Science has struggled to explain fully why an ice age occurs every 100,000 years. As researchers now demonstrate based on a computer simulation, not only do variations in insolation play a key role, but also the mutual influence of glaciated continents and climate.
Earth's climate cools roughly every 100,000 years, with vast areas of North America, Europe and Asia being buried under thick ice sheets, researchers said.
Eventually, the pendulum swings back: it gets warmer and the ice masses melt. While geologists and climate physicists found solid evidence of this 100,000-year cycle in glacial moraines, marine sediments and arctic ice, until now they were unable to find a plausible explanation for it.
Using computer simulations, a Japanese, Swiss and American team including Heinz Blatter, professor of physical climatology at ETH Zurich, has now managed to demonstrate that the ice-age/warm-period interchange depends heavily on the alternating influence of continental ice sheets and climate.
"If an entire continent is covered in a layer of ice that is 2,000 to 3,000 metres thick, the topography is completely different," said Blatter.
"This and the different albedo of glacial ice compared to ice-free Earth lead to considerable changes in the surface temperature and the air circulation in the atmosphere," said Blatter.
Moreover, large-scale glaciation also alters the sea level and therefore the ocean currents, which affects the climate.
These feedback effects between Earth and the climate occur on top of other known mechanisms, researchers said.
Because Earth's rotation and its orbit around the Sun periodically change slightly, the insolation also varies. If you examine this variation in detail, different overlapping cycles of around 20,000, 40,000 and 100,000 years are recognisable.
"It's the first time that the glaciation of the entire northern hemisphere has been simulated with a climate model that includes all the major aspects," said Blatter.
Researchers were also able to explain why ice ages always begin slowly and end relatively quickly. The ice-age ice masses accumulate over tens of thousands of years and recede within the space of a few thousand years.
It is not only the surface temperature and precipitation that determine whether an ice sheet grows or shrinks. Due to the aforementioned feedback effects, its fate also depends on its size, researchers said.