Increasing global vegetation browning hidden in overall vegetation greening: Insights from time-varying trends

Global vegetation dynamics are of critical importance for understanding changes in ecosystem structure and functioning and their responses to different natural and anthropogenic drivers. Under the background of rapid global warming, it is still unclear whether there were significant changes in the extent and intensity of global vegetation browning during the past three decades. Taking satellite-derived normalized difference vegetation index (NDVI) as the proxy of vegetation growth, we investigated spatiotemporal variances in global vegetation trends during the period 1982-2013 using the ensemble empirical mode decomposition (EEMD) method and two piecewise linear regression models. Our study suggests that increasing global vegetation browning is masked by overall vegetation greening. A >60% increase in browning area was found during the study period, and the results consistently indicate that the expansion of browning trends has accelerated since 1994. After the late 1990s, browning trends increased in all latitudinal bands in the Northern Hemisphere. This increase was particularly pronounced in the northern mid-low latitudes, where the greening trends stalled or even reversed. Areas with browning trends increased in all land cover types, although the increase processes varied substantially. During 1982-2013, although most vegetated lands exhibited overall greening trends, greening-to-browning reversals occurred on all continents and occupied a much larger area than browning-to-greening reversals. Greening trends prevailed before the turning points, and browning trends largely expanded and enhanced thereafter. The increased browning trends resulted in a slowdown of the increase in global mean NDVI since the early 1990s. Since drought is likely the main cause of the increasing browning trends, global vegetation growth is at risk of reversal from long-term greening to long-term browning in the warmer future.