Vegetation Greening Despite Weakening Coupling Between Vegetation Growth and Temperature Over the Boreal Region

The previously reported strong coupling between interannual variations in vegetation growth and mean growing season temperature in the extratropical (>25°N) Northern Hemisphere (NH) has displayed trends of progressive weakening in recent decades. However, the spatiotemporal patterns and drivers of the weakening coupling and its impacts on vegetation growth across the NH remain poorly understood. Here we address these issues using the vegetation index retrieved from remote sensing, global ecosystem model simulations, and gridded climate data. Our results reveal marked fading of the positive coupling between the mean growing season normalized difference vegetation index (NDVI_GS) and mean growing season temperature. Areas with significant positive coupling have shrunk from 31.2% to 20.0% over the periods of 1982–1997 and 1997–2012 across the extratropical NH. Vegetation productivity simulations from global ecosystem models did not capture the above mentioned patterns. There shows a continuous vegetation greening, with a greening rate of 0.0007–0.001 NDVI/year, despite the weakening of the positive coupling in the extratropical NH during the past few decades. Our results highlight that a comprehensive understanding of the underlying processes governing vegetation growth-climate coupling is urgently needed for accurate predictions of the future trajectories of the response of boreal terrestrial ecosystems to a warmer and more extreme climate regime.