Plants using the C-4 (Hatch-Slack) photosynthetic pathway are key for global food production and account for ca 25% of terrestrial primary productivity, mostly in relatively warm, dry regions. The discovery of modern naturally-occurring C-4 plant species at elevations up to 4500 m in Tibet and 3000 m in Africa and South America, however, suggests that C-4 plants are present in a wider range of environments than previously thought. Environmental conditions on the Tibetan Plateau, including high irradiance, rainfall focused in summer, and saline soils, can favor C-4 plants by offsetting the deleterious effects of low growing season temperature. We present evidence based on leaf wax carbon isotope ratios from Lake Qinghai that C-4 plants accounted for 50% of terrestrial primary productivity on the northeastern Tibetan Plateau throughout the Lateglacial and early Holocene. Despite cold conditions, C-4 plants flourished due to a combination of factors, including maximum summer insolation, pCO(2) ca 250 ppmv, and sufficient summer precipitation. The modern C-3 plant-dominated ecosystem around Lake Qinghai was established ca 6 thousand years ago as pCO(2) increased and summer temperature and precipitation decreased. C-4 plants were also intermittently abundant during the Last Glacial period; we propose that C-4 plants contributed a significant portion of local primary productivity by colonizing the exposed, saline Qinghai Lake bed during low stands. Our results contrast with state-of-the-art ecosystem models that simulate <0.5% C-4 plant abundance on the Tibetan Plateau in modern and past environments. The past abundance of C-4 plants on the Tibetan Plateau suggests a wider temperature range for C-4 plants than can be inferred from modern distributions and model simulations, and provides paleoecological evidence to support recent findings that C-4 plant evolution and distribution was determined by a combination of climatic and environmental factors (temperature, irradiance, precipitation amount and seasonality, and soil salinity). Moreover, this finding highlights the exceptional sensitivity of high-elevation ecosystems to environmental change, and provides critical benchmarks for ecosystem model validation.