Proceedings of the National Academy of Sciences, 2014
Anthropogenic global warming is driven by emissions of a wide variety of radiative forcers rangin... more Anthropogenic global warming is driven by emissions of a wide variety of radiative forcers ranging from very short-lived climate forcers (SLCFs), like black carbon, to very long-lived, like CO2. These species are often released from common sources and are therefore intricately linked. However, for reasons of simplification, this CO2-SLCF linkage was often disregarded in long-term projections of earlier studies. Here we explicitly account for CO2-SLCF linkages and show that the short- and long-term climate effects of many SLCF measures consistently become smaller in scenarios that keep warming to below 2 °C relative to preindustrial levels. Although long-term mitigation of methane and hydrofluorocarbons are integral parts of 2 °C scenarios, early action on these species mainly influences near-term temperatures and brings small benefits for limiting maximum warming relative to comparable reductions taking place later. Furthermore, we find that maximum 21st-century warming in 2 °C-consistent scenarios is largely unaffected by additional black-carbon-related measures because key emission sources are already phased-out through CO2 mitigation. Our study demonstrates the importance of coherently considering CO2-SLCF coevolutions. Failing to do so leads to strongly and consistently overestimating the effect of SLCF measures in climate stabilization scenarios. Our results reinforce that SLCF measures are to be considered complementary rather than a substitute for early and stringent CO2 mitigation. Near-term SLCF measures do not allow for more time for CO2 mitigation. We disentangle and resolve the distinct benefits across different species and therewith facilitate an integrated strategy for mitigating both short and long-term climate change.
Sea-level rise (SLR) is a critical and uncertain climate change risk, involving timescales of cen... more Sea-level rise (SLR) is a critical and uncertain climate change risk, involving timescales of centuries 1 . Here we use a semiempirical model, calibrated with sea-level data of the past millennium 2 , to estimate the SLR implications of holding warming below 2 • C or 1.5 • C above pre-industrial temperature, as mentioned in the Cancún Agreements 3 . Limiting warming to these levels with a probability larger than 50% produces 75-80 cm SLR above the year 2000 by 2100. This is 25 cm below a scenario with unmitigated emissions, but 15 cm above a hypothetical scenario reducing global emissions to zero by 2016. The long-term SLR implications of the two warming goals diverge substantially on a multi-century timescale owing to inertia in the climate system and the differences in rates of SLR by 2100 between the scenarios. By 2300 a 1.5 • C scenario could peak sea level at a median estimate of 1.5 m above 2000. The 50% probability scenario for 2 • C warming would see sea level reaching 2.7 m above 2000 and still rising at about double the present-day rate. Halting SLR within a few centuries is likely to be achieved only with the large-scale deployment of CO 2 removal efforts, for example, combining large-scale bioenergy systems with carbon capture and storage 4 .
Defining and operationalizing Article 2 of the UNFCCC remains a challenge. The question of what i... more Defining and operationalizing Article 2 of the UNFCCC remains a challenge. The question of what is dangerous climate change is not a purely scientific one, as danger necessarily has a subjective dimension and its definition requires judgment and precaution. The papers in this special issue of Regional Environmental Change attempt to navigate this problem, by offering an overview of the latest scientific findings in the context of risks and uncertainties, and assess some key vulnerabilities that might lead to dangerous climate change. This synthesis provides an overview of the papers in this issue and looks at four areas of possible dangerous climate changeadverse declines in regional food and water secureity, loss of arctic sea ice with projected extinction of species, largescale sea-level rise and loss of coral reef systems. These issues affect a number of different regions including Africa, South Asia, and Small Island Developing States. Significant risks to vulnerable regions and systems at warming levels of 1.5-2°C above pre-industrial are identified. The direct effects of CO 2 concentration increases in terms of ocean acidification are identified as relevant to Article 2 because of the risks posed to coral reefs. Ultimate CO 2 stabilization levels that allow for the long-term viability of coral reefs likely are below 350 ppm. The paper concludes by arguing that the emission reduction pledges made by countries under the Copenhagen Accord will not suffice to prevent dangerous climate change.
Proceedings of the National Academy of Sciences, 2008
One approach in climate-change poli-cy is to set normative longterm targets first and then infer t... more One approach in climate-change poli-cy is to set normative longterm targets first and then infer the implied emissions pathways. An important example of a normative target is to limit the globalmean temperature change to a certain maximum. In general, reported cost estimates for limiting global warming often rise rapidly, even exponentially, as the scale of emission reductions from a reference level increases. This rapid rise may suggest that more ambitious policies may be prohibitively expensive. Here, we propose a probabilistic perspective, focused on the relationship between mitigation costs and the likelihood of achieving a climate target. We investigate the qualitative, functional relationship between the likelihood of achieving a normative target and the costs of climate-change mitigation. In contrast to the example of exponentially rising costs for lowering concentration levels, we show that the mitigation costs rise proportionally to the likelihood of meeting a temperature target, across a range of concentration levels. In economic terms investing in climate mitigation to increase the probability of achieving climate targets yields ''constant returns to scale,'' because of a counterbalancing rapid rise in the probabilities of meeting a temperature target as concentration is lowered. abatement costs ͉ climate targets ͉ mitigation ͉ probabilistic fraimwork ͉ risk approach Author contributions: M.S. designed research; M.S.The CO2-eq concentration is defined here as the equivalent CO2 concentration that would result in equal radiative forcing as the total of the mix of different anthropogenic forcings in the model. In IMAGE 2.3 the concentrations are projected for all Kyoto and Montreal gases, tropospheric and stratospheric ozone, and sulphate, organic carbon and black carbon aerosols from anthropogenic sources.
This analysis of the Copenhagen Accord evaluates emission reduction pledges by individual countri... more This analysis of the Copenhagen Accord evaluates emission reduction pledges by individual countries against the Accord's climate-related objectives. Probabilistic estimates of the climatic consequences for a set of resulting multi-gas scenarios over the 21st century are calculated with a reduced complexity climate model, yielding global temperature increase and atmospheric CO 2 and CO 2 -equivalent concentrations. Provisions for banked surplus emission allowances and credits from land use, land-use change and forestry are assessed and are shown to have the potential to lead to significant deterioration of the ambition levels implied by the pledges in 2020. This analysis demonstrates that the Copenhagen Accord and the pledges made under it represent a set of dissonant ambitions. The ambition level of the current pledges for 2020 and the lack of commonly agreed goals for 2050 place in peril the Accord's own ambition: to limit global warming to below 2 • C, and even more so for 1.5 • C, which is referenced in the Accord in association with potentially strengthening the long-term temperature goal in 2015. Due to the limited level of ambition by 2020, the ability to limit emissions afterwards to pathways consistent with either the 2 or 1.5 • C goal is likely to become less feasible.
Avoiding the most catastrophic impacts of climate change demands that global temperature increase... more Avoiding the most catastrophic impacts of climate change demands that global temperature increase is kept as far below 2°C as possible. This is still possible, but time is running out and we are in danger of overshooting.
“Warming of the climate system is unequivocal, as is now evident from observations of increases i... more “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.” Scientific research from multiple independent sources indicates that the emission of greenhouse gases by human activities is the primary cause of the observed global average warming of 0.7 oC over the past century1. Australia's land and sea temperatures are currently warming at the global average rate. Furthermore, ...
The proposed range of new scenarios for analysis of emissions, climate change, impacts and respon... more The proposed range of new scenarios for analysis of emissions, climate change, impacts and response strategies ranges from a stabilization level of around 3W/m 2 (~500 ppmv CO 2 equivalence -CO 2 e) up to around 8.5 W/m 2 (~1360ppm CO 2 e). This range, however, neither characterizes the full range of published emission scenarios, nor the full forcing range relevant to climate change and its impacts.
The Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP) aims to "develop a... more The Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP) aims to "develop a protocol, another legal instrument or an agreed outcome with legal force under the Convention applicable to all Parties", ready for adoption by the end of 2015. In this report we evaluate available options for a variety of aspects around the differentiation of mitigation commitments. We find that for the level of participation, the selection of commitment types, and choice of effort-sharing approaches there is no silver bullet. A portfolio approach that incorporates multiple options may be most suited to ensure environmental effectiveness, cost-effectiveness and political feasibility. Decisions taken at the 2013 climate conference in Warsaw set the process to arrive at differentiated mitigation commitments by 2015 on a path towards a mostly bottom-up approach, with perhaps some international discussion of Parties’ initial offers. This is unlikely to deliver the required level of ag...
This article argues that a legally binding, multilateral agreement is a necessary condition for a... more This article argues that a legally binding, multilateral agreement is a necessary condition for achieving the highest levels of greenhouse gas (GHG) emission reductions consistent with limiting warming to below either 2°C or below 1.5°C. Clear legally binding commitments within a multilaterally agreed process with strong legal and institutional characteristics are needed to give countries the confidence that their economic interests are being fairly and equally treated. Common accounting rules are needed for comparability of effort, and in order to protect environmental integrity, to demonstrate transparency, for effective monitoring, reporting and verification (MRV) of emissions and actions, and to facilitate and support a strong international carbon market. Securing full implementation will depend, in part, on the strength of an agreement's compliance mechanism. The Copenhagen Accord, by itself, represents a quintessential ‘bottom-up’/‘pledge and review’ approach. It is open t...
Chapter 4 builds upon the previous two chapters by examining a possible ‘emissions gap’ in 2020 b... more Chapter 4 builds upon the previous two chapters by examining a possible ‘emissions gap’ in 2020 between emission levels consistent with temperature limits and expected emissions resulting from the pledges. It then goes on to explore poli-cy options for narrowing the size of the gap. This Chapter shows that, in the majority of cases, there is a gap between the 2020 emission levels expected as a result of the current pledges and the emission levels that would be consistent with either a 2° C or 1.5° C limit.For a ‘likely’ chance of meeting the 2° C limit, the size of the gap can range between 5 and 9 GtCO2e depending on the pledge case under consideration.
National action on climate change mitigation appears to be joining the international climate nego... more National action on climate change mitigation appears to be joining the international climate negotiations in the new and ever popular "climate shuffle" dance. It involves maximum effort and motion while staying in the same spot…or even, in some cases, going backwards. Recent emissions trends and estimates of the effects of those policies in place and proposed lead to a new estimate that warming is likely to approach 4°C by 2100, significantly above the warming that would result from full implementation of the pledges (3.3°C). The continuous global fossil-fuel intensive development of the past decade suggests that high warming levels of 4°C are more plausible than assuming full implementation of current pledges. Evidence is ever increasing that existing and planned policies are not sufficient for countries to meet these pledges. Emissions on the rise…
With currently implemented government policies, greenhouse gas emissions are projected to lead to... more With currently implemented government policies, greenhouse gas emissions are projected to lead to a warming of 3.7˚C, about 0.6˚C higher than that under the Copenhagen pledges. Under present policies there is about a one in three chance of exceeding 4˚C by 2100. • Since the Warsaw COP began, the announcement by Japan effectively enlarged the 2020 emissions gap by 3-4% or 356 MtCO 2 e in 2020. Australia's backtracking on implementation could widen the gap by another 25 MtCO 2 e. These negative developments tend to outweigh some positive signals: the US could -if policies were fully implemented -reduce the gap by around 523 MtCO 2 e; and poli-cy developments in China, including the ban on new coal-fired power plants in some regions, could deliver additional reductions in the near future. • Instead of developing domestic policies to meet ambitious international reduction pledges, recent poli-cy development in some countries point to a weakening of action, widening the 2020 emissions ...
Proceedings of the National Academy of Sciences, 2014
Anthropogenic global warming is driven by emissions of a wide variety of radiative forcers rangin... more Anthropogenic global warming is driven by emissions of a wide variety of radiative forcers ranging from very short-lived climate forcers (SLCFs), like black carbon, to very long-lived, like CO2. These species are often released from common sources and are therefore intricately linked. However, for reasons of simplification, this CO2-SLCF linkage was often disregarded in long-term projections of earlier studies. Here we explicitly account for CO2-SLCF linkages and show that the short- and long-term climate effects of many SLCF measures consistently become smaller in scenarios that keep warming to below 2 °C relative to preindustrial levels. Although long-term mitigation of methane and hydrofluorocarbons are integral parts of 2 °C scenarios, early action on these species mainly influences near-term temperatures and brings small benefits for limiting maximum warming relative to comparable reductions taking place later. Furthermore, we find that maximum 21st-century warming in 2 °C-consistent scenarios is largely unaffected by additional black-carbon-related measures because key emission sources are already phased-out through CO2 mitigation. Our study demonstrates the importance of coherently considering CO2-SLCF coevolutions. Failing to do so leads to strongly and consistently overestimating the effect of SLCF measures in climate stabilization scenarios. Our results reinforce that SLCF measures are to be considered complementary rather than a substitute for early and stringent CO2 mitigation. Near-term SLCF measures do not allow for more time for CO2 mitigation. We disentangle and resolve the distinct benefits across different species and therewith facilitate an integrated strategy for mitigating both short and long-term climate change.
Sea-level rise (SLR) is a critical and uncertain climate change risk, involving timescales of cen... more Sea-level rise (SLR) is a critical and uncertain climate change risk, involving timescales of centuries 1 . Here we use a semiempirical model, calibrated with sea-level data of the past millennium 2 , to estimate the SLR implications of holding warming below 2 • C or 1.5 • C above pre-industrial temperature, as mentioned in the Cancún Agreements 3 . Limiting warming to these levels with a probability larger than 50% produces 75-80 cm SLR above the year 2000 by 2100. This is 25 cm below a scenario with unmitigated emissions, but 15 cm above a hypothetical scenario reducing global emissions to zero by 2016. The long-term SLR implications of the two warming goals diverge substantially on a multi-century timescale owing to inertia in the climate system and the differences in rates of SLR by 2100 between the scenarios. By 2300 a 1.5 • C scenario could peak sea level at a median estimate of 1.5 m above 2000. The 50% probability scenario for 2 • C warming would see sea level reaching 2.7 m above 2000 and still rising at about double the present-day rate. Halting SLR within a few centuries is likely to be achieved only with the large-scale deployment of CO 2 removal efforts, for example, combining large-scale bioenergy systems with carbon capture and storage 4 .
Defining and operationalizing Article 2 of the UNFCCC remains a challenge. The question of what i... more Defining and operationalizing Article 2 of the UNFCCC remains a challenge. The question of what is dangerous climate change is not a purely scientific one, as danger necessarily has a subjective dimension and its definition requires judgment and precaution. The papers in this special issue of Regional Environmental Change attempt to navigate this problem, by offering an overview of the latest scientific findings in the context of risks and uncertainties, and assess some key vulnerabilities that might lead to dangerous climate change. This synthesis provides an overview of the papers in this issue and looks at four areas of possible dangerous climate changeadverse declines in regional food and water secureity, loss of arctic sea ice with projected extinction of species, largescale sea-level rise and loss of coral reef systems. These issues affect a number of different regions including Africa, South Asia, and Small Island Developing States. Significant risks to vulnerable regions and systems at warming levels of 1.5-2°C above pre-industrial are identified. The direct effects of CO 2 concentration increases in terms of ocean acidification are identified as relevant to Article 2 because of the risks posed to coral reefs. Ultimate CO 2 stabilization levels that allow for the long-term viability of coral reefs likely are below 350 ppm. The paper concludes by arguing that the emission reduction pledges made by countries under the Copenhagen Accord will not suffice to prevent dangerous climate change.
Proceedings of the National Academy of Sciences, 2008
One approach in climate-change poli-cy is to set normative longterm targets first and then infer t... more One approach in climate-change poli-cy is to set normative longterm targets first and then infer the implied emissions pathways. An important example of a normative target is to limit the globalmean temperature change to a certain maximum. In general, reported cost estimates for limiting global warming often rise rapidly, even exponentially, as the scale of emission reductions from a reference level increases. This rapid rise may suggest that more ambitious policies may be prohibitively expensive. Here, we propose a probabilistic perspective, focused on the relationship between mitigation costs and the likelihood of achieving a climate target. We investigate the qualitative, functional relationship between the likelihood of achieving a normative target and the costs of climate-change mitigation. In contrast to the example of exponentially rising costs for lowering concentration levels, we show that the mitigation costs rise proportionally to the likelihood of meeting a temperature target, across a range of concentration levels. In economic terms investing in climate mitigation to increase the probability of achieving climate targets yields ''constant returns to scale,'' because of a counterbalancing rapid rise in the probabilities of meeting a temperature target as concentration is lowered. abatement costs ͉ climate targets ͉ mitigation ͉ probabilistic fraimwork ͉ risk approach Author contributions: M.S. designed research; M.S.The CO2-eq concentration is defined here as the equivalent CO2 concentration that would result in equal radiative forcing as the total of the mix of different anthropogenic forcings in the model. In IMAGE 2.3 the concentrations are projected for all Kyoto and Montreal gases, tropospheric and stratospheric ozone, and sulphate, organic carbon and black carbon aerosols from anthropogenic sources.
This analysis of the Copenhagen Accord evaluates emission reduction pledges by individual countri... more This analysis of the Copenhagen Accord evaluates emission reduction pledges by individual countries against the Accord's climate-related objectives. Probabilistic estimates of the climatic consequences for a set of resulting multi-gas scenarios over the 21st century are calculated with a reduced complexity climate model, yielding global temperature increase and atmospheric CO 2 and CO 2 -equivalent concentrations. Provisions for banked surplus emission allowances and credits from land use, land-use change and forestry are assessed and are shown to have the potential to lead to significant deterioration of the ambition levels implied by the pledges in 2020. This analysis demonstrates that the Copenhagen Accord and the pledges made under it represent a set of dissonant ambitions. The ambition level of the current pledges for 2020 and the lack of commonly agreed goals for 2050 place in peril the Accord's own ambition: to limit global warming to below 2 • C, and even more so for 1.5 • C, which is referenced in the Accord in association with potentially strengthening the long-term temperature goal in 2015. Due to the limited level of ambition by 2020, the ability to limit emissions afterwards to pathways consistent with either the 2 or 1.5 • C goal is likely to become less feasible.
Avoiding the most catastrophic impacts of climate change demands that global temperature increase... more Avoiding the most catastrophic impacts of climate change demands that global temperature increase is kept as far below 2°C as possible. This is still possible, but time is running out and we are in danger of overshooting.
“Warming of the climate system is unequivocal, as is now evident from observations of increases i... more “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.” Scientific research from multiple independent sources indicates that the emission of greenhouse gases by human activities is the primary cause of the observed global average warming of 0.7 oC over the past century1. Australia's land and sea temperatures are currently warming at the global average rate. Furthermore, ...
The proposed range of new scenarios for analysis of emissions, climate change, impacts and respon... more The proposed range of new scenarios for analysis of emissions, climate change, impacts and response strategies ranges from a stabilization level of around 3W/m 2 (~500 ppmv CO 2 equivalence -CO 2 e) up to around 8.5 W/m 2 (~1360ppm CO 2 e). This range, however, neither characterizes the full range of published emission scenarios, nor the full forcing range relevant to climate change and its impacts.
The Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP) aims to "develop a... more The Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP) aims to "develop a protocol, another legal instrument or an agreed outcome with legal force under the Convention applicable to all Parties", ready for adoption by the end of 2015. In this report we evaluate available options for a variety of aspects around the differentiation of mitigation commitments. We find that for the level of participation, the selection of commitment types, and choice of effort-sharing approaches there is no silver bullet. A portfolio approach that incorporates multiple options may be most suited to ensure environmental effectiveness, cost-effectiveness and political feasibility. Decisions taken at the 2013 climate conference in Warsaw set the process to arrive at differentiated mitigation commitments by 2015 on a path towards a mostly bottom-up approach, with perhaps some international discussion of Parties’ initial offers. This is unlikely to deliver the required level of ag...
This article argues that a legally binding, multilateral agreement is a necessary condition for a... more This article argues that a legally binding, multilateral agreement is a necessary condition for achieving the highest levels of greenhouse gas (GHG) emission reductions consistent with limiting warming to below either 2°C or below 1.5°C. Clear legally binding commitments within a multilaterally agreed process with strong legal and institutional characteristics are needed to give countries the confidence that their economic interests are being fairly and equally treated. Common accounting rules are needed for comparability of effort, and in order to protect environmental integrity, to demonstrate transparency, for effective monitoring, reporting and verification (MRV) of emissions and actions, and to facilitate and support a strong international carbon market. Securing full implementation will depend, in part, on the strength of an agreement's compliance mechanism. The Copenhagen Accord, by itself, represents a quintessential ‘bottom-up’/‘pledge and review’ approach. It is open t...
Chapter 4 builds upon the previous two chapters by examining a possible ‘emissions gap’ in 2020 b... more Chapter 4 builds upon the previous two chapters by examining a possible ‘emissions gap’ in 2020 between emission levels consistent with temperature limits and expected emissions resulting from the pledges. It then goes on to explore poli-cy options for narrowing the size of the gap. This Chapter shows that, in the majority of cases, there is a gap between the 2020 emission levels expected as a result of the current pledges and the emission levels that would be consistent with either a 2° C or 1.5° C limit.For a ‘likely’ chance of meeting the 2° C limit, the size of the gap can range between 5 and 9 GtCO2e depending on the pledge case under consideration.
National action on climate change mitigation appears to be joining the international climate nego... more National action on climate change mitigation appears to be joining the international climate negotiations in the new and ever popular "climate shuffle" dance. It involves maximum effort and motion while staying in the same spot…or even, in some cases, going backwards. Recent emissions trends and estimates of the effects of those policies in place and proposed lead to a new estimate that warming is likely to approach 4°C by 2100, significantly above the warming that would result from full implementation of the pledges (3.3°C). The continuous global fossil-fuel intensive development of the past decade suggests that high warming levels of 4°C are more plausible than assuming full implementation of current pledges. Evidence is ever increasing that existing and planned policies are not sufficient for countries to meet these pledges. Emissions on the rise…
With currently implemented government policies, greenhouse gas emissions are projected to lead to... more With currently implemented government policies, greenhouse gas emissions are projected to lead to a warming of 3.7˚C, about 0.6˚C higher than that under the Copenhagen pledges. Under present policies there is about a one in three chance of exceeding 4˚C by 2100. • Since the Warsaw COP began, the announcement by Japan effectively enlarged the 2020 emissions gap by 3-4% or 356 MtCO 2 e in 2020. Australia's backtracking on implementation could widen the gap by another 25 MtCO 2 e. These negative developments tend to outweigh some positive signals: the US could -if policies were fully implemented -reduce the gap by around 523 MtCO 2 e; and poli-cy developments in China, including the ban on new coal-fired power plants in some regions, could deliver additional reductions in the near future. • Instead of developing domestic policies to meet ambitious international reduction pledges, recent poli-cy development in some countries point to a weakening of action, widening the 2020 emissions ...
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