Amazon trees kept a rainfall diary in their rings; it shows wet seasons grew up to 22% wetter while dry seasons lost up to 13.5% of their rain

Who would ever think of a tree as a bookkeeping machine? Well, in the western Amazon, scientists have discovered that enormous and ancient trees are quietly recording the history of our changing climate, one ring per year. What has been revealed from these rings, and what has recently been uncovered, is both fascinating and concerning.

According to a Communications Earth & Environment study titled “Tree ring isotopes reveal an intensification of the hydrological cycle in the Amazon,” scientists have examined oxygen isotopes within the yearly rings of two different Amazonian tree species, one which grows during wet periods, and the other during dry periods, finding out that there has been a dramatic change in the hydrology of the Amazon region since 1980, as the wet season became wetter, increasing by an estimated 15% to 22%, while the dry season became drier, decreasing by about 8% to 13.5%.

Trees don't lie, and this is how scientists cracked the code


But these aren't your ordinary trees. The cedar species Cedrela odorata, which grows in higher, non-floodplains, thrives during the wet season, while Macrolobium acaciifolium, a floodplain species, grows exclusively during the dry season, when floodwaters recede and its soil becomes accessible. Scientists used this information to reconstruct the history of seasonal rainfall dating back several decades, using the ratio of oxygen isotopes in the tree rings: basically, the chemical signature of the rainwater collected during each season, in two areas about 1,000 kilometers apart.

And the secret? It has something to do with what scientists call "Rayleigh distillation." Since moisture travels inland from the Atlantic, heavier water molecules (containing oxygen-18) preferentially rain out first. The more rainfall there is along that path, the more those heavy molecules are stripped away, leaving behind increasingly lighter isotopic signatures in the remaining vapor. That lighter signature then gets embedded in the tree's wood. In short: more rainfall means more depleted oxygen isotopes in the ring. And because of this, the chemical signature in the tree gives scientists an idea of how much it rained that season. Imagine your very own rain gauge that you don't have to replace batteries for.

Why this matters

If the wet-dry divide seems overly theoretical, it may help to think about its real-world implications. There have been four all-time Amazon River floods in the past 15 years: in 2009, 2012, 2021, and 2022. In turn, there have been three all-time low dry seasons for river levels in 2010, 2023, and 2024. As noted above, the highest and lowest extremes in Manaus's 122-year history of monitoring the river levels have appeared only over the past three years.

The Amazon stores carbon equivalent to an estimated 15 to 20 years of global anthropogenic greenhouse gas emissions, according to "Critical transitions in the Amazon forest system," a study published in the journal Nature in 2024. According to the same study, by 2050, 10% to 47% of Amazonian forests will be exposed to compounding disturbances that are severe enough to trigger unexpected ecosystem transitions and potentially exacerbate regional climate change. Once the Amazon is gone, we can expect more droughts in the south-western USA, more unstable climate patterns in the northern hemisphere, and higher global temperatures.

Deforestation is making a bad situation worse

It's important to note that changes in the water cycle of the Amazon region are not occurring in isolation. In fact, according to "How climate change and deforestation interact in the transformation of the Amazon rainforest," a study published in Nature Communications in September 2025, deforestation accounts for about 74.5% of the reduction in rainfall during the dry season in the Brazilian Amazon over the last 35 years. That means that the forest generates its own rain.

Deforestation can result in reduced moisture input through evapotranspiration and may contribute to lower rainfall, creating a vicious circle, especially considering all climate change effects on the region. Two factors, increasing natural intensity of the water cycle and the drying process driven by deforestation, are now acting simultaneously to put pressure on the Amazon region in multiple ways. A more intense rainy season leads to increased floods, damaging floodplains and riverside communities. The drying process, in turn, makes the forest more susceptible to fire. And fires have already reached a 14-year high in 2024.

A scientific method that changes everything

The value of the particular study in the Communications Earth & Environment lies not only in its results but in the methodology used. There is an insufficient number of meteorological observations in the Amazon, and no more than one rain gauge per 150,000 km² of forest area. While satellite data can be accessed, they have a comparatively short period of observations. Current climate grids tend to contradict each other.

The proposed methodology of analyzing oxygen isotope content in tree rings allows bypassing these shortcomings; it provides for an independent line of evidence based on atmospheric physics. The authors note that there is some uncertainty in their estimations due to the impact of parameters such as evapotranspiration, ocean temperature, and moisture dynamics on the model. However, even in the most conservative scenario, the trend points toward more extreme seasons, more often.

The clock is ticking, and so are the rings

This is a major conservation issue. The Amazon is being affected by both climate change and deforestation. The rings will keep growing. The record will continue to be kept. The real question is whether the people who can turn things around are listening. They have been keeping track for years. The changes are already underway.