Precipitation leads the long-term vegetation increase in the conterminous United States drylands

Drylands, encompassing over 40% of the conterminous United States (CONUS), are crucial to the global carbon cycle and highly susceptible to climate change. However, Earth system models offer conflicting projections of future drought and vegetation activity in North America, and in-depth analyses of the long-term changes in greenness and its relationship with underlying climate drivers, considering both spatial and temporal variations at the ecosystem scale, are lacking. This study analyzes 20 year (2001–2020) MODIS normalized difference vegetation index (NDVI) observations to assess greenness trends in CONUS drylands and their relationship with climate drivers at 1 km spatial resolution. Results indicate a large scale and systematic greening trend, particularly in the northern Great Plains (NGP) region. Using an empirical linear attribution approach and empirical orthogonal function analysis, we uncover varied relationships between greenness trends and climate drivers, particularly highlighting the dominant role of increased precipitation in driving the observed greening. Trend analysis reveals that while rain use efficiency (RUE) remains stable in most areas, increases in the NGP region suggest potential CO _2 fertilization effects, while decreases in southern states correlate with rising temperatures. We also develop an efficiency-based model featuring RUE which successfully reproduces historical NDVI, re-confirming the dominant influence of precipitation in local greenness interannual variability. However, CMIP6 projections for 2021–2040 under the ‘Regional Rivalry’ scenario (SSP370) paint a worrying picture, with projected browning in the NGP region and states near the 42°N latitude, contrasting recent greening trends. This potential reversal underscores the vulnerability of these ecosystems to future climate change, highlighting the need to consider both historical trends and future climate projections when assessing the resilience of drylands ecosystems. Overall, our work re-emphasizes the significance of water availability to drylands vegetation growth and contributes to a more comprehensive understanding of carbon-water cycling in arid and semi-arid regions.