While natural gas produces lower carbon dioxide emissions than diesel during combustion, if enough methane is emitted across the fuel cycle, then switching a heavy-duty truck fleet from diesel to natural gas can produce net climate damages (more radiative forcing) for decades. Using the Technology Warming Potential methodology, we assess the climate implications of a diesel to natural gas switch in heavy-duty trucks. We consider spark ignition (SI) and high-pressure direct injection (HPDI) natural gas engines and compressed and liquefied natural gas. Given uncertainty surrounding several key assumptions and the potential for technology to evolve, results are evaluated for a range of inputs for well-to-pump natural gas loss rates, vehicle efficiency, and pump-to-wheels (in-use) methane emissions. Using reference case assumptions reflecting currently available data, we find that converting heavy-duty truck fleets leads to damages to the climate for several decades: around 70-90 years for the SI cases, and 50 years for the more efficient HPDI. Our range of results indicates that these fuel switches have the potential to produce climate benefits on all time frames, but combinations of significant well-to-wheels methane emissions reductions and natural gas vehicle efficiency improvements would be required.

Total CO 2 emissions from the United States power sector increased over the period 1990-2005, but peaked soon after, and by 2015 they had declined by 20% compared to 2005. This study analyzes the supply-side drivers of the increasing trend up until 2005 as well as the factors across US states that enabled significant reductions in the following decade. Using index decomposition analysis, we show that the two main factors driving the CO 2 decrease were natural gas substituting for coal and petroleum, and large increases in renewable energy generation (primarily wind)-which were responsible for 60% and 30% of the decline respectively since 2005. Both effects were concentrated in states where low natural gas prices or a combination of federal tax credits, state energy policies, decreasing costs of renewables, and advantageous wind conditions drove significant reductions of CO 2 emissions-resulting in the overall national emissions decline.

While market-based climate policy in the United States has been dominated by emissions trading, including policies at the state level and in proposed major federal legislation, a number of recent U.S. climate proposals focus on a carbon tax (or fee). Just as emissions trading programs have included some form of “price containment” measures, carbon tax policies may include “emissions assurance” provisions or, as we refer to them here, “environmental integrity mechanisms” (EIMs), which are designed to provide greater certainty about the resulting quantity of emissions. We propose three political economy reasons—as well as one practical reason relating to international climate agreements—that EIMs are likely to be an important component of any politically successful carbon tax (or fee) legislation in the U.S. Congress.