Editing IPCC:AR6/WGI/TS (section)

===== Connecting Scenarios and Global Warming Levels =====

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In this Report, scenario-based climate projections are translated into GWLs by aggregating the ESM model response at specific GWLs across scenarios (see Figure TS.5 and Figure TS.6). The climate response pattern for the 20-year period around when individual simulations reach a given GWL are averaged across all models and scenarios that reach that GWL. The best estimate and ''likely'' range of the timing of when a certain GWL is reached under a particular scenario (or ‘GWL-crossing time’), however, is based not only on CMIP6 output, but on a combined assessment taking into account the observed warming to date, CMIP6 output and additional lines of evidence (see Cross-Section Box TS.1). Links to chapters 4.3.4, Cross-Chapter Box 11.1, Atlas.2, Interactive Atlas

Global warming levels are closely related to cumulative CO <sub>2</sub> (and in some cases CO <sub>2</sub> -equivalent) emissions. This Report confirms the assessment of the WGI contribution to AR5 and SR1.5 that a near-linear relationship exists between cumulative CO <sub>2</sub> emissions and the resulting increase in global surface temperature (Section TS.3.2). This implies that continued CO <sub>2</sub> emissions will cause further warming and associated changes in all components of the climate system. For declining cumulative CO <sub>2</sub> emissions (i.e., if negative net emissions are achieved), the relationship is less strong for some components, such as the hydrological cycle. The WGI report uses cumulative CO <sub>2</sub> emissions to compare climate response across scenarios and provides a link to the emissions pathways assessment in WGIII. The advantage of using cumulative CO <sub>2</sub> emissions is that it is an inherent emissions scenario characteristic rather than an outcome of the scenario-based projections, where uncertainties in the cause–effect chain from emissions to temperature change are important (Figure TS.4), for example, the uncertainty in ERF and TCR. Cumulative CO <sub>2</sub> emissions can also provide a link to the assessments of mitigation options. Cumulative CO <sub>2</sub> emissions do not carry information about non-CO <sub>2</sub> emissions, although these can be included with specific emissions metrics to estimate CO <sub>2</sub> -equivalent emissions. (Section TS.3.3) Links to chapters 1.3.2, 1.6, 4.6.2, 5.5, 7.6

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