A ton is not always a ton: A road-test of landfill, manure, and afforestation/reforestation offset protocols in the U.S. carbon market

•Protocols are the foundation of an offset program.•Using sample projects, we “road test” landfill, manure and afforestation protocols from 5 programs.•For a given project, we find large variation in the volume of offsets generated.•Harmonization of protocols can increase the likelihood that “a ton is a ton”.•Harmonization can enhance prospects for linking emission trading systems.

The outcome of recent international climate negotiations suggests we are headed toward a more fragmented carbon market, with multiple emission trading and offset programs operating in parallel. To effectively harmonize and link across programs, it will be important to ensure that across offset programs and protocols that a “ton is a ton”. In this article, we consider how sample offsets projects in the U.S. carbon market are treated across protocols from five programs: the Clean Development Mechanism, Climate Action Reserve, Chicago Climate Exchange, Regional Greenhouse Gas Initiative, and the U.S. EPA's former program, Climate Leaders. We find that differences among protocols for landfill methane, manure management, and afforestation/reforestation project types in accounting boundary definitions, baseline setting methods, measurement rules, emission factors, and discounts lead to differences in offsets credited that are often significant (e.g. greater than 50%). We suggest opportunities for modification and harmonization of protocols that can improve offset quality and credibility and enhance prospects for future linking of trading units and systems.