We emphasize that the analysis above should be considered conservative in its estimate of emissions. First, we reduced the emphasis on high-end estimates of global area by using gamma distributions to minimize the impact of especially high estimates. Second, we did not include any potential impacts on deep sediment C (>1 m depth), in part because of limited available science. These layers often contain more C per hectare than all the near-surface carbon combined [24] and have been found to be impacted by land-use change in the few cases studied [23] . This means that even our high-end scenario of 100% C loss upon conversion is actually much less than all of the ecosystem carbon. Third, the low-end scenario of 25% C loss upon conversion effectively assumes that all land-use changes in coastal systems across the entire globe could retain 75% of all near-surface carbon (if most C in disturbed systems is merely buried or redistributed) – an extremely conservative assumption. Fourth, we did not include the loss of annual sequestration of sediment carbon that occurs due to vegetation removal or hydrological isolation that reduces new sediment inputs.
Regarding other greenhouse gases such as methane (CH4) or nitrous oxide (N2O), excluding changes in these components is likely either a neutral or conservative approach. In highly saline wetlands (>18 ppt), sediment C sequestration rates exceed CH4 emission rates in CO2-equivalent units [55] , suggesting that the net effect of losing both sequestration and CH4 emissions with disturbance should be an increase in greenhouse gas emissions. In lower salinity wetlands (salinity 5–18 ppt), CH4 emissions and sequestration are approximately in balance [56] , except perhaps for oligohaline systems (<5 ppt) that are a small portion of the global area we evaluated. Finally, we conservatively did not consider evidence that common disturbances, such as conversion to shrimp ponds, that cause eutrophication have been shown to stimulate CH4 emissions [27] . Eutrophication is likely to also increase N2O emissions if the system receives high nitrate loading; otherwise it is not necessary to account for changes in N2O fluxes because emissions from anaerobic sediments are negligible in the absence of nitrate loading.
Regarding other greenhouse gases such as methane (CH4) or nitrous oxide (N2O), excluding changes in these components is likely either a neutral or conservative approach. In highly saline wetlands (>18 ppt), sediment C sequestration rates exceed CH4 emission rates in CO2-equivalent units [55] , suggesting that the net effect of losing both sequestration and CH4 emissions with disturbance should be an increase in greenhouse gas emissions. In lower salinity wetlands (salinity 5–18 ppt), CH4 emissions and sequestration are approximately in balance [56] , except perhaps for oligohaline systems (<5 ppt) that are a small portion of the global area we evaluated. Finally, we conservatively did not consider evidence that common disturbances, such as conversion to shrimp ponds, that cause eutrophication have been shown to stimulate CH4 emissions [27] . Eutrophication is likely to also increase N2O emissions if the system receives high nitrate loading; otherwise it is not necessary to account for changes in N2O fluxes because emissions from anaerobic sediments are negligible in the absence of nitrate loading.
Free full text: Click here
