TEMPO.CO, Zurich - Based on a computer simulation, an international team has managed to unravel the mystery behind why an ice age occurs every 100,000 years. Not only do variations in insolation play a key role, but also the mutual influence of glaciated continents and climate.
Ice ages and warm periods have alternated fairly regularly since the beginning of the earth’s history. The earth’s climate cools roughly every 100,000 years. When the earth cools, most of the parts in Northern America, Europe and Asia are buried underneath thick sheets of ice.
The pendulum swings back after another 100,000 years. The earth warms up and ice masses melt. Although 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 reasonable explanation for it.
Using computer simulations, a Japanese, Swiss and American team including Heinz Blatter, an emeritus 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," says Heinz, explaining this feedback effect. "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."
Large-scale glaciation also changes the sea level, thus the ocean currents as well, which also affects the climate.
As the scientists from Tokyo University, ETH Zurich and Columbia University demonstrated in their paper published in the journal Nature, these feedback effects between Earth and the climate occur on top of other known mechanisms. It has long been clear that the climate is greatly influenced by insolation on long-term time scales. Because the 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 recognizable.
Given the fact that the 100,000-year insolation cycle is relatively weak, scientists could not easily explain the 100,000-year-cycle of the ice ages with this information alone. With the aid of the feedback effects, however, this is now possible.
Using the model, the 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.
Now we know why: 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.
"The larger the ice sheet, the colder the climate has to be to preserve it," says Heinz. In the case of smaller continental ice sheets that are still forming, periods with a warmer climate are less likely to melt them. It is a different story with a large ice sheet that stretches into lower geographic latitudes: a comparatively brief warm spell of a few thousand years can be enough to cause an ice sheet to melt and bring the end of an ice age.
SCIENCEDAILY | NATURE | TJANDRA DEWI
