The study of humidity is important for better understanding the past climatic variations. However, there have been few long-term humidity reconstructions using tree-ring widths worldwide. Here, we present a June–September mean relative humidity (MRH69) reconstruction from AD 1837 to 2011 using Chinese pine trees (Pinus tabulaeformis Carr.) from the Yiwulü Mountain region in northeastern China. The reconstruction explained 39.8% of the instrumental variance during the calibration period. We found that precipitation only influenced the MRH69 on an annual scale (high frequency), but the mean maximum temperature (MMT) impacted the MRH69 both on an annual scale (high frequency) and decadal time scale (low frequency). The MRH69 reconstruction showed a consistently increasing trend from AD 1881 to 1990, after which it diminished sharply. More high humidity years occurred after the 1960s and more low humidity years occurred before the 1960s. In the study area, MRH69 decreased along with global warming after the 1980s. On the decadal scale, there were five high-value MRH69 intervals: 1842–1850, 1865–1871, 1898–1902, 1823–1927 and 1932–1997; there were five low-value intervals: 1851–1864, 1872–1897, 1903–1922, 1928–1931 and 1998–2006. In addition, the MRH69 reconstruction showed the large-scale representativeness of sea–land coupling. On the decadal scale, our MRH69 reconstruction varied synchronously with the Arctic Oscillation index (AOI), suggesting that the AOI might be linked with MRH69 in the Yiwulü Mountain region. Historical documents and meteorological data indicate that flooding (or drought) often occurred in the years with higher (or lower) humidity compared with the moving average values of the mean relative humidity (MRH).
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