Climate Research: When the Earth was a Snowball

By human standards, our little planet is incredibly old. The Earth has already been around a remarkable 4.5 billion years. However, life-friendly conditions only emerged much later. In general, the comparatively temperate climatic conditions under which modern humans have spread over the last 200,000 to 300,000 years are a relatively new phenomenon. Of course, this is by no means a reason to put them at risk carelessly, as is currently happening.

But that’s another story. This is supposed to be about a period more than 600 million years ago when the earth was an ice desert. Evidence of this has been discussed since the 1980s, and now scientists at University College London in northern Scotland have found uniquely good evidence. These allow one of these glaciation periods to be limited to the period between 717 and 658 million years before the present.

The evidence was found in a geological formation in northern Scotland. At the bottom there is a layer of carbonate rock that was formed at the bottom of a tropically warm ocean. Above this, the researchers were able to identify rocks that were formed from sediment from a very cold, icy sea. The pressure of later deposits turned both layers into rock over millions of years, which was eventually folded and brought to the surface by shifting continental plates.

Without greenhouse gases, the earth would be around 30 degrees Celsius colder on average.

–

To determine the age, the researchers took advantage of the special properties of the mineral zircon. This consists of a combination of the element zirconium with silicon and oxygen and can also include other elements when it is created. For example uranium, but definitely not lead. As a result, the mineral contains a uranium-lead clock, so to speak. The uranium initially enclosed in the zircon decays over time through nuclear fission, producing lead and gradually accumulating in the mineral. Its age – and thus the rock in which it is enclosed – can be determined from the ratio of lead to uranium. The researchers were therefore able to narrow down the glaciation very well from the dated sequence of tropical and Ice Age sediments. Her Study was published in the British “Journal of the Geological Society”.

But how could it happen that our planet is turning into a “snowball,” as it is called in the technical debate? The temperature on earth depends on three main factors. The incoming solar energy, the amount of this energy that is reflected and does not initially remain in the Earth system, and the amount of greenhouse gases in the atmosphere. The latter ensure that the heat radiation from the earth’s surface does not escape directly into space, but first warms the air. Without these greenhouse gases, the Earth would be on average around 30 degrees Celsius colder under current conditions, meaning it would be an ice planet.

nd.DieWoche – our weekly newsletter

With our weekly newsletter nd.DieWoche look at the most important topics of the week and read them Highlights our Saturday edition on Friday. Get your free subscription here.

That’s exactly how it was over 700 million years ago. The most important natural greenhouse gases are water vapor and the well-known carbon dioxide (CO₂). Water vapor is abundant in the atmosphere of a warm planet. The warmer, the more, or the less, the colder. A positive feedback that is kept in check by other mechanisms so as not to get out of hand in one direction or the other. CO₂ was abundant in the early atmosphere. As long as life is not present on a large scale – as was the case 700 million years ago – CO₂ mainly released by volcanism. Processes in the oceans and weathering in rivers remove the gas from the atmosphere.

These are very slow processes by human standards, but carbonate formation in the water and weathering can sometimes be faster as a continuous process because volcanism does not occur evenly. Periodically reduced solar radiation probably also contributed to putting the planet on the wrong track: the CO₂-Concentration in the atmosphere decreased more and more, and with it the temperature, which meant less water vapor. On Earth, which was no longer quite so tropical, ice formed at the poles and in the high mountains, so that the back radiation increased and the soil and water absorbed less solar energy. The temperature continued to drop until Earth eventually became an ice planet.

But how did she get out of this icing trap? The answer also lies in weathering and volcanism. On an ice planet it never or rarely thaws. Therefore there are no rivers and no CO₂-Degradation through weathering. However, volcanism continues on an ice earth, which is why the CO₂ could accumulate again in the atmosphere at that time. So much so that for a short time there was a kind of super greenhouse in which the ice finally disappeared.

This “snowball” episode had far-reaching effects: it triggered what some biologists call a “big bang” of evolution. Until the glaciation, life on Earth consisted primarily of single-celled organisms that fed on photosynthesis and inorganic trace substances available in water. But the harsher and nutrient-poor living conditions apparently gave evolution a boost. “Herbivores” came under drastically increased competitive pressure because less food was available. Four US scientists recently calculated this using different evolutionary models and came up with it a paper published in “Proceedings of the Royal Society B”. came to the conclusion that these extreme changes in environmental conditions probably formed the starting point for the development of all modern-day multicellular organisms. After the end of the glaciation, their ancestors would have spread explosively.

Subscribe to the “nd”

Being left is complicated.
We keep track!

With our digital promotional subscription you can read all issues of »nd« digitally (nd.App or nd.Epaper) for little money at home or on the go.
Subscribe now!