Tree rings reveal increasing rainfall seasonality in the Amazon

Scientists have used clues locked into tree rings to reveal major changes in the Amazon’s rainfall cycle over the last 40 years:  wet seasons are getting wetter and dry seasons drier.

Oxygen isotope signals in rings from two Amazon tree species allowed the international research team to reconstruct seasonal changes in rainfall for the recent past.

Publishing their findings today (17 June) in Communications Earth and Environment, the researchers reveal that wet season rainfall has increased by 15 - 22%, and dry season rainfall decreased by 5.8 - 13.5% since 1980.

The study is a result of a collaboration between the Universities of Leeds, Leicester and the National Institute for Amazon Research, in Brazil.

Co-author Dr. Bruno Cintra, now at the University of Birmingham, commented:  "The Amazon is a key component of the Earth's climate system. Understanding how its hydrological cycle is changing is essential for predicting future climate scenarios and developing effective conservation strategies. The upcoming COP30 in Belém, Brazil presents a critical opportunity for world leaders to take decisive action.”

Researchers believe this intensified seasonal cycle is caused by changes in temperatures of the surrounding Atlantic and Pacific oceans influencing the atmospheric circulation. While these changes are partly driven by natural variability, there are also strong indications that anthropogenic climate change plays a role.

Co-author Roel Brienen and Emanuel Gloor, from the University of Leeds, commented: “Our research demonstrates that the Amazon's hydrological cycle is becoming more extreme. 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.”

The study was based on oxygen isotope ratios from tree rings of Cedrela odorata and Macrolobium acaciifolium in the Amazon from 1980 to 2010 to reconstruct past wet and dry season rainfall variability. The researchers linked oxygen isotope changes to large-scale precipitation, estimating long-term rainfall changes and uncertainties using observed data, isotope models, and sensitivity analyses to atmospheric parameters.

Co-author Dr Arnoud Boom, from the University of Leicester, commented: “While traditional climate datasets may underestimate these changes, the tree ring isotope data offer a more integrated, large-scale perspective.

Our unique approach combining oxygen isotope ratios in tree rings from non-flooded (terra firme) and from flooded Amazon forests allowed us to separately estimate wet and dry seasons rainfall trends.”

The Amazon rainforest plays a critical role in global climate regulation, acting as a major carbon sink and participating in global atmospheric patterns. Observed changes in the rainfall cycle could have far-reaching effects on global climate stability.

Co-author Dr Jochen Schöngart from the National Institute for Amazon Research (INPA) in Manaus, Brazil, commented:  “These findings highlight that the Amazon is not simply drying or wetting overall but experiencing more extreme seasonal swings.

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

ENDS 

For more information, interviews, or an embargoed copy of the research paper, please contact the Press Office at University of Birmingham on pressoffice@contacts.bham.ac.uk  or +44 (0) 121 414 2772.

PHOTO CAPTIONS:

• Extreme river flood levels reach several meters depth, as indicated by the darker shade on the bark of this tree from seasonally flooded forests (credit Bruno B L Cintra, University of Birmingham).
• Clouds over the Amazon Basin bring moisture from the Atlantic Ocean until the Andes mountains, carrying large-scale climatic information that is recorded in the wood of Amazon trees that take up rainwater (credit Bruno B L Cintra, University of Birmingham).
• Tree ring sample being extracted from trunk (credit: Prof. Roel Brienen, University of Leeds)

Notes to editor:   

• The University of Birmingham is ranked amongst the world’s top 100 institutions. Its work brings people from across the world to Birmingham, including researchers, teachers and more than 8,000 international students from over 150 countries.
• ‘Independent evidence of hydrological cycle intensification in the Amazon from tree ring isotopes’ - Bruno B. L. Cintra, Emanuel Gloor, Jessica C. A. Baker, Arnoud Boom, Jochen Schöngart, Santiago Clerici, Kanhu Pattnayak, Roel J. W. Brienen is published by Communications Earth and Environment.

University of Leeds

• The University of Leeds is one of the largest higher education institutions in the UK, with more than 40,000 students from about 140 different countries. We are renowned globally for the quality of our teaching and research.
• We are a values-driven university, and we harness our expertise in research and education to help shape a better future for humanity, working through collaboration to tackle inequalities, achieve societal impact and drive change. 
• The University is a member of the Russell Group of research-intensive universities, and is a major partner in the Alan Turing, Rosalind Franklin and Royce Institutes www.leeds.ac.uk  

University of Leicester

• The University is led by discovery and innovation – an international centre for excellence renowned for research, teaching and broadening access to higher education. It is among the Top 30 universities in the Times Higher Education (THE)’s Research Excellence Framework (REF) 2021 rankings with 89% of research assessed as world-leading or internationally excellent, with wide-ranging impacts on society, health, culture, and the environment.
• In 2023, the University received an overall Gold in the Teaching Excellence Framework (TEF) 2023, making it one of a small number of institutions nationally to achieve TEF Gold alongside a top 30 REF performance. The University is home to more than 20,000 students and approximately 4,000 staff.