Editing IPCC:AR6/WGI/Chapter-2 (section)

== 2.1 Introduction ==

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This chapter assesses the evidence basis for large-scale past changes in selected components of the climate system. As such, it combines much of the assessment performed in Chapters 2 through 5 of the Fifth Assessment Report (AR5) WGI contribution ([[#IPCC--2013|IPCC, 2013]]) that, taken together, supported a finding of unequivocal recent warming of the climate system. The Sixth Assessment Report (AR6) WGI Report structure differs substantially from that in AR5 ([[IPCC:Wg1:Chapter:Chapter-1#1.1.2|Section 1.1.2]]). This chapter focuses upon observed changes in climate system drivers and changes in key selected large-scale indicators of climate change and in important modes of variability (Cross-Chapter Box 2.2), which allow for an assessment of changes in the global climate system in an integrated manner. This chapter is complemented by Chapters 3 and 4, which respectively consider model assessment/detection and attribution, and future climate projections for subsets of these same indicators and modes. It does not consider changes in observed extremes, which are assessed in Chapter 11. The chapter structure is outlined in the visual abstract (Figure 2.1).

Use is made of paleoclimate, in situ, ground- and satellite-based remote sensing, and reanalysis data products where applicable ([[IPCC:Wg1:Chapter:Chapter-1#1.5|Section 1.5]]). All observational products used in the chapter are detailed in Annex I, and information on data sources and processing for each figure and table can be found in the associated chapter Table 2.SM.1 available as an electronic supplement to the chapter. Use of common periods ranging from 56 million years ago through to the recent past is applied to the extent permitted by available data ([[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] and Cross-Chapter Box 2.1). In all cases, the narrative proceeds from as far in the past as the data permit through to the present. Each sub-section starts by highlighting the key findings from AR5 and any relevant AR6-cycle Special Reports (SROCC, SR1.5, SRCCL), and then outlines the new evidence-basis arising from a combination of: (i) new findings reported in the literature, including new datasets and new versions of existing datasets; and (ii) recently observed changes, before closing with a new summary assessment.

Trends, when calculated as part of this assessment, have wherever possible been calculated using a common approach following that adopted in Box 2.2 of Chapter 2 of AR5 ([[#Hartmann--2013|Hartmann et al., 2013]]). In addition to trends, consideration is also made of changes between various time slices/periods in performing the assessment ([[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] and Cross-Chapter Box 2.1). Statistical significance of trends and changes are assessed at the two-tailed 90% confidence (''very likely'') level unless otherwise stated. Limited use is also made of published analyses that have employed a range of methodological choices. In each such case the method/metric is stated.

There exist a variety of inevitable and, in some cases, irreducible uncertainties in performing an assessment of the observational evidence for climate change. In some instances, a combination of sources of uncertainty is important. For example, the assessment of global surface temperature over the instrumental record in [[#2.3.1.1.3|Section 2.3.1.1.3]] considers a combination of observational-dataset and trend-estimate uncertainties. Furthermore, estimates of parametric uncertainty are often not comprehensive in their consideration of all possible factors and, when such estimates are constructed in distinct manners, there are often significant limitations to their direct comparability ([[#Hartmann--2013|Hartmann et al., 2013]], their Box 2.1).

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