The Sacramento-San Joaquin Delta provides an ideal ecosystem to understand restored peatland ecosystem greenhouse gas fluxes. Marshes are important sinks for carbon, and in the Delta, 7,000 years of organic matter buildup had produced a carbon-rich peat layer up to 15 meters deep. Much of this carbon was removed in the last 150 years through levee building, drainage, and subsidence. Subsided delta peatlands are now being re-flooded to harness multiple environmental benefits. The net climate benefit associated with restoration, however, is highly uncertain, as restoring drained peatlands can also lead to increased emissions of methane, the second-most important greenhouse gas to climate change.