About 56 million years ago, Earth underwent 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 subjects of ongoing scientific debate.
The most plausible drivers include large volcanic activity in the North Atlantic, thawing of Antarctic permafrost, and the sudden release of methane from the ocean floor. These processes likely worked in concert to drive the rapid carbon surge and climate response observed in the geological record.
The principal evidence for this ancient climate upheaval comes from marine sediments, but to fully understand the period and prepare for what continued climate change might bring, researchers also study terrestrial changes. Reconstructing land-based ecosystems is essential to paint a complete picture of PETM dynamics.
To trace how vegetation and climate shifted on land, a research team examined fossil pollen preserved in rocks worldwide from that era.
V. Korasidis
Research led by Vera Korasidis of the University of Melbourne and Scott Wing of the Smithsonian Institution’s National Museum of Natural History, and published in Science Paleoceanography and Paleoclimatology, indicates that higher atmospheric CO2 concentrations played a pivotal role in altering Earth’s climate and biology. The study highlights the connection between carbon levels and ecological change, including plant communities.
The authors warn that a similar rise in CO2 could occur in the coming centuries due to human activities, potentially triggering comparable climate and ecological responses.
Fossil pollen, preserved in rocks for millions of years, enables reconstructions of ancient flowering patterns and past climates. By sampling fossil material from 38 PETM regions across most continents except Antarctica, scientists show that plant communities during the PETM 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 was reshaping global vegetation, with the specific plant species involved varying by region.
Plants move, and with them animals
When plant migration is discussed, it means seeds dispersed by wind, water, or animals establishing new populations where conditions are more favorable. In the PETM, plants tended to prosper at higher, cooler latitudes than in warmer zones.
Plants can migrate more than 500 meters a year, enabling long journeys over thousands of years.
In the northern hemisphere, for instance, certain swamp ecosystems in what is now 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.
V. Korasidis
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 similar to those seen 56 million years ago 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. Scientists note that such shifts have implications for agriculture and the ability to grow crops in certain regions as warming continues.
Understanding PETM as a case study of a warmer climate helps illuminate possible futures. Korasidis asks 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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Reference work: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021PA004325
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