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The last ice age shows: The climate system reacts sensitively to our greenhouse gas emissions (symbol image)
Photo: Grafissimo/Getty Images
A third of the world's land area has disappeared under glacial ice.
A gigantic mass of ice, thousands of meters thick, covers all of Scandinavia, most of the British Isles and northern Germany down to the Berlin area.
The Alps also lie under a dense network of ice streams, from which only the highest mountain peaks protrude and which reach into the foothills of the Alps, where Bern, Munich and Vienna are today.
The huge Rhine glacier digs the Lake Constance basin out of the landscape.
Where there is no ice in Europe, woolly mammoths and woolly rhinos graze, and saber-toothed tigers and cave lions hunt.
Our ancestors, anatomically modern
homo sapiens
like us, lived as hunter-gatherers in caves and huts, using simple stone tools.
This is what happened at the height of the last ice age, the last glacial maximum around 20,000 years ago.
A group of American colleagues led by Matthew Osman from the University of Arizona recently succeeded in reconstructing the global climate over the last 24,000 years in unprecedented detail.
They linked more than 500 data series from all over the world with simulation calculations and solved some of the previously unsolved mysteries of recent climate history.
The existence of ice ages has been discussed by scholars since the 19th century at the latest, and Goethe also commented on it.
In the 1920s, Milutin Milanković was finally able to explain its cause, the cycles of the earth's orbit around the sun.
During these tens of thousands of years long cycles, the position of the Earth's axis in space and the shape of the Earth's orbital ellipse change.
I've been fascinated by these Milanković cycles ever since I learned about them in geography class as a high school student.
The study by Osman and colleagues now found that there must have been a cooling of 6 to 8 degrees Celsius at the height of the ice age.
Regionally, the cooling was even greater, up to 20 degrees, especially over the ice masses in the northern hemisphere.
The extent of glacial global cooling contains the
first lesson
for the future.
Because it allows an assessment of the so-called climate sensitivity, i.e. the sensitivity of the earth's climate to disturbances.
The stronger the reaction of the climate in the past, the stronger it will be to the greenhouse gases we produce.
In a separate publication, the US colleagues calculated the climate sensitivity from the ice age data: according to this, a doubling of the amount of CO2 in the atmosphere would lead to global warming of 2.5 to 4.5 degrees.
In a first study in 2006, we received a range of 1.2 to 4.3 degrees.
The industrialized countries in particular have already increased the CO2 concentration by half by using fossil fuels.
We are leaving the climatic range to which animal and plant species have adapted for thousands of years
The
second lesson
from the ice age results from the further global temperature development of the Osman study.
What is new is the finding that a “hump” in the Holocene curve that was present in earlier reconstructions – with a slight cooling since the middle Holocene – has now disappeared.
This hump has long been a discrepancy between data and computer simulations, known as the "holocene conundrum."
Here, as is so often the case, the computer models were right: the hump turns out to be the result of inaccurate averaging from the limited, unequally distributed data points.
This makes it even clearer than before that we are already moving further and further away from the experience of the civilization history of
homo sapiens
.
The past decade was most likely already more than half a degree warmer than the warmest decade of the previous 24,000 years, and a full 1.5 degrees warmer than the Holocene mean temperature.
We are also leaving the climatic range to which animal and plant species and entire ecosystems have adapted over thousands of years - and at a breathtaking pace compared to the slow Milanković cycles.
In the meantime, my colleagues at the Potsdam Institute can even successfully simulate the ice age cycles of the past three million years in the climate model, driven solely by the Milanković cycles.
The beginning of the next ice age can also be predicted, because these Earth orbital cycles can be calculated in advance with astronomical precision.
It would be due in 50,000 years.
I say "would be" because according to the calculations, this ice age will probably not happen - because even then there will still be enough of the excessive CO₂ already caused by the fossil fuel economy in the air to prevent this ice age.
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The
third lesson
from the ice ages: when it gets warmer, CO₂ comes out of the oceans like sparkling water on the stove.
This feedback massively amplified the effect of the Milanković cycles and ensured that the ice ages are global.
Today, the world's oceans still absorb a quarter of the CO₂ that we blow into the air, slowing down global warming.
But that should change with further warming and thus accelerate global warming, because experience has shown that a cold ocean (for physical, chemical and biological reasons) can hold more CO₂ than a warmer one.
At the transition from the last ice age, sea levels rose by 120 meters
The
fourth lesson
from the ice age: apart from the global increase in temperature, the second most important pattern of change since the ice age is a see-saw between north and south: when it is particularly cold in the north of the earth, it becomes warm in the south and vice versa.
This is partly a result of the Milanković cycles, but with more rapid climate changes it is a result of drastic fluctuations in the Atlantic circulation, which pumps vast amounts of heat from the southern hemisphere across the equator to the far north.
This heat pump began to sputter due to meltwater entering during the warming in the Holocene - a concern that is becoming more and more relevant now due to the ongoing global warming.
The
fifth lesson
from the Ice Age is also related to meltwater masses and has long been well documented.
During the transition from the last ice age to the Holocene, the sea level rose by 120 meters due to ice melt, flooding many flat coastal areas - for example the southern North Sea.
Even today, sea levels are rising faster and faster as a result of warming.
And we still have enough ice on the continents to raise global sea levels by another 65 meters.
We therefore cannot afford to lose even a few percent of this ice mass.
The last ice age shows that the climate system reacts sensitively to our greenhouse gas emissions, just as it reacted to the Earth's orbital cycles in the past.
The global warming that is already being caused has already reached geological proportions and will very likely change the earth significantly for tens of thousands of years.
And there are devastating consequences for mankind, which are not just theory, but empirical values from the past.
These include a destabilization of ocean currents and ice masses and a massive rise in sea levels.
In order to keep these consequences within a reasonably manageable framework, an emergency brake on greenhouse gas emissions is now necessary due to decades of political hesitation.
