Tuesday, October 11, 2016

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 with respect to the food available in the region. Here is what Dr. Sittler and his collaborators found out:

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

Lemmings are terribly reproductive with four gestations during an arctic summer. A big population of lemmings presents welcome food to ermines. However, the population of the latter increases slowly with the increasing food supply for ermines have an unusually long gestation period of nine months. As the ermine population increases and that of the lemmings is eaten their population eventually decreases to a level that there is no longer enough food for all those ermines. Then, when the ermine population decreases due to starvation that of the lemmings starts to rise again*. This interaction formed a periodic cycle of four years until 2000 when a dramatic change in the population of lemmings was observed that is attributed to climatic change.
*A colleague wrote me that such a timely behavior is described by the Lotka–Volterra equations, a system of coupled multi-parameter differential equations with periodic solutions. Thanks, Walter.

Greenland suffers from an accentuated climatic change as the two photos below show. Snow and ice in the mountains decreased dramatically between 1989 and 2007.

Following the year 2000 the Sittler's research team observed another anomaly: an increasing number of polar bears going ashore in their quest for food.

The ice floes from where the polar bears used to hunt seals, their favorite food, practically disappeared at the shore near Sittler's camp starting around 2000. The aggressiveness of those hungry polar bears becomes 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 about the build-up of Greenland's kilometer-thick ice cap showing 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 are 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 Milankovitch cycles. It was noted that 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 last 150 years that value has risen to 350 ppm caused by 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 solved, a new one to be understood.

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

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