Preserving Tibet's unique history and cultural heritage relies on the sustainability of the Tibetan croplands, which are characterized by highland barley, the only cereal crop cultivated over 4000 m above sea level. Yet it is unknown how these croplands will respond to climate change. Here, using yield statistics from 1985 to 2015, we found that the impact of temperature anomalies on the Tibetan crop yield shifted from nonsignificant (P > 0.10) in the 1980s and 1990s to significantly negative (P < 0.05) in recent years. Meanwhile, the apparent sensitivity of the crop yield to temperature anomalies almost doubled, from (–0.13 ± 0.20) to (–0.22 ± 0.14) t·ha^–1·^°C^–1. The emerging negative impacts of higher temperatures suggest an increasing vulnerability of Tibetan croplands to warmer climate. With global warming scenarios of +1.5 or +2.0 ^°C above the pre-industry level, the temperature sensitivities of crop yield may further increase to (–0.33 ± 0.10) and (–0.51 ± 0.18) t·ha^–1·^°C^–1, respectively, making the crops 2–3 times more vulnerable to warmer temperatures than they are today.