Why are the ice ages happening?

Imagine the Chicago skyline. Now imagine it under almost 3 km of ice. That's what the landscape looked like in the height of the last ice age.

In the context of the recent geological history of the Earth, this would not have been so unusual. Over the past 2.6 million years (or so-called Quaternary), the planet has undergone more than 50 ice ages, with warmer interglacial periods in between.

But what causes periodic expansion of ice caps and glaciers? Glacial times are influenced by a complex and interdependent set of factors, involving the Earth's position in the solar system and more local influences, such as carbon dioxide levels. Scientists are still trying to understand how this system works, especially because man-made climate change may have permanently broken the cycle.[[[[Has the Earth been so hot already?]

Scarcely a few centuries ago, scientists began to recognize traces of the cold past. In the mid-nineteenth century, Swiss and American naturalist Louis Agassiz documented the marks left by glaciers on Earth, such as displaced rocks and clusters of giant debris, known as moraines, which he suspected old glaciers. pushed over long distances.

In the late nineteenth century, scientists named four ice ages that occurred during the Pleistocene epoch, which lasted from about 2.6 million years up to about 11,700 years ago. It was only decades later, however, that researchers realized that these cold periods were much more regular.

A major breakthrough in the understanding of the cycles of glaciation took place in the 1940s, when the Serbian astrophysicist Milutin Milankovich proposed what is now called the Milankovitch cycles, information on the movement of Earth that are still used to explain the climate change.

Milankovitch described the three main ways in which the orbit of the Earth varies with respect to the sun, said Mark Maslin, professor of paleoclimatology at University College London, at Live Science. These factors determine how much solar radiation (in other words, heat) reaches the planet.

First, there is the eccentric shape of Earth's orbit around the sun, which ranges from almost circular to elliptical over a 96,000-year cycle. "The reason she has this bulge is because Jupiter, which accounts for 4% of the mass of our solar system, has a significant gravitational effect, displacing the Earth's orbit backwards then towards the back, "explained Maslin.

Second, there is the tilt of the Earth, which is why we have seasons. The inclined axis of The rotation of the earth means that one hemisphere always leans away from the sun (causing winter) while the other leans toward the sun (causing the summer). The angle of this inclination varies over a cycle of about 41,000 years, which changes the magnitude of the seasons, Maslin said. "Yes [the axis] is more straight, so of course the summers will be less hot and the winter will be a little colder. "

Third, there is the oscillation of the inclined axis of the Earth, which moves as if it 's been a spinning top. "What happens is that the kinetic moment of the Earth turns very fast once a day and causes the axis to flicker Maslin also said. This flicker occurs every 20,000 years.

Milankovitch determined that the orbital conditions of the cool summers were particularly important precursors for the ice ages. "You will always have ice in winter," said Maslin. "To create an ice age, some of these ice must survive during the summer."

But, to move to the ice age, orbital phenomena are not enough. The actual causality of an ice age is the fundamental feedback in the climate system, Maslin said. Scientists still mock how various environmental factors influence glaciation and deglaciationbut recent research suggests that levels of greenhouse gases in the atmosphere play an important role.

For example, scientists at the Institute for Climate Impact Research (PIK) in Potsdam, Germany, have shown that the beginnings of the ice age were mainly due to: decreased carbon dioxide and that the dramatic increase of carbon dioxide in the atmosphere, due to anthropogenic emissions, has probably suppressed the onset of the next ice age up to 100,000 years.

"Like no other force on the planet, the ice ages shaped the global environment and thus determined the development of human civilization," said Hans Joachim Schellnhuber, then director of PIK and co-author of the IKF. one of these studies, in a statement. in 2016. "For example, we owe our fertile soil to the last ice age that also shaped today's landscapes, leaving behind glaciers and rivers, forming fjords, moraines and lakes. the humanity that emits burn fossil fuels which determines the future development of the planet ".

Original article on Science live.