The Paleogene Period, from 66 to 23 million years ago, began the Cenozoic Era after the Cretaceous extinction event. This period saw significant climate transformations, shaping Earth’s modern environment. Understanding Paleogene climate offers insights into Earth’s past warmth and long-term climate change.
General Climate Characteristics
The Paleogene Period had a much warmer, more humid climate than today, with significantly less ice at the poles and no permanent ice sheets during the Paleocene. The average global temperature during the Paleocene was around 24–25 °C, considerably higher than the modern average of 14 °C. Warmth extended to high latitudes, where subtropical and tropical flora thrived in now-temperate or cold regions. For instance, palm trees grew as far north as Greenland, and crocodilians inhabited areas above the Arctic Circle during the Eocene. The latitudinal temperature gradient was much weaker, meaning less difference between equatorial and polar temperatures.
Major Climate Events
While the Paleogene was broadly warm, it experienced notable climatic fluctuations.
Paleocene-Eocene Thermal Maximum (PETM)
The most prominent was the Paleocene-Eocene Thermal Maximum (PETM), a brief interval of extreme global warming around 55.8 million years ago. During the PETM, global average temperatures rapidly rose by 5–8 °C, with sea surface temperatures in the high-latitude Arctic reaching as warm as 23 °C. This rapid warming, lasting 100,000 to 200,000 years, was linked to a massive carbon release into the ocean and atmosphere.
Eocene-Oligocene Transition (EOT)
After the PETM, global temperatures gradually declined, though the early Eocene remained warm, sometimes called the Eocene Climatic Optimum. A significant shift occurred during the Eocene-Oligocene Transition (EOT) 34 million years ago. This transition marked substantial global cooling, shifting Earth from an ice-free greenhouse world to an icehouse climate. During the EOT, the first major continent-scale ice sheets began to form on Antarctica, leading to a global temperature drop of up to 5 °C and a widespread extinction in marine life.
Drivers of Paleogene Climate Change
Paleogene temperatures and climatic shifts were influenced by several geological and atmospheric factors.
High CO2 Levels
High atmospheric carbon dioxide (CO2) levels played a primary role, contributing to a strong greenhouse effect. Concentrations of CO2 during the Paleogene are estimated to have been around 1000-2000 parts per million (ppm), significantly higher than present-day levels. Volcanism was a major source for these high CO2 levels, contributing to the period’s warmth.
Continental Drift and Ocean Currents
Changing continental configurations and ocean currents also impacted global heat distribution. As continents drifted, new oceanic gateways opened or closed, altering global water flow. For example, the opening of the Drake Passage between South America and Antarctica during the Eocene-Oligocene Transition allowed the Antarctic Circumpolar Current to form. This current isolated Antarctica from warmer waters, contributing to cooling and ice sheet establishment.
Reconstructing Ancient Climates
Scientists piece together the climate of millions of years ago using various proxy records found in geological archives. Analyzing fossilized plants and animals provides direct evidence of past environmental conditions. For instance, leaf margin analysis relies on the observation that woody plant species with smooth-edged leaves increase in warmer climates, while toothed leaves are more common in cooler regions. By examining fossil leaf assemblages, researchers can estimate mean annual temperatures for ancient ecosystems.
Geochemical analyses, particularly of oxygen isotopes in marine sediments, are another important tool. The ratio of heavier oxygen-18 to lighter oxygen-16 in the shells of ancient marine organisms, such as foraminifera, provides information about past ocean temperatures and global ice volume.
Additionally, geological evidence like ancient soils and coal deposits offers clues about past precipitation patterns and vegetation. These diverse methods collectively allow scientists to reconstruct the complex climatic history of periods like the Paleogene.