Regional Emergence of Water-Related Browning in a Greening World.

The Earth is greening in many regions under elevated atmospheric CO2 concentrations, along with increasing temperature and land use changes. However, despite the continued rise in CO2, greening has stagnated or even reversed in some regions, suggesting a reduced capacity of the land surface to act as a carbon sink. Here, we show that declining water availability and rising atmospheric water demand have coincided with regional browning trends over recent decades in some tropical regions that have historically acted as prominent carbon sinks. A regression analysis considering a balanced set of water- and energy-related variables alongside land cover change and climate extremes confirms that both water availability and atmospheric water demand are important contributors to inter-annual variability in Leaf Area Index (LAI) there. Earth system models mostly reproduce the observed spatial extent of browning and concurrent related coinciding water changes in the multi-model mean, but results from individual models differ strongly. Our findings provide a new constraint for related model development and highlight the need for enhanced monitoring and consideration of observation-based water availability trends as an emerging driver of vegetation in future analyses and model development.