What can the fossil record reveal about historical climate change? Climate change has been a recurrent theme throughout Earth’s history. Yet, these historical climatic shifts were consistently associated with geological or biological processes rather than occurring arbitrarily or in a cyclical pattern.
Typical triggers of climate change include:
- Elevated CO2 levels due to increased volcanic activity.
- Abrupt shifts in tectonic plates.
- Alterations in Earth’s orbital patterns.
- Oxidative phenomena from photosynthetic life forms (such as algae).
- Fluctuations in solar radiation.
(Currently, none of these factors are at play. The only phenomenon observed today is the rise in CO2 and other greenhouse gases; this time, volcanic activity is not the culprit.)
So, when did these phenomena occur, and what were their impacts?
End-Permian Hyperthermal Event (~252 Million Years Ago)
This period predates the dinosaur era. The End-Permian Hyperthermal Event, marked by a global temperature increase of 6–11°C, was concurrent with a surge in CO2 levels due to significant volcanic activity in what is today known as Siberia. This event led to heightened ocean acidification and marine anoxia (depleted oxygen levels in water).
This climate event precipitated the most devastating extinction episode in Earth’s history. Paleontologists estimate that 81% of marine species (including corals and fish) and 70% of terrestrial species (such as precursors to mammals) were eradicated. Imagine erasing 75% of the species from a child’s animal encyclopedia to visualize this.
End-Triassic Hyperthermal Event (~201 Million Years Ago)
At this juncture, dinosaurs already inhabited Earth just before the Jurassic Period commenced. Similar to the End-Permian event, volcanic eruptions were responsible.
These eruptions increased atmospheric CO2 and sulfur dioxide levels, leading to a global temperature rise of +6°C. This resulted in rising sea levels and more acidic oceans.
This was confirmed through the study of gas bubbles trapped in ancient volcanic rocks, where, in a laboratory environment, researchers determined the volume of carbon dioxide released by each bubble. Astonishingly, the quantities were comparable to current greenhouse gas emissions trajectories.
The aftermath was dire, with about three-quarters of all species dying out. All Triassic archosaurs, except dinosaurs, pterosaurs, and crocodiles, became extinct. Marine life suffered the most severe losses.
Toarcian Oceanic Anoxic Event (~183 Million Years Ago)
By this time, numerous dinosaurs roamed the Earth, while plesiosaurs and ichthyosaurs thrived in the oceans. Once again, a significant increase in volcanic activity, likely spurred by disruptions in the global carbon cycle, led to rising sea levels and higher seawater temperatures.
This prompted an explosion in marine primary productivity, causing algae to proliferate uncontrollably. Their subsequent death and deposition on the ocean floor increased methane emissions, exacerbating global warming and rising sea levels. These deposits also contributed to the formation of what we now refer to as crude oil.
Crude oil consists of hydrocarbons formed from the decomposition of carbon-based plant and animal remains accumulated in ancient seabeds and lakes and then buried under intense pressures and temperatures for millions of years. — US Department of Energy
As temperatures soared, permafrost melted, releasing additional methane and contributing to a temperature increase of up to +5°C.
These events culminated in a massive extinction event that was particularly disastrous for corals, with an estimated 49% of all coral genera being annihilated.
Cretaceous Oceanic Anoxic Event 1a (~120 Million Years Ago)
During the Cretaceous, the zenith of dinosaur domination, Triceratops, and Tyrannosaurus rex traversed the planet. Research indicates that the Cretaceous Oceanic Anoxic Event 1a was precipitated by severe volcanic eruptions, discharging immense quantities of carbon dioxide into the atmosphere. This resulted in a greenhouse effect, leading to a global temperature increase of +4°C. Such rising temperatures had far-reaching impacts on marine environments.
A principal outcome of this episode was widespread oceanic anoxia or a significant reduction in oxygen levels. Elevated temperatures diminished the ocean’s capacity to retain oxygen, fostering conditions detrimental to marine survival. This oxygen scarcity adversely affected marine ecosystems, triggering widespread extinctions among species, including plankton, benthic creatures (those residing on the ocean floor), and marine reptiles.
Cretaceous Oceanic Anoxic Event 2a (~94 Million Years Ago)
Still, within the dinosaur epoch, it is posited that this event was prompted by a combination of elements, with volcanic activity again playing a critical role. Massive volcanic eruptions expelled considerable amounts of carbon dioxide, enhancing the greenhouse effect and raising global temperatures by +4°C. These warmer conditions altered oceanic currents and decreased the seawater’s oxygen-carrying capacity.
This occurrence had deep-seated effects, primarily on marine ecosystems. Marine species faced significant challenges due to oxygen scarcity in vast oceanic regions, affecting nutrient dynamics and the carbon cycle, thereby altering the chemical composition of the oceans. This event precipitated extensive effects on various organisms, from plankton to benthic fauna, leading to notable extinctions among marine life.
The Paleocene-Eocene Thermal Maximum (PETM) (~56 Million Years Ago)
In the mammalian era, the PETM holds a particular intrigue for me as a Paleontologist, having examined numerous fossils from this timeframe.
The precise origin of the PETM remains unclear, but the consensus among scientists suggests a swift influx of greenhouse gases into the atmosphere as the likely cause. This influx, possibly from volcanic activity and methane release from seabed deposits, significantly increased greenhouse gas levels, notably carbon dioxide and methane, instigating a marked greenhouse effect and a subsequent +5°C surge in global temperatures.
This rapid and significant climatic shift led to extensive ecological alterations, characterized by significant changes in ocean circulation, sea level variations, and precipitation patterns.
Marine ecosystems were particularly hit hard, with mass extinctions observed among some foraminifera—critical players in healthy marine ecosystems—and other aquatic species. Terrestrial environments also transformed, marked by vegetation shifts and the proliferation of specific plant types.
A unique sediment layer distinguishes the PETM, called the “clay layer” or “boundary clay,” as a geological marker of this significant climatic phenomenon.
Then and Now
As evident, climate change has occurred in the past. These episodes were not arbitrary but were precipitated by extraordinarily high levels of greenhouse gas emissions.
Although active volcanoes still exist today, their impact pales in comparison to the events of the past, where vast expanses of the Earth’s crust discharged toxic gases into the atmosphere. The most severe of these, the End-Permian volcanic activity, was particularly disastrous, resulting in a sixfold surge in atmospheric CO2 levels.
Geologists estimate that every 500 years, the End-Triassic volcanic event spewed approximately 100,000 km^3 of lava. Intriguingly, the amount of CO2 emitted by these volcanoes over these 500 years is comparable to the anticipated emissions from all human activities in the 21st century.
If we aim to draw lessons from history, we must acknowledge the potential for cascading impacts of elevated greenhouse gas levels in the atmosphere to wreak havoc on contemporary ecosystems. The rise in temperatures represents just the beginning, with disruptions in ocean circulation, ocean acidification, and anoxic events looming as potential threats.
Some might argue that the repercussions on ecosystems are of no concern to them, believing these changes will not impact human lives. Yet, we are integral components of these ecosystems. Alterations in precipitation patterns will influence our agriculture. The extinction of species will have ramifications on food production and water supplies.
We are only beginning to grasp the potential consequences of what may seem like a “minor” shift in global temperatures on the planet as we know it. Fortunately, studying fossil records allows us to view the past as an extensive experiment. Decoding historical climate events enables us to enhance our understanding of current conditions and foresee future scenarios.