About 56 million years ago, Earth experienced a dramatic climate shift. A massive release of carbon into the oceans and atmosphere raised global temperatures by roughly 5 to 8 degrees Celsius and pushed sea levels higher. This event, known as the Paleocene-Eocene Thermal Maximum, unfolded over tens of thousands of years, and its causes and consequences remain topics of active scientific discussion today.
Leading explanations point to several concurrent forces: intense volcanic activity in the North Atlantic, thawing of Antarctic permafrost, and a sudden release of methane from the ocean floor. It is now understood that these processes together triggered a rapid surge in carbon and a notable climate response preserved in the geological record.
The strongest evidence for this ancient upheaval comes from marine sediments, but to gain a fuller picture of PETM dynamics researchers also study land-based changes. Reconstructing terrestrial ecosystems is essential to understand how climate shifts affected plant communities and the broader biosphere during this interval.
To trace how vegetation and climate shifted on land, researchers examined fossil pollen preserved in rocks worldwide from that era.
Research indicates that higher atmospheric CO2 concentrations played a pivotal role in shaping Earth’s climate and biology. The study highlights the connection between carbon levels and ecological change, including plant communities. The findings also raise concerns about a future where rising CO2 could drive similar climatic and ecological responses if human activities continue at the current pace.
Fossil pollen, preserved in rocks for millions of years, enables reconstructions of ancient flowering patterns and past climates. By sampling fossil material from PETM regions across most continents, scientists show that plant communities during this period differed from those that existed before it in the same regions. This pollen-based approach demonstrates how plant assemblages migrated and shifted in response to climate forcing. Changes in flower composition driven by large-scale plant migrations illustrate that climate change reshaped global vegetation, with the specific plant species involved varying by region.
Plants move, and with them animals
When plant migration is discussed, it refers to seeds dispersed by wind, water, or animals establishing new populations where conditions are more favorable. In the PETM, plants tended to flourish at higher, cooler latitudes than in warmer zones. Species could migrate more than 500 meters each year, enabling long journeys over thousands of years.
In the northern hemisphere, some swamp ecosystems in areas that are now part of Wyoming gave way to subtropical dry forests with different leaf forms. In the southern hemisphere, humid temperate forests were replaced by subtropical palm-dominated communities. These shifts show that PETM produced warmer, wetter conditions near the poles and seasonal drying at mid-latitudes across both hemispheres.
Climate model simulations complemented the pollen work by mapping the geographic reach of these changes. The models reproduced the expansion of temperate climates toward higher latitudes, the retreat of very cold climates, and the spread of temperate and tropical conditions into mid-latitudes. These results reinforce the link between CO2-driven warming and broad ecological reorganization during PETM.
The researchers emphasize that if current CO2 levels keep rising, warming and thawing permafrost could release additional carbon, potentially triggering new vegetation shifts and altering regional ecosystems again. The capacity of vegetation to migrate will depend on several factors, including the pace of climate change and the availability of suitable corridors for movement. Wherever plants go, the animals that depend on them will follow, and in many cases this includes humans. Such shifts have implications for agriculture and the ability to grow crops in certain regions as warming continues.
Studying PETM as a case study of a warmer climate helps illuminate possible futures. The big question is whether societies are ready to relocate or adapt in response to climate-driven ecosystem changes or if proactive action is needed now to mitigate negative outcomes.
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