![]() On the other hand, the axis of this ellipsis turns in the plane of Earth's orbit. Finally, Earth's elliptical orbit around the sun changes in a cycle of around 100,000 years in two respects: on the one hand, it changes from a weaker elliptical (circular) form into a stronger one. Moreover, Earth's axis gyrates in a cycle of 26,000 years, much like a spinning top. The explanation for the cyclical alternation of ice and warm periods stems from Serbian mathematician Milutin Milankovitch (1879-1958), who calculated the changes in Earth's orbit and the resulting insolation on Earth, thus becoming the first to describe that the cyclical changes in insolation are the result of an overlapping of a whole series of cycles: the tilt of Earth's axis fluctuates by around two degrees in a 41,000-year cycle. 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 herald the end of an ice age. In the case of smaller continental ice sheets that are still forming, periods with a warmer climate are less likely to melt them. "The larger the ice sheet, the colder the climate has to be to preserve it," says Blatter. Due to the aforementioned feedback effects, its fate also depends on its size. Now we know why: it is not only the surface temperature and precipitation that determine whether an ice sheet grows or shrinks. The ice-age ice masses accumulate over tens of thousands of years and recede within the space of a few thousand years. Using the model, the researchers were also able to explain why ice ages always begin slowly and end relatively quickly. "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," says Blatter. The model not only takes the astronomical parameter values, ground topography and the physical flow properties of glacial ice into account but also especially the climate and feedback effects. The researchers obtained their results from a comprehensive computer model, where they combined an ice-sheet simulation with an existing climate model, which enabled them to calculate the glaciation of the northern hemisphere for the last 400,000 years. With the aid of the feedback effects, however, this is now possible. Given the fact that the 100,000-year insolation cycle is comparatively weak, scientists could not easily explain the prominent 100,000-year-cycle of the ice ages with this information alone. If you examine this variation in detail, different overlapping cycles of around 20,000, 40,000 and 100,000 years are recognisable. Because Earth's rotation and its orbit around the sun periodically change slightly, the insolation also varies. It has long been clear that the climate is greatly influenced by insolation on long-term time scales. "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." Moreover, large-scale glaciation also alters the sea level and therefore the ocean currents, which also affects the climate.Īs 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. "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 Blatter, explaining this feedback effect. 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. 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. Eventually, the pendulum swings back: it gets warmer and the ice masses melt. ![]() Ice ages and warm periods have alternated fairly regularly in Earth's history: Earth's climate cools roughly every 100,000 years, with vast areas of North America, Europe and Asia being buried under thick ice sheets.
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