Abstract. Satellite data reveal widespread changes of 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 biome-level that can be scaled into 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 of 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). Our results do not support previously published accounts of dominant global-scale effects of CO₂ fertilization. Altogether, our analysis reveals a slowing down of greening and strengthening of browning trends, particularly in the last two 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 slow-down in the terrestrial carbon sink. Models need to account for this effect to realize plausible climate projections of the 21^st century.

中文翻译：

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自然植被的绿化趋势随着大气中CO _2的进一步增加而减慢

摘要。卫星数据揭示了地球植被的广泛变化。由于土地管理，人为密集的区域大多绿化。另一方面，自然植被在所有大陆上都呈现出绿化和褐变的格局。与人为碳排放相关的因素，例如CO ₂施肥，气候变化以及随之而来的干扰，例如火灾和干旱，被认为是自然植被变化的主要驱动力。目前缺乏在生物群落水平上的严格区域归因，可以将其归纳为观察到的变化背后的全球情况。在这里，我们分析了全球叶片面积指数（LAI，1981–2017）的数十年卫星观测的不同数据集，以及植被变化的其他代理，并确定了几个具有重要长期变化的集群。使用基于过程的模型模拟（地球系统和土地表面模型），我们使用因果反事实理论，在概率环境中，弄清了人为碳排放对LAI的影响。该分析突出表明了气候变化对许多生物群落的影响-北部生态系统的变暖（绿化）和热带生物群落的降雨异常（褐变）。我们的结果不支持先前发表的关于CO在全球范围内占主导地位的影响的描述₂施肥。总而言之，我们的分析表明，尤其是在最近的二十年中，绿化的速度有所放缓，而褐化的趋势有所加强。大多数模型都大大低估了新出现的植被褐变，特别是在热带雨林中。这些生产性生态系统中的叶面积减少可能是陆地碳汇速度下降的早期迹象。模型需要考虑到这种效果，实现了21似是而非的气候预测^ST世纪。