Timeline of events
- 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.
Video 1: The movement of the tectonic plates played a crucial role in enabling the current ice age. This short video clip is a time series showing position of continents in the present. It then goes back in time 100 million years before returning to the present.
- 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. 1).
- 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).
Fig. 1: As iron in the ocean increased (red line), it fertilised the phytoplankton, which withdrew large quantities of CO₂ (blue line) from of the atmosphere.
- 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. 2).
- 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) ‘oceanic pump’ 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.
Fig. 2: A satellite view over Antarctica reveals a frozen continent surrounded by icy waters. The sea ice extent is in light blue. Moving northward, away from Antarctica, the water temperatures rise slowly at first and then rapidly across a sharp gradient. The Antarctic Circumpolar Current (ACC) maintains this boundary. The two black lines indicate the long-term position of the southern and northern front of the ACC (Image credit: The Conversation).
