A Mystery Solved

At the Museumsgesellschaft yesterday evening, Red Baron listened to a talk by Professor Benoît Sittler of the Institut für Naturschutz und Landschaftsökologie in Freiburg: Grönland im Griff des Klimawandels (Greenland in the grip of climatic change). Dr. Sittler, an Alsacien, worked from 1988 until his retirement in 2014 for the Karupelv-Valley Project in North-East Greenland National Park at an outpost on the eastern coast. Sittler studied, among other ecological topics, the lemming mystery.

There is a strong belief that when a lemming population becomes too big, the animals commit collective suicide to normalize their number for the food available in the region. Here is what Dr. Sittler and his collaborators found out:

The population of lemmings is closely related to that of one of their predators, the ermines. The story goes like this:

Lemmings are terribly reproductive, with four gestations during an arctic summer. A significant population of lemmings presents welcome food to ermines. However, their number increases only slowly with the increasing food supply, for ermines have an unusually long gestation period of nine months. The ermine population increases with more lemmings eaten, but their number will eventually decrease to a low level as there is no longer enough food for all of them. So, when the ermine population declines due to starvation, the lemmings population rises again*. This interaction formed a periodic cycle of four years until 2000 when a dramatic change in the population of lemmings was observed due to climate change.
*A colleague wrote to me that the Lotka–Volterra equations, a system of coupled multi-parameter differential equations with periodic solutions, describe such timely behavior. Thanks, Walter.


The two photos below show that Greenland is suffering from accentuated climatic change. Snow and ice in the mountains decreased dramatically between 1989 and 2007.

In 2000, Sittler's research team observed another anomaly: an increasing number of polar bears went ashore in their quest for food.

Starting around 2000, the ice floes from where the polar bears used to hunt seals, their favorite food, practically disappeared at the shore near Sittler's camp. The aggressiveness of those hungry polar bears became dangerous, so the researchers had to protect their tents with a high-voltage charged fence during the night.

Greenland is losing ice at a rapid rate. Icebergs are calving from the front fold line of glaciers:

Here is a graph showing the build-up of Greenland's kilometer-thick ice cap and how its weight depresses the land level.

With the lemming mystery solved, another mystery is shown in the following diagram:

Temperature black, CO₂ concentration blue

When analyzing deep borings of Greenland's ice cap, glacial and warm periods have been observed over the last 400,000 years. The periodic cycle of about 100,000 years is attributed, among other things, to changes in the position of the Earth in comparison to the Sun, now known as the Milankovitch cycle. At the same time, the CO₂ concentration of the air in the ice borings varying from 180 to 280 ppm runs nicely in parallel with the temperature, as the diagram shows.

In fact, over the last 10,000 years, the mean concentration of CO₂ in the atmosphere stayed around 280 ppm, but during the previous 150 years, that value rose to 350 ppm due to human activities. This increase above 280 ppm is said to be responsible for the observed global warming.

What can we learn from the above long-term graph? What caused the CO₂ concentration to vary with temperature? One mystery has been solved, but a new one must be understood.

N.B. All pictures are © Dr. Benoît Sittler

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