Warmer wetter climate causes unexpected increases in ultraviolet radiation penetration and carbon mineralization in high-elevation lakes

Climate warming is greening alpine landscapes, yet its net effects on high-elevation lakes are unknown because of complex interactions between dissolved organic matter (DOM) subsidies, ultraviolet radiation (UVR) fluxes, and catchment hydrology. Here a survey of 131 Qinghai-Tibetan lakes shows that UVR penetration in freshwaters is correlated to low levels of aromatic DOM, nutrients, and CO₂, whereas aliphatic and bioavailable DOM regulate transparency in saline basins. Laboratory experiments reveal that differences in optical properties among lakes are due to DOM mineralization by UVR, resulting in a 26-fold increase in CO₂ production. Rapid DOM photo-oxidation also explains 40-years of regional lacustrine change in which water clarity has increased by up to 1.3 m per decade in 1,152 lakes, despite a 66% increase in terrestrial DOM sources and enhanced runoff. Our findings suggest that DOM mineralization by UVR creates a large overlooked alpine source of CO₂ emissions in the world’s largest plateau that can increase with further atmospheric warming.