Tiergarten Nürnberg

As their German name (Eisbär = ice bear) indicates, polar bears rely on sea ice. It is here that the seals, their main prey, live. In spring, seals give birth to their young on the pack ice, they dig breathing holes and they rest before hunting for fish in the sea. If the pack ice melts, polar bears will not only lose their shelters but their entire hunting grounds.

Seals are easiest to hunt in spring and summer. This is the time when polar bears have to make up for the weight loss of the long polar winter. And this is why they are particularly hard hit by the decrease in pack ice during the summer. Bears which originally preferred to roam in areas beyond the coast must now follow the sea ice further and further to the north – were there is less food to be found. The females lose contact with their traditional birth dens on land. Other polar bears who habitually spent their summers on the coast now lose valuable hunting time in spring and autumn, because the sea ice withdraws earlier or comes back later.

Scientists assume that polar bears lose ten kilograms of body weight for each week in which they cannot hunt on pack ice. As a consequence of this lack of nutrition, their reproduction rate goes down, and the cubs' chance of survival decreases. The increasing number of cases of cannibalism in Alaska and Canada could also be linked to the lack of food.

Between 1978 and 2005, sea ice decreased by an annual average of 8 percent, with a summer average of 22 percent. In summer 2007, the ice on the North Polar Sea covered the smallest area since documentation began. Pack ice laid down in several annual layers is increasingly replaced by thin ice which has developed in only a year.
Research by the Alfred-Wegener-Institut (AWI) for Polar and Oceanographic Research in Bremerhaven showed that, in 2007, the pack ice in the central Arctic basin was only about one metre thick, half the thickness of 2001. "Currently we are observing an accelerated retreat of ice" says oceanographer, Ursula Schauer, who played a leading role in the measurements on board the research vessel, "Polarstern".

Sea ice is influenced by very many factors. "Therefore nobody can predict exactly how the ice in the Arctic will develop", says Schauer. But there are many clues that the Arctic Ocean might be completely ice-free in less than 50 years. According to model calculations presented in the 2007 UN Climate Report, the Arctic summer ice will have completely disappeared by the end of the 21st century.

This would have dramatic consequences for the white bears. Steven Amstrup, director of the Polar Bear Project at the Alaska Science Center in Anchorage, fears that their number will decrease by two thirds by the middle of the century.

Unfortunately, the future extent of the summer pack ice is very difficult to predict. Any one influence follows strict laws of physics, but the inter-linking and feed-back between various climate factors is extremely complicated, and in some instances has not been researched at all. Ice is not just ice. There is a reason why the communities of the Inuit and Cree have over 80 words for sea ice.

Everything is connected to everything else. Because of its salt content, sea water only freezes at –1.86 centigrade, and salt is excluded when the water crystallizes. It then collects in channels and hollows (pockets of brine) which make sea ice look like Swiss cheese. Some of the salt gets into the sea water, increasing its density.

The heavy, cold water sinks to the bottom of the sea and triggers the North Atlantic circulation of water, one of the motors of world wide climate events. In principle, cold deep water flows to the South, and is replaced by warm surface water. If the pack ice melts, a large amount of freshwater is created which could considerably disturb this circulation. The melting of large areas of ice on Greenland would have similar consequences. At present, this disturbance cannot be detected, but a decrease of about 25 percent in water circulation is predicted by the end of the 21st century.

Wind and currents also have a role to play. They push the ice together into pack ice floes which can be up to 120 metres thick. This "shunting" creates variable thicknesses in the ice. The thicker the ice layer, the more it insulates, and the less heat can be exchanged between the ocean and the atmosphere. This control loop also influences global weather events.

Then there is the so-called Albedo Effect. The albedo is a measure of the ability of surfaces to reflect radiation. A layer of ice reflects about five times more sun radiation than water. The less ice, the less heat can be reflected into outer space, the more the earth heats up. Snow has a higher albedo than ice, dry snow a higher albedo than wet snow, thick ice a higher value than thin ice etc.
Because of the complexity of correlations and the huge climate changes in the course of the earth's history, scientists are finding it hard to prove that the present global warming was caused by humans and therefore may be affected by them. Lobbyists, amateur researchers and scientists hell-bent on publicity keep repeating the opposite. And all the time the weather is doing summersaults. So, for example, Greenland's Disko Bay, where in 2007 politicians were looking woefully at melting icebergs, is currently experiencing the harshest winter for the last ten years.

One should not be too impressed by doubters. In 1998, the United Nations established the reliable "Intergovernmental Panel on Climate Change" (IPCC). The IPCC does not do its own research, but evaluates the findings of climatologists all over the world. Over 3,700 experts collaborated on the fourth assessment report of 2007. The report is based on data collected before 2005. "That's why the wording is weaker than it would be if it was based on current research", emphasises Schauer.

The UN Report includes the following core statements:

The authors reach the conclusion that very probably humans were responsible for global warming. "The report is reliable", explains oceanographer Schauer. "The warming we measure cannot be explained by anything other than the greenhouse effect."

Schauer is referring to the “anthropogenic greenhouse effect” - the increase in so called greenhouse gases collecting in the atmosphere because of human activities. These gases include carbon dioxide, methane, laughing gas, fluorinated hydrocarbons, some fluorine compounds and ozone. The "climate killer" is carbon dioxide, present in the largest quantities, and created by burning fossil fuels (coal, crude oil, natural gas) and bio mass (wood, plant oils).

Every human being exhales about 350 kilograms of carbon dioxide per year. If you add up the carbon dioxide exhaled by all the organisms on earth, it comes to about 550 billion tons per year. However, this has no influence on climate, since roughly the same amount of carbon dioxide is extracted from the air by photosynthesis. But there are a number of other sources (such as volcano eruptions) which increase the "natural" carbon dioxide content of the atmosphere. For at least 650,000 years, this value was below 0.028 percent (280 ppm).

As early as 1859, the Irish physicist, John Tyndall (1820-1893) discovered the importance of carbon dioxide for the earth's heat balance. He explained that it was not oxygen and nitrogen, but water vapour, carbon dioxide and ozone which were responsible for the absorption of radiant heat. Years before, the French physicist, Jean Baptiste Joseph Fourier (1768-1830) suspected that the earth's atmosphere was able to store the heat of the sun like in a greenhouse.

Window panes are a good model for explaining the function of greenhouse gases. Visible light can go through the glass almost unhindered. An object inside the glass is heated up by the light and now emits long-wave infrared radiation. Because the thermal radiation cannot get through the glass, the temperature inside the house rises.

Greenhouse gases have the same effect as a window pane, because they stop long-wave thermal radiation from escaping to outer space. Without the atmosphere with its "natural greenhouse effect", the average temperature on earth would not be +15 degree centigrade, but –18 degrees centigrade.

So the problem is not carbon dioxide itself, but the increase in the atmosphere’s carbon dioxide content caused by humans. The knowledge that we can change the climate, is nothing new. As early as the end of the 19th century, the Swedish analytical chemist, Svante Arrhenius (1859-1927), examined the question of how the burning of fossil fuels might affect the climate. However, he did not foresee any problems then. He is reported to have said: "The increase of carbon dioxide will permit future generations to live under warmer skies."

His grandson, Gustav Arrhenius, proposed the construction of a carbon dioxide measuring station on the volcano Mauna Loa on Hawaii. Since 1958, continual measurements have been taken there. In 1973, the Club of Rome published a report on the world situation entitled "The Limits to Growth" - which reached a wide public. Based on the measurements on Mauna Loa, scientists predicted a carbon dioxide content of 380 ppm for 2000. This value was exceeded in 2006, and in 2007 the measured value was 383.72 ppm.

Today, there are further predictions for climate change, and the chances are quite good, that politicians will finally take them seriously. The Potsdam Institute for Climate Impact Research (PIK) names 16 possible anthropogenic "tipping elements" which might completely upset global climates. Five of them concern the Arctic:

With human assistance some polar bears might be able to survive even these disaster scenarios. Homo sapiens will be able to adapt, or at least the wealthier sections of the earth's population will. The question is whether we have to take the risk of a completely changed world, while alternatives are available.

Mathias Orgeldinger