17 August 2025
Guaranisuchus munizi Bld. Nobu Tamura CC BY SA 3.0
Earth’s climate has always shifted, but some past periods offer stark warnings. The Paleocene-Eocene Thermal Maximum (PETM), a period of intense warming around 56 million years ago, is a prime example. While a natural event, its speed and impact, particularly on oceans and ecosystems, hold crucial lessons for today. The critical difference? Today’s changes are happening many times faster.
The PETM: A Geological Heatwave
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Around 56 million years ago, roughly 9 million years after the dinosaurs’ extinction, Earth experienced significant warming. This event, known as the PETM, saw temperatures rise by 4 to 8 degrees Celsius over 10,000 years. Scientists believe astronomical influences may have triggered it, leading to an estimated 3,000 to 10,000 billion tons of carbon, largely methane, entering the oceans and atmosphere. This surge caused widespread consequences, a pace considered exceptionally rapid by geological standards and often referred to as “hyperthermals.”
Figure 1. Reconstruction of Earth’s temperature from 60 million years ago to the present, with future expectations based on greenhouse gas emission scenarios.
Impact on Life and Oceans
The influx of carbon during the PETM led to ocean acidification. This had devastating effects, causing the extinction of approximately 40 percent of deep-sea organisms called Benthian Foraminifera. Coral reefs also declined, and many species migrated towards cooler regions. Some animals exhibited dwarfism as an evolutionary response to heat stress and reduced nutrient availability. While not a mass extinction event, the PETM permanently altered ecosystems, with many deep-sea species disappearing.
Lessons from the Past
Research into extreme climate events like the PETM helps us understand climate sensitivity, feedback loops, and ecosystem resilience. The PETM demonstrates how rapid carbon releases can trigger intense warming, ocean acidification, and significant ecological shifts. It took hundreds of thousands of years for natural processes, like rock weathering, to reabsorb the released carbon. This highlights the immense timescale of Earth’s natural cleanup mechanisms.
Figure 2 (A) Carbon emission speeds during the PETM (orange) versus a potential human emission scenario (5,000 billion tons in 500 years; gray). (B) Ocean calcite levels, indicating acidity; lower levels mean more acidic oceans. Ocean acidification is currently progressing faster than during the PETM.
Unprecedented Speed of Change
The most striking difference between the PETM and today’s climate crisis is the speed of change. While the PETM’s carbon release unfolded over thousands of years, current climate change is occurring within centuries. Human activities, primarily burning fossil fuels and deforestation, have released about 720 billion tons of carbon. Under certain scenarios, human emissions could match or even exceed the PETM’s scale, with potentially 5,000 billion tons emitted in a fraction of the time. This accelerated ocean acidification, progressing much faster than during the PETM, leaves ecosystems, including humanity, with significantly less time to adapt.
In Earth’s 4.54 billion-year history, no period is known for such rapid and extensive carbon emissions. We are venturing into uncharted territory. Therefore, the PETM, despite its differences, serves as our most critical analogy.
