A collapse of tropical forests around 252 million years ago may have kept Earth in an extremely hot climate for approximately five million years, according to a new study published in the journal Nature Communications.
Researchers from the University of Leeds and the China University of Geosciences in Wuhan reconstructed ancient vegetation patterns using fossil records and incorporated the findings into a climate model. Their results suggest that the widespread loss of forests during the Permian-Triassic mass extinction weakened natural processes that remove carbon dioxide from the atmosphere, prolonging global warming.
The extinction occurred near the end of the Permian period, when Earth’s continents were joined into the supercontinent Pangea. Massive volcanic eruptions in what is now Siberia released large quantities of carbon dioxide, triggering a major climate crisis. However, scientists have long debated why extreme temperatures persisted for millions of years after the volcanic emissions began.
Under normal conditions, natural processes such as rock weathering and carbon burial help remove carbon dioxide from the atmosphere. The researchers argue that the collapse of tropical vegetation disrupted these processes, preventing the climate from cooling as quickly as expected.
Fossil evidence reveals widespread forest loss
The research team compiled fossilised plant remains and pollen records from locations across the world, including the Kuznetsk Basin in Siberia, the Karoo Basin in South Africa, Australia’s Sydney Basin and the Prince Charles Mountains in Antarctica.
The findings indicate that plant species losses reached 86% at low and middle latitudes, compared with 66% closer to the poles. Tall lowland forests, whose canopies once reached around 50 metres, were replaced by much shorter vegetation, with plants ranging from approximately five centimetres to two metres in height.
Large-leaved rainforest plants known as gigantopterids disappeared, while smaller lycopods became widespread across tropical regions. Fossil evidence also points to a prolonged decline in land animal populations during the earliest Triassic.
The researchers used modern plant communities with similar structures and climatic conditions as substitutes for extinct vegetation to estimate how much carbon ancient forests absorbed annually.
Their estimates suggest that global plant productivity fell from around 54 billion to 63 billion metric tonnes of carbon per year in the late Permian to approximately 13 billion to 20 billion tonnes in the earliest Triassic a decline of about 70%.
Loss of forests weakened natural carbon removal
The study identifies two major ways forests help regulate Earth’s climate.
First, vegetation can accumulate in wetlands and swamps, where it is buried over time. This process stores carbon in sediments and can eventually contribute to coal formation, keeping carbon out of the atmosphere for long periods.
Second, plant roots and soil organisms help break down rocks and release acids that accelerate the weathering of silicate minerals. This process contributes to the long-term removal of atmospheric carbon dioxide, which can ultimately be stored as carbonate in ocean sediments.
When forests disappeared, both carbon storage and weathering slowed. The researchers also identified a prolonged gap in coal formation following the Permian Triassic boundary, indicating that peat accumulation had largely ceased across the planet for millions of years.
Climate modelling reveals prolonged extreme temperatures
To investigate the effects of vegetation loss, the team used the SCION Earth system model, which combines climate simulations with a global carbon cycle model.
In a control simulation where vegetation remained unchanged, the model showed that temperatures would have begun falling within a few hundred thousand years after the initial volcanic carbon dioxide spike.
However, when the researchers incorporated fossil-based estimates of vegetation collapse, the simulated climate remained extremely hot for around five million years.
The model produced atmospheric carbon dioxide concentrations of approximately 7,000 parts per million and equatorial surface temperatures of 33°C to 34°C during this period.
The authors noted that the model has relatively low climate sensitivity, meaning the actual carbon dioxide concentrations and temperature changes during the ancient event may have differed from the simulated values.
The modelling also suggested that weakened rock weathering played a greater role in sustaining the prolonged warming than reduced carbon burial on land. Changes in the carbon cycle partly offset the effects of reduced vegetation burial, whereas the decline in weathering lacked a comparable counterbalancing process.
The researchers said the findings highlight the importance of tropical forests in regulating the global carbon cycle. Although the ancient event unfolded under conditions very different from those of today, it offers insight into the potential consequences of a large scale collapse of forest ecosystems.
Professor Benjamin Mills of the University of Leeds a co-author of the study warned that the ancient record demonstrates how the loss of tropical forests can have long-lasting effects on the climate system.
Modern rainforests absorb carbon and support processes that influence the movement of carbon between land, oceans and the atmosphere. Extensive forest degradation could weaken these functions, potentially making it more difficult for the planet to recover from elevated carbon dioxide levels.
However, the researchers cautioned that the ancient findings cannot be transferred directly to the present. The study used modern plant communities as substitutes for extinct species, and the physiology of ancient vegetation remains uncertain. The model also excluded some processes that could have influenced carbon dioxide levels and temperatures during the Early Triassic.
The study’s authors said further research is needed to test the approach across other geological periods and improve understanding of how vegetation changes influence long-term climate patterns.
The research, titled “Early Triassic super-greenhouse climate driven by vegetation collapse,” provides evidence that the loss of forests can affect the climate far beyond the immediate period of ecological destruction. It also highlights the potential importance of protecting present day tropical forests, whose role in carbon storage and climate regulation extends well beyond the ecosystems themselves.