Why did the Earth freeze during a period of record-high atmospheric carbon dioxide levels?
In short: The Ordovician Period is famous for its greenhouse climate and flooded continents, yet it ended with a sudden, brutal ice age. This Hirnantian glaciation triggered one of history's largest mass extinctions, forcing marine life to adapt or vanish as sea levels plummeted and global temperatures crashed in a geological heartbeat.
The Hirnantian Age, spanning 445.2 to 443.4 million years ago, represents the final chapter of the Ordovician Period. For most of the Ordovician, the Earth was a greenhouse world. Numerical models suggest atmospheric carbon dioxide levels were several times higher than today, fostering warm climates from the Equator to the poles. This warmth, combined with rapid seafloor spreading, drove global sea levels to such heights that massive portions of continents like North America were submerged under shallow, sediment-rich seas.
Despite this pervasive warmth, the period concluded with a dramatic climatic shift. While the Northern Hemisphere remained largely ice-free due to the distribution of landmasses, the supercontinent Gondwana—which straddled the South Pole—became the epicenter of an intense, brief glaciation. This event was not merely a minor cooling; it was a severe environmental crisis. The rapid expansion of glaciers at the Katian-Hirnantian boundary caused a massive drop in sea levels and a sharp decline in tropical ocean temperatures, estimated at 10 degrees Celsius.
The biological consequences were catastrophic. The Ordovician had been a time of extraordinary diversification, known as the Ordovician radiation, which established nearly every modern marine invertebrate phylum. However, the Hirnantian glaciation precipitated the second-largest mass extinction in Earth's history. As glaciers advanced and retreated, the resulting instability in sea levels and temperatures decimated marine ecosystems. This cooling event stands as a stark reminder of how quickly a stable, warm climate can collapse into a glacial state, fundamentally altering the trajectory of life on Earth.
Opens the Intellect+ feed with this card first. Swipe up for the next one.
Sources
- Summary by Intellect · source: Hirnantian Stage | Ordovician, Carboniferous & Glaciation | Britannica
- Also: Hirnantian glaciation
To cite this card, link to https://intellectplus.co.uk/app/nature-hirnantian-glaciation.
How we source, check and correct cards: Editorial standards.
General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.
Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?
Related
More in Geology · More in Nature · Latest · A–Z · All topics
