What Ancient Pollen Tells Us About Future Climate Change

Changes in plant ranges from pre-PETM to PETM mapped to simplified Köppen climate types. Credit: University of Melbourne

About 56 million years ago, the Earth’s climate underwent a major climate transition. A huge release of carbon into the ocean and atmosphere lifted atmospheric carbon dioxide (CO2) concentrations, which means an increase in temperatures of 5 to 8°C and a rise in sea levels.

Sound familiar?

This event, called Paleocene-Eocene Thermal Maximum (PETM), took place over a few tens of thousands of years, but the causes and consequences of this transition are still widely debated.

Some of the hypothetical causes for the huge carbon release include massive volcanic activity in the North Atlantic, sudden release of methane from the ocean floor, or melting of permafrost or peat in Antarctica.

The evidence for PETM comes mostly from ancient marine sediments, but if we are to learn from this period what might happen as a result of our climate crisis of change, we also need to understand what happened on the ground.

To date, little information is available on how the PETM climate has changed life on earth, so our research team used fossil pollen preserved in ancient rocks to reconstruct the evolution of land vegetation and climate during this period.

Our new research, led by myself and Dr. Scott Wing in the Department of Paleobiology at the Smithsonian’s National Museum of Natural History and published in the journal Paleooceanography and Paleoclimatology, demonstrates that an increase in the concentration of atmospheric CO2 played a major role in changing Earth’s climate and plant life.

We could see a similar increase in the coming centuries due to anthropogenic (human-caused) increase in CO2.

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 seen under a microscope.

The distinct appearance of pollen has evolved to facilitate the pollination strategies employed by plants. Because each species has unique pollen, this means we can compare fossil pollen with modern pollen to find a match, as long as the plant family has not disappeared.

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

To give more confidence in this hypothesis, we avoided data from groups of plants that we knew evolved after the PETM, as these species may not have settled into the same climate preference that they did. today.

The pollen preserved in the rocks for tens of millions of years makes it possible to reconstruct both ancient floral communities and past climates.

What Ancient Pollen Tells Us About Future Climate Change

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

For the first time, we have applied this approach worldwide, to fossil samples from 38 PETM sites from all continents except Antarctica. This new pollen analysis shows that PETM plant communities are distinct from pre-PETM plant communities at the same sites.

These changes in floral composition, due to mass plant migrations, indicate that vegetation changes resulting from climate change were global, although the types of plants involved varied by region.

When we talk about plant migration, we mean the movement of plants, as seeds that spread grow better in one place and climate than in another, in this case at higher and colder latitudes compared to other places. lower and warmer latitudes.

Plants can migrate over 500 meters each year, so over thousands of years they can move great distances.

For example, in the northern hemisphere, the bald cypress swamps of Wyoming in the United States were suddenly replaced by seasonally dry subtropical palm-dominated forests. Similarly, in the southern hemisphere, humid temperate podocarp forests have been replaced by subtropical palm forests.

We assigned each species a climate-based category, called Köppen climate type. Examples of this include tropical rainforest, arid desert, hot temperate summer, and polar tundra.

This tells us that the PETM brought warmer and wetter climates towards the poles in both hemispheres, but seasonally warmer and drier climates at mid-latitudes.

To explore the geographic extent of these changes, we worked with Dr. Christine Shields of the US National Center for Atmospheric Research and Dr. Jeffrey Kiehl of the University of California to run climate model simulations.

The data used to create these simulations comes from the Community Earth System Model (version CESM1.2).

These simulations closely matched the climate data we found in pollenincluding the expansion of temperate climates away from cold climate types towards the poles as well as the expansion of temperate and tropical climates at mid-latitudes.

So if our current CO2 levels continue to rise warming and melting the permafrost which could release more stored carbon into the atmosphere as it may have done 56 million years ago we will see these massive vegetation changes again in response to dramatic changes in local climatic conditions.

The ability of vegetation to migrate will depend on many factors, including the rate of climate change and the availability of suitable migration areas for these plants.

Where the plants go, the animals that depend on them (if they can) will go too – perhaps in some cases humans included.

Understanding this massive change to our planet that has occurred as a result of global warming gives us insight into our potential future. Are we ready to physically leave our homes, as these ancient forests did, to adapt to climate change or can we work together now to avoid the adverse consequences of a warming world?

Changes in vegetation have shaped global temperatures over the past 10,000 years

More information:
Vera A. Korasidis et al, Global changes in land vegetation and continental climate during the Paleocene-Eocene Thermal Maximum, Paleooceanography and paleoclimatology (2022). DOI: 10.1029/2021PA004325

Quote: What Ancient Pollen Tells Us About Future Climate Change (May 6, 2022) Retrieved May 6, 2022 from https://phys.org/news/2022-05-ancient-pollen-future-climate.html

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