Approximately 56 million years ago, Earth underwent a period of rapid global warming. Scientists have named this event the Paleocene-Eocene Thermal Maximum, or PETM for short. Long before humans even existed, Earth’s climate warmed by over 4 °C (and perhaps by as much as 8 °C). From a geological perspective, the PETM occurred out of the blue (in just 20,000 years) and lasted for just the blink of an eye (200,000 years).
For many scientists, the PETM is one of the closest analogues to modern day climate change, making it an invaluable case study for understanding the impacts that much higher carbon dioxide levels can have on the planet.
An important piece of evidence indicating that Earth was once much warmer than it is today comes from the fossil record. All over the world, the fossilized remains of tropical plants can be found well outside the hot, humid equatorial areas where these kinds of plants are found today. In Canada, for example, remains of towering redwoods and lush ferns have been found near the Arctic Circle. The planet as a whole must have been much, much warmer than it is now for tropical plants to thrive so close to the north pole.
But there is also good evidence showing that not all life on Earth was flourishing during the PETM. In analyzing deep-sea ocean cores, scientists found that upwards of half of Earth’s marine species disappeared during this time.
So what caused the PETM, and what made it so bad for life on Earth?
As it turns out, scientists know what caused the PETM, but they don’t agree on exactly why it happened in the first place. What we know is that suddenly, around 56 million years ago, greenhouse gas concentrations in the atmosphere began to rise.
The data suggests that greenhouse gases were being pumped into the atmosphere at a rate of about 1 billion metric tons per year (a metric ton = 1000 kilograms, or 2200 pounds). By analyzing the atomic structure inside greenhouse gas molecules trapped in deep-sea cores, scientists determined that these greenhouse gases came from organic sources, meaning the remains of plants.
These emissions continued for thousands of years. And as greenhouse gas concentrations rose in the atmosphere, so too did the temperature of the planet.
Sound familiar? This is pretty close to the same thing we see happening today, only today we know why greenhouse gas emissions are on the rise: human activities. Only we are emitting greenhouse gases into the atmosphere at a rate of approximately 10 billion metric tons per year, which is about 10 times faster than emission rates during the PETM.
But why was the PETM so devastating for life on Earth? After all, warmer temperatures sound like a good thing.
Ocean life is extremely sensitive to small changes in temperature and water chemistry. When the tropical oceans warm by just 1 °C, coral reefs begin to die in a process called coral bleaching. Reefs are a foundation for ocean-going life, and their demise has catastrophic and lasting implications. At the same time, warmer water contains less dissolved oxygen, which is needed by fish and other species to breathe. Finally, an increase in atmospheric carbon dioxide concentrations slowly turns the oceans more and more acidic (carbon dioxide + water = carbonic acid), which causes further harm.
Presently, our planet is on target to warm by over 2 °C by the end of this century. The same magnitude of warming took place over many thousands of years during the PETM, which gave life a fighting chance to adapt to the changing conditions. Today’s much faster warming poses a much greater challenge to Earth’s animals, plants and ecosystems.
Today the PETM is a reminder: climate change is dangerous for life and the planet. If over half of the ocean’s life disappeared under high temperatures before, then it will likely happen again if we don’t soon reduce greenhouse gas emissions. With this knowledge comes great responsibility: we need to press our leaders for rapid and large reductions in greenhouse gas emissions and more global cooperation on green energy projects.
Let’s not let climate history repeat itself.
Excuse the interruption…
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Ryan Smith, Prairie Climate Centre