Fossils of mammoths from the North Sea floor, and palm trees that once grew around the North Pole. We sense that climate change and sea level changes are of all times. But is current climate change a natural phenomenon that we as humans cannot do much about?

Appy Sluijs from Utrecht University answers. He is a professor of paleo-oceanography and researches how and why the climate changed in the earthly past.

Isn't climate change of all times?

The climate on earth has changed countless times. But the change we are now experiencing is special for three reasons. For example, it is the first time that greenhouse gases are released into the air on such a large scale through the burning of fossil fuels, ie the use of coal, oil and gas.

In addition, the speed at which the CO2 concentration and temperature are currently rising is unparalleled; climate change from the past was almost always much slower than the current one. Finally, the current warming is taking place worldwide, as KNMI's Geert Jan van Oldenborgh showed earlier in this series. That is a difference with natural climate fluctuations from the past centuries, which often only played regional; in one place it warmed up, while it cooled elsewhere.

It is also clear that climate change in the past did have a major effect. Not only on the appearance of the planet, but also on life. For example, in periods when the CO2 concentration rose slowly due to natural causes, ice caps melted and sea levels rose. And then it became more extreme again, with major droughts or floods. That had major consequences for life on earth. Many species left or died out. Just like what is happening now. The fact that there were similar climate changes in the past does not mean that climate change will not affect us.

As a researcher, there is one climate change from the past that fascinates me the most. You could call it my favorite piece of earth history: 56 million years ago - on the border between paleocene and eocene geological eras - there was a sharp climate change that is similar to current global warming.

It was already warm on earth in the preceding period - the CO2 concentration was higher than today - and then, in a period of a few thousand years, about 5 degrees were added. Although the current climate change is considerably faster, geologically speaking it is incredibly fast.

The cause of that rapid warming? Possibly a huge eruption of methane from the seabed - a powerful greenhouse gas. The release of methane lasted several thousands of years. For comparison: we currently emit ten times as much greenhouse gases per year as the seabed at the time.

A fossil of a leaf from the warm eocene in the Arctic (Spitsbergen) . (Photo: polar researcher Marc Cornelissen)

The result? Palm trees and mangroves then grew along the edges of the Arctic Ocean. So it never froze on the North Pole. Because the sea water expanded due to global warming and the small amount of ice that melted in Antarctica at that time, the sea level rose. It was at least 60 meters higher than now. Some species died out, but for others it was an opportunity; fossils show a strong evolution of primates, of which we also belong, which replaced other species. We would not have been there without this heat record.

How do we know this?

Measurements of the temperature with the help of thermometers show us well what has happened to the climate in the last century. But they don't go back far enough to see what happened in the past. So we have to reduce that in other ways: temperature reconstructions. We do this, for example, with drilling in ice caps that are thousands of years old.

Those kilometers-thick ice masses consist of layers of compressed snow. That's why tiny ice bubbles are still trapped in that ice. By analyzing those air bubbles, we can measure the CO2 concentration of the former air - more than a million years ago.

To go even further back, and for example to study the eocene, we use layers that are deposited on the bottom of oceans. In it we find fossils of single-celled organisms and molecules that come from all kinds of different organisms. Because one organism likes warmer water than another, we can use the fossils to say something about variations in the former sea water temperature.

The fossil of a tropical alga (Apectodinium augustum) that Appy Sluijs found during his research in soil deposits of the Arctic Ocean. The fossil is 56 million years old - the border between the paleocene and the eocene. (Photo: Appy Sluijs)

But nowadays we mainly measure the chemical composition of the fossils. At higher temperatures, the precise chemical composition of lime pellets made by organisms in seawater changes. The chemical composition of a fossil sediment from the sediment is therefore a measure of the temperature of the seawater in which it was once made. As a result, we can now really say how warm the sea water was in the past.

From such research we know, for example, that the global temperature has hardly varied globally on average over the last ten thousand years. Regionally this is different. In England and Greenland it was quite hot in parts of the Middle Ages. This was because more heat was being supplied from the tropics by the Atlantic Gulf Stream. But in other places during the same period it was actually colder.

Climate fluctuations from the relatively recent earthly past were therefore mainly local phenomena - without much influence on the global average temperature. So it is really special that the global temperature has risen by 1 degree on average in the last century.

Conclusion: climate change is all-time, but current times are exceptional.

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See also: Climate demand: How is it possible that the temperature rises due to greenhouse gases?

See also: Climate change: How much warmer does it actually get?