GLOBAL TRENDS IN WINTER CROPLAND EXPANSION UNDER CLIMATE CHANGE

Abstract

Climate change is increasingly reshaping global agricultural systems, with profound implications for food security and land-use sustainability. The primary research question of this dissertation is how winter cropland extent, an essential component of global cereal production, and in particular the primary niche for winter wheat, has changed in the 21st Century under the pressure of climate change, and what spatial and temporal patterns can be observed across different geographical scales. Winter cropland, as defined by the WorldCereal project and the Food and Agriculture Organization, refers to cropland used for the production of winter crop types that grow during the winter season according to local agro-ecological conditions. These crops are typically planted in the fall, remain dormant or emerge during winter, and mature for harvest in spring or early summer. To address research questions, the dissertation first conducts a global analysis of climate velocity as an essential climate variable linking spatio-temporal environmental change to shifts in climate conditions within the land-cover and land-use systems. A novel optical-flow-based climate velocity estimation method was developed to overcome key limitations of traditional approaches and to improve robustness and environmental realism when applied to remote-sensing-based land-cover and land-use data. Results from this analysis demonstrate that croplands experience the highest climate velocity among vegetated land-cover classes, with pronounced hotspots in North America and Africa, and reveal elevated vulnerability of cereal-producing regions relative to other agricultural systems. Then, to quantify historical changes in winter cropland extent, the dissertation introduces a novel continental-scale winter cropland mapping framework. This allowed the production of four-year aggregated winter cropland extent maps for the 2000–2003 and 2016–2019 periods, as well as winter cropland change maps at 250 m spatial resolution. Using these products, a stratified random sampling approach was applied to estimate winter cropland area and associated uncertainty at global, regional, and latitudinal scales based on 13,090 manually interpreted sample pixels. The results indicate that global winter cropland extent expanded by 44%, increasing from 283.73 ± 13.35 Mha in 2000–2003 to 407.61 ± 17.09 Mha in 2016–2019, which is equivalent of 10% of global cropland area. Major expansion hotspots are identified in Europe and Asia, where winter cropland area increased by 52.93 ± 8.94 Mha and 40.34 ± 8.23 Mha, respectively. In addition, the analysis reveals a pronounced northward shift of winter cropland in the Northern Hemisphere, with winter cropland change strongly correlated with latitude and moderately correlated with trends in cumulative biologically active temperature during the winter–spring season, indicating a significant role of climate change in this process, as well as in the emergence of cultivation in high-latitude regions. Together, these results provide a consistent global baseline for monitoring the extent and shifting geography of winter cropland, identifying potential future niches for wheat expansion, and improving understanding of climate adaptation, food-system resilience, and land-use sustainability under ongoing environmental change. The datasets and methodologies developed in this dissertation offer valuable resources for future analyses of climate impacts on land cover, cropland dynamics, and global food security, and for improving projections of long-term cereal production under climate change.