Rivers are a key component of the global carbon cycle. They receive vast quantities of terrestrial carbon, of which a large fraction is ultimately exported to the coastal ocean. Our review of previously published assessments reveals that substantial uncertainties remain with regard to the spatial distribution and speciation of the carbon export. Accurate quantification of the relative contributions of dissolved, particulate, organic and inorganic carbon to the total amounts is, however, of crucial importance for the coupling between the terrestrial and marine carbon cycles. Breaking down existing spatially explicit assessments over large river basins, we find a disagreement in flux estimates that exceeds two orders of magnitude for more than half of the basins. Using machine-learning techniques in combination with a multi-model ensemble and an updated database of observations, we overcome the inconsistencies in existing assessments and narrow down uncertainties in riverine carbon exports. Our revised assessment yields a global riverine export of 1.02 ± 0.22 (2σ) PgC yr⁻¹. This carbon flux is partitioned into 0.52 ± 0.17, 0.30 ± 0.14, 0.18 ± 0.04 and 0.03 ± 0.02 PgC yr⁻¹ of dissolved inorganic, dissolved organic, particulate organic and particulate inorganic carbon, respectively. We estimate the carbon contribution through groundwater export to be minor (0.016 PgC yr⁻¹). Our assessment suggests an underestimation of the land-to-ocean carbon flux by 0.24 PgC yr⁻¹ by the Intergovernmental Panel on Climate Change (IPCC) and calls for a revision of the oceanic carbon budget.

河流是全球碳循环的关键组成部分。它们接收了大量的陆地碳，其中很大一部分最终被输送到沿海海洋。我们对之前发布的评估的回顾表明，碳输出的空间分布和形态仍然存在很大的不确定性。然而，准确量化溶解碳、颗粒碳、有机碳和无机碳对总量的相对贡献对于陆地和海洋碳循环之间的耦合至关重要。通过对大型河流流域现有的空间明确评估进行分析，我们发现超过一半流域的通量估计存在超过两个数量级的分歧。利用机器学习技术与多模型集成和更新的观测数据库相结合，我们克服了现有评估中的不一致之处，并缩小了河流碳出口的不确定性。我们修订后的评估得出全球河流出口量为 1.02 ± 0.22 (2 σ ) PgC yr ⁻¹ 。该碳通量分别被分配为0.52±0.17、0.30±0.14、0.18±0.04和0.03±0.02PgCyr ^-1的溶解无机碳、溶解有机碳、颗粒有机碳和颗粒无机碳。我们估计通过地下水出口产生的碳贡献很小（0.016 PgC yr ^-1 ）。我们的评估表明，政府间气候变化专门委员会 (IPCC) 将陆地到海洋的碳通量低估了 0.24 PgC yr ^-1 ，并呼吁修订海洋碳预算。