Extreme weather and climate events—like heatwaves and heavy rainfall—have profound impacts on ecosystems, infrastructure, and society. To track these extremes, scientists typically compare current conditions to a historical “climate baseline,” usually a 30-year period such as 1981–2010. In line 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 adopted the latest baseline of 1991–2020 as recommended by WMO.

WMO State of the Global Climate 2023 uses 1991–2020 as the baseline period. (Image by WMO)

But how does this change in baseline affect our assessment of extreme event changes?

A new study published on March 29 in Journal of Geophysical Research: Atmospheres provides the first comprehensive look at how baseline updates influence the detection of extreme climate events across China. Led by a Ph.D. student Li Lan from the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, and the University of Chinese Academy of Sciences (UCAS), the study reveals that changing the baseline significantly alters the intensity, trend, and timing of detectable climate signals across Chinese mainland.

The research revealed that using different baselines can significantly change how we interpret extreme climate events. “Over Chinese mainland, when we updated the baseline from 1981–2010 to 1991–2020, we found that cold extremes appeared 10% to 38% more frequently. On the other hand, hot extremes became 11% to 32% less prominent,” explained Li. “For heavy precipitation events, we observed fewer occurrences, but the intensity slightly increased in specific regions of China such as Northeast China.”

The change in baseline also impacts the “Time of Emergence” (ToE)—the point when climate change signals clearly stand out from natural variability. High-temperature ToEs were delayed by up to 8 years, while cold extremes and intense rainfall events emerged earlier under the new baseline.

“The fact that updating baselines affects detection of extreme events is because the background climate is changing over time,” added Zhang Wenxia, a co-author of the study and an associate professor at IAP.

“Failing to account for the effect of baseline updates would lead to misjudgments in risk assessments and planning, especially in vulnerable areas,” noted Prof. Zhou Tianjun, the study’s corresponding author and a professor at IAP and UCAS. “Our results are based on the analysis of extreme event changes across the Chinese mainland. We call for international collaboration to harmonize climate monitoring standards and to better understand regional differences in baseline sensitivity.”