## 3 Results and Discussion
### Geographical Patterns of Trend Types
#### 3.2.1 Monotonic Change  
A wide range of studies previously quantified monotonic NDVI changes in specific regions, or globally. For illustration and comparison, Figure 4 shows the magnitude of monotonic change (1982-2006). Figure 3(b) confirms that vegetation activity in certain regions showed a monotonic behavior, even if trend shifts are accounted for. Examples include the northern Great Plains, parts of eastern Europe and eastern China. In other regions, however, the assumption of monotonicity (Figure 4) may have overlooked a possible interruption (e.g., western India and Asia Minor) or may have averaged out separate periods with significant change, resulting in unjust classification of the region as stable (e.g., southern Argentina, northern Kazakhstan).  
Conspicuous greening was found in large parts of the Northern Hemisphere during the last decades of the previous century [2]. With a 12 year longer dataset, indications for this phenomenon are still present. Parts of Europe and Asia--mainly between 40 and 60 degrees latitude--show signs of a continuous increase in vegetation activity, without significant interruptions. The northern part of the Great Plains formed the largest region with monotonic greening in the Americas. It is beyond the capacity of this study to interpret driving processes, but literature suggests that an increase in precipitation boosted crop yields in these regions [41]. Similarly, large crop-land areas in eastern China (e.g., Shandong province) and in the Indo-Gangetic Plain showed monotonic increase in vegetation activity.  
The tundra and boreal forests were largely excluded from our analysis, based on GIMMS quality flags (_i.e._, long snow cover). These regions, however, were previously found to be least affected by discontinuities in vegetation activity trends [8]. Under warming conditions, monotonic greening has been reported in tundra regions, while boreal forests showed signs of (late-summer) browning, possibly drought-induced [42]. Recently, these patterns seem to have amplified rather than reversed [43].  
Extensive monotonic green-up was furthermore found in a latitudinal belt south of the Sahel, roughly in the sub-humid zone around 5\({}^{\circ}\)N. This coincides with the southern part of the documented \"Sahelian  
Figure 4: Magnitude of NDVI change, quantified using a linear model under the assumption of monotonic change. The source data, with respect to this study, is an earlier version of the GIMMS record with a shorter time span (1982–2006). Reprinted with permission from [PERSON] _et al._[12].  
green-up\" [44, 45, 46] and covers (parts of) Nigeria, Togo, Ghana, among other countries. The semi-arid Sahel itself rather showed an early-2000s reversal towards reduced vegetation activity after two decades of prosperous growth. In the northern and driest parts of the Sahel, in turn, the greening pattern persisted, despite a setback around the same time.  
A monotonic decrease in vegetation activity appeared to have sustained in substantially fewer regions. Few examples include the border region between Paraguay and Bolivia and some hotspots in northern China (Heilongjiang province). Most of the browning trends, however, were interrupted by a burst in vegetation activity.