Summary
In geological terms, Earth is still in an Ice Age called the Quaternary Period, which began ~2.6 million years ago. Confused? Don’t be. See ‘What’s in a name?’ below. The exact timing and relative contributions of the chain of events that led to the current Ice Age are somewhat uncertain, but the most recent scientific evidence suggests that the (now rapidly weakening*) Antarctic Circumpolar Current (ACC) played a significant role:
...a strengthening of an ocean pump in the waters around Antarctica sucked carbon dioxide out of the air and sent it plunging to the abyss, cooling the planet and intensifying the ice ages. – Voosen, 2024
Once events were set in motion, through feedbacks and tipping points these forcings compounded one another like a slow-motion toppling of dominoes that began some 100 million years ago (Video 1).
* Physical measurements confirm these changes (to the ACC) are already well underway. As Antarctica melts, more freshwater flows into the oceans. This disrupts the sinking of cold, salty, oxygen-rich water to the bottom of the ocean. From there this water normally spreads northwards to ventilate the far reaches of the deep Indian, Pacific and Atlantic Oceans. But that could all come to an end soon. In our lifetimes. – England et al, 2023
What’s covered in this page:
Ice Age or Glacial? What’s in a name and why it matters
The names we use to describe places, times, events, and processes matter because they communicate accurate information. Many of the names used to describe Geological times and processes are unfamiliar or not well understood. The term ‘ice age‘ for example conveys a compelling image of a time (age) when the planet was much colder. The term ‘glacial’ evokes images of frozen rivers. This is why ‘ice age’ rather than ‘glacial’ is often used colloquially to describe short stretches of time—geologically speaking—when conditions were colder than what people considered ‘normal’. The ‘Little Ice Age’ for example wasn’t an Ice Age. Caused mostly by large volcanic eruptions, it was a cold snap that lasted a few hundred years; barely a blip in geological time.
Geological names and processes
- Time: We mark time in incrementally smaller units: centuries, years, days, hours, seconds. Geological time is also marked in incrementally smaller unit: eons, eras, periods, epochs, ages. However, these units are not equal lengths of time. Rather, they describe conditions on Earth that are distinct in the geological record, often ending in catastrophic events after, which a new name is given.
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Glacier: ice that flows like a river. If enough glaciers join together they can form an ice sheet.
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Ice sheet: spreads out in all directions from a central dome. By definition an ice sheet covers an area of at least 50,000 square kilometers.
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Ice Ages: geological periods during which there was at least one ice sheet larger than 50,000 square kilometres on the planet. Ice Ages last for many millions of years. Today the Greenland and Antarctic ice sheets together cover almost 16 million square kilometers. So, by definition, Earth is in an Ice Age. This is the Quaternary Period (Fig. 2).
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- Glacials: geological epochs that happen during Ice Ages when glaciers march out from the ice sheets, creating much larger and sometimes new ice sheets that cover vast areas of the planet.
- Interglacials: geological epochs when the glaciers retreat back to just a few ice sheets in polar regions, just as they exist today in Green;and and Antarctica. Both Glacials and Interglacials can last for tens to hundreds of thousands of years during multi-million year long Ice Ages. The last interglacial, the Holocene Epoch began 11,700 years ago (Fig. 2).
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- Stadials: colder but relatively short stretches of time during glacial epochs where ice sheets extend even further. The Last Glacial Maximum (LGM) was a stadial that began around ~33,000 years ago and reached its peak (coldest) ~26,000-19,000 years ago.
- Interstadials: warmer but relatively short stretches of time during glacial epochs where some ice sheets contact.
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The more than 1.5 trillion tonnes of carbon dioxide humans have emitted into the atmosphere since the industrial revolution are expected to cause enough warming to disrupt this long-term glacial cycle…[this] may have been high enough to delay the advance of the ice sheets by 50,000 years. – New Scientist 2025
Timeline of events 100 million years ago – present day
Around 100 million year ago, the Indian tectonic plate left the supercontinent Gondwana. When it collided with the Eurasian plate 35-50 million years later, it pushed up the steep Tibetan Plateau and formed the Himalayan Mountains (Video 1). These new steep mountains were chemically weathered by rain. This is because carbon dioxide (CO₂) in the atmosphere mixes with rainwater (H₂O) to become a weak carbonic acid (H₂CO₃).
While the rainwater was only slightly acidic, over millions of years it was enough to gradually erode the mountains. The CO₂ was locked inside river waters as calcium carbonate (CaCO₃) that flowed down into the sea. With increasing amounts of CO₂ being taken out of the atmosphere, more heat from the sun was able to escape back into space because of the greenhouse effect.
Over millions of years iron and other nutrients were blown and washed down from the land—which was slowly becoming colder and drier—and into the ocean. Here, the iron fertilised tiny free-swimming short-lived microscopic plants called phytoplankton, enabling them to reproduce in extraordinary numbers, taking CO₂ out of the atmosphere (Fig. 4).
Phytoplankton are primary producers; everything in the ocean feeds on them either directly or by bigger marine creatures eating the smaller animals that graze on them. When they died, they fell to the deep ocean floor and were eventually buried. This locked the carbon away so it couldn’t enter the atmosphere (see the carbon cycle). Around 200 gigatonnes of CO₂ was eventually removed from the atmosphere over this period, possibly due to this process.
While this was happening (and is still happening today), around 34-20 million years ago, both the South American and Australian tectonic plates also separated from Gondwana and headed north (Video 1) This left the Antarctic plate isolated. Ocean currents that once flowed between the tropics and the poles, carrying heat to Antarctica, were partially blocked by a newly forming Antarctic Circumpolar Current (ACC) an oceanic pump that helps draw down CO₂ and store in the depths of the ocean (Fig. 5).
Isolated from the warm waters, Antarctica grew colder and colder. Snow turned into ice that became glaciers which began spreading out over the continent as ice sheets.
By 2.6 million years ago, the South American plate had joined the North American plate. This separated the Pacific and Atlantic oceans from one another by closing the gap at Panama, further isolating Antarctica, where the glaciers and ice sheets merged to become an ice cap. The joining of South and North American continental plates also changed how the Northern Hemisphere oceanic thermohaline current worked. This cooled the northern parts of Eurasia and America. By then the Himalayan Mountains were also covered in snow and glaciers. The increased albedo created a feedback effect, cooling the area even more. Glaciers and ice sheets grew into ice caps and spread over Eurasia and North America. The Quaternary Ice Age had begun.
1.5 million years ago, the ‘oceanic pump’ Antarctic Circumpolar Current (ACC) strengthened. This sucked carbon dioxide out of the air and sent it plunging to the abyss, cooling the planet.
The timing of Milankovitch Cycles amplified the effects of cooling. While it was most likely the primary climate forcing that led to the initial cooling, subsequent events outlined above played a key role in drawing down and storing large volumes of CO₂ in the ocean.
Today, the ACC helps act as a planetary thermostat, keeping Antarctica cool. But the ACC is now failing.
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