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Around 56 million years ago, Earth’s climate underwent a major climatic transition. A huge release of carbon into the ocean and atmosphere raised atmospheric carbon dioxide (CO₂) concentrations – which meant temperatures going up by 5 to 8°C and rising sea levels.
Do you sound familiar?
This event is called the Paleocene–Eocene Thermal Maximum(PETM) occurred over a few tens or thousands of years. However, the causes and effects of this transition remain a matter of debate.
Some possible causes of the large carbon release include volcanic activity in North Atlantic, sudden release of methane at the ocean floor, melting of permafrost in Antarctica, and massive volcanic activity.
The majority of evidence for the PETM comes from ancient marine sediments. But, if this period is to be used as a learning tool, we will need to see what might happen. Climate change crisis currentlyIt is important to understand what happened on the ground.
Antarctic ice covers fossil forests
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There is not much information available about how the PETM climate affected life on land. Our research team used fossil pollen from around the globe that was preserved in ancient rocks to reconstruct how climate and terrestrial vegetation changed over this period.
Our New researchLed by me and Dr Scott Wingat the Department of Paleobiology Smithsonian’s National Museum of Natural HistoryPublished in the journal Paleoceanography & Paleoclimatology, demonstrates that an increase in the concentration of atmospheric CO₂ played a major role in shifting Earth’s climate and plant life.
We could see a similar increase in the coming centuries as a result of anthropogenic (that is caused by humans) increases in CO₂.
A recently developed approach was used to understand how terrestrial plants changed and moved over the period. It is based on fossilized pollen that has been preserved in ancient rock materials. It uses the distinct appearance of pollen grains, which can be seen under a microscope to determine their species.
The distinct appearance and function of pollen evolved in order to help plants with their pollination strategies. 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.
Fossil pollen can therefore be confidently assigned for many modern plant families. Each modern plant has its own climate requirements. We assume their ancient relatives also had similar climates.
To increase confidence in this assumption, we omitted data from plant group members that we knew had evolved after PETM. These species might not have settled into a similar climate preference as they do today.
When the Earth’s magnetic fields flipped
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We can reconstruct past climates and ancient floral communities by using pollen that has been preserved in rocks for tens or millions of years.
This is the first time that we have used this approach globally to fossil samples taken from 38 PETM sites across all continents, with the exception of Antarctica. This new pollen analysis reveals that PETM plant communities are different from pre-PETM plant communities at these same sites.
These changes in floral composition are a result massive plant migrations. However, the types of plants involved vary by region.
When we say plant migration we mean plant movement, as the seeds that are spread grow better in one place and climate than in another – in this case at higher, cooler latitudes over lower, warmer ones.
Plants can move over 500 m each year, so over thousands, they can travel huge distances.
In the Northern Hemisphere for example, the bald-cypress swamps in Wyoming, USA were replaced by palm-dominated, dry subtropical forests. Similar happened in the Southern Hemisphere where wet-temperate podocarp trees were replaced with subtropical palm forests.
Each species was assigned a category based upon its climate. Köppen climate type. Examples include tropical rainforest, temperate summer, polar tundra, and arid desert.
A new thermometer for studying the past climate
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This shows that the PETM brought warmer and drier climates towards the poles of both hemispheres but a warmer and dryer climate to the mid-latitudes.
We worked with Dr Christine ShieldsFrom the US National Centre for Atmospheric ResearchAnd Dr Jeffrey KiehlThe University of California to run climate model simulations.
These simulations were created using data derived from Community Earth System Model (version CESM1.2)
These simulations closely matched climate data we found in pollen. They included the expansion and contraction of temperate climates towards the poles at the expense or tropical climates in the mid-latitudes.
So, if our current CO₂ levels continue to rise, warming and melting permafrost which could release more stored carbon to the atmosphere as it may have done 56 million years ago, we will once again see these mass shifts in vegetation in response to dramatic changes in local climate conditions.
How well the vegetation can migrate depends on many factors, including how fast climate change is happening and whether there are suitable areas for them to move.
Where the plants go, so too will the animals that rely on them (if they can) – perhaps in some cases humans included.
Understanding the massive shift in our planet’s climate has given us a glimpse into our future. Is it possible to work together to prevent the negative consequences of climate change?
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