Abstract
Significant land “greening” in the northern-extratropical latitudes (NEL) has been documented from satellite observations during the past three decades, and has broad implications for regional surface energy, water and carbon budgets, and ecosystem services (Myneni et al., 1997; Zhou et al., 2001; Lucht et al., 2002; Pan et al., 2011; Mao et al., 2013; Xu et al., 2013; Buitenwerf et al., 2015; Liu et al., 2015). Discernable human impacts on the Earth’s climate system have been revealed by using statistical frameworks of detection-attribution (Hegerl et al., 2007; Bindoff et al., 2013). These impacts, however, were not previously identified on the “northern greening”, due to the lack of long-term observational records, possible bias of satellite data and different algorithms used to calculate vegetation greenness, and the lack of suitable simulations from global coupled Earth System Models (ESMs). Here, we have overcome these challenges in order to attribute changes in NEL vegetation activity. We have used two 30-year long remote sensing-based Leaf Area Index (LAI) datasets (Zhu et al., 2013; Raret et al., 2013), externally-forced and unforced simulations from 19 coupled ESMs with interactive vegetation, and a formal detection and attribution algorithm (Allen et al, 2003; Ribes et al., 2013). Our findings reveal that the observed record is consistent with the simulations with anthropogenic forcings, where the greenhouse gases forcing play a dominant role, but not with that expected from internal climate variability and natural forcings only. Therefore, and for the first time, these results provide clear evidence of a discernible human fingerprint on large-scale terrestrial vegetation activity.
