Cropland encroachment on ecological land in Mainland Southeast Asia leads to massive carbon emissions

• Encroachment of cropland on ecological land has resulted in massive carbon emissions. • Carbon emissions from cropland encroachment into ecological land are decreasing. • Under SSP1-RCP2.6, emissions of 5.79 × 10 8 t CO 2 can be offset. Mainland Southeast Asia (MSEA) is essential to the global carbon cycle. Balancing cropland expansion and ecological land protection amid population and economic growth is urgent. This study simulated MSEA’s land use patterns under SSP1-RCP2.6, SSP2-RCP4.5, and SSP5-RCP8.5 using the PLUS model with data from 1992 to 2022. Based on historical and future land use data and carbon density data for each land use type, the spatial and temporal changes in the encroachment of cropland on ecological land (E&C) and the resulting carbon emissions were analysed using the carbon stock loss method. The results showed that cropland encroached on 100,413.27 km 2 of ecological land in MSEA over 30 years, causing 27.44 × 10 8 t of CO 2 emissions, which accounts for 17.46 % of the total greenhouse gas emissions (data source: Our World in Data) in MSEA during the same period. Among these, Myanmar and Vietnam are the main contributing countries. Taking 2012 as the node, the E&C in MSEA is categorised into the early stage (1992–2012) and the late stage (2012–2022). Earlier, 57,253.86 km 2 of ecological land was encroached, causing a carbon stock loss of 4.29 × 10 8 t and 15.73 × 10 8 t of CO 2 emissions. Later, cropland encroached on 54,549.09 km 2 of ecological land, causing a carbon stock loss of 4.03 × 10 8 t and 14.81 × 10 8 t of CO 2 emissions. CO 2 emissions from E&C accounted for 19.01 % and 18.44 % of total greenhouse gas emissions in the early and late stage, respectively. The largest carbon stock loss is in forests and shrublands. Under SSP1-RCP2.6, converting cropland to ecological land increases carbon stocks by 1.58 × 10 8 t, offsetting 5.79 × 10 8 t of CO 2 emissions. However, under SSP2-RCP4.5 and SSP5-RCP8.5, carbon stocks decreased by 1.59 × 10 8 t and 0.78 × 10 8 t, leading to CO 2 emissions of 5.83 × 10 8 t and 2.87 × 10 8 t, respectively. MSEA countries should adopt SSP1-RCP2.6, manage cropland expansion, balance economic development with ecological preservation, and strengthen international cooperation to address climate change.