Dynamic environmental impacts of technologies

The impacts of technologies depend on various factors, including the background state of the environment.

Here we explore how the intensity and decay of impacts influences the relative attractiveness of technologies. We focus on comparing the impacts of various greenhouse gases, and develop new models to evaluate technologies.

Papers focus on the development of new metrics to compare the impacts of greenhouse gases and technologies to one another and testing these metrics against climate policy goals. We also investigate how technology portfolios can optimally adapt to a changing background environmental state. These methods are used to study vehicle emissions of long- and short-lived climate forcers.

New metric development: We are working on revised CO₂-equivalency metrics that take into account how close in time a technology is used to when critical climate thresholds are expected to be reached. This work focuses on comparing methane and carbon dioxide emissions across various transportation fuels and electricity generation options (Figure 1).

Figure 1. Assessments of the climate benefits of alternative fuels depend on the time horizon over which impacts are evaluated, for example 100 years (A) or 20 years (B). Alternative fuels emit multiple gases during their life cycle – including methane, which has a high initial climate impact but relatively short atmospheric lifetime (C). As a result, the impacts of methane-heavy fuels like natural gas are initially high but decay more quickly than those of methane-light fuels like gasoline (D).

Edwards MR, Trancik JE, Climate impacts of energy technologies depend on emissions timing, Nature Climate Change, 2014, Vol. 4, pp. 347-352 link.
Edwards MR, McNerny J, Trancik JE, Testing emissions equivalency metrics against climate policy goals, Environmental Science & Policy, 2016, Vol. 66, pp. 191-198 link.

Portfolio optimization: In this work we formulate dynamic technology choice as a simplified forward-looking multi-period portfolio optimization problem, maximizing energy consumption over a planning horizon in the presence of a policy constraint. Optimal portfolios require switching from relatively methane-heavy technologies to methane-light ones as the policy target approaches. The benefit of this method is numerically quantified using various transportation technology pairs (Figure 2). The optimal results are compared to choices relying on the standard GWP-based method which does not allow for dynamic technology selection (Figure 2C-D).

Figure 2. Greenhouse gas emission intensities of alternative transportation technologies and the benefit of using dynamic portfolio choice method in the presence of a policy constraint. Emission intensities of carbon dioxide (A), methane and nitrous oxide (B). Energy consumption allowed using individual technologies, their optimal switching portfolios and the optimal GWP-based portfolio (C). Gain in energy consumption using the optimal switching portfolio and GWP100 over the methane-light (D) and methane-heavy (E) technology in each pair.

Roy M, Edwards MR, Trancik JE, Methane mitigation timelines to inform energy technology evaluation, Environmental Research Letters, 2015, Vol. 10, 114024 link.

Trade-offs between short-lived and long-lived climate forcers: In this work we model the tradeoffs between short-lived and long-lived emission reductions across a range of vehicle technology options, life cycle emission intensities, and equivalency metrics. We discuss these trade-offs in the context of emission trends for light- and heavy-duty transport over the period 2000–2030, and compare current and estimated future vehicle emission intensities (e.g. under a decarbonized electricity mix) to intensity targets derived from climate policy goals. While short-lived vehicle emissions have decreased, significant reductions in CO₂ will be required by mid-century to meet climate change mitigation targets. The short-lived emission intensities of some low-CO₂ technologies are significantly 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

Edwards MR, Klemun MM, Hyung CK, Wallington TJ, Winkler SL, Tamor MA, Trancik JE, Vehicle emissions of short-lived and long-lived climate forcers: trends and tradeoffs, Faraday Discussions, 2017, Issue 0 link.