Forest ecosystems are vital to the global carbon cycle, yet their long-term aboveground carbon (AGC) dynamics remain uncertain. Here, we integrate multi-source satellite observations with probabilistic deep learning models to reconstruct a harmonized, uncertainty-aware global forest AGC record from 1988 to 2021 at 0.25-deg. We find that, although global forests sequestered 6.2 PgC, moist tropical and boreal forests have progressively transitioned toward carbon sources since the early 2000s. This shift coincides with a strengthening negative correlation between tropical AGC variability and atmospheric CO2 growth rates (r = -0.63 in 2011-2021), suggesting tropical forests increasingly modulate the global carbon cycle. Notably, in the Brazilian Amazon, the contribution of intact forests to the year-to-year variations in AGC losses increased from 33% in the 1990s to 76% in the 2010s, surpassing that of deforested areas (from 60% to 13%). Our findings highlight the vulnerability of carbon stocks in key biomes and provide a benchmark to track emerging sink-source shifts under anthropogenic climate change.

翻译：森林生态系统对全球碳循环至关重要，但其长期地上碳动态仍存在不确定性。本研究整合多源卫星观测与概率深度学习模型，重构了1988年至2021年0.25度分辨率下、具有一致性及不确定性量化的全球森林地上碳数据。研究发现：尽管全球森林累计固碳6.2 PgC，但自21世纪初以来，湿润热带与北方森林已逐渐向碳源转变。这一转变与热带地上碳变异性与大气CO2增长率之间负相关性增强（2011-2021年相关系数r=-0.63）同步发生，表明热带森林正日益主导全球碳循环调节。值得注意的是，在巴西亚马逊地区，完整森林对地上碳损失年际变化的贡献率从20世纪90年代的33%跃升至2010年代的76%，超过同期森林砍伐区域的贡献率（从60%降至13%）。研究结果揭示了关键生物群落碳储量的脆弱性，并为追踪人为气候变化背景下新兴的碳汇-碳源转变提供了基准参照。