In 1908, locals in Cape Cod, Mass., built an earthen dike across the Herring River to curtail its flow into the surrounding wetlands. Their goal was to clamp down on the number of mosquitoes. The dike destroyed the original salt marsh, replacing it with woodland, shrub, and impounded wetlands. But the community’s efforts over a century ago did far more than dry out the landscape. According to a new study, impounded wetlands can undergo an important change, shifting from carbon sinks into methane sources. The transformation turns these landscapes from ones that help mitigate climate change into ones that exacerbate it.