Madagascar is home to seven species of baobab trees, of which six are found nowhere else on the planet. Many of the trees have been alive for well over 1,000 years. The ancient trees have become symbols of Madagascar itself. They’re also gifts to climate science.
Imagine these trees as filing cabinets for climate history. Every year a tree grows, it lays down a new ring, and locked inside that ring are chemical fingerprints that reveal how much rain fell that year.
These records could tell society what it needs to know about the climate history of Madagascar. But until now, this information was hidden in the trunks of the trees.
We are a team of applied palaeoecologists and climate scientists who set out to provide the first ever tree ring rainfall record reconstructed from isotopes in baobab rings in Madagascar.
Isotopes are different forms of the same chemical element that have the same number of protons but a different number of neutrons in their nucleus. Baobab trees absorb carbon dioxide, which contains three carbon isotopes: carbon-12 (light, most common), carbon-13 (heavy, stable but less common), and carbon-14 (rare and radioactive).
The chemistry is elegant: in dry years, trees absorb more of a heavier form of carbon because they close their stomata to conserve water, which leads to higher absorption of ¹³C relative to normal conditions. In wet years, that signal drops. String enough of these signals together, and combine them with radiocarbon dating that establishes how old the trees are, and you have a natural rain gauge running through the centuries – the first of its kind ever produced for Madagascar.
Our research aimed to increase Madagascar’s palaeoclimate records. These are records of temperature, rainfall and climate that span centuries and millennia. When used along with other palaeoecological proxies such as pollen, they help to show how ecosystems function and change over hundreds of years.
Read more: Madagascar’s ancient baobab forests are being restored by communities – with a little help from AI
Most of the palaeoclimate records in Madagascar are found in mineral deposits like speleothems, stalagmites and sediments, and to date there are not many. Although scientists have previously used this method to generate a 1,000-year rainfall record from baobabs for South Africa, this hadn’t been tried in Madagascar.
Our goal wasn’t just academic. This kind of long-term evidence has direct, practical relevance for biodiversity conservation, land management, and building resilience to global warming.
From the field to the lab
We went to four baobab trees across south-western Madagascar, the driest corner of the island, and inserted a long borer into the trees to extract samples of the core. This didn’t harm the trees. Afterwards, the hole was sealed with a tree sealer to prevent damage by insects or fungi.
We sub-sampled each core along its length so that we could analyse isotopes at different points in time. Over 2,000 samples were sent to the Mammal Research Institute Laboratory at the University of Pretoria to analyse their isotopes and radiocarbon dating was conducted at iThemba LABS. From this, we were able to reconstruct a continuous rainfall record stretching back to the year 1300 – more than 700 years of climate history, written in wood.
The record tells a dramatic story. South-western Madagascar had its wettest era between 1350 and 1450. This was followed by a prolonged and brutal dry spell from 1600 to 1750. From 1750 until today, south-west Madagascar has been in a long-term trend of declining rainfall.
We didn’t stop there. We also took sediment samples and analysed charcoal deposits, pollen grains and isotopes preserved in nearby wetlands. These too are natural archives that track changes in fire and vegetation over the same period. We laid these side by side with the baobab tree samples – and something important emerged.
Humans and the climate have changed Madagascar
When set out next to the rainfall record (wet and dry periods) obtained from the baobabs, the pollen and charcoal samples we took revealed that evergreen and deciduous trees were reduced through drought, and over time replaced by grass. Human farming practices helped maintain grass-dominated landscapes through fire and vegetation clearance.
In other words, we found that changes to landscape in south-west Madagascar were not caused by humans or climate alone. Drought and human activity reshaped the land in tandem.
We also found that, remarkably, the landscape proved resilient. As rainfall declined, drought-adapted plant species moved in to replace water-hungry ones. Humans (who are believed to have settled in Madagascar approximately 2,000 years ago) also moved away from hunter gathering, and began farming cattle and rice, except for the Mikea communities who adopted seasonal practices of agriculture and hunter-gathering activities.
This shows that people actively found new ways to survive increasingly unpredictable rains by adapting their livelihood practices alongside the changing landscape.
What this means for Malagasy today
This might sound like a story about a remote island in the past but its implications are immediate and global.
By pinpointing exactly when Madagascar’s most arid region was wet, dry and everything in between over 700 years, scientists now have a powerful new baseline for evaluating what might happen with today’s climate change.
Our findings from baobab trees, combined with pollen and charcoal samples, all created a picture of the landscape cover in south-west Madagascar over centuries. The use of these long-term data (palaeoclimate, vegetation and charcoal records) across the island contributed to existing research that debunks the colonial narrative that Madagascar was fully forested before humans settled and destroyed the forests. Instead, our findings also show the presence of more open vegetation and that people and the landscape adapted to the changing climate together.
What needs to happen next
Our previous research has provided scientists and governments with the information they need to understand how ecosystems might respond to global warming and how people may adapt their livelihood strategies in times of drought.
This new research shows that there’s a deep connection between human societies and the natural world. Building resilience to climate change now means understanding how that relationship has played out over centuries, not just decades.
This will help integrated conservation and livelihood strategies to become climate-informed, and for communities to get the support they need to continue to adapt and find new and different ways to survive in a warmer and drier climate.
Read more: Baobab trees trace the African diaspora across the Indian Ocean
Our study would be combined with others from across southern Africa, connecting Madagascar’s record to datasets from Botswana, South Africa and beyond. This will help scientists piece together the full regional climate picture.
This research must feed into policy. Long-term ecological data of this kind has direct relevance to global goals around climate action, biodiversity protection, poverty reduction and international scientific collaboration. The past has a great deal to teach us – if we take the time to decode and read it.
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