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Assessments of the climate impacts of energy technologies and other emissions sources can depend strongly on the equivalency metric used to compare short- and long-lived greenhouse gas emissions. However, the consequences of metric design choices are not fully understood, and in practice, a single metric, the global warming potential (GWP), is used almost universally. Many metrics have been proposed and evaluated in recent decades, but questions still remain about which ones perform better and why. Here, we develop new insights on how the design of equivalency metrics can impact the outcomes of climate policies. We distill the equivalency metric problem into a few key design choices that determine the metric values and shapes seen across a wide range of different proposed metrics. We examine outcomes under a hypothetical 1.5 or 2 ∘C policy target and discuss extensions to other policies. Across policy contexts, the choice of time parameters is particularly important. Metrics that emphasize the immediate impacts of short-lived gases such as methane can reduce rates of climate change but may require more rapid technology changes. Differences in outcomes across metrics are more pronounced when fossil fuels, with or without carbon capture and storage, play a larger role in energy transitions. By identifying a small set of consequential design decisions, these insights can help make metric choices and energy transitions more deliberate and effective at mitigating climate change.

Methane leaks in natural gas systems are low-hanging fruit for near-term, locally driven climate policy. Recent work suggests this emissions source is larger than previously believed and that repairing a small number of high emitters can cost-effectively reduce system-wide leakage. How successful are these repairs on the ground? Here, we assess the effectiveness of repair policies in the Massachusetts distribution system. Our analysis leverages state-wide utility data, on-site empirical measurements, stakeholder interviews, and document and legal analysis. We use these mixed methods to investigate the rate of repair failure, where a gas utility identifies and fixes a leak, but on-site emissions are not eliminated. We find that repair failures are relatively common, yet they are repeatedly neglected in policy. By not accounting for repair failures, policy may overestimate the effectiveness of distribution system repairs in meeting local greenhouse gas reduction targets. These results also underscore the importance of data transparency for monitoring and verifying subnational climate policies.

Approaches that root national climate strategies in local actions will be essential for all countries as they develop new nationally determined contributions under the Paris Agreement. The potential impact of climate action from non-national actors in delivering higher global ambition is significant. Sub-national action in the United States provides a test for how such actions can accelerate emissions reductions. We aggregated U.S. state, city, and business commitments within an integrated assessment model to assess how a national climate strategy can be built upon non-state actions. We find that existing commitments alone could reduce emissions 25% below 2005 levels by 2030, and that enhancing actions by these actors could reduce emissions up to 37%. We show how these actions can provide a stepped-up basis for additional federal action to reduce emissions by 49%—consistent with 1.5 °C. Our analysis demonstrates sub-national actions can lead to substantial reductions and support increased national action., Climate action from local actors is vital in achieving nationally determined contributions under the Paris Agreement. Here the authors show that existing commitments from U.S. states, cities and business could reduce emissions 25% below 2005 levels by 2030, with expanded subnational action reducing emissions by 37% and federal action by up to 49%.

A global phaseout of unabated coal use is critical to meeting the Paris climate goals. This transition can potentially lead to large amounts of stranded assets, especially in regions with newer and growing coal fleets. Here we combine plant-level data with a global integrated assessment model to quantify changes in stranded asset risks across locations and over time. With new plant proposals, cancellations, and retirements over the past five years, global committed emissions in 2030 from existing and planned coal plants declined by 3.3 GtCO2 (25%). While these emissions are now roughly in line with near-term (2030) Nationally Determined Contributions (NDCs) to the Paris Agreement, they remain far off track from longer-term climate goals. Building all proposed coal plants in the pipeline leads to a 24% (503 GW) increase in capacity and a 55% ($520 billion) increase in stranded assets under 1.5°C. Stranded asset risks fall disproportionately on emerging Asian economies with newer and growing coal fleets.

More than half of current coal power capacity is in China. A key strategy for meeting China’s 2060 carbon neutrality goal and the global 1.5 °C climate goal is to rapidly shift away from unabated coal use. Here we detail how to structure a high-ambition coal phaseout in China while balancing multiple national needs. We evaluate the 1037 currently operating coal plants based on comprehensive technical, economic and environmental criteria and develop a metric for prioritizing plants for early retirement. We find that 18% of plants consistently score poorly across all three criteria and are thus low-hanging fruits for rapid retirement. We develop plant-by-plant phaseout strategies for each province by combining our retirement algorithm with an integrated assessment model. With rapid retirement of the low-hanging fruits, other existing plants can operate with a 20- or 30-year minimum lifetime and gradually reduced utilization to achieve the 1.5 °C or well-below 2 °C climate goals, respectively, with complete phaseout by 2045 and 2055., A key strategy for meeting China’s 2060 carbon neutrality goal and the global 1.5 °C climate goal is to rapidly shift away from unabated coal use. Here, the authors detail how to structure a high-ambition, plant-by-plant coal phaseout in China while balancing multiple national needs.

Research over the past decade has highlighted the climate impacts of natural gas leakage throughout the supply chain, from production to processing to transmission and distribution. Urban methane emissions from local distribution and end-users initially received less attention, yet they can be a significant emissions source in areas with aging infrastructure. This study provides the first empirical evidence on the impact of local gas leaks on nearby house prices. Using high-resolution geographical data in property transactions and a difference-in-differences method, we find that gas leaks significantly reduce nearby house prices by 2.61% ($11,700) on average in MA. Our estimation is robust to the different boundary definitions of the control group. More importantly, the estimated benefits of fixing gas leaks are larger than the average costs per repair reported by utilities in MA.

Methane leaks in natural gas systems are low-hanging fruit for near-term, locally driven climate policy. Recent work suggests this emissions source is larger than previously believed and that repairing a small number of high emitters can cost-effectively reduce system-wide leakage. How successful are these repairs on the ground? Here, we assess the effectiveness of repair policies in the Massachusetts distribution system. Our analysis leverages state-wide utility data, on-site empirical measurements, stakeholder interviews, and document and legal analysis. We use these mixed methods to investigate the rate of repair failure, where a gas utility identifies and fixes a leak, but on-site emissions are not eliminated. We find that repair failures are relatively common, yet they are repeatedly neglected in policy. By not accounting for repair failures, policy may overestimate the effectiveness of distribution system repairs in meeting local greenhouse gas reduction targets. These results also underscore the importance of data transparency for monitoring and verifying subnational climate policies.

Growing momentum for decentralized climate policy and the falling costs of low‐carbon technologies are creating new climate change mitigation opportunities for subnational actors. Here we discuss how research can best support these subnational efforts to allow limited resources to stretch further. To stimulate this discussion, we identify four research priorities. (1) Innovation mechanisms examines local policy opportunities for technology improvement to achieve high returns on investments. (2) Co‐benefits analyzes the non‐climate benefits of emissions reductions to highlight how local policies can affect communities directly. (3) Emissions monitoring develops rapid, low‐cost, local measurement strategies to allow communities to assess and weigh in on the emissions impacts of local energy systems. (4) Decision levers reframes large‐scale analyses into more targeted and actionable metrics for local policy decisions. This piece was informed and inspired by a set of interviews we conducted with representatives in business, government, NGOs, and educational institutions actively engaged in local climate action, and by our own research. This article is categorized under: The Carbon Economy and Climate Mitigation > Policies, Instruments, Lifestyles, Behavior Policy and Governance > Private Governance of Climate Change

Momentum to phase out unabated coal use is growing globally. This transition is critical to meeting the Paris climate goals but can potentially lead to large amounts of stranded assets, especially in regions with newer and growing coal fleets. Here we combine plant-level data with a global integrated assessment model to quantify changes in global stranded asset risks from coal-fired power plants across regions and over time. With new plant proposals, cancellations, and retirements over the past five years, global net committed emissions in 2030 from existing and planned coal plants declined by 3.3 GtCO2 (25%). While these emissions are now roughly in line with initial Nationally Determined Contributions (NDCs) to the Paris Agreement, they remain far off track from longer-term climate goals. Progress made in 2021 towards no new coal can potentially avoid a 24% (503 GW) increase in capacity and a 55% ($520 billion) increase in stranded assets under 1.5 °C. Stranded asset risks fall disproportionately on emerging Asian economies with newer and growing coal fleets. Recent no new coal commitments from major coal financers can potentially reduce stranding of international investments by over 50%.

Evaluating technology options to mitigate the climate impacts of road transportation can be challenging, particularly when they involve a tradeoff between long-lived emissions (e.g., carbon dioxide) and short-lived emissions (e.g., methane or black carbon). Here we present trends in short- and long-lived emissions for light- and heavy-duty transport globally and in the U.S., EU, and China over the period 2000–2030, and we discuss past and future changes to vehicle technologies to reduce these emissions. We model the tradeoffs between short- and long-lived emission reductions across a range of technology options, life cycle emission intensities, and equivalency metrics. While short-lived vehicle emissions have decreased globally over the past two decades, significant reductions in CO2will be required by mid-century to meet climate change mitigation targets. This is true regardless of the time horizon used to compare long- and short-lived emissions. The short-lived emission intensities of some low-CO2technologies are higher than others, and thus their suitability for meeting climate targets depends sensitively on the evaluation time horizon. Other technologies offer low intensities of both short-lived emissions and CO2.

The climate impacts of energy technologies are frequently assessed using equivalency metrics, which convert emissions of multiple greenhouse gases to a common scale. Numerous metrics have been proposed that incorporate, in different ways, information about the time-dependent impacts of gases. However, more focus has been placed on proposing metrics than on testing their performance in real-world use cases. Here we present a testing approach that simulates how metrics would affect the selection of energy technology portfolios that comply with a CO2-equivalent emissions cap. Unintended radiative forcing outcomes can occur, emphasizing the need to test metrics in a practical context. We demonstrate the approach for policies designed to limit radiative forcing and discuss extensions to limits on temperature or economic impacts. Metric performance is evaluated by (i) how much actual radiative forcing overshoots the intended stabilization level and (ii) the level of energy consumption permitted. We use this testing approach to study a variety of metrics based on an estimated radiative forcing stabilization time under two climate policy goals. We find that these goal-inspired metrics, if chosen carefully, can exhibit performance improvements over the standard global warming potential (GWP) while maintaining its transparency and ease-of-use. These alternative metrics can significantly reduce the overshoot in radiative forcing observed with the GWP, at a small cost in energy consumption. Moreover, simple metrics can exhibit similar performance improvements to more complex ones.

There has been a rapid growth in the use of natural gas and biofuels. This study evaluates energy technologies against climate targets and shows that the impact of technologies that result in the emission of both methane and carbon dioxide depends on their time of use. The changes in climate impacts over time are large, with several technologies that are widely considered mitigation options becoming unfavourable within two decades.

Energy technologies emitting differing proportions of methane (CH[subscript 4]) and carbon dioxide (CO[subscript 2]) vary significantly in their relative climate impacts over time, due to the distinct atmospheric lifetimes and radiative efficiencies of the two gases. Standard technology comparisons using the global warming potential (GWP) with a fixed time horizon do not account for the timing of emissions in relation to climate policy goals. Here we develop a portfolio optimization model that incorporates changes in technology impacts based on the temporal proximity of emissions to a radiative forcing (RF) stabilization target. An optimal portfolio, maximizing allowed energy consumption while meeting the RF target, is obtained by year-wise minimization of the marginal RF impact in an intended stabilization year. The optimal portfolio calls for using certain higher-CH[subscript 4]-emitting technologies prior to an optimal switching year, followed by CH[subscript 4]-light technologies as the stabilization year approaches. We apply the model to evaluate transportation technology pairs and find that accounting for dynamic emissions impacts, in place of using the static GWP, can result in CH[subscript 4] mitigation timelines and technology transitions that allow for significantly greater energy consumption while meeting a climate policy target. The results can inform the forward-looking evaluation of energy technologies by engineers, private investors, and policy makers., MIT Energy Initiative, Massachusetts Institute of Technology. Charles E. Reed Faculty Initiative Fund, New England University Transportation Center (DOT Grant DTRT12-G-UTC01), National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)

Energy technologies emitting differing proportions of methane and carbon dioxide vary in their relative climate impacts over time, due to the different atmospheric lifetimes of the two gases. Standard technology comparisons using the global warming potential (GWP) emissions equivalency metric do not reveal these dynamic impacts, and may not provide the information needed to assess technologies and emissions mitigation opportunities in the context of broader climate policy goals. Here we formulate a portfolio optimization model that incorporates changes in technology impacts as a radiative forcing (RF) stabilization target is approached. An optimal portfolio, maximizing allowed energy consumption while meeting the RF target, is obtained by year-wise minimization of the marginal RF impact in an intended stabilization year. The optimal portfolio calls for using certain higher methane-emitting technologies prior to an optimal switching year, followed by methane-light technologies as the stabilization year approaches. The model is applied to transportation technology pairs and shows that, by accounting for the differing decay behavior of gases, optimal switching portfolios allow energy consumption gains of up to 50% compared to relying on the higher methane-emitting technology alone. The results allow for estimates of target timelines for mitigating methane emissions from energy technologies.

Nature Climate Change 4, 348–352 (2014); published online 25 April 2014; corrected after print 25 April 2014. In the print version of this Letter, the y axes in Fig. 4a,b should have been labelled 'Impact (g CO2-eq km−1)'. In addition, the first sentence of the author contributions should have read 'J.