The fastest growing type of offset on the global carbon market subsidizes the distribution of efficient cookstoves in developing countries to reduce greenhouse gas emissions, but a new study finds that the credits overestimate the stoves' carbon savings by a factor of 10.

The overestimation undermines efforts to counteract carbon emissions to slow climate change, since companies use these offsets to meet climate targets and to sell products labeled as "carbon neutral" instead of making real reductions in greenhouse gas emissions. It also undermines trust in the carbon market, and therefore the market's ability to support the long-term financing of efficient stoves.

The conclusions come from the first comprehensive, quantitative quality assessment of any type of offset project, in which researchers from the University of California, Berkeley, compared five methodologies for evaluating the emission reductions of cookstoves to published studies and their own independent analysis. The study is published in the journal Nature Sustainability.

"Clean cooking now figures very centrally on national decarbonization and sustainable development goal strategies, global funding cycles and the political agenda of national leaders," said Daniel Kammen, the James and Katherine Lau Distinguished Professor of Sustainability at UC Berkeley. "Integrating the science, human rights and economics of clean cooking is now critically linked to both social justice and climate strategies around the planet."

This study, which was shared in preprint form last year, has received substantial attention from the cooking-health, carbon market, sustainable development and national climate action communities, he noted.

"Cookstove offset methodologies are currently being revised and if our recommendations are adopted could become a rare project type that offset buyers can trust," said Barbara Haya, an expert on offset quality and director of the Berkeley Carbon Trading Project in UC Berkeley's Goldman School of Public Policy.

Cookstove offsets have become popular because roughly 2.4 billion people around the world cook with smoky solid fuels or kerosene, contributing to 2 to 3 million premature deaths annually and approximately 2% of global greenhouse gas (GHG) emissions. Providing efficient cookstoves now represents the fastest growing project type on the voluntary carbon market, with the second largest share of issued credits in the first ten months of 2023.

Estimated correctly, carbon offsets have the potential to support the free or subsidized distribution of efficient stoves that reduce time spent collecting firewood or the cost of purchasing fuel, said first author Annelise Gill-Wiehl, a UC Berkeley Ph.D. candidate in the Energy and Resources Group who has conducted extensive household energy fieldwork in East Africa. Furthermore, certain stoves can reduce smoke enough to save lives.

The UC Berkeley study not only documents the extent of the quality issues on the offset market, but also offers specific recommendations to align cookstove methodologies with current science and Sustainable Development Goal progress. A companion website provides guidance for buyers and developers on how to trade in quality credits that can substantially improve health.