Antarctica on ice

Tectonic influences on climate

Our world was not always the way we experience it. Climates of the past have been vastly different.

In the early Cenozoic, during the Early Eocene Climatic Optimum (~52–50 million years ago), temperatures were up to 14 °C warmer than today, polar regions were nearly ice-free with lush forests in Antarctica and sea levels were up to ~70 meters higher than today. CO₂ levels ranged from 800–2000 ppm during this time.

Major changes began to occur at the close of the Eocene and beginning of the Oligocene, 33.7 million years ago. The Earth shifted from this warm wet, 'greenhouse' world, to a cooler, drier 'icehouse' world. The first evidence for permanent ice on Antarctica starts around this time.

Cool conditions for some 10 million years were followed by an excursion towards a warmer world that peaked at around 15 million years ago in the Middle Miocene - the Middle Miocene Climatic Optimum. At this time, CO₂ levels were between 400 and 600 ppm, the world was 3 to 6 °C warmer than today and there is fossil evidence for trees and shrubs in Antarctica.

From this time, the trend has been towards a cooler Earth.

CO₂ fell to ~300 ppm or lower by the Late Miocene due in part to increased weathering of the Himalayas, burial of carbon in deep-sea sediments due to expanded ocean productivity. Sea levels dropped and Antarctica became the icy continent.

Summary of Cenozoic climate record. From Zachos et al. (2008). See full text in the journal Nature at https://www.nature.com/articles/nature06588.

Packing 50 million years of climate history into a few paragraphs doesn't do the science behind these observations justice, but it sets the scene for one aspect of this evolution that I want to focus on.

In a review of the effect of the Pacific Ocean on global climate in the Cenozoic, Lyle et al. (2009) observed:

Most of the prominent changes in Cenozoic climate appear to be related to changes in Pacific Ocean circulation linked to changes inflow through ocean gateways.

Let's trace some elements of the tectonic dance of the continents in the southern hemisphere to see how the opening of two seaways contributed to the profound shifts in global climate during the Cenozoic as a result of the formation of the Antarctic Circumpolar Current.

Oceanic and mantle gateways in the (a) Eocene/Oligocene and (b) today. In the Eocene the Panama, Gibraltar, Middle East, and Indonesia oceanic gateways were open. The global oceanic circulation was more equatorial, and Earth was characterized by a warmer climate. b) Present continental configuration. Tropical gateways are closed or restricted (Indonesia) and the Southern Ocean is open. Figure from Martos and Catalan (2024).

World’s worst sea with global impact

The final instalment of the isolation of Antarctica was the rifting of South America from the Antarctic Peninsula and the formation of the world's most notoriously rough sea - Drake's Passage and the Scotia Sea. I covered this a little in a previous article on the South Sandwich Islands.

Drake's Passage began opening around 34 million years ago, but in detail, the process was a little stop-start. A paper by Lagabrielle et al. (2009) provides a detailed treatment.

In summary, there was a seaway between southern South America and the Antarctic Peninsula in the Eocene, ~40 million years ago, but flows were restricted as the continents remained relatively close to each other.

Between ~29–22 and 14 million years ago, the region underwent a period of constriction, with the formation of major transform fault zones in the north Scotia Sea, uplift of the Tiera del Fuego, and a related spreading centre in the southern Scotia Sea.

In relation to these periods of tectonic complexity around the southern tip of South America, Lagabrielle et al. (2009) note:

This period of active deformation in southern South America also corresponds to a period of the global climate having two anomalies well known from the isotopic records: the Late Oligocene Warming, around 26 Ma and the Mid-Miocene Climatic Optimum which ended between 15 and 14 Ma.

Not that this is the only cause of these climatic shifts, but restrictions in the flow of water through Drake's Passage was certainly very significant contributor to these climate 'excursions'.

By 10 million years ago, the two continents had parted ways for good and the full force of the Antarctic Circumpolar Current was being felt.

Unsuspecting tourists who flock to the port of Punta Arenas for their trip of a lifetime can find out the hard way what happens when three oceans collide - the south-eastern Pacific, the Atlantic and the Southern Ocean - as they sail cross the Scotia Sea.

Antarctic Circumpolar Current

Why is the Antarctic Circumpolar Current so important for global climate?

Prior to opening of the Drake's Passage, warm ocean currents reached Antarctica, possibly from the South Atlantic or the Indian Ocean. These currents would have helped to moderate Antarctic winters and limit the extent of the ice sheet.

Once the Antarctic Circumpolar Current was established, cold waters were able to circulate around the continent, having an insulating effect and preventing large volumes of warmer waters from the northern oceans reaching the continent.

The current is the strongest oceanic current in the world as a consequence of it being uninterrupted and also being in part influenced by the the Coriolis force - the rotation of the globe itself helps to push the Southern Ocean along its full circumambulation of the great southern continent.

On top of thermally insulating Antarctica, the Antarctic Circumpolar Current is also so important for connecting the world's oceans and helping to mix them keeping global ocean salinity and chemistry similar.

The cold and dense waters off Antarctica generally sink and then spread out across other ocean systems acting like a giant air conditioner, bringing cool waters and moderating climate across the globe.

Visualisation of the Antarctic Circumpolar Current from Wikipedia.

Variability of the Antarctic Circumpolar Current

A recent review by Lamy et al. (2024) emphasises that the "strength and position of the ACC and its associated oceanic fronts are controlled by wind stress, interaction of flow with the deep-ocean bathymetry and buoyancy forcing" and as a consequence, the strength and position of the current is far from set in stone.

Further increases in CO₂ and changes in ocean circulation that result will have some effect on the shape and effectiveness of the current. They comment that their findings:

provide geological evidence in support of further increasing ACC flow with continued global warming. If true, a future increase in ACC flow with warming climate would mark a continuation of the pattern observed in instrumental records, with probable negative consequences for the future Southern Ocean uptake of anthropogenic CO2.

The ability of the Southern Ocean to absorb CO₂ is affected by its chemistry, which is affected by its interaction with other ocean waters. Less interaction means less mixing.

That CO₂ will have to go somewhere else - either remain in the atmosphere or be absorbed in other ocean waters and other such carbon sinks.

Global climate is a complex chemical system underpinned by complex tectonic process that extend back into deep time.

Visualisation of eddies and whorls of oceanic circulation of the Antarctic Circumpolar Current in the Drake's Passageway. This is a clip of a larger figure from Lamy et al. (2024). Shown is the simulated ocean velocity at 100-m water depth (blue = weak; white = strong).