It’s been called the largest natural climate event of the Cenozoic Era, it set off a series of environmental changes that paved the way for our modern world, and it’s a critical case study in what happens when a huge rise in atmospheric Carbon triggers rapid warming of the global climate. This episode, we discuss the Paleocene-Eocene Thermal Maximum.
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Dawn of the Age of Mammals
The Paleocene Epoch spans the first 10 million years of the Cenozoic Era (66-56 million years ago). Like the Cretaceous Period before it, it was a warm world where forests grew across the unfrozen poles. Unlike the Cretaceous, this was a world almost entirely devoid of dinosaurs. The Paleocene was home to an early diversity of modern birds (including early penguins, the enormous Gastornis, and more) as well as some famously large reptiles like Titanoboa and Carbonemys. But the most celebrated denizens of the Paleocene are mammals. During this time, we see the earliest ecosystems dominated by mammals, including early cousins of our modern hoofed mammals, mammalian carnivores, primates, and more.
The Paleocene Epoch is book-ended by two of the most dramatic events in Earth history: it starts with the end-Cretaceous mass extinction that saw the end of so many of our favorite ancient reptiles, and it ends with the Paleocene-Eocene Thermal Maximum (PETM), the largest natural climate change event of the Cenozoic.
The PETM is a hyperthermal, a brief period of globally warming climate. It isn’t the only hyperthermal in Earth history, but it stands out for its intensity: at ~56 million years ago, global temperatures rose an estimated 5-8°C over a geologically short period of perhaps 10,000-20,000 years or less, then returned to normal in under 200,000 years. This event marks the boundary between the Paleocene and Eocene Epochs, and it is one of most studied ancient events by scientists hoping to better understand modern human-caused climate change.
Even before this climate event was recognized, paleontologists could tell from the fossil record that this was a time of change. Notably, the Paleocene-Eocene boundary features a mass extinction of benthic foraminifera (seafloor-dwelling microorganisms) and a major change in mammal diversity that includes some of the earliest appearances of modern mammal groups like Perissodactyls and Primates.
In the early 1990s, geologists and paleontologists began thoroughly investigating the signs of PETM climate change. The dramatic rise in temperature left its mark in the form of evidence such as changing oxygen isotopes ratios, leaf shapes, and microbe biomarkers, all of which change in response to rising temperatures. From these data, scientists have estimated sea surface temperatures as high as 35°C in some places, and some polar areas might have been warmer than today’s tropics.
Early evidence of the cause of this warming came from Carbon isotopes. Scientists observed changing ratios of Carbon-12 and Carbon-13 in ancient sediments and fossils, pointing to a dramatic rise in Carbon in the atmosphere in the form of greenhouse gases like carbon dioxide and methane, which trap heat and warm the Earth. It’s estimated that during the PETM, the rise of atmospheric Carbon was on the order of thousands of gigatons (not dissimilar to estimates of human-induced Carbon rise today).
Most studies of the PETM revolve around some major questions. For example: where did that Carbon come from? Top candidates include volcanic activity (at this time, rifting of northern continents created the North Atlantic Igneous Province, which would have produced lots of Carbon-rich gases) and the release of Carbon from methane-rich ocean sediments or organic-rich permafrost (which could be triggered by tectonic shifts, changing ocean circulations, or by the onset of rising temperatures). Another major question is the timing of the event: plenty of research is devoted to estimating exactly how long it took for temperatures and Carbon levels to rise, and exactly how long it took to recover.
Impacts of Climate Change
As you might imagine, rapidly rising global temperatures and Carbon levels had some major impacts on the world. In the ocean, seafloor organisms suffered major losses (especially benthic foraminifera), some shelled organisms and corals declined due to ocean acidification, and many warm-water species spread to higher latitudes. On land, weather patterns seem to have shifted, with some regions growing wetter and others dryer, leading various animals and plants to move into or out of changing habitats.
At this time, many plants and animals also moved between North America and Eurasia, probably because the warming climate allowed many species to access the northern connections between these continents. This shuffle of species is also accompanied by radiations in mammals, giving rise to the kinds of mammals that would come to dominate the globe for the rest of the Cenozoic.
One peculiar effect of the PETM is that mammals shrank! A 2012 study found that one group of early horses averaged around 5.5kg (12 lbs) before the PETM but only 4kg (9lbs) during it, and a similar trend is seen in many other groups. This shrinking might have been a result of warming temperatures (small mammals shed heat more easily) or low food availability (plants might have struggled to produce certain proteins in a high-CO2 atmosphere).
In many ways, the PETM might have set the stage for the world as we know it, pushing Early Cenozoic ecosystems to transition to more familiar forms. But the PETM is also extremely relevant to our modern world because it helps us understand climate change. Right now, we are seeing rapidly rising temperatures due to a major influx of Carbon into the atmosphere, this time thanks to human activity. The PETM isn’t a perfect comparison to our current situation, but it can help us answer some critical questions: How do ecosystems respond to rising temperatures? How does climate change affect weather patterns? Which natural processes will counteract climatic change, and which ones will exacerbate it? And from there, we can try to understand how all this change will impact us, our society, and our culture.
When global warming made our world super-hot. A non-technical overview of the PETM, with lots of links to more info.
The Paleocene-Eocene Thermal Maximum. A technical review of the PETM from 2011.
Life and death at the Chicxulub impact crater: a record of the Paleocene-Eocene Thermal Maximum: A technical study from 2020.
Temperature changes across the Paleocene-Eocene Thermal Maximum. This technical study from 2020 is an example of how researchers study temperature change and includes discussion of possible causes of the PETM.
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