Hydrological Extremes in the Upper Yangtze River Over the Past 700 yr Inferred From a Tree Ring δ18O Record.

Ongoing global warming has a strong influence on regional hydrological cycles. The upper reaches of the Yangtze River (UYR) are highly vulnerable to extreme hydrological droughts and floods, but a lack of long‐term streamflow observations has limited our understanding on global warming influences. Here, we establish an annually resolved and absolutely dated tree‐ring oxygen (δ18O) isotope chronology and use it to reconstruct the summer streamflow history of the UYR between 1260 and 2017. The UYR experienced 79 extreme floods and 107 extreme hydrological droughts over this period. We found a trend toward dry conditions in the region with tendencies for an increased drought probability since around 1850s. Especially, the frequency of hydrological droughts in recent decades has been outside the envelope of natural variability of extreme hydrological droughts. The temporal evolution of hydrological extremes is closely linked to variability of the Indian summer monsoon (ISM) at decadal scales. The UYR has experienced more frequent droughts during dry spells and experienced more frequent floods during wet spells of the ISM. The recent increase in the frequency of hydrological droughts is consistent with the observed trend toward a weaker ISM and increasing temperatures. Weaker transport of low‐level moisture and greater evaporation caused by global warming have contributed to the increase in extreme hydrological droughts. Our results thus provide a long‐term context for observed hydrological changes and suggest that the UYR will experience more frequent and severe extreme events as a consequence of increasing greenhouse gas emissions. Plain Language Summary: We present a 700 yr reconstruction of summer streamflow for the upper Yangtze River (UYR) in China using a tree‐ring δ18O record. Based on this reconstruction, we found an increase of extreme hydrological droughts since the onset of global warming around 1850s, whereas extreme floods have decreased. The temporal pattern of hydrological extremes is significantly linked with the Indian summer monsoon intensity. This study suggests that UYR would experience more frequent hydrological extremes under the context of global warming. Key Points: Reconstruction of hydrological extremes in the upper Yangtze River over the past 700 yr using tree ring oxygen isotopeExtreme hydrological droughts have increased significantly under the background of ongoing global warmingRecent increase in extreme hydrological droughts is related to the weakening of India's summer monsoon due to increasing greenhouse gas emissions [ABSTRACT FROM AUTHOR]