<p>Retrogressive thaw slumps are an increasingly prevalent form of abrupt permafrost thaw that can reshape ecosystem carbon exchange, yet their impacts remain poorly quantified. Here we show, using chamber measurements of fluxes at five young slumps during the 2024 growing season and a complementary August 2025 survey at five young and two older slumps, that thaw slumps shift alpine grasslands on the interior Tibetan Plateau from a carbon sink toward a carbon source. Exposed, vegetation-free surfaces halve respiration but reduce gross primary productivity by approximately four-fifths, producing a large increase in net CO₂ release relative to undisturbed ground. CH₄ uptake occurs but is too small to offset these losses. Cross-age comparisons show that the CO₂ anomaly diminishes with time, with net exchange trending toward undisturbed levels. Together these results support a single-peak trajectory: expansion of exposed areas drives a source state, followed by partial recovery as vegetation re-establishes. These findings highlight the critical role of thaw slumps in reshaping alpine grassland carbon dynamics and underscore the need to incorporate abrupt thaw and its age dependence into carbon–climate models under ongoing permafrost degradation.</p>