Before the ice ages began roughly 2.6 million years ago, Earth was in the Pliocene epoch, a stretch of time running from about 5.3 to 2.6 million years ago. The planet was warmer, sea levels were dramatically higher, and the continents were slowly shifting into positions that would eventually plunge the world into repeated cycles of glaciation. The Pliocene wasn’t a tropical paradise everywhere, but compared to the frozen swings that followed, it was a remarkably mild and stable period.
A Warmer World With Higher Seas
During the mid-Pliocene warm period, around 3 to 3.3 million years ago, global average temperatures were more than a degree Celsius above modern levels. That might sound modest, but the effects were enormous. High-latitude regions like the Arctic were significantly warmer than today, and the Eurasian side of the Arctic Ocean experienced what scientists call “Atlantification,” where warm Atlantic water flooded in and kept spring sea ice concentrations 30 to 35 percent lower than current levels. Parts of the Arctic may have been seasonally ice-free.
With less ice locked up on land, sea levels stood far higher than today. Estimates from geological evidence in Patagonia, the Mediterranean, and South Africa suggest the ocean was roughly 20 to 28 meters above its present level. That’s enough to drown most modern coastal cities. Greenland was likely ice-free, much of West Antarctica had melted, and even marine-based portions of the East Antarctic ice sheet had retreated. Charles Darwin himself noted ancient shorelines high above the modern coast during his voyage on the Beagle, evidence that would puzzle scientists for over 150 years.
CO2 Levels Strikingly Close to Today’s
One reason the Pliocene gets so much attention from climate scientists is its atmospheric carbon dioxide. During the mid-Pliocene warm period, CO2 ranged from about 330 to 391 parts per million. For context, pre-industrial CO2 was around 280 ppm, and today’s levels have surpassed 420 ppm. The Pliocene is the last time Earth’s atmosphere held CO2 concentrations comparable to what we have now, which makes it a natural laboratory for understanding where modern climate might be heading.
Megafauna and Green Landscapes
The Pliocene and the early Pleistocene that followed supported an extraordinary diversity of large animals. At least 50 species of giant herbivores roamed the planet, each weighing over 1,000 kilograms. These included 16 species of elephants and their relatives, 9 species of rhinoceros, and 7 species of large even-toed animals like camels, bovids, and hippos. In the Americas, the lineup was even stranger: giant ground sloths, armored glyptodonts the size of small cars, and massive toxodonts that looked like a cross between a hippo and a rhino. Only 9 of those 50 megaherbivore species survive today, all of them in Africa and Asia.
Predators matched their prey in scale. Fifteen species of mammal megacarnivores stalked the landscape, including several sabertooth cats weighing 200 to 300 kilograms, a North American lion relative tipping the scales at 430 kilograms, and the short-faced bear of North America at a staggering 720 kilograms. These animals shaped their environments profoundly. In northern Eurasia, mammoths, horses, and bison maintained vast grasslands known as mammoth steppe, keeping the ground dry and productive through their grazing and trampling.
Landscapes that are barren today were lush during warmer phases. The Sahara, now the world’s largest hot desert, was periodically covered in vegetation, rivers, and enormous lakes during wetter climate windows. Fossils of rhinos, giraffes, and hippos have been found at North African sites, suggesting the region was ecologically connected to sub-Saharan Africa. Plants, too, co-evolved with their giant consumers. Some American fruit species still produce fruits too large for any living animal to eat and disperse, likely adapted for megafauna that no longer exist.
Our Ancestors in the Pliocene
This was also the world in which early human ancestors lived. Australopithecus afarensis, the species that includes the famous fossil “Lucy,” inhabited eastern Africa between about 3.85 and 2.95 million years ago, squarely within the Pliocene. These small-brained but upright-walking hominins moved between woodlands and grasslands, taking advantage of the diverse environments that the warmer, wetter climate supported. Their ability to walk on two legs and still climb trees gave them access to resources across multiple habitat types.
What Tipped the World Into Ice Ages
The Pliocene didn’t end abruptly. It cooled gradually, driven by a combination of geological and astronomical forces that reinforced each other over millions of years.
One major trigger was the closing of the Central American Seaway. As the Isthmus of Panama rose from the ocean floor between about 13 and 4 million years ago, it cut off the exchange of water between the Pacific and the Caribbean. This reorganized ocean circulation across the entire Atlantic, strengthening the Gulf Stream and boosting the production of deep water in the North Atlantic. Paradoxically, this warmer current carried more moisture northward, where it fell as snow at high latitudes, feeding the growth of massive ice sheets.
Layered on top of these geological changes were shifts in Earth’s orbit. Three types of orbital variation, collectively called Milankovitch cycles, control how much sunlight reaches different parts of the planet at different times of year. The shape of Earth’s orbit stretches and contracts over roughly 100,000-year cycles. The tilt of Earth’s axis wobbles between 22.1 and 24.5 degrees over about 41,000 years. And the direction the axis points traces a slow circle every 26,000 years or so. Between 1 and 3 million years ago, these cycles triggered glacial advances and retreats at roughly 41,000-year intervals. During the Pliocene itself, glacial and interglacial swings operated on a similar 40,000-year rhythm, with sea-level oscillations of up to 13 meters.
As CO2 levels gradually declined and orbital geometry aligned to reduce summer warmth in the Northern Hemisphere, ice sheets began growing on Greenland and northern North America. By 2.6 million years ago, the planet had crossed a threshold. The relatively stable warmth of the Pliocene gave way to the Pleistocene, an epoch defined by dramatic swings between glacial and interglacial periods that would reshape landscapes, drive mass extinctions, and ultimately set the stage for modern humans.