Abstract
Abstract. Satellite data reveal widespread changes in Earth's vegetation cover. Regions intensively attended to by humans are mostly greening due to land management. Natural vegetation, on the other hand, is exhibiting patterns of both greening and browning in all continents. Factors linked to anthropogenic carbon emissions, such as CO2 fertilization, climate change, and consequent disturbances such as fires and droughts, are hypothesized to be key drivers of changes in natural vegetation. A rigorous regional attribution at the biome level that can be scaled to a global picture of what is behind the observed changes is currently lacking. Here we analyze different datasets of decades-long satellite observations of global leaf area index (LAI, 1981–2017) as well as other proxies for vegetation changes and identify several clusters of significant long-term changes. Using process-based model simulations (Earth system and land surface models), we disentangle the effects of anthropogenic carbon emissions on LAI in a probabilistic setting applying causal counterfactual theory. The analysis prominently indicates the effects of climate change on many biomes – warming in northern ecosystems (greening) and rainfall anomalies in tropical biomes (browning). The probabilistic attribution method clearly identifies the CO2 fertilization effect as the dominant driver in only two biomes, the temperate forests and cool grasslands, challenging the view of a dominant global-scale effect. Altogether, our analysis reveals a slowing down of greening and strengthening of browning trends, particularly in the last 2 decades. Most models substantially underestimate the emerging vegetation browning, especially in the tropical rainforests. Leaf area loss in these productive ecosystems could be an early indicator of a slowdown in the terrestrial carbon sink. Models need to account for this effect to realize plausible climate projections of the 21st century.
Highlights
Satellite observations reveal widespread changes in terrestrial vegetation across the entire globe
We find that CO2 fertilization is an important driver of greening in some biomes but cannot be established as a dominant causal driver in many others
The complete time series of LAI3gV1 was generated using an artificial neural network trained on data of the overlap period of the Collection 6 Terra Moderate Resolution Imaging Spectroradiometer (MODIS) LAI dataset (2000–2017) and the latest version of the GIMMS group Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data (NDVI3g)
Summary
Satellite observations reveal widespread changes in terrestrial vegetation across the entire globe. The greening and browning trends reflect changes in the abundance of green leaves, and the rate and amount of photosynthesis. Others have identified regions of declining trends in leaf area (Goetz et al, 2005; Verbyla, 2011). The drivers underlying these long-term vegetation changes, remain under debate. In the light of nearly 40 years of continuous satellite observations, we reassess the driver attribution of natural vegetation changes in a new cause-and-effect framework
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