Tree rings show Amazon rainfall is getting more extreme

The Amazon Basin is no longer following its familiar seasonal rhythm. A new study shows that over the past four decades, the region’s wet months have grown markedly wetter, while its dry months are turning even drier.

The discovery comes from an unlikely archive: the annual growth rings of rainforest trees. Their chemistry records the isotopic “fingerprint” of rainwater year by year.

Rings that remember rain

Every downpour carries a distinct ratio of oxygen isotopes. Heavy rains flush lighter isotopes out of clouds first, enriching the water in heavier variants.

When a tree absorbs that moisture, the isotope mix becomes locked into its wood. By extracting pencils of core wood from trunks and reading those ratios, scientists can reconstruct past rainfall with month-by-month resolution.

For the new analysis, researchers sampled Cedrela odorata, a mahogany relative that grows on well-drained terra-firme soils, and Macrolobium acaciifolium, a species rooted in seasonally flooded forests.

Those complementary habitats allowed the team to profile both wet- and dry-season water inputs. Their isotope record spans 1980 to 2010.

The verdict is striking: wet-season rainfall in the Amazon has climbed between 15 and 22 percent, while dry-season rainfall has dropped by roughly 6 to 14 percent. The shift is large enough to affect river levels, forest physiology, and regional weather far beyond the Amazon.

Oceans’ influence on rainfall patterns

What is driving the divergence? The scientists point to changing sea-surface temperatures in both the tropical Atlantic and Pacific.

Warm anomalies in these oceans alter wind belts, convection, and therefore the intensity of rain systems that sweep across South America.

Some of that variability is natural, cycling over decades. But the authors warn that greenhouse warming is very likely amplifying the pattern.

“Our research demonstrates that the Amazon’s hydrological cycle is becoming more extreme,” said co-authors Roel Brienen and Emanuel Gloor from the University of Leeds.

“Increased wet season rainfall can lead to more frequent and severe flooding, while reduced dry season rainfall exacerbates drought conditions, impacting forest health and biodiversity.”

Seeing deeper than standard datasets

Standard rainfall gauges and satellite products often underplay long-term trends because they cover limited areas or short periods. Isotope ratios aggregate the signal over large catchments, providing what the authors call a “basin-integrated” view.

“While traditional climate datasets may underestimate these changes, the tree ring isotope data offer a more integrated, large-scale perspective,” explained co-author Arnoud Boom of the University of Leicester.

By combining upland and floodplain samples, the team isolated changes unique to wet and dry months. That dual approach revealed a previously unrecognized “bimodality” in Amazon rainfall trends.

Amazon’s pulse grows more erratic

Seasonal extremes bring real-world consequences. In wetter months, swollen rivers inundate low-lying towns, destroy crops, and erode banks. In drier months, the receding waters limit transport, concentrate pollutants, and leave isolated villages without reliable fish stocks.

The study’s co-author is Jochen Schöngart of Brazil’s National Institute for Amazon Research (INPA).

“These findings highlight that the Amazon is not simply drying or wetting overall but experiencing more extreme seasonal swings,” he said.

“This is of relevant concern as the intensification of the hydrological cycle impacts the functioning of ecosystems, water, and food security of millions of traditional and indigenous people.”

Changing Amazon endangers climate future

The ramifications go far beyond local hardship. The Amazon acts as a giant carbon sink. Longer and hotter dry seasons stress trees, raising mortality and potentially tipping the forest from absorbing CO2 to emitting it.

More intense wet seasons, on the other hand, can destabilize soils and release methane from flooded areas.

“The Amazon is a key component of the Earth’s climate system,” said lead author Bruno Cintra, a scientist at the University of Birmingham. “Understanding how its hydrological cycle is changing is essential for predicting future climate scenarios and developing effective conservation strategies.”

He points to the 2025 UN climate summit in Belém – COP30 – as “a critical opportunity for world leaders to take decisive action.”

Tree rings reveal rainfall trends

The researchers relied on the National Ecological Observatory Network’s deep-coring archive to calibrate tree-ring isotopes against rain gauges, reanalysis grids, and atmospheric models.

Sensitivity tests confirmed that temperature or evaporation could not alone explain the isotope trends; rainfall changes in the Amazon were the dominant driver.

Although the record ends in 2010, satellite data suggest the divergence has persisted – and may have accelerated – in recent years. That makes the study a baseline for assessing how future warming, deforestation, and fires will interact with ocean-driven variability.

Amazon stability demands urgent action

Policy implications are clear: conserving intact forest buffers against drought by maintaining local moisture recycling, while curbing global emissions limits further ocean-atmosphere shifts.

The experts argue that accurate Amazon rainfall projections will require integrating deep tree-ring records with modern sensor networks.

“Urgent actions to mitigate climate change and simultaneously adapt the livelihood and traditional management of the populations are required,” Schöngart said.

The rings have spoken; the challenge now is to translate their warning into meaningful protection for the world’s largest rainforest – and the climate system it helps stabilize.

The study is published in the journal Nature Communications Earth & Environment.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–