China's lake expansion amplified rapid CO2 emissions.

CO2 emissions from lakes play a crucial role in the global carbon cycle, yet their long-term dynamics remain poorly constrained. Here, leveraging a compiled lake CO2 flux dataset, machine learning model, and lake area data, we demonstrated a nationwide increase in CO2 emissions across China's lakes, from 11.25 teragrams of C per year in 2000 to 13.94 teragrams of C per year in 2021 with an abrupt shift after 2010. This transition coincided with rapid lake expansion, gaining at a pace of 71 to 462 square kilometers per year across regions. Large lakes (>50 square kilometers) dominated national emissions, accounting for ~62% of the total, despite large regional variability (33 to 79%). Lakes exhibited divergent responses to climate extremes, with CO2 flux variations of 4 to 48% across regions. The CO2 emissions in China's lakes offset ~12% of national wetland carbon sinks, revealing the critical role in terrestrial carbon budget. Our findings highlighted the urgent need for sustained and high-resolution monitoring to refine lake carbon budget and support climate mitigation policy formulation.