Ice ages follow a distinctive rhythm, at least in recent Earth history. A look at the climate history of our planet shows this: huge masses of ice have repeatedly grown and shrunk on the mainland of the Northern Hemisphere. The most recent Ice Age came to an end about 10,000 years ago, the second youngest about 130,000 years ago, the third youngest about 220,000 years ago. Even in the time before that, it seems that another ice age ended about every 100,000 years. However, this rhythm can only be found in the Younger Pleistocene. If you look further back – up to 2.6 million years – you will find a different, faster climate rhythm of around 41,000 years.
Why did the fast clock of ice ages end and went into a slow? This is one of the most important questions of paleoclimatology. She has been doing researchers for decades. The answer to this is by no means just an academic pointed free. It is about how reliable climate models are, says Russell Drysdale from the University of Melbourne. "If you can map this change, this increases our trust that you can also represent future development."
So far, this has not been successful or at most with various assumptions. Drysdale and other scientists are currently trying to reconstruct the earth's history in a particularly exciting era. It is the time ago between 1.2 and 0.8 million years, it is referred to as "mid-pleistocene transition" (MPT): the transition from the 41,000 to 100,000 world in the middle Pleistocene. The climate changes at that time can be seen from various deposits in the deep sea. But the data contains uncertainties and partially contradict each other. The more precisely the climate events are broken down, the better it is to clarify the enigmatic change, the researchers hope.
So far, they have "worked back" about 800,000 years and have reliable climate data for this period. They come from ice cores extracted in Antarctica. The paleotemperature can be determined on the basis of trace gases such as carbon dioxide, but also isotopes of oxygen and hydrogen. These results are supplemented with data of oxygen isotopes from deep-sea sediments. Dripstones are also important climate archives, which, in addition to the oxygen isotopes, contain small amounts of uranium – and thus help to accurately time striking climate changes via radiometric age dating.
The end of ice ages is clearly recognizable in this data because there are abrupt changes within a few millennia. Warm times, so -called interglacials, follow the phases between the ice ages. Eleven of them have been identified in the past 800,000 years. When looking at the timeline, it becomes clear: The 100,000 rhythm is on the one hand an average indication that has a certain range. On the other hand, the data contains two interglacial more than would be expected in a purely mathematical manner during this period. In addition we come later.
Earth's orbit changes with the same frequency
Why the interglacial roughly return every 100,000 years is one of the unsolved problems of climate research. In principle, the changing earth orbit could be responsible with the same frequency. It changes between a more circular and more elliptical path. This is referred to as the eccentricity. As a result, the sun's rays in the northern hemisphere varies. However, the effect is too small to explain the advance and withdrawal of the ice masses. The change from 41,000 to 100,000 rhythm cannot be explained.
There are other Earth parameters that lead to alternating solar radiation and have become known as Milanković cycles. This includes precession – the gyroscopic motion of the Earth's axis is involved here - with a periodicity of 19,000 and 23,000 years, respectively. The inclination of the Earth's axis relative to its orbital plane around the Sun also changes, with a period of 41,000 years.
This rhythm can be found in the climate data from 2.6 to 1.0 million years before today, but it should be noted: there are fewer data from this time, and the signal is weaker. Only in three quarters of the cases there was a real interglacial, characterized by the disappearance of the large landing masses in North America and Eurasia, such as Peter Köhler from the Alfred Wegener Institute (AWI) in Bremerhaven and Roderik van de Wal close a modeling study in »Nature Communications«.
Does an additional cooling play a role?
What happened then, why did the rhythm change from the 41,000 to the 100,000 in the recent past? There are various attempts at explanation, which Constanijn Berends from the University of Utrecht and Peter Köhler and colleagues summarized last year in "Review of Geophysics". Many assume that it became cooler on average during the Pleistocene. There is clear evidence for this assumption in climate archives, although some data on the CO2 content of the atmosphere and carbon isotopes in sediments are contradictory, the authors write.
At a lower temperature, according to the consideration, ice shields would no longer melt completely in times of stronger sunlight - after 41,000 years. The interglacial fails, so to speak. The cold period continues and only ends in the next round after 82,000 years, possibly only in the third round after 123,000 years. Or in the phase in between, after all, the climate is not only influenced by the change in the sun's rays by the Milanković cycles, but also by various feedback processes. Among other things, this includes the ice-albedo feedback: the bright masses reflect more solar radiation than ordinary ground or sea water. As a result, the temperature drops, and the ice mass grows beyond the edges and the height, where it is even colder and the conditions are all the better. A glacier can also survive a phase of increased sunlight in this way. Conversely, the melt can be accelerated when the ice masses become unstable, breaking, losing height and more dark areas are heated up in the neighborhood.
Other studies see a connection with octreatic circulation. The colder the surface water is, the more CO2 it can absorb from the air, which increases the cooling effect. The process is further tightened during far -reaching icing when the glacier dust that has been used promotes the growth of phytoplankton, which in turn absorbs further CO2 from the air. Analyzes of sediment kernels from the deep sea indicate that currents changed during the MPT. How strongly they influence the rhythm of the cold times or whether they are only one consequence is open.
The researchers hope to use further data from climate archives to crack the "MPT enigma," as Köhler puts it. "The decisive factor will be ice cores from Antarctica, which provide information beyond the 800,000 years reached so far, for example on the CO2 content of the atmosphere." In this and the next two seasons, the relevant shifts should be achieved, he says. In addition, some time is still needed for measurements, data analysis and publication – but the goal is close. "If you combine these with measurements from deep-sea sediments, this gives a more accurate picture.«
Russell Drysdale continues his archive work in the Tuscan karst cave Antro del Corchia. Using dripstones, he and other researchers dated climate change up to 960,000 years before today. He was there again in autumn and took more rehearsals. "We can decrease up to 1.4 million years and dated changes in this time," he says.
Next ice age falls completely out of rhythm
So more and more pieces of the puzzle come together. The researchers' detective work is not necessarily easier. The CO2 content is largely the same, but the isotope data, from which temperatures, salt content and precipitation can be read, vary depending on the location. The models that can replace the change from the 41,000 to 100,000 world must be correspondingly complex.
It is not excluded that the strict distinction could be eliminated in the future. "Some people think that we are basically dealing with a 41,000-year rhythm that continues to this day," says Köhler. Only that recently interglacial ones have failed more often due to a wide variety of interactions. The eccentricity with its 100,000-meter clock would be less powerful than previously suspected. "This idea can certainly be taken seriously," says the AWI researcher, "but it has not yet been accepted in the entire research community.«
Most likely the next ice age will not follow any of the rhythms mentioned. It should have started shortly before the start of the industrial revolution, but high CO2 content and a low eccentricity of the earth's railway prevented that, wrote Andrey Ganopolski and his colleagues from the Potsdam Institute for Climate Impact Research (PIK) 2016 in "Nature". The beginning of the next icing would therefore shift by several millennia - without humanity. If one takes into account the amounts of greenhouse gases and their climate effect that have been released since then, it will take at least 100,000 years before the northern hemisphere is again covered by huge ice shields.
