Extreme weather and climate events—such as heatwaves and heavy rainfall—have significant effects on ecosystems, infrastructure, and society. To monitor these extremes, scientists usually compare current conditions to a historical "climate baseline," typically a 30-year period like 1981 to 2010. In accordance with World Meteorological Organization (WMO) guidance, this baseline is updated every decade to reflect recent climate changes. Since 2024, China's official climate assessments have utilized the latest baseline of 1991 to 2020 as recommended by the WMO.

But how does changing the baseline affect our assessment of extreme events?

A recent study published in the Journal of Geophysical Research: Atmospheres offers a comprehensive analysis of how updates to the baseline influence the detection of extreme climate events across China. The study, led by LI Lan, a Ph.D. student at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, and the University of Chinese Academy of Sciences, highlights that modifying the baseline significantly alters the intensity, trends, and timing of detectable climate signals across the Chinese mainland.

This study found that using different baselines can substantially change the interpretation of extreme climate events. When updated the baseline from 1981–2010 to 1991–2020, the researchers discovered that cold extremes occurred 10% to 38% more frequently. Conversely, hot extremes became 11% to 32% less prominent.

"For heavy precipitation events, we noted fewer occurrences, yet the intensity slightly increased in certain regions of China, such as Northeast China," said LI.

The change in baseline also affects the "Time of Emergence" (ToE)—the point at which climate change signals become distinguishable from natural variability. High-temperature ToEs were delayed by up to eight years, while cold extremes and intense rainfall events emerged earlier under the updated baseline.

"Neglecting the effects of baseline updates could lead to misjudgments in risk assessments and planning, especially in vulnerable areas," noted Prof. ZHOU Tianjun, the study's corresponding author. "Our findings are based on an analysis of extreme event changes across the Chinese mainland. We advocate for international collaboration to harmonize climate monitoring standards and to better understand regional differences in baseline sensitivity."