Recent leveling off of vegetation greenness and primary production reveals the increasing soil water limitations on the greening Earth

Abstract

Global vegetation photosynthesis and productivity have increased substantially since the 1980s, but this trend is heterogeneous in both time and space. Here, we categorize the secular trend in global vegetation greenness into sustained greening, sustained browning and greening-to-browning. We found that by 2016, increased global vegetation greenness had begun to level off, with the area of browning increasing in the last decade, reaching 39.0 million km2 (35.9% of the world’s vegetated area). This area is larger than the area with sustained increasing growth (27.8 million km2, 26.4%); thus, 12.0% ± 3.1% (0.019 ± 0.004 NDVI a−1) of the previous earlier increase has been offset since 2010 (2010–2016, P < 0.05). Global gross primary production also leveled off, following the trend in vegetation greenness in time and space. This leveling off was caused by increasing soil water limitations due to the spatial expansion of drought, whose impact dominated over the impacts of temperature and solar radiation. This response of global gross primary production to soil water limitation was not identified by land submodels within Earth system models. Our results provide empirical evidence that global vegetation greenness and primary production are offset by water stress and suggest that as global warming continues, land submodels may overestimate the world’s capacity to take up carbon with global vegetation greening.