A new research study into Earth’s dramatic climatic transition about 56 million years ago has provided evidence on its consequent impact on terrestrial vegetation and climate, as well as insights into Earth’s future as a result of the ongoing climate change. The study demonstrated that an increase in the concentration of atmospheric CO2 played a major role in shifting Earth's climate and plant life.

The research was led by Vera Korasidis of the University of Melbourne and Dr Scott Wing at the Department of Paleobiology at the Smithsonian's National Museum of Natural History, and published in the Paleoceanography and Paleoclimatology journal.

According to Korasidis, this event – called the Paleocene–Eocene Thermal Maximum (PETM) – witnessed a huge release of carbon into the ocean and atmosphere, which raised atmospheric carbon dioxide (CO2) concentrations and led to temperatures going up by 5 to 8°C and rising sea levels.

While PETM happened over the course of a few tens of thousands of years, the causes and consequences of this transition remain as hypothesis with evidence based on ancient marine sediments. With little information on how the PETM climate changed life on land, the research team used globally distributed fossil pollen preserved in ancient rocks to reconstruct how terrestrial vegetation and climate changed across this period, allowing them to reconstruct both ancient floral communities, and past climates.

“To understand how terrestrial vegetation changed and moved during this period, we used a recently developed approach based on fossil pollen preserved in ancient rock deposits. It uses the distinct, species-specific appearance of pollen grains observed using a microscope,” Korasidis explained.

“The distinct appearance of pollen evolved to assist with pollination strategies employed by plants. Because each species has unique pollen, it means we can compare fossil pollen with modern pollen to find a match – as long as the plant family hasn't gone extinct.

“As a result, fossil pollen can be confidently assigned to numerous modern plant families. Each of these modern plants has specific climatic requirements, and we make the assumption that their ancient relatives needed a similar climate.”

The research team collected fossil samples from 38 PETM sites from every continent except Antarctica. Analysis of the pollen samples showed that the PETM plant communities were distinct from pre-PETM plant communities at the same sites.

These shifts in floral composition caused by massive plant migrations indicated that changes in vegetation as a result of climate change were global. Plants can migrate over 500 metres each year, which means they can move huge distances over thousands of years.

The study revealed that in the Northern Hemisphere, the bald-cypress swamps of Wyoming in the US were suddenly replaced with palm-dominated seasonally dry subtropical forests. Likewise, in the Southern Hemisphere, wet-temperate podocarp forests were replaced by forests of subtropical palms.

This meant that the PETM brought warmer and wetter climates towards the poles in both hemispheres, but warmer and more seasonally dry climates to the mid-latitudes.

The research team worked with Dr Christine Shields from the US National Centre for Atmospheric Research and Dr Jeffrey Kiehl at the University of California to run climate model simulations and find the geographic extent of these shifts. These simulations closely matched the climate data the researchers had found in their pollen study.

Applying the conclusions to the current climate crisis, Korasidis said the rising CO2 levels today could lead to the release of more stored carbon into the atmosphere, resulting in mass shifts in vegetation.

“Understanding this massive shift on our planet that came as a result of a warming climate gives us an insight into our potential future. Are we prepared to physically move from our homes, like these ancient forests did, to adapt to climate change or can we work together now to avoid the adverse consequences of a warming world?” Korasidis concluded.

Image: The research uses the distinct, species-specific appearance of pollen grains observed using a microscope. Credit: University of Melbourne