Scientists claim to have found the "missing link" in the process that led to an ice age on Earth.
The melting of icebergs in Antarctica is key, according to the Cardiff University team, triggering a series of chain reactions that plunge the earth into prolonged periods of cold temperatures.
The results were published in today nature by an international consortium of scientists from universities around the world.
It has long been known that ice age cycles are controlled by periodic changes in the sun's orbit, which changes the amount of solar radiation that reaches the earth's surface.
So far, however, it has been a mystery how small fluctuations in solar energy can trigger such dramatic changes in the earth's climate.
In their study, the team suggests that if the Earth's orbit around the Sun is just right, the Antarctic icebergs will melt further and further away from Antarctica, shifting large amounts of freshwater from the Southern Ocean to the Atlantic.
As the Southern Ocean becomes salty and the North Atlantic fresher, ocean circulation patterns begin to change dramatically on a large scale, drawing CO2 from the atmosphere and reducing what is known as the greenhouse effect.
This in turn brings the earth into ice age conditions.
As part of their study, the scientists used various techniques to reconstruct past climatic conditions. This included identifying tiny fragments of the Antarctic rock that fell into the open ocean from melting icebergs.
The rock fragments were extracted from sediments obtained as part of Expedition 361 of the International Ocean Discovery Program (IODP). They represent over 1.6 million years of history and are one of the longest detailed archives of Antarctic icebergs.
The study found that these deposits, known as ice-rafted debris, consistently lead to changes in deep-sea circulation that have been reconstructed from the chemistry of tiny deep-sea fossils called foraminifera.
The team also used new climate model simulations to test their hypothesis. It found that large amounts of fresh water could be moved by the icebergs.
Study lead author Aidan Starr of the School of Earth and Environmental Sciences at Cardiff University said, "We were amazed that this lead-lag relationship existed at the beginning of every ice age over the past 1.6 million years in the South Ocean's leading role and Antarctica in global climate has been speculated, but it was very exciting to see them so clearly in geological evidence. "
Professor Ian Hall, co-author of the study and co-chief scientist of the IODP expedition, also from the School of Earth and Environmental Sciences, said: "Our results provide the missing link to how Antarctica and the Southern Ocean responded to the rhythms of nature Climate system associated with our orbit around the sun. "
For the past 3 million years, the earth has regularly encountered glacial conditions, but is currently in an interglacial period where temperatures are warmer.
However, given the increased global temperatures due to anthropogenic CO2 emissions, the researchers suggest that the natural rhythm of the ice age cycles may be disrupted as the Southern Ocean is likely to get too warm for icebergs in Antarctica to move far enough to accommodate the changes the ocean circulation is required for an ice age to develop.
Professor Hall believes the results can be used to understand how our climate may respond to anthropogenic climate change in the future.
"Just as we see an increase in Antarctic continent mass loss and iceberg activity in the Southern Ocean as a result of warming associated with current human greenhouse gas emissions, our study underscores the importance of understanding iceberg trajectories and melting patterns in developing the most robust predictions of their future impacts on the ocean circulation and the climate, "he said.
Professor Grant Bigg of the University of Sheffield's Department of Geography, who contributed to the iceberg model simulations, said: "The pioneering modeling of icebergs within the climate model is critical in identifying and supporting the ice raft debris hypothesis of the impact of iceberg meltwater on Antarctica that marked the beginning of the Advancing the glacier cycle. "
The study, led by Cardiff University, was funded by the NERC.