Determination of end of the Ice Age? – Niels Bohr Institute - University of Copenhagen

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Determination of end of the Ice Age?

Hello ice core researcher

I am interested in the newly publicized research that dates the end of the last Ice age to 11,710 years ago coinciding with a sudden warming period. I can't reconcile these results with sea level rises that show ice sheets beginning to melt ~18,000 years ago.

Lambeck 2004 has charted sea level changes for the past 20,000 years or more. His work suggests that the peak ice of the last ice age occurred when sea levels were lowest (120 meters below current sea level ~18,000 BP.)

The Lambeck chart below shows sea level changes for the past 20,000 years. Clearly massive ice melts occurred from 18,000 BP to 12,000 BP, which suggests that sufficient warming took place from 18,000 BP to 12,000 BP to melt most of the ice sheets of North America and Europe. This seems to contradict the findings of your research. Can you explain this contradiction please?

Kind regards J.S.

Answer from ice core researcher Jørgen Peder Steffensen, Ph.D. Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen

Part of your question and one of the reasons for your confusion stems from the fact that the term "Ice Age" is not well defined and people use it in different contexts.

Here in Northern Europe (or Scandinavia) the term ice age is often used for the period of time where either ice or arctic climate conditions (as reflected in European pollen records) existed far down in Northern Europe (Britain, The Netherlands, Denmark Germany and Poland).

The sea level record reflects the build up and decay of massive ice masses mainly in North America and Northern Europe. Records from both Antarctica and Greenland ice cores and Marine sediments and sea level records show that the last glacial (ice age) culminated about 20,000 years ago with the largest ice sheets.

This was the time when a big glacier also went down the Hudson river valley in New York and my little country, Denmark, was half covered with ice. At about 18,000 years ago the ice sheets began to retreat and the ice cores from both poles confirm that warming began. The ice sheet left Denmark some 16,000 years ago.

However in the North during the ice age climate was also subjected to fast climate oscillations, i.e. a flip-flop from cold to very cold conditions. This flip-flop or bipolar see saw continued to occur during the slow and gradual warming from 18,000 to 11,000 years ago. In Europe and Greenland there was a flip 14,700 years ago which gave Europe almost present day climate. Then 12,700 years ago the climate in Europe and Greenland flopped and reverted back to arctic conditions for 1000 years. This is called the Younger Dryas, as Dryas plant pollen are found all over in European sediments. Dryas is a tundra plant. Then finally 11,703 years before 2000 AD the climate flipped back into a warmer mode where it has remained ever since. You can see the effects of this flip-flop as steps in the sea level curve you have attached.

To summarize, the term ice age is in the eye (or mind) of the beholder. In Denmark the ice age ended 11,703 years ago as arctic conditions vanished for good. Globally however, the ice age began to go away 18,000 years ago and was completely gone 8,000 or 9,000 years ago as the last pieces to the Scandinavian and Canadian ice sheets melted away.

In a geological sense, we are still in an ice age as there is still ice in Antarctica and Greenland. The Antarctic ice sheet is roughly 35 million years old, and the Greenland one is roughly 1.5 million years old. Before 35 million years ago there were no ice sheets at all. We have to go back to the Permian period some 270 millions years back to find another period on Earth with significant ice volumes.

Kind regards
Jørgen Peder Steffensen