The Changing Amazon Hydrological Cycle-Inferences From Over 200 Years of Tree-Ring Oxygen Isotope Data

Changes to the Amazon hydrological cycle have important consequences for world's largest tropical forest, and the biodiversity it contains. However, a scarcity of long-term climate data in the region makes it hard to contextualize recent observed changes in Amazon hydrology. Here, we explore to what extent tree-ring oxygen isotope (delta O-18(TR)) chronologies can inform us about hydrological changes in the Amazon over the past two centuries. Two delta O-18(TR) records from northern Bolivia and the Ecuadorian Andes are presented. The Ecuador record spans 1799-2012 (n = 16 trees) and the Bolivia record spans 1860-2014 (n = 32 trees), making them the longest delta O-18(TR) records from the Amazon, and among the most highly-replicated delta O-18(TR) records from the tropics to date. The two chronologies correlate well at interannual and decadal timescales, despite coming from sites more than 1,500 km apart. Both delta O-18(TR) records are strongly related to interannual variation in Amazon River discharge measured at Obidos, and accumulated upwind precipitation, suggesting a common climatic driver. In both records a strong increase in delta O-18(TR) was observed up until approximately 1950, consistent with positive trends in the few other existing delta O-18 proxy records from across the Amazon. Considering all possible drivers of this long-term increase, a reduction in rainout fraction over the basin driven by rising sea surface temperatures in the North Atlantic is suggested as the most likely cause. The upward trend in delta O-18(TR) reverses over the past 1-2 decades, consistent with the observed strengthening of the Amazon hydrological cycle since approximately 1990.

Automated summary

Changes in the Amazon hydrological cycle have significant implications for the world's largest tropical forest and its biodiversity. However, the lack of long-term climate data in the region hinders our understanding of recent changes in Amazon hydrology. In this study, researchers analyze tree-ring oxygen isotope chronologies to shed light on hydrological changes in the Amazon over the past two centuries. The results reveal a strong correlation between tree-ring oxygen isotope records and interannual variation in Amazon River discharge and upwind precipitation. The findings suggest that rising sea surface temperatures in the North Atlantic may be the main driver behind the long-term increase in tree-ring oxygen isotope values, while recent decades have seen a reversal of this trend due to the strengthening of the Amazon hydrological cycle.