Accelerated land surface greening caused by earlier permafrost thawing.

Persistent and above-average warming has advanced the start of spring permafrost thawing, intensifying climate warming through carbon feedbacks. However, the extent to which variations in spring permafrost thawing contribute to greening trends in permafrost-affected areas (i.e., increases in vegetation greenness) over time remains unclear, limiting our understanding of the ecological consequences of permafrost degradation. Analyzing 40-year freeze/thaw data and multiple satellite-derived greenness indicators, we identify widespread increases in the sensitivity of spring greenness to spring permafrost thawing based on moving-window analyses, indicating that advances in spring permafrost thawing have played a progressively stronger role in promoting spring greening, particularly in boreal forests and tundra regions underlain by continuous permafrost. In addition to the region-specific climate and permafrost conditions, we uncover biogeophysical pathways accounting for the increase in sensitivity of spring greenness to spring permafrost thawing, including reduced albedo, earlier vegetation phenology, and enhanced soil moisture infiltration. Notably, state-of-the-art Dynamic Global Vegetation Models consistently underestimate both the magnitude and variability of sensitivity of spring greenness to spring permafrost thawing. These findings highlight the temporal changes in vegetation responses to freeze-thaw dynamics, necessitating improved model projections concurrent with climate change.