Banking on banking: does “when” flexibility mask the costs of stringent climate policy?

Banking and borrowing emission allowances provide temporal flexibility in cap-and-trade systems, which can enhance the economic efficiency of environmental policy while adhering to the same cumulative emission budget. This paper investigates the role of temporal (“when”) flexibility from emission banking provisions under an economy-wide cap-and-trade policy in the USA. The current literature on meeting deep decarbonization targets almost exclusively assumes unlimited banking, which may bias policy recommendations and have important consequences for R&D prioritization and model development. Numerical experiments using the energy-economic model US Regional Energy, GHG, and Economy (US-REGEN) indicate that assumptions about banking materially impact cost and emission pathways in meeting long-term targets like 80% reductions by 2050 relative to 2005 levels. Given the stringency of long-run targets and convexity of marginal abatement costs, the cost-minimizing time path for mitigation with banking suggests that 2025 abatement should exceed the pledged level under the Paris Agreement (42% instead of 26–28%) to reduce future costs. Total policy costs are approximately 30% higher when banking is excluded; however, political economy barriers and uncertainty may limit the use of banking provisions despite their appeal on economic efficiency grounds. Banking on policy implementation with unlimited temporal flexibility may distort insights about the pace, extent, and economic impacts of future energy transitions associated with long-term abatement targets, especially for more stringent climate policies.