Historical and contemporary subsidence in the San Joaquin-Sacramento Delta, California indicates that subsidence rates associated with drainage of organic soils have declined over time. Contemporary measurements of carbon flux into the atmosphere can be used to predict contemporary rates of permanent subsidence. This correspondence indicates that most subsidence is caused by carbon oxidation. The current contribution of atmospheric carbon from the Delta is 2 x 10(12) gm C/yr. This estimate is a factor of 3-4 less than previous estimates and reflects the declining rate of CO2 production in the Delta over the last several decades. Estimates of current production of CO2 from other drained agricultural lands that are based upon time-averaged historical rates of subsidence are also likely to be too large.
