Melting icebergs in the Antarctic are key to the process that leads to an ice age on Earth, scientists have said.

It has long been known that ice age cycles are paced by periodic changes to the Earth’s orbit of the sun, which changes the amount of solar radiation that reaches the Earth’s surface.

But until now, it has been a mystery as to how small variations in solar energy can trigger dramatic shifts in the climate on Earth.

Scientists at Cardiff University now believe that when the orbit of the Earth around the sun is just right, Antarctic icebergs begin to melt further and further away from Antarctica.

This shifts huge amounts of freshwater away from the Southern Ocean and into the Atlantic. As the Southern Ocean becomes saltier and the North Atlantic gets fresher, large-scale ocean circulation patterns begin to change.

Carbon dioxide is pulled out of the atmosphere, reducing the so-called greenhouse effect, and this pushes the Earth into ice age conditions, scientists said.

The findings, from a consortium of scientists from universities around the world, have been published in the journal Nature.

Such a leading role for the Southern Ocean and Antarctica in global climate has been speculated but seeing it so clearly in geological evidence was very exciting

Professor Ian Hall, from the School of Earth and Environmental Sciences at Cardiff University, said: “Our results provide the missing link into how Antarctica and the Southern Ocean responded to the natural rhythms of the climate system associated with our orbit around the sun.”

As part of the study, scientists used multiple techniques to reconstruct past climate conditions. This included identifying tiny fragments of Antarctic rock dropped in the ocean by melting icebergs.

The rock fragments were obtained from sediments recovered by the International Ocean Discovery Program Expedition 361, representing more than 1.6 million years of history and one of the longest detailed archives of Antarctic icebergs.

Scientists found that these deposits, known as ice-rafted debris, appeared to consistently lead to changes in deep ocean circulation.

They also used new climate model simulations to test their hypothesis and found that large volumes of freshwater could be moved by the icebergs.

Aidan Starr, also from the School of Earth and Environmental Sciences, said: “We were astonished to find that this lead-lag relationship was present during the onset of every ice age for the last 1.6 million years.

“Such a leading role for the Southern Ocean and Antarctica in global climate has been speculated but seeing it so clearly in geological evidence was very exciting.”

Over the past three million years, the Earth has regularly plunged into ice age conditions.

At present, it is currently situated within an interglacial period where temperatures are warmer.

But researchers suggest that due to increased global temperatures, the natural rhythm of ice age cycles may be disrupted.

This is because the Southern Ocean will likely become too warm for Antarctic icebergs to travel far enough to trigger the changes in ocean circulation required for an ice age to develop.

Professor Grant Bigg, from the University of Sheffield’s Department of Geography, said: “The ground-breaking modelling of icebergs within the climate model is crucial for identifying and supporting the ice-rafted debris hypothesis of Antarctic iceberg meltwater impacts which are leading glacial cycle onsets.”

The study was funded by the Natural Environment Research Council.