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ICE AGE | An Unusual Suspect:

Did the Gulf Stream Trigger the Ice Age?

Intensification of the Gulf Stream may have triggered the onset of the Ice Age

After the blistering heights in temperature of the early Eocene, around 55 million years ago, the world's climate experienced a gradual cooling, albeit with warm temperature reversals here and there, which eventually culminated in the brutal descent into the Pleistocene Ice Age just under 2.6 million years ago. The effects of this great age of ice are written all over the island of Ireland, and how it shaped and sculpted this land into the form we are familiar with today is well documented. However, when it comes to one fundamental aspect of this defining age a deep mystery still persists – just what caused the Ice Age to begin?

In general, many aspects of the gradual cooling during the Cenozoic (66 million years ago–Present) are quite well understood, with major parts played by, for example, the isolation of Antarctica in the late Eocene and the establishment of its first great Cenozoic ice sheets in the early Oligocene, just under 34 million years ago, while changes in ocean circulation with the closing of ancient oceans like the Tethys Sea which once separated Africa and Eurasia have also been implicated, as has the rise and weathering of new mountain chains. However, exactly what was responsible for the specific onset of the Pleistocene Ice Age is as yet unclear.

Some experts, for instance, have placed emphasis on the rise of the Himalayas in the later Cenozoic as the main driver of the descent into this Age of Ice, as this monstrous mountain chain greatly altered airflow patterns in the atmosphere and also offered a vast new surface of rock to be battered by wind and rain, this chemical weathering process removing massive quantities of carbon dioxide from the atmosphere. However, although processes such as the Himalayan emergence certainly contributed to the cooling of the global climate, there is a growing body of evidence that suggests the onset of the Ice Age itself may have been triggered by a familiar and quite unexpected source – the Gulf Stream.

The Gulf Stream is generally held to be one of the main determinants of Ireland's climate, the warm waters transported northeastwards from the Gulf of Mexico warming Ireland's climate so that it enjoys markedly higher temperatures than other parts of the world at the same latitude such as Newfoundland in Canada. Although the manner in which the Gulf Stream produces this temperature difference is not as simple as it sounds, and there has even been heated debate over just how much the Gulf Stream is responsible for the warming of northwest Europe, all experts would agree that the contribution it makes to this region is one of warmth. It seems very odd, then, that it should be implicated in the onset of a period of deep cold like the Ice Age but the argument that it did just that is quite compelling.

The roots of this story lie in the early Pliocene, around five million years ago, when a world-altering event was taking place thousands of kilometres from Irish shores. At this time, the continents of North America and South America, which had been entirely isolated from each other for most of the Cenozoic, began to converge, the plates they sit upon colliding with the Caribbean plate to form what would eventually become the Isthmus of Panama.

Map of the Central American Seaway, which separated North and South America before the formation of the Isthmus of Panama

Compared to the two American continental giants it joins together, this isthmus is very thin, but the rise of this thin strip of land during the Pliocene (c. 5.3–2.6 million years ago) would set in train a number of changes in ocean currents which would have global consequences.

Today, the main driver of the global ocean circulation system, sometimes referred to as the Ocean Conveyor, is the difference in the amount of salt in the Pacific and Atlantic Oceans, as the sinking of cold, salty waters, which are denser, determines the pattern of global ocean currents. However, before the rise of the Isthmus of Panama, the salt content of the Pacific and the Atlantic was roughly the same, the surface waters of the Pacific able to flow through the Central American Seaway and mix with those of the Atlantic. As the isthmus began to rise and the Central American Seaway became increasingly shallow, though, a global ocean circulation system more like that of today began to emerge.

Modern global ocean circulation system, with warm, shallow currents in red, and cold, deep, and salty currents in blue

As the mixture of Pacific and Atlantic waters was less and less able to take place, the salt content of the two oceans began to diverge. The waters of the Caribbean Sea and the tropical Atlantic became more salty due to evaporation, while the Pacific became less salty as this water vapour from the Atlantic and Caribbean was carried westwards over the emerging isthmus by the trade winds where it fell as rain, increasing the amount of fresh water in the Pacific. As this continued over thousands of years, the Pacific became less and less salty while the Atlantic became ever more so.

The formation of the Isthmus of Panama not only caused the salt content of the Atlantic to rise but intensified the Gulf Stream, and it was these two factors together which may have altered the Earth's climate. Initially, though, it was not a descent into cold which these changes heralded but an ascent into warmth, a time called the Mid-Pliocene Warm Period (c. 3.3–3.0 million years ago) when the Earth was warm enough for camels to live on Ellesmere Island, Canada, inside the Arctic Circle.

During the Mid-Pliocene Warm Period, camels thrived in Ellesmere Island inside the Arctic Circle

The newly stronger Gulf Stream would have conveyed vast amounts of warm, salty waters to the northern reaches of the world where they were cooled by Arctic winds making them dense enough to sink to the bottom of the ocean. From here they undertook a great odyssey at depth through the oceans, picking up a vast cargo of carbon dioxide and nutrients, before finally upwelling in the North Pacific. Here, these rich upwelling waters could have spurred a great bloom of phytoplankton, but even a great sea of these tiny organisms might not have been enough to use all of the carbon dioxide in the upwelling water and so massive quantities of this greenhouse gas were able to filter into the atmosphere, causing the planet to warm.

However, the intensification of the Gulf Stream was a double-edged blade, climatically speaking, and the same process causing the Earth to warm may have eventually also led to its descent into the Ice Age.

The transportation of more moisture to the North Atlantic would have increased the levels of precipitation in this region, causing the levels of fresh water in the Arctic Ocean to increase through the falling of rain and snow and the emptying of Siberian rivers into its waters. This increase in fresh water could have promoted a descent into colder conditions in a number of ways.

For instance, it would have promoted the formation of sea ice in the Arctic, which would have reflected great amounts of heat and sunlight back into space. This sea ice would also have prevented heat from the ocean filtering into the atmosphere above. Finally, these fresher, less cold and less salty Arctic waters may have affected the global ocean circulation system which had been established, as their flow into the North Atlantic disrupted the transportation of warmth to this region.

With such a scenario unfolding, it would only have taken a slight nudge from any direction to send the climate toppling over and descending into an Ice Age, and this is exactly what seems to have happened.

Between around 3.1 and 2.5 million years ago, the tilt of the Earth's axis changed so that it became less than the 23.45 degrees of today, which reduced the amount of heat and sunlight striking the Northern Hemisphere. Summers became colder and colder and each year less of the winter snows melted, eventually leading to the growth of great ice sheets.

This deepening northern glaciation also seems to have shut off the final heat pump in the North Pacific. Around 2.7 million years ago, a freshwater barrier formed in the upper levels of the ocean which blocked the upwelling of the deep, carbon dioxide-laden waters to the surface, thus sundering the supply of carbon dioxide to the atmosphere. To borrow an image from the geologist Jan Zalasiewicz, this shattered the last pane in the global greenhouse and ensured our planet would become an icehouse world.

The Gulf Stream, then, which today helps to keep Ireland warm, may also have been instrumental in its descent into cold. Thus, this theory highlights the complex nature of the Earth's climate, a stage filled with actors whose roles may change from one scene to the next. All is not always as it seems, when even an agent of warmth can become a bringer of ice.

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