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    “Once in a Millennium” – Scientists Shed New Light on Unusual Super Rainstorm in China

    By Science China PressNo Comments5 Mins Read
    Dark Rain Storm Clouds
    The “7.20” super rainstorm, an intense precipitation event in Zhengzhou, China, was reconstructed using land snail shells by Dr. Hong Yan’s team. This innovative approach provides a closer look at historical weather patterns and suggests a potential shift in the frequency and intensity of such events due to climate change.

    Using snail shells, Dr. Hong Yan’s team reconstructed the intense “7.20” rainstorm in Zhengzhou, suggesting that such extreme weather events might become more common with global warming.

    In July 2021, Zhengzhou, central China was battered by an extreme rainstorm (then named “7.20” super rainstorm). Local daily precipitation was over 300mm and the region experienced nearly an entire year’s worth of precipitation within three days.

    Evaluating Historical Weather Events

    Although the “7.20” super rainstorm has been regarded as a “once-in-a-millennium” weather event in meteorological statistics, this designation is based on instrumental data from the past hundred years. With rapid global warming, the climate background of extreme weather events is also changing rapidly. Therefore, whether “7.20” super rainstorm is a “once-in-a-millennium” disaster or a new normal event in a warmer world is still a pressing question to clarify. Given this uncertainty, obtaining extreme weather event variability under various climate conditions could greatly improve our understanding of the mechanisms and dynamics of extreme weather events.

    However, due to the short time span of most modern instrumental data (no more than 100 years) and the coarse temporal resolution of most current paleoclimate records (ranging from years to millennia), it is fair to say that our knowledge about the intrinsic relationship between extreme weather variability and climate background is almost non-existent up to now.

    Zhengzhou-Xingyang Area Local Setting
    Figure 1. (a) Map showing Eastern Asia’s prevailing monsoon climates and the sampling site location. The topographic map comes from https://www.gebco.net/data_and_products/gridded_bathymetry_data/, and the red-filled circle and the white-filled circle represent the sampling site and Beijing, respectively. Abbreviations: ISM, Indian Summer Monsoon; EASM, East Asia Summer Monsoon; EAWM, East Asia Winter Monsoon. (b) The multi-year average monthly T, P, and δ18Oprecipitation in Zhengzhou (IAEA, http://www.iaea.org/water). The grey bar represents the growing seasons for Cathaica fasciola at Zhengzhou. The monthly mean T > 10oC from April to October. (c) Land snail Cathaica fasciola and the schematic flow chart for the cutting and sampling of snail shells. The red lines represent the sampling path of the subsamples for GSMS determination; The yellow arrows represent the direction of sampling, which is opposite to the growth direction. The enlarged section shows the sampling spots of SIMS analysis. Credit: ©Science China Press

    Paleoclimatic Reconstruction of Extreme Weather

    Now, the team of Dr. Hong Yan from the Institute of Earth Environment, Chinese Academy of Sciences has reconstructed the “7.20” super rainstorm using local land snail shells. Four modern land snail shells (Cathaica fasciola) were collected in the Zhengzhou-Xingyang area, one of the centers hit by the “7.20” super rainstorm (Figure 1).

    Ultra-High Resolution δ18Oshell Record Super Rainstorm
    Figure 2. (a) 0918-1 and 1218-1 δ18Oshell records determined by SIMS. The black arrow represents the direction of growth. (b) Daily precipitation records were obtained from the National Meteorological Information Center (http://data.cma.cn/). (c) Details in the dashed rectangle of (a). The comparison between the measured δ18Oshell amplitude of 0918-1 and 1218-1 resulted from the “7.20” rainstorm. (d) The probability density distribution for the δ18Oshell amplitudes of the theoretical and measured values. (e) Mean values and 90 % confidence intervals for the theoretical and measured δ18Oshell amplitudes caused by the “7.20” rainstorm. Credit: ©Science China Press

    Weekly and daily resolved snail shell δ18O records from June to September of 2021 were obtained by gas-source mass spectrometry (GSMS) and secondary ion mass spectrometry (SIMS). The daily resolved records show a dramatic negative shift between June 18th and September 18th, which is very likely to be related to the “7.20” super rainstorm according to the established age framework. Furthermore, the theoretical precipitation amounts of “7.20” super rainstorm calculated by the amplitude of this δ18O shift using the flux balance model closely approaches the precipitation amount observed at the local meteorological station (Figure 2).

    Technology Roadmap for Paleoclimate Interpretation
    Figure 3. This methodology involved several steps. Step 1: Carefully select representative loess sections and establish their chronology through dating methods. Collect loess powder samples and fossil snail samples from these sections with a specific resolution. Determine the magnetic susceptibility (MS) and mean grain size (MGS) of the loess powder samples to establish a comprehensive framework for local climate change patterns over time. Conduct taxonomic identification and count on the snail samples to establish the snail population-age profile. Step 2: Randomly select 138 snail shells from the designated period and perform SEM analysis and/or X-ray diffraction (XRD) analysis on them to assess whether diagenetic alteration has occurred. The well-preserved shells would then be checked for the physical marker. Step 3: SIMS δ18O determination will be conducted on the shell with the physical marker. The obtained SIMS δ18O profile will be applied for the rainstorm fingerprint analysis to investigate the frequency of extreme rainstorms with various magnitudes in the given period. Credit: ©Science China Press

    Implications for Climate Science

    This study is the first to demonstrate that terrestrial synoptic scale extreme rainstorm events can be quantitively reconstructed by a natural paleoclimate archive. The application of current work on fossil snail shells in the older sedimentary strata may offer an opportunity to reveal the frequency and intensity of synoptic scale extreme rainstorms under different climate backgrounds (Figure 3). These paleoweather information sources may prove to be essential sources to help understand the law and dynamics of regional extreme rainstorms, especially for calibrating numerical climatic models and predicting the future trend of extreme rainstorms.

    Reference: “Quantitative reconstruction of a single super rainstorm using daily resolved δ18O of land snail shells” by Guozhen Wang, Jibao Dong, Tao Han, Chengcheng Liu, Fan Luo, Haotian Yang, Miaohong He, Guoqiang Tang, Nanyu Zhao, Qian Zhang, Gang Xue, John Dodson, Qiuli Li and Hong Yan, 18 April 2024, Science Bulletin.
    DOI: 10.1016/j.scib.2024.04.037

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