Permafrost degradation reverses warming-induced earlier vegetation green-up

Global warming generally advances vegetation spring phenology, yet plants in permafrost regions exhibit contrasting trends, including some starting to have later greening. The role of permafrost degradation in this divergence remains uncertain. Leveraging satellite vegetation indices, permafrost degradation indicators, and climate data from the Northern Hemisphere (period 1982 to 2020), we identify drivers of slowing trends of earlier Green-Up Dates (GUD), including where reversing. We demonstrate that rather than temperature, permafrost degradation has become a dominant factor influencing GUD in 21.7% of locations. Evidence from these nearly four decades in the recent past confirms some places exhibit a strong nonlinearity where the impact of increasing permafrost degradation on vegetation GUD is characterized by an initial increase followed by a decrease after a tipping period. The tipping period timing varies significantly across different biomes, which we discover relates to root depths. Melting permafrost supports vegetation growth, but if the active layer falls below root depth, the opposite occurs, explaining why GUD reversal is first observed in grasslands with short roots. Our results illustrate a tipping period has already been triggered by climate change. Our findings will guide land model development and constrain future predictions of permafrost vegetation growth and carbon sequestration.