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 (delta O-18) 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.