About 56 million years ago, Earth’s climate underwent a major climate transition. The massive release of carbon into the ocean and atmosphere increases the concentration of carbon dioxide (CO₂) in the atmosphere – meaning a temperature rise of 5 to 8°C and rising sea levels.
This event, called the Paleocene–Eocene Thermal Maximum (PETM), occurred over several tens of thousands of years, but the causes and consequences of this transition are widely debated.
Some hypotheses for the causes of large carbon releases include massive volcanic activity in the North Atlantic, sudden release of methane from the ocean floor or melting of ice sheets or peat in Antarctica.
Evidence for PETM comes mostly from ancient marine sediments, but if we are to learn from this period what may have happened as a result of our current climate change crisis, we also need to understand what happened on land.
To date, little information is available on how the PETM climate changed life on land, so our research team used globally distributed fossil pollen preserved in ancient rocks to reconstruct how terrestrial vegetation and climate changed during this period.
Our new research, led by Dr Scott Wing and I in the Department of Paleobiology at the Smithsonian’s National Museum of Natural History and published in the journal Paleoceanography and Paleoclimatology, shows that increasing concentrations of CO₂ in the atmosphere played a major role in shifting the Earth. climate and plant life.
We could see similar increases in the coming centuries as a result of anthropogenic (human-caused) increases 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 and species-specific appearance of pollen grains observed using a microscope.
The different appearances of pollen evolved to aid the pollination strategies used by plants. Since each species has its own unique pollen, that means we can compare fossil pollen with modern pollen to find a match – as long as the plant family isn’t extinct.
As a result, fossil pollen can be confidently assigned to many modern plant families. Each of these modern plants has certain climatic requirements, and we make the assumption that their ancient relatives needed the same climate.
To further confirm this assumption, we avoided data from plant groups that we know to have evolved after PETM, because these species may not have had the same climatic preferences as they have today.
Pollens preserved in rocks for tens of millions of years allow us to reconstruct ancient flower communities, and past climates.
For the first time, we have applied this approach worldwide, on fossil samples from 38 PETM sites from every continent except Antarctica. This new pollen analysis showed that the PETM plant community differed from the pre-PETM plant community at the same site.
This shift in flower composition due to massive plant migration shows that changes in vegetation due to climate change are global, although the types of plants involved vary by region.
When we say plant migration, we mean the movement of plants, because dispersed seeds grow better in one place and climate than in another – in this case at higher and colder latitudes over lower and warmer latitudes.
Plants can migrate more than 500 meters each year, so over thousands of years, they can move great distances.
For example, in the Northern Hemisphere, the bald spruce swamps of Wyoming in the US were suddenly replaced with a seasonally dry subtropical forest dominated by palm trees. Similarly, in the Southern Hemisphere, humid podocarp forests are being replaced by subtropical palm forests.
We assign each species a category based on climate, which is called the Köppen climate type. Examples include tropical rainforests, arid deserts, temperate summers and polar tundra.
This tells us that the PETM brings warmer and wetter climates towards the poles in both hemispheres, but seasonally warmer and drier climates to mid-latitudes.
To explore the extent of this geographic shift, we worked with Dr Christine Shields of the US National Center for Atmospheric Research and Dr Jeffrey Kiehl at the University of California to run climate model simulations.
The data used to create this simulation comes from the Community Earth System Model (CESM1.2 version).
These simulations fit very well with the climatic data we found in pollen, including the expansion of temperate climates at the expense of cold climate types towards the poles and expansion of temperate and tropical climates in mid-latitudes.
So, if our current CO₂ levels continue to rise, warming and melting ice sheets that could release more stored carbon into the atmosphere as was possible 56 million years ago, we will once again see this mass shift of vegetation in response to dramatic changes in climate change. local climatic conditions.
How well vegetation can migrate will depend on many factors, including the speed of climate change and the availability of suitable migratory areas for these plants.
Where the plants go, so do the animals that depend on them (if they can) – perhaps in some cases including humans.
Understanding the massive changes on our planet that are occurring as a result of climate warming gives us insight into our potential future. Are we ready to physically move from our homes, as these ancient forests did, to adapt to climate change or can we work together now to avoid the disastrous consequences of global warming?