Wetlands represent the largest component of the terrestrial biological carbon pool [Dixon and Krankina, 1995] and thus play an important role in global carbon cycles [Sahagian and Melack, 1988]. Most global carbon budgets, however, have focused on dry land ecosystems that extend over large areas and have not accounted for the many small, scattered carbon-storing ecosystems such as mangrove swamps and salt marshes [Atjay et al., 1979; Olson et al., 1983]. Syntheses that do include wetlands typically exclude tidal saline wetlands (TSWs) because there have been no empirically based estimates of their carbon storage potential.
 In this study we used published and our own unpublished data to estimate the amount of carbon stored globally in soils of salt marshes and mangrove swamps. We then examine spatial patterns in carbon density and storage with respect to climate parameters, as well as local variability, to determine which are important controls.
 Tidal saline wetlands, i.e., salt marshes and mangrove swamps, are found on sheltered marine coastlines. The former, dominated by herbaceous vegetation, exist in climates ranging from arctic to subtropical. Mangrove swamps replace salt marshes in the subtropics, around 25°N and S and are dominated by woody vegetation [Mitsch and Gosselink, 2000]. Mangrove swamps and salt marshes are intertidal ecosystems; in order to persist, their surface elevations must increase with rising sea level.
 Both types of wetlands are noted for exceptional rates of production, rivaling that of productive agricultural lands [Odum, 1959]. Root to shoot ratios of salt marsh plants range from 1.4 to 50 [see review in the work of Smith et al., 1979], thus a large portion of the primary production is found in belowground biomass that contributes to vertically extensive deposits, as great as 8 m deep [e.g., Scott and Greenberg, 1983]. Mangrove deposits can attain comparable depths [e.g., Woodroffe et al., 1993]. In mangrove swamps, peat formation primarily occurs through deposition and slow turnover of mangrove roots as aboveground tissues rapidly decay or are transported from the system [Middleton and McKee, 2001].
 The global importance of wetlands as carbon sinks is widely recognized [Adams et al., 1990; Watson et al., 2000]. Because of their great expanse, the role of peatlands as carbon sinks has received the greatest attention by researchers [Roulet, 2000], who report rates of soil carbon sequestration from 20 to 30 g C m−2 yr−1. However, decomposition of peatland soils results in high rates of CH4 flux [Bartlett and Harriss, 1993], reducing their value as a means to moderate greenhouse warming. The soil chemistry and carbon accumulation patterns of TSWs differ in several respects from those of peatlands or other freshwater wetlands. For one thing, carbon concentrations in TSWs are often lower than in peatlands, since tidal wetlands can receive significant inputs of fine-grained minerals (through tidal exchange with adjacent coastal waters), which dilute the inputs of organic matter from above- and belowground production. On the other hand, rates of soil accumulation in tidal wetlands tend to be higher than in peatlands, so net carbon sequestration is potentially substantial. Perhaps most important is that the presence of abundant sulfate in TSW soils hinders CH4 production, so these ecosystems are considered to be negligible sources of CH4, if not CH4 sinks [Bartlett and Harris, 1993; Magenheimer et al., 1996; Giani et al., 1996]. Studies of gas fluxes in TSWs suggest that emissions of the greenhouse gas N2O are also negligible [Smith et al., 1983; DeLaune et al., 1990].