Roel J. W. Brienen

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 (δ 18 O TR ) chronologies can inform us about hydrological changes in the Amazon over the past two centuries. Two δ 18 O 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 δ 18 O TR records from the Amazon, and among the most highly‐replicated δ 18 O 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 δ 18 O TR records are strongly related to interannual variation in Amazon River discharge measured at Óbidos, and accumulated upwind precipitation, suggesting a common climatic driver. In both records a strong increase in δ 18 O TR was observed up until approximately 1950, consistent with positive trends in the few other existing δ 18 O 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 δ 18 O TR reverses over the past 1–2 decades, consistent with the observed strengthening of the Amazon hydrological cycle since approximately 1990.