IPCC:AR6/SR15/Chapter-1 - Revision history

Laura: /* 1.1 Assessing the Knowledge Base for a 1.5°C Warmer World */

2026-05-29T13:34:01Z

1.1 Assessing the Knowledge Base for a 1.5°C Warmer World

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	== Box 1.1 The Anthropocene: Strengthening the Global Response to 1.5°C Global Warming ==		== Box 1.1 The Anthropocene: Strengthening the Global Response to 1.5°C Global Warming ==

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Laura at 13:17, 29 May 2026

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Laura: /* Cross-Chapter Box 4: Sustainable Development and the Sustainable Development Goals */

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Cross-Chapter Box 4: Sustainable Development and the Sustainable Development Goals

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	== 1.5 Assessment Frameworks and Emerging Methodologies that Integrate Climate Change Mitigation and Adaptation with Sustainable Development ==		== 1.5 Assessment Frameworks and Emerging Methodologies that Integrate Climate Change Mitigation and Adaptation with Sustainable Development ==

Laura: /* Lead Authors */

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Lead Authors

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	~~======~~ Lead Authors ~~======~~		'''Lead Authors'''

	* Diana Liverman (United States)		* Diana Liverman (United States)

Laura: /* 1.4.3 Transformation, Transformation Pathways, and Transition: Evaluating Trade-Offs and Synergies Between Mitigation, Adaptation and Sustainable Development Goals */

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1.4.3 Transformation, Transformation Pathways, and Transition: Evaluating Trade-Offs and Synergies Between Mitigation, Adaptation and Sustainable Development Goals

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	== Cross-Chapter Box 4: Sustainable Development and the Sustainable Development Goals ==		== Cross-Chapter Box 4: Sustainable Development and the Sustainable Development Goals ==

Laura: /* Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C */

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Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C

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	'''Technological''': What technologies are available to support transformation?		'''Technological''': What technologies are available to support transformation?

	'''Economic''' : What economic conditions could support transformation?		'''Economic''': What economic conditions could support transformation?

	'''Socio-cultural''': What conditions could support transformations in behaviour and lifestyles? To what extent are the transformations socially acceptable and consistent with equity?		'''Socio-cultural''': What conditions could support transformations in behaviour and lifestyles? To what extent are the transformations socially acceptable and consistent with equity?

Laura: /* Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C */

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Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C

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	This Cross-Chapter Box describes the concept of feasibility in relation to efforts to limit global warming to 1.5°C in the context of sustainable development and efforts to eradicate poverty and draws from the understanding of feasibility emerging within the IPCC (IPCC, 2017) <sup>[[#fn:r258\|258]]</sup>. Feasibility can be assessed in different ways, and no single answer exists as to the question of whether it is feasible to limit warming to 1.5°C. This implies that an assessment of feasibility would go beyond a ‘yes’ or a ‘no’. Rather, feasibility provides a frame to understand the different conditions and potential responses for implementing adaptation and mitigation pathways, and options compatible with a 1.5°C warmer world. This report assesses the overall feasibility of limiting warming to 1.5°C, and the feasibility of adaptation and mitigation options compatible with a 1.5°C warmer world, in six dimensions:		This Cross-Chapter Box describes the concept of feasibility in relation to efforts to limit global warming to 1.5°C in the context of sustainable development and efforts to eradicate poverty and draws from the understanding of feasibility emerging within the IPCC (IPCC, 2017) <sup>[[#fn:r258\|258]]</sup>. Feasibility can be assessed in different ways, and no single answer exists as to the question of whether it is feasible to limit warming to 1.5°C. This implies that an assessment of feasibility would go beyond a ‘yes’ or a ‘no’. Rather, feasibility provides a frame to understand the different conditions and potential responses for implementing adaptation and mitigation pathways, and options compatible with a 1.5°C warmer world. This report assesses the overall feasibility of limiting warming to 1.5°C, and the feasibility of adaptation and mitigation options compatible with a 1.5°C warmer world, in six dimensions:

	'''Geophysical''' : What global emission pathways could be consistent with conditions of a 1.5°C warmer world? What are the physical potentials for adaptation?		'''Geophysical''': What global emission pathways could be consistent with conditions of a 1.5°C warmer world? What are the physical potentials for adaptation?

	'''Environmental-ecological''' : What are the ecosystem services and resources, including geological storage capacity and related rate of needed land-use change, available to promote transformations, and to what extent are they compatible with enhanced resilience?		'''Environmental-ecological''': What are the ecosystem services and resources, including geological storage capacity and related rate of needed land-use change, available to promote transformations, and to what extent are they compatible with enhanced resilience?

	'''Technological''' : What technologies are available to support transformation?		'''Technological''': What technologies are available to support transformation?

	'''Economic''' : What economic conditions could support transformation?		'''Economic''' : What economic conditions could support transformation?

	'''Socio-cultural''' : What conditions could support transformations in behaviour and lifestyles? To what extent are the transformations socially acceptable and consistent with equity?		'''Socio-cultural''': What conditions could support transformations in behaviour and lifestyles? To what extent are the transformations socially acceptable and consistent with equity?

	'''Institutional''' : What institutional conditions are in place to support transformations, including multi-level governance, institutional capacity, and political support?		'''Institutional''': What institutional conditions are in place to support transformations, including multi-level governance, institutional capacity, and political support?

	Assessment of feasibility in this report starts by evaluating the unavoidable warming from past emissions (Section 1.2.4) and identifying mitigation pathways that would lead to a 1.5°C world, which indicates that rapid and deep deviations from current emission pathways are necessary (Chapter 2). In the case of adaptation, an assessment of feasibility starts from an evaluation of the risks and impacts of climate change (Chapter 3). To mitigate and adapt to climate risks, system-wide technical, institutional and socio-economic transitions would be required, as well as the implementation of a range of specific mitigation and adaptation options. Chapter 4 applies various indicators categorised in these six dimensions to assess the feasibility of illustrative examples of relevant mitigation and adaptation options (Section 4.5.1). Such options and pathways have different effects on sustainable development, poverty eradication and adaptation capacity (Chapter 5).		Assessment of feasibility in this report starts by evaluating the unavoidable warming from past emissions (Section 1.2.4) and identifying mitigation pathways that would lead to a 1.5°C world, which indicates that rapid and deep deviations from current emission pathways are necessary (Chapter 2). In the case of adaptation, an assessment of feasibility starts from an evaluation of the risks and impacts of climate change (Chapter 3). To mitigate and adapt to climate risks, system-wide technical, institutional and socio-economic transitions would be required, as well as the implementation of a range of specific mitigation and adaptation options. Chapter 4 applies various indicators categorised in these six dimensions to assess the feasibility of illustrative examples of relevant mitigation and adaptation options (Section 4.5.1). Such options and pathways have different effects on sustainable development, poverty eradication and adaptation capacity (Chapter 5).
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	'''Dynamic effects:''' Conditions that are highly dynamic and vary over time, especially under potential conditions of overshoot or no overshoot. Some dimensions might be more time sensitive or sequential than others (i.e., if conditions are such that it is no longer geophysically feasible to avoid overshooting 1.5°C, the social and institutional feasibility of avoiding overshoot will be no longer relevant). Path dependencies, risks of legacy lock-ins related to existing infrastructures, and possibilities of acceleration permitted by cumulative effects (e.g., dramatic cost decreases driven by learning-by-doing) are all key features to be captured. The effects can play out over various time scales and thus require understanding the connections between near-term (meaning within the next several years to two decades) and long-term implications (meaning over the next several decades) when assessing feasibility conditions.		'''Dynamic effects:''' Conditions that are highly dynamic and vary over time, especially under potential conditions of overshoot or no overshoot. Some dimensions might be more time sensitive or sequential than others (i.e., if conditions are such that it is no longer geophysically feasible to avoid overshooting 1.5°C, the social and institutional feasibility of avoiding overshoot will be no longer relevant). Path dependencies, risks of legacy lock-ins related to existing infrastructures, and possibilities of acceleration permitted by cumulative effects (e.g., dramatic cost decreases driven by learning-by-doing) are all key features to be captured. The effects can play out over various time scales and thus require understanding the connections between near-term (meaning within the next several years to two decades) and long-term implications (meaning over the next several decades) when assessing feasibility conditions.

	'''Spatial effects''' : Conditions that are spatially variable and scale dependent, according to context-specific factors such as regional-scale environmental resource limits and endowment; economic wealth of local populations; social organisation, cultural beliefs, values and worldviews; spatial organisation, including conditions of urbanisation; and financial and institutional and governance capacity. This means that the conditions for achieving the global transformation required for a 1.5°C world will be heterogeneous and vary according to the specific context. On the other hand, the satisfaction of these conditions may depend upon global-scale drivers, such as international flows of finance, technologies or capacities. This points to the need for understanding feasibility to capture the interplay between the conditions at different scales.		'''Spatial effects''': Conditions that are spatially variable and scale dependent, according to context-specific factors such as regional-scale environmental resource limits and endowment; economic wealth of local populations; social organisation, cultural beliefs, values and worldviews; spatial organisation, including conditions of urbanisation; and financial and institutional and governance capacity. This means that the conditions for achieving the global transformation required for a 1.5°C world will be heterogeneous and vary according to the specific context. On the other hand, the satisfaction of these conditions may depend upon global-scale drivers, such as international flows of finance, technologies or capacities. This points to the need for understanding feasibility to capture the interplay between the conditions at different scales.

	With each effect, the interplay between different conditions influences the feasibility of both pathways (Chapter 2) and options (Chapter 4), which in turn affect the likelihood of limiting warming to 1.5°C. The complexity of these interplays triggers unavoidable uncertainties, requiring transformations that remain robust under a range of possible futures that limit warming to 1.5°C.		With each effect, the interplay between different conditions influences the feasibility of both pathways (Chapter 2) and options (Chapter 4), which in turn affect the likelihood of limiting warming to 1.5°C. The complexity of these interplays triggers unavoidable uncertainties, requiring transformations that remain robust under a range of possible futures that limit warming to 1.5°C.

Laura: /* Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C */

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Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C

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	With each effect, the interplay between different conditions influences the feasibility of both pathways (Chapter 2) and options (Chapter 4), which in turn affect the likelihood of limiting warming to 1.5°C. The complexity of these interplays triggers unavoidable uncertainties, requiring transformations that remain robust under a range of possible futures that limit warming to 1.5°C.		With each effect, the interplay between different conditions influences the feasibility of both pathways (Chapter 2) and options (Chapter 4), which in turn affect the likelihood of limiting warming to 1.5°C. The complexity of these interplays triggers unavoidable uncertainties, requiring transformations that remain robust under a range of possible futures that limit warming to 1.5°C.
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Laura: /* 1.4.2 Governance, Implementation and Policies */

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1.4.2 Governance, Implementation and Policies

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	== Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C ==		== Cross-Chapter Box 3: Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature Increases to 1.5°C ==

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	This chapter frames the context, knowledge-base and assessment approaches used to understand the impacts of 1.5°C global warming above pre-industrial levels and related global greenhouse gas emission pathways, building on the IPCC Fifth Assessment Report (AR5), in the context of strengthening the global response to the threat of climate change, sustainable development and efforts to eradicate poverty.		This chapter frames the context, knowledge-base and assessment approaches used to understand the impacts of 1.5°C global warming above pre-industrial levels and related global greenhouse gas emission pathways, building on the IPCC Fifth Assessment Report (AR5), in the context of strengthening the global response to the threat of climate change, sustainable development and efforts to eradicate poverty.

	'''Human-induced warming reached approximately 1°C (''likely'' between 0.8°C and 1.2°C) above pre-industrial levels in 2017, increasing at 0.2°C (''likely'' between 0.1°C and 0.3°C) per decade (''high confidence'').''' Global warming is defined in this report as an increase in combined surface air and sea surface temperatures averaged over the globe and over a 30-year period. Unless otherwise specified, warming is expressed relative to the period 1850–1900, used as an approximation of pre-industrial temperatures in AR5. For periods shorter than 30 years, warming refers to the estimated average temperature over the 30 years centred on that shorter period, accounting for the impact of any temperature fluctuations or trend within those 30 years. Accordingly, warming from pre- industrial levels to the decade 2006–2015 is assessed to be 0.87°C (''likely'' between 0.75°C and 0.99°C). Since 2000, the estimated level of human-induced warming has been equal to the level of observed warming with a ''likely'' range of ±20% accounting for uncertainty due to contributions from solar and volcanic activity over the historical period (''high confidence'' ). {1.2.1}		'''Human-induced warming reached approximately 1°C (''likely'' between 0.8°C and 1.2°C) above pre-industrial levels in 2017, increasing at 0.2°C (''likely'' between 0.1°C and 0.3°C) per decade (''high confidence'').''' Global warming is defined in this report as an increase in combined surface air and sea surface temperatures averaged over the globe and over a 30-year period. Unless otherwise specified, warming is expressed relative to the period 1850–1900, used as an approximation of pre-industrial temperatures in AR5. For periods shorter than 30 years, warming refers to the estimated average temperature over the 30 years centred on that shorter period, accounting for the impact of any temperature fluctuations or trend within those 30 years. Accordingly, warming from pre- industrial levels to the decade 2006–2015 is assessed to be 0.87°C (''likely'' between 0.75°C and 0.99°C). Since 2000, the estimated level of human-induced warming has been equal to the level of observed warming with a ''likely'' range of ±20% accounting for uncertainty due to contributions from solar and volcanic activity over the historical period (''high confidence''). {1.2.1}

	'''Warming greater than the global average has already been experienced in many regions and seasons, with higher average warming over land than over the ocean (''high confidence'' ).''' Most land regions are experiencing greater warming than the global average, while most ocean regions are warming at a slower rate. Depending on the temperature dataset considered, 20–40% of the global human population live in regions that, by the decade 2006–2015, had already experienced warming of more than 1.5°C above pre-industrial in at least one season (''medium confidence'' ). {1.2.1, 1.2.2}		'''Warming greater than the global average has already been experienced in many regions and seasons, with higher average warming over land than over the ocean (''high confidence'').''' Most land regions are experiencing greater warming than the global average, while most ocean regions are warming at a slower rate. Depending on the temperature dataset considered, 20–40% of the global human population live in regions that, by the decade 2006–2015, had already experienced warming of more than 1.5°C above pre-industrial in at least one season (''medium confidence''). {1.2.1, 1.2.2}

	'''Past emissions alone are ''unlikely'' to raise global-mean temperature to 1.5°C above pre-industrial levels (''medium confidence'' )''' , but past emissions do commit to other changes, such as further sea level rise (''high confidence'' ). If all anthropogenic emissions (including aerosol-related) were reduced to zero immediately, any further warming beyond the 1°C already experienced would ''likely'' be less than 0.5°C over the next two to three decades (''high confidence'' ), and ''likely'' less than 0.5°C on a century time scale (''medium confidence'' ), due to the opposing effects of different climate processes and drivers. A warming greater than 1.5°C is therefore not geophysically unavoidable: whether it will occur depends on future rates of emission reductions. {1.2.3, 1.2.4}		'''Past emissions alone are ''unlikely'' to raise global-mean temperature to 1.5°C above pre-industrial levels (''medium confidence'')''' , but past emissions do commit to other changes, such as further sea level rise (''high confidence''). If all anthropogenic emissions (including aerosol-related) were reduced to zero immediately, any further warming beyond the 1°C already experienced would ''likely'' be less than 0.5°C over the next two to three decades (''high confidence''), and ''likely'' less than 0.5°C on a century time scale (''medium confidence''), due to the opposing effects of different climate processes and drivers. A warming greater than 1.5°C is therefore not geophysically unavoidable: whether it will occur depends on future rates of emission reductions. {1.2.3, 1.2.4}

	'''1.5°C emission pathways are defined as those that, given current knowledge of the climate response, provide a one- in-two to two-in-three chance of warming either remaining below 1.5°C or returning to 1.5°C by around 2100 following an overshoot.''' Overshoot pathways are characterized by the peak magnitude of the overshoot, which may have implications for impacts. All 1.5°C pathways involve limiting cumulative emissions of long-lived greenhouse gases, including carbon dioxide and nitrous oxide, and substantial reductions in other climate forcers (''high confidence'' ). Limiting cumulative emissions requires either reducing net global emissions of long-lived greenhouse gases to zero before the cumulative limit is reached, or net negative global emissions (anthropogenic removals) after the limit is exceeded. {1.2.3, 1.2.4, Cross-Chapter Boxes 1 and 2}		'''1.5°C emission pathways are defined as those that, given current knowledge of the climate response, provide a one- in-two to two-in-three chance of warming either remaining below 1.5°C or returning to 1.5°C by around 2100 following an overshoot.''' Overshoot pathways are characterized by the peak magnitude of the overshoot, which may have implications for impacts. All 1.5°C pathways involve limiting cumulative emissions of long-lived greenhouse gases, including carbon dioxide and nitrous oxide, and substantial reductions in other climate forcers (''high confidence''). Limiting cumulative emissions requires either reducing net global emissions of long-lived greenhouse gases to zero before the cumulative limit is reached, or net negative global emissions (anthropogenic removals) after the limit is exceeded. {1.2.3, 1.2.4, Cross-Chapter Boxes 1 and 2}

	'''This report assesses projected impacts at a global average warming of 1.5°C and higher levels of warming.''' Global warming of 1.5°C is associated with global average surface temperatures fluctuating naturally on either side of 1.5°C, together with warming substantially greater than 1.5°C in many regions and seasons (''high confidence'' ), all of which must be considered in the assessment of impacts. Impacts at 1.5°C of warming also depend on the emission pathway to 1.5°C. Very different impacts result from pathways that remain below 1.5°C versus pathways that return to 1.5°C after a substantial overshoot, and when temperatures stabilize at 1.5°C versus a transient warming past 1.5°C (''medium confidence'' ). {1.2.3, 1.3}		'''This report assesses projected impacts at a global average warming of 1.5°C and higher levels of warming.''' Global warming of 1.5°C is associated with global average surface temperatures fluctuating naturally on either side of 1.5°C, together with warming substantially greater than 1.5°C in many regions and seasons (''high confidence''), all of which must be considered in the assessment of impacts. Impacts at 1.5°C of warming also depend on the emission pathway to 1.5°C. Very different impacts result from pathways that remain below 1.5°C versus pathways that return to 1.5°C after a substantial overshoot, and when temperatures stabilize at 1.5°C versus a transient warming past 1.5°C (''medium confidence''). {1.2.3, 1.3}

	'''Ethical considerations, and the principle of equity in particular, are central to this report, recognizing that many of the impacts of warming up to and beyond 1.5°C, and some potential impacts of mitigation actions required to limit warming to 1.5°C, fall disproportionately on the poor and vulnerable (''high confidence'' ).''' Equity has procedural and distributive dimensions and requires fairness in burden sharing both between generations and between and within nations. In framing the objective of holding the increase in the global average temperature rise to well below 2°C above pre-industrial levels, and to pursue efforts to limit warming to 1.5°C, the Paris Agreement associates the principle of equity with the broader goals of poverty eradication and sustainable development, recognising that effective responses to climate change require a global collective effort that may be guided by the 2015 United Nations Sustainable Development Goals. {1.1.1}		'''Ethical considerations, and the principle of equity in particular, are central to this report, recognizing that many of the impacts of warming up to and beyond 1.5°C, and some potential impacts of mitigation actions required to limit warming to 1.5°C, fall disproportionately on the poor and vulnerable (''high confidence'').''' Equity has procedural and distributive dimensions and requires fairness in burden sharing both between generations and between and within nations. In framing the objective of holding the increase in the global average temperature rise to well below 2°C above pre-industrial levels, and to pursue efforts to limit warming to 1.5°C, the Paris Agreement associates the principle of equity with the broader goals of poverty eradication and sustainable development, recognising that effective responses to climate change require a global collective effort that may be guided by the 2015 United Nations Sustainable Development Goals. {1.1.1}

	'''Climate adaptation refers to the actions taken to manage impacts of climate change by reducing vulnerability and exposure to its harmful effects and exploiting any potential benefits.''' Adaptation takes place at international, national and local levels. Subnational jurisdictions and entities, including urban and rural municipalities, are key to developing and reinforcing measures for reducing weather- and climate-related risks. Adaptation implementation faces several barriers including lack of up-to-date and locally relevant information, lack of finance and technology, social values and attitudes, and institutional constraints (''high confidence'' ). Adaptation is more ''likely'' to contribute to sustainable development when policies align with mitigation and poverty eradication goals (''medium confidence'' ). {1.1, 1.4}		'''Climate adaptation refers to the actions taken to manage impacts of climate change by reducing vulnerability and exposure to its harmful effects and exploiting any potential benefits.''' Adaptation takes place at international, national and local levels. Subnational jurisdictions and entities, including urban and rural municipalities, are key to developing and reinforcing measures for reducing weather- and climate-related risks. Adaptation implementation faces several barriers including lack of up-to-date and locally relevant information, lack of finance and technology, social values and attitudes, and institutional constraints (''high confidence''). Adaptation is more ''likely'' to contribute to sustainable development when policies align with mitigation and poverty eradication goals (''medium confidence''). {1.1, 1.4}

	'''Ambitious mitigation actions are indispensable to limit warming to 1.5°C while achieving sustainable development and poverty eradication (''high confidence'' ).''' Ill-designed responses, however, could pose challenges especially – but not exclusively – for countries and regions contending with poverty and those requiring significant transformation of their energy systems. This report focuses on ‘climate-resilient development pathways’, which aim to meet the goals of sustainable development, including climate adaptation and mitigation, poverty eradication and reducing inequalities. But any feasible pathway that remains within 1.5°C involves synergies and trade-offs (''high confidence'' ). Significant uncertainty remains as to which pathways are more consistent with the principle of equity.<br />		'''Ambitious mitigation actions are indispensable to limit warming to 1.5°C while achieving sustainable development and poverty eradication (''high confidence'').''' Ill-designed responses, however, could pose challenges especially – but not exclusively – for countries and regions contending with poverty and those requiring significant transformation of their energy systems. This report focuses on ‘climate-resilient development pathways’, which aim to meet the goals of sustainable development, including climate adaptation and mitigation, poverty eradication and reducing inequalities. But any feasible pathway that remains within 1.5°C involves synergies and trade-offs (''high confidence''). Significant uncertainty remains as to which pathways are more consistent with the principle of equity.<br />
	{1.1.1, 1.4}		{1.1.1, 1.4}

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	It is frequently asked whether limiting warming to 1.5°C is ‘feasible’ (Cross-Chapter Box 3 in this chapter). There are many dimensions to this question, including the warming ‘commitment’ from past emissions of greenhouse gases and aerosol precursors. Quantifying commitment from past emissions is complicated by the very different behaviour of different climate forcers affected by human activity: emissions of long-lived greenhouse gases such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O) have a very persistent impact on radiative forcing (Myhre et al., 2013) <sup>[[#fn:r153\|153]]</sup>, lasting from over a century (in the case of N <sub>2</sub> O) to hundreds of thousands of years (for CO <sub>2</sub> ). The radiative forcing impact of short-lived climate forcers (SLCFs) such as methane (CH <sub>4</sub> ) and aerosols, in contrast, persists for at most about a decade (in the case of methane) down to only a few days. These different behaviours must be taken into account in assessing the implications of any approach to calculating aggregate emissions (Cross-Chapter Box 2 in this chapter).		It is frequently asked whether limiting warming to 1.5°C is ‘feasible’ (Cross-Chapter Box 3 in this chapter). There are many dimensions to this question, including the warming ‘commitment’ from past emissions of greenhouse gases and aerosol precursors. Quantifying commitment from past emissions is complicated by the very different behaviour of different climate forcers affected by human activity: emissions of long-lived greenhouse gases such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O) have a very persistent impact on radiative forcing (Myhre et al., 2013) <sup>[[#fn:r153\|153]]</sup>, lasting from over a century (in the case of N <sub>2</sub> O) to hundreds of thousands of years (for CO <sub>2</sub>). The radiative forcing impact of short-lived climate forcers (SLCFs) such as methane (CH <sub>4</sub>) and aerosols, in contrast, persists for at most about a decade (in the case of methane) down to only a few days. These different behaviours must be taken into account in assessing the implications of any approach to calculating aggregate emissions (Cross-Chapter Box 2 in this chapter).

	Geophysical warming commitment is defined as the unavoidable future warming resulting from physical Earth system inertia. Different variants are discussed in the literature, including (i) the ‘constant composition commitment’ (CCC), defined by Meehl et al. (2007) <sup>[[#fn:r154\|154]]</sup> as the further warming that would result if atmospheric concentrations of GHGs and other climate forcers were stabilised at the current level; and (ii) and the ‘zero emissions commitment’ (ZEC), defined as the further warming that would still occur if all future anthropogenic emissions of greenhouse gases and aerosol precursors were eliminated instantaneously (Meehl et al., 2007; Collins et al., 2013) <sup>[[#fn:r155\|155]]</sup>.		Geophysical warming commitment is defined as the unavoidable future warming resulting from physical Earth system inertia. Different variants are discussed in the literature, including (i) the ‘constant composition commitment’ (CCC), defined by Meehl et al. (2007) <sup>[[#fn:r154\|154]]</sup> as the further warming that would result if atmospheric concentrations of GHGs and other climate forcers were stabilised at the current level; and (ii) and the ‘zero emissions commitment’ (ZEC), defined as the further warming that would still occur if all future anthropogenic emissions of greenhouse gases and aerosol precursors were eliminated instantaneously (Meehl et al., 2007; Collins et al., 2013) <sup>[[#fn:r155\|155]]</sup>.
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	Emissions of many different climate forcers will affect the rate and magnitude of climate change over the next few decades (Myhre et al., 2013) <sup>[[#fn:r180\|180]]</sup>. Since these decades will determine when 1.5°C is reached or whether a warming greater than 1.5°C is avoided, understanding the aggregate impact of different forcing agents is particularly important in the context of 1.5°C pathways. Paragraph 17 of Decision 1 of the 21st Conference of the Parties on the adoption of the Paris Agreement specifically states that this report is to identify aggregate greenhouse gas emission levels compatible with holding the increase in global average temperatures to 1.5°C above pre-industrial levels (see Chapter 2). This request highlights the need to consider the implications of different methods of aggregating emissions of different gases, both for future temperatures and for other aspects of the climate system (Levasseur et al., 2016; Ocko et al., 2017) <sup>[[#fn:r181\|181]]</sup>.		Emissions of many different climate forcers will affect the rate and magnitude of climate change over the next few decades (Myhre et al., 2013) <sup>[[#fn:r180\|180]]</sup>. Since these decades will determine when 1.5°C is reached or whether a warming greater than 1.5°C is avoided, understanding the aggregate impact of different forcing agents is particularly important in the context of 1.5°C pathways. Paragraph 17 of Decision 1 of the 21st Conference of the Parties on the adoption of the Paris Agreement specifically states that this report is to identify aggregate greenhouse gas emission levels compatible with holding the increase in global average temperatures to 1.5°C above pre-industrial levels (see Chapter 2). This request highlights the need to consider the implications of different methods of aggregating emissions of different gases, both for future temperatures and for other aspects of the climate system (Levasseur et al., 2016; Ocko et al., 2017) <sup>[[#fn:r181\|181]]</sup>.

	To date, reporting of GHG emissions under the UNFCCC has used Global Warming Potentials (GWPs) evaluated over a 100-year time horizon (GWP <sub>100</sub> ) to combine multiple climate forcers. IPCC Working Group 3 reports have also used GWP <sub>100</sub> to represent multi-gas pathways (Clarke et al., 2014) <sup>[[#fn:r182\|182]]</sup>. For reasons of comparability and consistency with current practice, Chapter 2 in this Special Report continues to use this aggregation method. Numerous other methods of combining different climate forcers have been proposed, such as the Global Temperature-change Potential (GTP; Shine et al., 2005) <sup>[[#fn:r183\|183]]</sup> and the Global Damage Potential (Tol et al., 2012; Deuber et al., 2013) <sup>[[#fn:r184\|184]]</sup>.		To date, reporting of GHG emissions under the UNFCCC has used Global Warming Potentials (GWPs) evaluated over a 100-year time horizon (GWP <sub>100</sub>) to combine multiple climate forcers. IPCC Working Group 3 reports have also used GWP <sub>100</sub> to represent multi-gas pathways (Clarke et al., 2014) <sup>[[#fn:r182\|182]]</sup>. For reasons of comparability and consistency with current practice, Chapter 2 in this Special Report continues to use this aggregation method. Numerous other methods of combining different climate forcers have been proposed, such as the Global Temperature-change Potential (GTP; Shine et al., 2005) <sup>[[#fn:r183\|183]]</sup> and the Global Damage Potential (Tol et al., 2012; Deuber et al., 2013) <sup>[[#fn:r184\|184]]</sup>.

	Climate forcers fall into two broad categories in terms of their impact on global temperature (Smith et al., 2012) <sup>[[#fn:r185\|185]]</sup> : long-lived GHGs, such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O), whose warming impact depends primarily on the total cumulative amount emitted over the past century or the entire industrial epoch; and short-lived climate forcers (SLCFs), such as methane and black carbon, whose warming impact depends primarily on current and recent annual emission rates (Reisinger et al., 2012; Myhre et al., 2013; Smith et al., 2013; Strefler et al., 2014) <sup>[[#fn:r186\|186]]</sup>. These different dependencies affect the emissions reductions required of individual forcers to limit warming to 1.5°C or any other level.		Climate forcers fall into two broad categories in terms of their impact on global temperature (Smith et al., 2012) <sup>[[#fn:r185\|185]]</sup> : long-lived GHGs, such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O), whose warming impact depends primarily on the total cumulative amount emitted over the past century or the entire industrial epoch; and short-lived climate forcers (SLCFs), such as methane and black carbon, whose warming impact depends primarily on current and recent annual emission rates (Reisinger et al., 2012; Myhre et al., 2013; Smith et al., 2013; Strefler et al., 2014) <sup>[[#fn:r186\|186]]</sup>. These different dependencies affect the emissions reductions required of individual forcers to limit warming to 1.5°C or any other level.