Tree rings of the Amazon Basin keep records of the region’s rainfall

The Amazon Basin contains the largest rainforest in the world, known for its rich biodiversity and its importance in the world’s oxygen and carbon cycles. It also has an outsized impact on the water cycles of South America and beyond. Understanding how climate change is affecting the hydrology of the Amazon is therefore a key priority for climate scientists. However, modern measurements of annual rainfall in the region do not provide the historical context needed to explain a recent increase in rainy season rainfall.

In your Journal of Geophysical Research: Biogeosciences Study, Baker et al. Use more than 200 years of tree-ring oxygen isotope data as a window into the region’s hydrological past. Oxygen isotopes can serve as a proxy for historical precipitation amounts, since heavier isotopes are more likely to be washed out of the atmosphere by precipitation in years with heavier precipitation. This means that rings formed in years with less precipitation should have a higher proportion of heavy oxygen isotopes than in wetter years.

The dataset compiled in this study is the longest record of oxygen isotopes from the Amazon basin to date, and one of the longest records of its kind for any rainforest in the world. To collect it, the researchers collected tree ring cores and slices from more than 50 trees growing in the lowland rainforest of Bolivia and the montane forest of Ecuador. Samples collected from Ecuador provided dates from 1799 to 2012, while data from samples from Bolivia covered the period 1860 to 2014.

Records from the two sites matched closely, suggesting the trees were capturing large-scale climate signals that weren’t just limited to the regions where the trees were located. Also, year-to-year variations in oxygen isotopes agreed well with modern hydrological data, indicating a reliable record of past changes.

Because precipitation and sea surface temperatures are linked in the region, the researchers also compared their results to reconstructed sea surface temperature records. Their data set shows that annual rainfall in the Amazon decreased as sea surface temperature rose between about 1800 and 1950. After that, the relationship between the two became more fragile. In recent decades, the trend has reversed, a finding consistent with modern records showing that the Amazon’s hydrological cycle has strengthened in recent years.

This story is republished courtesy of Eos hosted by the American Geophysical Union. Read the original story here.