Advanced vegetation green-up onset in regions with cooling air temperatures in the Northern Hemisphere: Drivers and impacts on productivity

Climate warming was considered the primary driver of the advanced vegetation green-up onset date (VGD) in the Northern Hemisphere. However, the continuous advancement of VGD at the hemispheric scale during the hiatus in spring warming remains unexplained. Using high-quality satellite and multiple environmental datasets, we revealed two causes of VGD advancement during 2000–2014 (a period when a warming hiatus occurred) over the mid-to-high latitudes of the Northern Hemisphere. Firstly, VGD advanced despite pre-VGD cooling air temperature, which was associated with enhanced radiation, increased precipitation, higher soil temperature, elevated soil moisture, and earlier snowmelt dates, in 20.4 % of the study area, mostly located in northern North America, eastern Europe, and northeastern Asia. The VGD advancements resulted in widespread increases in spring vegetation greenness and gross primary productivity in these areas. Secondly, in some areas with weak pre-VGD air warming, strong sensitivity of VGD to air temperature resulted in substantial VGD advancement (e.g., eastern Canada, southern Europe, and central China); conversely, in some areas with pre-VGD cooling air, weak temperature sensitivity resulted in limited VGD delay (e.g., Canadian Prairies, southeastern Russia, and northern Ural region). These findings underscore the importance of considering factors beyond air temperature as well as the significance of the spatial correspondence between phenological temperature sensitivity and changes in air temperature when attributing large-scale phenological changes. These insights are crucial for understanding phenological responses to climatic changes and their implications for terrestrial carbon cycling.