Relative to agricultural systems, high-carbon ecosystems – such as forests, peatlands, and mangroves – store large amounts of carbon in relatively small areas. Agricultural expansion often comes at the expense of high-carbon ecosystems, contributing to climate change. The food system is connected to these challenges. Ensuring no further agricultural expansion occurs in high-carbon ecosystems is a substantial climate change mitigation opportunity. The estimated costs of avoiding deforestation range from US$1.1 to US$395 billion per year, depending on growth scenarios and carbon prices; this is a bargain compared to the leverage these systems have on climate change and its social costs. Individuals, indigenous people, policies, institutions, and investments are all agents of change and will have to work together to avoid further land conversion.