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Eocene-Oligocene Cooling: Geological Drivers Over CO2

By Lewis Loflin | Published May 20, 2025

Thirty-three million years ago, Earth’s climate shifted from a warm Eocene greenhouse to a cooler, drier Oligocene, dropping global temperatures by ~4–5°C and replacing forests with grasslands. Geological forces—Antarctica’s isolation, ice buildup, and oceanic changes—drove this, amplified by water vapor loss and CO2 sequestration in cooling oceans, not atmospheric CO2 decline. Today, high CO2 fuels vegetation growth, increasing fire risk in arid regions like California and Chile (see our wildfire analysis). This page explores the 33 MYA cooling, prioritizing geology over CO2-centric narratives.

Drake Passage and Antarctic Circumpolar Current

The opening of the Drake Passage ~34 MYA, as Antarctica separated from South America, formed the Antarctic Circumpolar Current (ACC), isolating Antarctica from warm equatorial waters. This cooled Southern Ocean waters by ~5–7°C, triggering the Oi-1 glaciation (~33.7 MYA). Sediment cores from the Scotia Sea (ODP Site 696) confirm the ACC’s onset, driving ~2–3°C of global cooling. By altering atmospheric circulation, it dried mid-latitudes, as seen in dust-rich sediments from Nebraska’s Ogallala Group.

Eocene Epoch ocean currents 55.8 – 33.9 MYA when mammals thrived.

Click for a larger image.

Antarctic Ice Buildup

Rapid ice sheet growth during Oi-1 locked vast water volumes in Antarctica, lowering sea levels by ~50–70 meters. Glacial deposits in Prydz Bay and δ18O spikes (ODP Site 689) show this ice buildup. Increased albedo (reflectivity) amplified cooling by ~1–2°C, while water sequestration dried continents, reducing precipitation. This favored open landscapes, as pollen records from Mississippi’s Gulf Coast show forest-to-grassland shifts.

Water Vapor Loss from Forest Decline

Water vapor, the dominant greenhouse gas (~60–70% of the effect), plummeted as forests died. Cooling and drying from the ACC and ice, plus CO2’s drop to ~600–800 ppm, stressed C3 plants (trees), shrinking forests. Reduced transpiration cut atmospheric water vapor by ~10–20% in mid-latitudes (deuterium isotopes, Tanzania), amplifying cooling by ~1–2°C. This feedback, seen in Rhine Graben pollen shifts, was more significant than CO2 decline, mirroring how CO2-driven vegetation affects climate today.

Oceanic CO2 Dissolution and Sequestration

Cooling oceans, driven by the ACC, dissolved more CO2, acting as a carbon sink. A ~5°C ocean temperature drop increased CO2 solubility by ~10–15%, removing ~100–200 Gt of carbon (CCD shifts, Site 744). Phytoplankton blooms, fueled by nutrient upwelling, buried carbon in sediments, further lowering CO2. This feedback, seen in δ13C records, stressed forests and amplified cooling, but was secondary to the ACC and ice.

Factor Mechanism Cooling Contribution Evidence
Drake Passage/ACC Isolated Antarctica, cooled oceans ~2–3°C ODP Site 696 cores
Antarctic Ice Albedo, water lockup ~1–2°C Prydz Bay deposits
Water Vapor Loss Forest decline, reduced transpiration ~1–2°C Deuterium isotopes
Oceanic CO2 Dissolution, sequestration ~0.5–1°C CCD shifts, Site 744
Atmospheric CO2 Weathering, volcanism decline ~0.5–1°C δ13C, stomatal indices
Expanding Plant Growth

Conclusion

The Eocene-Oligocene cooling was driven by geological forces—the Drake Passage’s ACC, Antarctic ice buildup, water vapor loss from forest decline, and oceanic CO2 sequestration—not atmospheric CO2, which played a minor role. These factors cooled Earth by ~4–5°C and dried continents, much like how high CO2 today fuels vegetation growth, increasing fire risk in arid regions (ar4.htm). By prioritizing geology over CO2 fixation, we gain a clearer view of Earth’s climate past, challenging overhyped narratives with reason and evidence.

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