Climate impacts of energy technologies depend on emissions timing

Abstract

Energy technologies emit greenhouse gases with differing radiative efficiencies and atmospheric lifetimes [1][2][3]. Standard practice for evaluating technologies, which uses the global warming potential (GWP) to compare the integrated radiative forcing of emitted gases over a fixed time horizon [4], does not acknowledge the importance of a changing background climate relative to climate change mitigation targets [5,6]. Here we demonstrate that the GWP misvalues the impact of CH 4 -emitting technologies as mid-century approaches, and we propose a new class of metrics to evaluate technologies based on their time of use. The instantaneous climate impact (ICI) compares gases in an expected radiative forcing stabilization year, and the cumulative climate impact (CCI) compares their integrated radiative forcing up to a stabilization year. Using these dynamic metrics, we quantify the climate impacts of technologies and show that high-CH 4 -emitting energy sources become less advantageous over time. The impact of natural gas for transportation, with CH 4 leakage, exceeds that of gasoline within 1-2 decades for a commonly-cited 3 W/m 2 stabilization target. The impact of algae biodiesel overtakes that of corn ethanol within 2-3 decades, where algae co-products are used to produce biogas and corn co-products are used for animal feed. The proposed metrics capture the changing importance of CH 4 emissions as a climate threshold is approached, thereby addressing a major shortcoming of the GWP for technology evaluation [7,8].Comparing the climate impacts of energy technologies is challenging because they emit differing types and quantities of greenhouse gases, most notably CH 4 and CO 2 , and these gases have dissimilar properties (Fig. 1a,b). Present approaches to technology evaluation use an equivalency metric to convert emissions to their CO 2 -equivalent value [1][2][3]9]. The most common metric is the global warming potential (GWP(τ )), which takes the ratio of the time-integrated radiative forcing of pulse non-CO 2 and CO 2 emissions over a fixed time horizon (τ ), typically 100 years. The GWP(100) was initially intended as a placeholder [10], in large part because of its sensitivity to the arbitrarily selected time horizon [7] (Fig. 1c,d), but it remains the standard metric for technology evaluation.

Methane mitigation timelines to inform energy technology evaluation

“…The marginal RF impact can be determined for a range of scenarios leading to stabilization at a given target RF level (section S1). This formulation of the technology choice problem is equivalent to applying the instantaneous climate impact (ICI) emissions equivalency metric proposed in earlier work [40]. Here we demonstrate the sizable benefits of using this approach to plan for technology transitions given a global RF target.…”

An overview of biogas production and utilization at full-scale wastewater treatment plants (WWTPs) in the United States: Challenges and opportunities towards energy-neutral WWTPs

“…Wastewater treatment was the 8th largest anthropogenic source of CH 4 emissions (12.8 million metric tons of CO 2 equivalent) in the US in 2012 [12]. CH 4 as a GHG has more than 20-200 times the radiative forcing per gram of CO 2 depending on evaluation emission time horizon [13].…”

Timelines for Mitigating Methane Emissions from Energy Technologies

“…S1). This formulation of the technology choice problem is equivalent to applying the instantaneous climate impact (ICI) emissions equivalency metric proposed in earlier work [40]. Here we demonstrate the sizable benefits of using this approach to plan for technology transitions given a global RF target.…”