Guest Post by Dr. Pål Prestrud, Director of CICERO, the Centre for International Climate and Environmental Research
Outside my office building here in Oslo, the capital of Norway, the ground and the roofs of the buildings are still covered with half a meter of white snow now in mid March. It has been like this for four months now. In the hills behind my office building the World Nordic Ski Championship is just finished. A big event for most Norwegians. I would guess it is difficult to imagine how it is for people living close to the equator.
Many people who live in the world’s northern and mountain regions would be astounded that the question in the title would even be asked. The answer is obvious: ice and snow are for fun! Frozen water is a perfect medium for Homo ludens, the playful human. Skiing, skating, sledding, snowmobiling, and ice-sculpting are activities many northerners enjoy and appreciate. But many also consider snow and ice a nuisance. It makes driving and walking difficult and the daily life more burdensome.
But snow and ice are much more important than for fun and recreation. Snow and ice are integral parts of the global climate system and have a cooling effect on the Earth. We all know from our own experiences that white surfaces stays cooler than black surfaces when they are hit by sun radiation. The reason is that much more of the sun radiation is reflected by a white surface than a black surface. The portion of the radiation that is reflected from the Earth’s surface is called albedo.
At this time of the year approximately 15% (50-60 millions km2) of the Earth’s surface is covered by white snow and ice which reflects 80-90 % of the incoming sun radiation. Snow and ice-free ground and oceans will absorb 80-90 % of the radiation and transform it into heat. In recent years the minimum extent (September) of sea ice in the Arctic has decreased by 11 percent per decade, while the maximum extent (March) has decreased by three percent. Similarly, the extent of snow cover on the Northern Hemisphere has decreased by seven percent the last 20-30 years. The sea ice extent in the Antarctica has shown an insignificant increase in the same period. Models project that the Arctic sea ice during summer could be gone in 50 years. Projections of changes in the snow-cover extent are much more uncertain, because a warmer Arctic means higher air humidity and potentially more precipitation as snow.
The tropics receive much more heat from the sun than the polar areas. The laws of physics tell us that the temperature differences will attempt to even themselves out through the transport of heat to the cold areas. Thus, there is a continuous transport of heat from the tropics to the polar regions by atmospheric winds and ocean currents. A warmer Arctic or Antarctica means that less of the tropical heat has to be removed by this transport, and consequently, the tropics may also get warmer when the polar areas warm up. This amplifying effect on the global temperature if the extent of snow and ice is reduced is an example of what the climate scientists call a positive feedback on the global climate.
And that is exactly what we are observing. The rate of warming of the Arctic is about twice the rate of the global warming. Snow and ice are melting. The climate models predict that the Arctic will warm up more rapidly than the rest of the globe because of this feedback process.
But there is also another potential positive feedback in the Arctic that could be of considerable significance for the global climate. Permanently frozen ground or permafrost makes up approximately 20-25 million km2 of the Northern Hemisphere. The uppermost part of the permafrost contains frozen organic material from vegetation that is being decomposed very slowly due to the low temperatures, and low biodiversity and number of microorganisms and invertebrates in the soil. These deep-frozen storages of organic material may decompose at a much higher rate if permafrost melts and the temperature increases. Then CO2 and CH4 will be released to the atmosphere, increasing the greenhouse effect and trapping more of the heat radiation from the Earth’s surface, and amplify global warming.
It is difficult to estimate the amount of carbon in the organic material in the permafrost, but it is in the order of one to two times more than in the atmosphere. In addition, there are huge amounts of methane (CH4) in hydrates deep in or under the permafrost with much more carbon than is found in the atmosphere. Because the organic material from dead vegetation is stored in the surface of the permafrost, much less warming is needed to release greenhouse gasses from this source than from the methane hydrates. The potential, however, for a significant increase in the concentration of greenhouse gases in the atmosphere is much bigger from the methane hydrates than from the organic material.
It has to be emphasized that there are large uncertainties in our understanding on how the natural carbon cycle will be affected by melting permafrost and releases of greenhouse gasses from natural sources. Presently, nature is taking up about 55% of the antropogenic emissions of greenhouse gasses. Whether an increase in the emissions from natural sources like those in the permafrost will remain entirely in the atmosphere or only parts of it, is highly uncertain.
The sea level rises by 3-3.5 mm per year. About half of this comes from the melt-water of alpine glaciers and polar ice-sheets. The potential sea level rise if all the ice on land melts is 60-70 metres. It is extremely unlikely that this will happen in a short time. But melting of only a small part of it – let us say less than five percent – may also cause considerable problems for coastal nations. The most recent scientific results indicate that the Greenland and Antarctic ice sheets are losing in the order of 3-400 gigatons of ice per year, which contributes to a sea level rise of 1.3 mm per year. The loss of ice from these ice sheets has increased by approximately 30 gigatons per year in the last decade. During the last inter-glacial (120 000 years ago), when the temperature was 2-3 degrees Celsius higher than today, the sea level was 5-6 metres higher than today.
Melting ice on land and on sea also have the potential to influence the big ocean currents. For example, the so-called thermohaline circulation is one of the drivers of the North Atlantic current which brings huge amounts of heat to northwest Europe and makes the living conditions there comfortable. When surface waters freeze, the water gets saltier. Cold and salty water is heavy and will start sinking. This process is called thermohaline circulation and takes place in the northernmost part of the North Atlantic. The downward flowing cold water flows to the South along the bottom of the western part of the North Atlantic. This process helps to pull more warm surface water to the north. Input of light fresh water from melting glaciers in the area and the movement of the freezing zone to the North may influence the thermohaline circulation and the North Atlantic current. Again our understanding of the process and how it may be altered is highly uncertain, and this makes it difficult to predict what will happen in the future during a global warming.
Snow and ice are crucial for the ecosystems and people inhabiting the regions regularly covered by snow and ice. The cultures and traditional way of living for indigenous peoples in these areas are strongly influenced by snow and ice. “We have a right to stay cold” as the president of the Inuit Circumpolar Conference stated it a few years ago. In many southern and dry regions people are dependent on the water supply from melting glaciers and snow in the mountains. The glaciers serve as water reservoirs for dry seasons. This is particularly important in parts of Peru and Central Asia, but also in the Himalayan region, which is called the water towers of Asia. Snow and ice are important for the big rivers which supply water for several hundreds of millions of people. How important is, however, difficult to quantify.