Editing IPCC:AR6/WGI/TS (section)

==== TS.1.3.2 Global Warming Levels and Cumulative CO <sub>2</sub> Emissions ====

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'''Quantifying geographical response patterns of climate change at various global warming levels (GWLs), such as 1.5°C or 2°C above the 1850–1900 period, is useful for characterizing changes in mean climate, extremes and climatic impact-drivers. Global warming levels are used in this Report as a dimension of integration independent of the timing when the warming level is reached and of the emissions scenario that led to the warming. For many climate variables the response pattern for a given GWL is consistent across different scenarios. However, this is not the case for slowly responding processes, such as ice-sheet and glacier mass loss, deep ocean warming, and the related sea level rise. The response of these variables depends on the time it takes to reach the GWL, differs if the warming is reached in a transient warming state or after a temporary overshoot of the warming level, and will continue to evolve, over centuries to millennia, even after global warming has stabilized. Different GWLs correspond closely to specific cumulative CO <sub>2</sub> emissions due to their near-linear relationship with global surface temperature. This Report uses 1.0°C, 1.5°C, 2.0°C, 3.0°C and 4.0°C above 1850–1900 conditions as a primary set of GWLs. Links to chapters 1.6.2, 4.2.4, 4.6.1, 5.5, Cross-Chapter Box 11.1, Cross-chapter Box 12.1'''
For many indicators of climate change, such as seasonal and annual mean and extreme surface air temperatures and precipitation, the geographical patterns of changes are well estimated by the level of global surface warming, independently of the details of the emissions pathways that caused the warming, or the time at which the level of warming is attained. GWLs, defined as a global surface temperature increase of, for example, 1.5°C or 2°C relative to the mean of 1850–1900, are therefore a useful way to integrate climate information independently of specific scenarios or time periods. Links to chapters 1.6.2, 4.2.4, 4.6.1, 11.2.4, Cross-Chapter Box 11.1

The use of GWLs allows disentangling the contribution of changes in global warming from regional aspects of the climate response, as scenario differences in response patterns at a given GWL are often smaller than model uncertainty and internal variability. The relationship between the GWL and response patterns is often linear, but integration of information can also be done for non-linear changes, like the frequency of heat extremes. The requirement is that the relationship to the GWL is broadly independent of the scenario and relative contribution of radiative forcing agents. Links to chapters 1.6, 11.2.4, Cross-Chapter Box 11.1

The GWL approach to integration of climate information also has some limitations. Variables that are quick to respond to warming, like temperature and precipitation, including extremes, sea ice area, permafrost and snow cover, show little scenario dependence for a given GWL, whereas slow-responding variables such as glacier and ice-sheet mass, warming of the deep ocean and their contributions to sea level rise, have substantial dependency on the trajectory of warming taken to reach the GWL. A given GWL can also be reached for different balances between anthropogenic forcing agents, such as long-lived greenhouse gas and SLCF emissions, and the response patterns may depend on this balance. Finally, there is a difference in the response even for temperature-related variables if a GWL is reached in a rapidly warming transient state or in an equilibrium state when the land–sea warming contrast is less pronounced. In this Report, the climate responses at different GWLs are calculated based on climate model projections for the 21st century (see Figure TS.5), which are mostly not in equilibrium. The SSP1-1.9 scenario allows assessing the response to a GWL of about 1.5°C after a (relatively) short-term stabilization by the end of the 21st century. Links to chapters 4.6.2, 9.3.1.1, 9.5.2.3, 9.5.3.3, 11.2.4, Cross-Chapter Box 11.1, Cross-Chapter Box 12.1

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'''Figure TS.5 |''' '''Scenarios, global warming levels, and patterns of change.''' ''The intent of this figure is to show how scenarios are linked to global warming levels (GWLs) and to provide examples of the evolution of patterns of change with global warming levels.'' (a) Illustrative example of GWLs defined as global surface temperature response to anthropogenic emissions in unconstrained Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations, for two illustrative scenarios (SSP1-2.6 and SSP3-7.0). The time when a given simulation reaches a GWL, for example, +2°C, relative to 1850–1900 is taken as the time when the central year of a 20-year running mean first reaches that level of warming. See the dots for +2°C, and how not all simulations reach all levels of warming. The assessment of the timing when a GWL is reached takes into account additional lines of evidence and is discussed in Cross-Section Box TS.1. (b) Multi-model, multi-simulation average response patterns of change in near-surface air temperature, precipitation (expressed as percentage change) and soil moisture (expressed in standard deviations of interannual variability) for three GWLs. The number to the top right of the panels shows the number of model simulations averaged across including all models that reach the corresponding GWL in any of the five Shared Socio-economic Pathways (SSPs). See Section TS.2 for discussion. Links to chapters Cross-Chapter Box 11.1

Global warming levels are highly relevant as a dimension of integration across scientific disciplines and socio-economic actors and are motivated by the long-term goal in the Paris Agreement of ‘holding the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels’. The evolution of aggregated impacts with temperature levels has also been widely used and embedded in the WGII assessment. This includes the ‘Reasons for Concern’ (RFC) and other ‘burning ember’ diagrams in IPCC WGII. The RFC framework has been further expanded in SR1.5, SROCC and SRCCL by explicitly looking at the differential impacts between half-degree GWLs and the evolution of risk for different socio-economic assumptions. Links to chapters 1.4.4, 1.6.2, 11.2.4, 12.5.2, Cross-Chapter Box 11.1, Cross-Chapter Box 12.1

SR1.5 concluded that ‘climate models project robust differences in regional climate characteristics between present-day and global warming of 1.5°C, and between 1.5°C and 2°C’. This Report adopts a set of common GWLs across which climate projections, impacts, adaptation challenges and climate change mitigation challenges can be integrated, within and across the three Working Groups, relative to 1850–1900. The core set of GWLs in this Report are 1.0°C (close to present day conditions), 1.5°C, 2.0°C, 3.0°C and 4.0°C. Links to chapters 1.4, 1.6.2, Cross-Chapter Box 1.2, Table 1.5, Cross-Chapter Box 11.1

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===== 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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