Unmask temporal trade-offs in climate policy debates

103

Bioenergy is booming as nations seek to cut their greenhouse gas emissions. The European Union declared biofuels to be carbon-neutral, triggering a surge in wood use. But do biofuels actually reduce emissions? A molecule of CO 2 emitted today has the same impact on radiative forcing whether it comes from coal or biomass. Biofuels can only reduce atmospheric CO 2 over time through post-harvest increases in net primary production (NPP). The climate impact of biofuels therefore depends on CO 2 emissions from combustion of biofuels versus fossil fuels, the fate of the harvested land and dynamics of NPP. Here we develop a model for dynamic bioenergy lifecycle analysis. The model tracks carbon stocks and fluxes among the atmosphere, biomass, and soils, is extensible to multiple land types and regions, and runs in ≈1s, enabling rapid, interactive policy design and sensitivity testing. We simulate substitution of wood for coal in power generation, estimating the parameters governing NPP and other fluxes using data for forests in the eastern US and using published estimates for supply chain emissions. Because combustion and processing efficiencies for wood are less than coal, the immediate impact of substituting wood for coal is an increase in atmospheric CO 2 relative to coal. The payback time for this carbon debt ranges from 44-104 years after clearcut, depending on forest type-assuming the land remains forest. Surprisingly, replanting hardwood forests with fast-growing pine plantations raises the CO 2 impact of wood because the equilibrium carbon density of plantations is lower than natural forests. Further, projected growth in wood harvest for bioenergy would increase atmospheric CO 2 for at least a century because new carbon debt continuously exceeds NPP. Assuming biofuels are carbon neutral may worsen irreversible impacts of climate change before benefits accrue. Instead, explicit dynamic models should be used to assess the climate impacts of biofuels.

Section: Introductionmentioning

confidence: 99%

Does replacing coal with wood lower CO ₂ emissions? Dynamic lifecycle analysis of wood bioenergy

Sterman

Siegel²,

Rooney‐Varga

2018

103

“…Alongside its long history of use in successive IPCC assessments, its central role in the Kyoto Protocol has contributed to GWP 100 becoming a de facto standard in the policy arena (Plattner et al 2009). However, there is extensive literature covering the limitations of metrics (Fuglestvedt et al 2003, Forster et al 2007, Shine et al 2007, Jenkins et al 2018 and GWP in particular (Shine et al 2005, Ocko et al 2017. This motivates the question as to whether the use of metrics, especially GWP 100 , serves the purpose of the Paris Agreement well enough.…”

Section: Introductionmentioning

confidence: 99%

Guidance on emissions metrics for nationally determined contributions under the Paris Agreement

Denison

Forster

2019

Many nationally determined contributions (NDCs) under the Paris Agreement follow the established practice of specifying emissions levels in tonnes of CO 2 equivalent emissions. The Global Warming Potential (GWP) is the emissions metric used most often to aggregate contributions from different greenhouse gases (GHGs). However, the climate impact of pathways expressed in this way is known to be ambiguous. For this reason, alternatives have been proposed but the ambiguity has not been quantified in the context of the Paris Agreement. Here we assess the variation in temperature using pathways consistent with the ambition of limiting temperature increases to well below 2°C. These are taken from the IPCC Special Report on Global Warming of 1.5°C (SR15). The CO 2 emission levels are adjusted so that the pathways all have the same total CO 2 equivalent emissions for a given emissions metric but have different proportions of short-lived and long-lived pollutants. We show that this difference affects projections by up to 0.17°C when GWP 100 is used. Options of reducing this ambiguity include using a different emissions metric or adding supplementary information in NDCs about the emissions levels of individual GHGs. We suggest the latter on the grounds of simplicity and because it does not require agreement on the use of a different emissions metric.

“…Although most commitments under the Paris Agreement use the GWP(100) to compare CO 2 and non-CO 2 GHGs in CO 2 -equivalent terms, alternative metrics are a subject of active debate in policy and environmental impact research [60,70,84]. For example, recent research has called for reporting CO 2 -equivalent emissions using multiple metrics to better represent the effects of different time horizons, physical and economic impact indicators, and modeling uncertainties [70,71]. Here we show how multiple metrics can be used to evaluate the benefits and drawbacks of policies that assign differing levels of importance to mitigating CH 4 .…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…decision makers to understand the impacts of choosing this over other metrics. This importance is reflected in an active debate about which metrics to use in policy [70,71]. Because of this we consider in our analysis not just one but a set of metrics.…”

Section: Co 2 -Equivalent Emissions Impacts Of a Co 2 -Focused Policymentioning

confidence: 99%

Timelines for mitigating the methane impacts of using natural gas for carbon dioxide abatement

Klemun

Trancik

2019

Reducing carbon dioxide (CO 2 ) emissions through a reliance on natural gas can create a hidden commitment to methane (CH 4 ) leakage mitigation. While the quantity of CH 4 leakage from natural gas has been studied extensively, the magnitude and timing of the CH 4 mitigation required to meet climate policy goals is less well understood. Here we address this topic by examining the case of US electricity under a range of baseline natural gas leakage rate estimates and emissions equivalency metrics for converting CH 4 to CO 2 -equivalent emissions. We find that CH 4 emissions from the power sector would need to be reduced by 30%-90% from today's levels by 2030 in order to meet a CO 2 -equivalent climate policy target while continuing to rely on natural gas. These CH 4 emissions reductions are greater than the required CO 2 reductions under the same policy. Alternatively, expanding carbon-free sources more rapidly could meet the 2030 target without reductions in natural gas leakage rates. The results provide insight on an important policy choice in regions and sectors using natural gas, between emphasizing a natural gas supply chain clean-up effort or an accelerated transition toward carbon-free energy sources.