Updating the Baseline Period Affects the Detection of Extreme Climate Change in China

Extreme climate events significantly impact ecosystems and society. The assessments of extreme events often rely on percentile-based indices using a 30-year baseline period. In China, the Blue Book on Climate Change has traditionally been based on the 1981–2010 baseline period, and it began to use the 1991–2020 baseline period since 2024. However, the impact of baseline changes on assessing extremes in China remains unclear. This study examines how baseline period updates influence the detection of long-term climate change in China, particularly in estimating the Time of Emergence (ToE) for climate change signals. The results show that for temperature extremes, updating the baseline period leads to more (10%∼38%) cold extremes identified by 10th percentile indices and fewer (−32%∼−11%) warm extremes identified by 90th percentile indices across China. It slows the increase in identified warm extremes and accelerates the decrease in cold extremes. It delays the ToE for warm events and advances the ToE for cold events. For precipitation extremes, the update leads to fewer (−12%∼−1%) but more intense (at most 4%) extreme precipitation events identified by 95th and 99th percentile indices across China with slower increases in frequency and faster rises in intensity. The baseline period update advances the ToE for extreme precipitation intensity and delays it for frequency. The update of the baseline significantly affects the assessment of changes in climate extremes in China due to the background warming and wetting in 1991–2020 compared to 1981–2010.