What Caused Past Climate Change Events?

And what were the consequences? It’s time to talk fossils.

What does the fossil record tell us about climate change in the past? We know that climate change has been common in the Earth’s history. However, all these past climatic events can be linked to a geological or biological process. They didn’t happen randomly or cyclically.

Common climate change-inducing events include increased CO2 from increased vulcanism, sudden tectonic plate movements, changes in Earth’s orbit, oxidative events from photosynthetic organisms (i.e., algae), or variations in solar radiation.

(None of these are happening at the moment, by the way. The only event we see in the present is increased CO2 and other greenhouse gas concentrations. And vulcanism is not to blame this time.)

So, when did these events happen, and what were their consequences?

End-Permian Hyperthermal Event (~252 Million Years Ago)

This is a point in time before the age of the dinosaurs. The End-Permian Hyperthermal Event was characterized by global warming of 6–11°C and coincided with an increase in CO2 caused by extreme vulcanism in what we now call Siberia. Following this event, we also observe increased ocean acidification and marine anoxia (low oxygen in the water).

This climatic episode led to Earth’s most traumatic extinction event. Paleontologists have estimated that 81% of marine species (e.g., corals and fish) and 70% of terrestrial species (e.g., early mammal ancestors) went extinct. To picture what that means, think of a kid’s animal encyclopaedia and imagine what the world would look like if 75% of that book were removed.

End-Triassic Hyperthermal Event (~201 Million Years Ago)

Now, we are at a point when some dinosaurs were already populating the Earth, right before the Jurassic Period. Similarly to what happened during the End-Permian event, vulcanism was to blame.

These volcanoes increased the amount of CO2 and sulfur dioxide in the atmosphere, causing a global warming event of +6°C. Sea levels began to rise, and the oceans became more acidic.

Researchers could measure this by studying bubbles of gas trapped in ancient volcanic rocks; in a laboratory setting, they determined how much carbon dioxide each burst released. To their surprise, the amount was similar to what we are heading towards with greenhouse emissions.

And the consequences weren’t pretty. About three-quarters of all species died out. All Triassic archosaurs, apart from dinosaurs, pterosaurs, and crocodiles, went extinct. Marine wildlife took the biggest toll.

Toarcian Oceanic Anoxic Event (~183 Million Years Ago)

Many dinosaurs now roam the earth, and plesiosaurs and ichthyosaurs swim freely in the oceans. Again, massive volcanism likely triggered by a global carbon-cycle perturbation caused a major rise in sea levels and elevated seawater temperatures.

All this caused a major increase in marine primary productivity, so algae started growing out of control. As these died, they deposited on the ocean floor, increasing methane emissions and further accentuating global warming and rising sea levels. These depositions also contributed to the creation of what we call crude oil.

Crude oil is a mixture of hydrocarbons (molecules of carbon and hydrogen) formed from the decomposition of carbon-based plant and animal organisms that accumulated in ancient seabed and lakes, buried under high pressures and high temperatures for millions of years. — US Department of Energy

As the Earth got warmer, the permafrost melted, releasing even more methane and increasing temperatures up to +5°C.

These events were followed by a massive mass extinction that was unbelievably catastrophic for corals. Paleontologists have estimated that 49% of all coral genera were wiped out.

Cretaceous Oceanic Anoxic Event 1a (~120 Million Years Ago)

The Cretaceous was the golden age of the dinosaurs, with Triceratopses and T-rexes roaming the Earth. Scientific evidence suggests that the Cretaceous Oceanic Anoxic Event 1a was also triggered by intense volcanic activity that released vast amounts of carbon dioxide into the atmosphere, leading to a greenhouse effect and subsequent global warming of +4°C. The rising temperatures had cascading effects on the oceans.

One of the primary consequences of this event was widespread oceanic anoxia or oxygen depletion. The warmer temperatures reduced the ocean’s ability to hold oxygen, creating conditions where marine life struggled to survive. The lack of oxygen negatively impacted marine ecosystems, causing mass extinctions among species, including plankton, benthic organisms (those from the bottom of the ocean), and marine reptiles.

Cretaceous Oceanic Anoxic Event 2a (~94 Million Years Ago)

We are still in the age of dinosaurs. Research suggests that this event may have been instigated by a combination of factors, with volcanic activity playing a pivotal role. Large-scale volcanic eruptions release substantial amounts of carbon dioxide into the atmosphere, contributing to a greenhouse effect and elevating global temperatures by +4°C. The warmer conditions impacted ocean circulation and reduced the capacity of seawater to hold oxygen.

This event had profound consequences, primarily affecting marine ecosystems. Marine life faced considerable challenges. The lack of oxygen in large parts of the oceans also influenced nutrient cycling and the carbon cycle, impacting the oceans’ chemistry. This event had cascading effects on various organisms, from plankton to benthic fauna, and is associated with significant extinctions among marine species.

The Paleocene-Eocene Thermal Maximum (PETM) (~56 Million Years Ago)

This happened during the age of mammals. I’m especially excited about this event because, as a Paleontologist, I’ve studied many fossils from this period.

The exact cause of the PETM is not definitively established, but scientific consensus points towards the rapid release of greenhouse gases into the atmosphere. This release may have resulted from volcanic activity and methane from the ocean floor. The increased concentration of greenhouse gases, particularly carbon dioxide and methane, led to a potent greenhouse effect, causing a spike in global temperatures by +5°C.

Consequently, the Earth experienced a rapid and substantial increase in temperature, leading to widespread shifts in ecosystems. The event was associated with significant changes in ocean circulation, sea level fluctuations, and precipitation patterns.

Marine life was particularly affected, with evidence of mass extinctions among some foraminifera — crucial in healthy marine ecosystems — and other marine organisms. Terrestrial ecosystems also transformed, with shifts in vegetation and the spread of certain plant groups.

The PETM is characterized by a distinctive layer of sediment known as the “clay layer” or “boundary clay.” This layer serves as a geological indicator of this significant climatic event.

Then and now

As you can see, climate change has indeed happened in the past. However, these events were not random but caused by exceptionally high greenhouse emissions.

While it is true that we still have active volcanoes, they are nothing compared to what happened during those events, with miles upon miles of Earth’s crust releasing toxic gases into the atmosphere. The worst of them, the end-Permian vulcanism, was the most catastrophic, with a Six-fold increase in atmospheric CO2.

Geologists have estimated that 100,000 km3 of lava was released every 500 years during the End-Triassic vulcanism event. Interestingly, the volcanoes released as much CO2 during these 500 years’ eruptions as the expected emissions from all human activity in the 21st century.

If we are willing to learn from the past, we need to see that the cascading effects of increased greenhouse gases in the atmosphere could devastate our modern ecosystems. Rising temperatures are only the tip of the iceberg, with changes in ocean circulation, ocean acidification, and anoxic events on the horizon.

Some may claim that they don’t care about what happens to the ecosystems; they think it won’t affect us. However, WE are part of these ecosystems. If rainfall is affected, our crops will be affected. If species go extinct, it will also affect food production and water availability.

We can only begin to picture the consequences that a seemingly “small” change in global temperature could have on the Earth as we know it. Luckily, studying fossil records allows us to treat the past as a massive experiment. Deciphering the past will help us better understand the present and predict the future.