It has been well recognized that converting wetlands to cropland results in loss of soil organic carbon (SOC), while less attention was paid to concomitant changes in methane (CH4) and nitrous oxide (N2O) emissions. Using datasets from the literature and field measurements, we investigated loss of SOC and emissions of CH4 and N2O due to marshland conversion in northeast China. Analysis of the documented crop cultivation area indicated that 2.91 Mha of marshland were converted to cropland over the period 1950–2000. Marshland conversion resulted in SOC loss of ∼240 Tg and introduced ∼1.4 Tg CH4 and ∼138 Gg N2O emissions in the cropland, while CH4 emissions reduced greatly in the marshland, cumulatively ∼28 Tg over the 50 years. Taking into account the loss of SOC and emissions of CH4 and N2O, the global warming potential (GWP) at a 20-year time horizon was estimated to be ∼180 Tg CO2_eq. yr−1 in the 1950s and ∼120 Tg CO2_eq. yr−1 in the 1990s, with a ∼33% reduction. When calculated at 100-year time horizon, the GWP was ∼73 Tg CO2 _eq. yr−1 in the 1950s and ∼58 Tg CO2_eq. yr−1 in the 1990s, with a ∼21% reduction. It was concluded that marshland conversion to cropland in northeast China reduced the greenhouse effect as far as GWP is concerned. This reduction was attributed to a substantial decrease in CH4 emissions from the marshland. An extended inference is that the declining growth rate of atmospheric CH4 since the 1980s might be related to global loss of wetlands, but this connection needs to be confirmed.