Disentangling the relative effects of climate change and anthropogenic activities on paddy expansion in the northern Sanjiang Plain of China

Since the changes in paddy areas would significantly affect food security and agricultural sustainability, it has received worldwide attention guided by the United Nations 2030 Sustainable Development Goals. However, the process of paddy field change was complex, which was affected by both climate change and anthropogenic activities. Most existing studies focused on the spatial–temporal evolution of paddy field in different regions, while the quantitative separation of climatic and anthropogenic contributions to paddy field expansion was rare but imperative. This study adopted the patch-generating land use simulation (PLUS) model to simulate the paddy field expansion under different scenarios. Based on satellite-based land use data, meteorological data, socioeconomic data, physical geographical data, and transport data, it proposed four indicators, including ACI (anthropogenic contribution index), CCI (climatic contribution index), RII (relative importance index), and CEI (combined effects index) to quantitatively disentangle the climatic and anthropogenic contribution at different scales, and further determined the relative importance and combined effects of climate change and anthropogenic activities. These indicators were applied to the case of northern Sanjiang Plain of China and found that the total area of paddy fields in this area expanded greatly from 2638.88 km2 in 1995 to 19363.80 km2 in 2020. 1) At the regional scale (the northern Sanjiang Plain of China), the effect of climate change was slightly larger than that of anthropogenic activities on the paddy field expansion, with an inhibitory interactive effect of climate change and anthropogenic activities. 2) At the city scale (Hegang, Jiamusi and Shuangyashan), the effect of climate change was slightly stronger than anthropogenic activities in Hegang and Jiamusi, whereas Shuangyashan was the opposite. 3) At the county scale (15 counties), paddy field expansion of the central NSP was dominated by anthropogenic activities, while the northern part was disturbed more by climate change. 4) At the grid scale (30-m resolution), 51.17% of the grids had anthropogenic activities contributing more than climate change to paddy field expansion, and 88.60% of the grids presented an inhibitory interactive effect of climate change and anthropogenic activities. Thus, the indicators established for in this study can not only provide a better understanding of the relative effects of climate change and anthropogenic activities on paddy field expansion, but also give some policy implications for sustainable agricultural practice in both China and other developing countries.