Mean air temperature (MAT) has risen continuously but increase in vegetation growth starts to slow down in recent decades at northern high latitudes, challenging the classic view that warming enhances productivity in cold regions. Such out-of-sync between MAT and vegetation growth is poorly understood. Here, based on three widely-used climate databases, we show that although there was a coherent increase in MAT during the past three decades over northern lands (> 50 ºN), the fastest warming season had shifted from spring (e.g., +0.71 to +0.21 ºC decade-1 from 1981‒2000 to 1991‒2010) to autumn (e.g.,+0.52 to +0.59 ºC decade-1). In parallel with the slowdown of spring warming, the increasing trend of vegetation greenness (represented by the satellite-derived normalized difference vegetation index; NDVI) was dampened from 1982‒2001 (+0.21 decade-1) to 1991‒2010 (+0.02 decade-1).
Results/Conclusions
The dynamic of annual NDVI was associated with that of growing season length (GSL), which was driven by the start of growing season (SOS) and the end of growing season (EOS). We further apply the partial correlation analysis and detect a sensitive advancement of SOS to spring warming (0.17day/ ºC) but a neutral relationship between EOS and autumn temperature (0.058day/ ºC). The findings indicate that although temperature keeps increasing, the shifted warming seasons from spring to autumn would suppress ecosystem productivity via reducing the warming sensitivity of vegetation phenology. Further, the Gross Primary Productivity (GPP) in northern high latitude simulated by five models were tested overrating effects of autumn warming to plant growth. This study implies that the seasonal non-uniform climate warming would cause the northern lands losing its current carbon sink capacity faster than expected.