climate, changes and variability. This assessment is the basis for understanding what impacts of anthropogenic climate change may already be occurring and informs our confidence in climate projections. Moreover, an understanding of the amount of human-induced global warming to date is key to assessing our status with respect to the Paris Agreement goals of holding the increase in global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C ( [[#UNFCCC--2016|UNFCCC, 2016]] ).

The evidence of human influence on the climate system has strengthened progressively over the course of the previous five IPCC assessments, from the Second Assessment Report that concluded ‘the balance of evidence suggests a discernible human influence on climate’ through to the Fifth Assessment Report (AR5) which concluded that ‘it is ''extremely likely'' that human influence caused more than half of the observed increase in global mean surface temperature (GMST) from 1951 to 2010’ (see also Sections 1.3.4 and 3.3.1.1). The AR5 concluded that climate models had been developed and improved since the Fourth Assessment Report (AR4) and were able to reproduce many features of observed climate. Nonetheless, several systematic biases were identified ( [[#Flato--2013|Flato et al., 2013]] ). This chapter additionally builds on the assessment of attribution of global temperatures contained in the IPCC Special Report on Global Warming of 1.5°C (SR1.5; [[#IPCC--2018|IPCC, 2018]] ), assessments of attribution of changes in the ocean and cryosphere in the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#IPCC--2019b|IPCC, 2019b]] ), and assessments of attribution of changes in the terrestrial carbon cycle in the IPCC Special Report on Climate Change and Land (SRCCL, [[#IPCC--2019a|IPCC, 2019a]] ).

This chapter assesses the evidence for human influence on observed large-scale indicators of climate change that are described in Cross-Chapter Box 2.2 and assessed in Chapter 2. It takes advantage of the longer period of record now available in many observational datasets. The assessment of the human-induced contribution to observed climate change requires an estimate of the expected response to human influence, as well as an estimate of the expected climate evolution due to natural forcings and an estimate of variability internal to the climate system (internal climate variability). For this we need high quality models, primarily climate and Earth system models. Since AR5, a new set of coordinated model results from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project Phase 6 (CMIP6; [[#Eyring--2016a|Eyring et al., 2016a]] ) has become available. Together with updated observations of large-scale indicators of climate change (Chapter 2), CMIP simulations are a key resource for assessing human influence on the climate system. Pre-industrial control and historical simulations are of most relevance for model evaluation and assessment of internal variability, and these simulations are evaluated to assess fitness-for-purpose for attribution, which is the focus of this chapter (see also ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.4|Section 1.5.4]] ). This chapter provides the primary evaluation of large-scale indicators of climate change in this Report, and is complemented by other fitness-for-purpose evaluations in subsequent chapters. CMIP6 also includes an extensive set of idealized and single forcing experiments for attribution ( [[#Eyring--2016a|Eyring et al., 2016a]] ; [[#Gillett--2016|Gillett et al., 2016]] ). In addition to the assessment of model performance and human influence on the climate system during the instrumental era up to the present-day, this chapter also includes evidence from paleo-observations and simulations over past millennia ( [[#Kageyama--2018|Kageyama et al., 2018]] ).

Whereas in previous IPCC assessment reports the comparison of simulated and observed climate change was done separately in a model evaluation chapter and a chapter on detection and attribution, in AR6 these comparisons are integrated together. This has the advantage of allowing a single discussion of the full set of explanations for any inconsistency in simulated and observed climate change, including missing forcings, errors in the simulated response to forcings, and observational errors, as well as an assessment of the application of detection and attribution techniques to model evaluation. Where simulated and observed changes are consistent, this can be interpreted both as supporting attribution statements, and as giving confidence in simulated future change in the variable concerned (see also Box 4.1). However, if a model’s simulation of historical climate change has been tuned to agree with observations, or if the models used in an attribution study have been selected or weighted on the basis of the realism of their simulated climate response, this information would need to be considered in the assessment and any attribution results correspondingly tempered. An integrated discussion of evaluation and attribution supports such a robust and transparent assessment.

This chapter starts with a brief description of methods for detection and attribution of observed changes in [[#3.2|Section 3.2]] , which builds on the more general introduction to attribution approaches in the Cross-Working Group Box on Attribution in Chapter 1. In this chapter we assess the detection of anthropogenic influence on climate on large spatial scales and long temporal scales, a concept related to, but distinct from, that of the emergence of anthropogenically-induced climate change from the range of internal variability on local scales and shorter time scales ( [[IPCC:Wg1:Chapter:Chapter-1#1.4.2.2|Section 1.4.2.2]] ). The following sections address the climate system component by component, in each case assessing human influence and evaluating climate models’ simulations of the relevant aspects of climate and climate change. This chapter assesses the evaluation and attribution of global, hemispheric, continental and ocean basin-scale indicators of climate change in the atmosphere and at the Earth’s surface ( [[#3.3|Section 3.3]] , cryosphere ( [[#3.4|Section 3.4]] , ocean ( [[#3.5|Section 3.5]] , and biosphere ( [[#3.6|Section 3.6]] , and the evaluation and attribution of modes of variability (Section 3.7), the period of slower warming in the early 21st century (Cross-Chapter Box 3.1) and large-scale changes in extremes (Cross-Chapter Box 3.2). Model evaluation and attribution on sub-continental scales are not covered here, since these are assessed in the [[IPCC:Wg1:Chapter:Atlas|Atlas]] and in Chapter 10, and extreme event attribution is not covered since it is assessed in Chapter 11. [[#3.8|Section 3.8]] assesses multivariate attribution and integrative measures of model performance based on multiple variables, as well as process representation in different classes of models. The chapter structure is summarized in Figure 3.1.

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[[File:5740b4b8d000c2b2169a33a9df5bc211 IPCC_AR6_WGI_Figure_3_1.png]]

'''Figure 3.1 | Visual guide to Chapter 3.'''

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