###### Abstract  
Variations in individual phenological events in response to global change have received considerable attentions. However, the development of phenological stages is relatively neglected, especially based on in situ observation data. In this study, the rate of vegetation greenup (Vgreenup) across the Northern Hemisphere was examined for different plant functional types (PFTs) by using eddy covariance flux data from 40 sites (417 site-years). Then, the controls of climatic variables on the spatial distribution of Vgreenup across PFTs were further investigated. The mean Vgreenup was 0.22 \(\pm\) 0.11 g C m\({}^{-2}\) day\({}^{-2}\) across all sites, with the largest and lowest values observed in cropland and evergreen needle-leaf forest, respectively. A strong latitude dependence by Vgreenup was observed in both Europe and North America. The spatial variations of Vgreenup were jointly regulated by the duration of greenup (Dgreenup) and the amplitude of greenup (Agreenup). However, the predominant factor was Dgreenup in Europe, which changed to Agreenup in North America. Spring climatic factors exerted significant influences on the spatial distribution of Vgreenup across PFTs. Specifically, increasing temperature tended to shorten Dgreenup and promote Agreenup simultaneously, resulting in an acceleration of Vgreenup. Dyrness had a depression effect on Vgreenup for the whole study area, as exhibited by a lower Vgreenup with increasing vapor pressure deficit or decreasing soil moisture. However, Vgreenup in North America was only significantly and positively correlated with temperature. Without the limitation of other climatic factors, the temperature sensitivity of Vgreenup was higher in North America (0.021 g C m\({}^{-2}\) day\({}^{-2}\) C\({}^{-1}\)) than in Europe (0.015 g C m\({}^{-2}\) day\({}^{-2  
by altering vegetation photosynthesis, respiration, and even autumn leaf senescence [16]. Vgreenup also influences vegetation cover and can further provide feedback to the climate system by regulating the exchange of water and energy between the land surface and atmosphere [17; 18]. Therefore, the dynamics of greenup development and the underlying mechanism need to be thoroughly investigated to improve regional and global carbon cycles modeling performances. Increasing Vgreenup has been reported for the Northern Hemisphere and the globe based on remote sensing observations [19; 20; 21]. Using the leaf area index (LAI) dataset, [PERSON] et al. [19] found that the global average Vgreenup had increased by 0.003 m\({}^{2}\) m\({}^{-2}\) day\({}^{-1}\) in response to climate change and human land management from 1982-2015, whereas the average Dgreenup had shortened by 1.4 days, and Agreenup had increased by 0.116 m\({}^{2}\) m\({}^{-2}\). Another study also showed an increasing trend of Vgreenup induced by the warming at high latitudes above 60\({}^{\circ}\) N, from 1982-2016, based on LAI data [20]. Despite the globally accelerated Vgreenup, some areas experienced decreasing Vgreenup. For example, Vgreenup declined in Northeast China between 1982 and 2015 due to the impact of frost [22]. Therefore, Vgreenup will not exhibit identical trends under distinct environments, so it is crucial to reveal the underlying mechanism. Though the relationships between Vgreenup and some climatic factors (such as temperature, precipitation, and radiation) have been revealed, more comprehensive research including additional climatic factors is still needed to better understand the driving mechanism underlying Vgreenup. For example, dryness and wind speed were confirmed to have significant effects on vegetation phenology and productivity [23; 24]. Although remote sensing data have the advantages of extensive spatial coverage and high temporal frequencies, their results can be highly biased by data quality, due to snow and cloud contamination, sensor degradation bias, and the mixed pixel problem, especially in sparse vegetation areas [2].