Different trends of vegetation activity over northern extratropics during two multidecadal warming periods in the 20th century

There have been two periods (1916–1945 and 1982–2014) of rapid and comparable rates (0.054 °C/year vs. 0.055 °C/year) of increase in Global Mean Temperature (GMT). However, little is known about different trends in vegetation activity over the northern extratropics during these two periods, especially for vegetation dynamics in response to the earlier global warming in the 20th century. In this study, we find that although the global warming rates are comparable between the two warming periods, regional warming over the northern extratopics has significant differences. Meanwhile, vegetation greening trends are significantly different, with the latter period being 3–4 times greater than the former. Further analysis of the leading causes reveals that the impacts of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) on regional climate are critical to these differences. In 1916–1945, the in-phase combination of AMO and PDO offsets each other's impact on the climate factors and weakens the greening trend, while in 1982–2014, the effects of AMO and PDO are synergetic for the out-of-phase combination and enhance the greening trend. Our findings highlight the role of internal climate variability in modulating vegetation dynamics under global warming and suggest that future projections of vegetation change may be improved by considering decadal and multidecadal climate oscillations.