In the summer of 2022, China's Yangtze River Valley (YRV) experienced an unprecedented heatwave with severe impacts on human society and ecosystems. Traditionally, the extremity of such hot events has been evaluated against a fixed historical baseline (e.g., 1979-2022), suggesting record-breaking magnitudes. However, recent research underscores the significance of the chosen baseline in quantifying these extremes.

In a study published on June 7 in Environmental Research Letters, researchers from the Institute of Atmospheric Physics of the Chinese Academy of Sciences have examined how using a fixed versus a 10-year moving baseline alters the perceived extremity of the 2022 heatwave.

The results show that using a moving baseline, which accounts for recent climatic adaptation, the record-breaking daily hot extremes were confined to the Sichuan Basin in the upper reaches of the YRV.

"Fixed baselines compare extremes to a static historical period (e.g., 1981-2010), capturing the continuous warming trend and increasing the perceived magnitude of extremes," said LI Lan, lead author of the study. "In contrast, moving baselines account for the time-evolving climatology and societal adaptation to warming, providing a nuanced understanding of extremes."

The researchers found that while fixed baselines showed record-breaking daily hot extremes across the upper and middle reaches of the YRV in 2022, moving baselines identified such extremes only in the Sichuan Basin, with a magnitude of 2.52 standard deviations. Furthermore, this event was not the most extreme historically, as China has faced approximately 13 events of greater magnitudes since 1971.

Future projections suggest that under high-emission scenarios, extremely hot events similar to the 2022 Sichuan Basin event could occur every two to twelve years between 2081 and 2100, affecting up to 25% of continental China with magnitudes greater than 5 standard deviations. However, with a moving baseline, the projected changes in extremity are minimal, suggesting that continuous adaptation to warming could mitigate future risks.

"The moving baseline assumes prompt adaptation of humans and ecosystems to background warming," said Prof. ZHOU Tianjun, corresponding author of the study. "However, adaptation alone, such as improving infrastructure and health systems, is not enough. Reducing greenhouse gas emissions is crucial. Climate change adaptation and mitigation must work together to address global warming effectively."