[MUSIC] Welcome back to the arctic. In this lesson, we'll be learning about the arctic cryosphere, the portion of the Earth's surface that consists of snow and ice. In the previous lesson, we talked about atmospheric and oceanic circulation. Now we'll learn about how ice forms on the Earth's oceans and on land and how it relates to the atmosphere and ocean. One of the first types of ice that comes to mind when thinking about polar regions is sea ice. Sea ice can be found covering the Arctic Ocean and also surrounding Antarctica. This ice grows during winter and melts during summer, though in the Arctic Ocean, much of it remains all year. In total, about 15% of the world's oceans are covered by sea ice over the course of a year. The Arctic Ocean is covered by sea ice for much of the year, with the ice waxing in winter, and waning in summer. What do you think is the southern limit of sea ice in the Northern Hemisphere during the winter? Is it A, the Arctic Circle, the latitude of 66 degrees north. B, the entrances of the Arctic Ocean from the Atlantic and Pacific sides. C, the shallow continental shelves of the Eurasian and North American Arctic. Or, D, depending on water conditions, sea ice can extend farther south than the Arctic Ocean proper. Answer D is correct. In the Northern Hemisphere, sea ice is not confined to the Arctic Ocean alone. It can actually occur farther south. There are examples of winter sea ice in the Bering Sea, the Baltic Sea of northern Europe, north of the Japanese island of Hokkaido, off Canada's east coast, and even the Gulf of Saint Lawrence. But what is sea ice anyway? Well, simply put, sea ice is frozen sea water. It forms when conditions are cool enough for sea water to freeze. The various salts dissolved in sea water makes sea water taste salty. A salinity of 35, for example, means that 35 grams of salts are dissolved in one kilogram or about a liter of water. It is this salinity that lowers the freezing temperature of the ocean. The saltier the water, the lower the freezing temperature. At a salinity of 35, for example, the freezing point is at -1.8 degrees Celsius. This is very different to fresh water which freezes at 0 degrees Celsius. Ice has the unique property that it is less dense frozen than when it was a liquid. This means that sea ice floats on top of the ocean. There are many classifications of sea ice. New ice forms when conditions are cool enough for sea water to freeze. The salinity of the water must be low to prevent density driven sinking. In calm conditions, long needle or plate-like crystals develop in the top centimeters of the water. This is frazil ice. If frazil ice continues to grow, crystals combine into grease ice, giving the ocean an oily appearance. Grease ice may grow, particularly under windy conditions, into spongy white lumps known as shuga. When affected by wind and waves, grease ice and shuga can grow additional frazil crystals along the margins until circular pieces with upturned rims are formed. This pancake ice can merge into larger pieces, and may eventually form a continuous cover of sea ice. When sea ice accumulates into a continuous but bendable crust up to ten centimeters thick, it is called nilas ice. When the sea ice grows thicker than ten centimeters, it is known as young ice and will no longer flex with moving waves, often breaking into fragments. Once ice crust is formed on the ocean surface, the ice will grown downwards by freezing water onto its base. This will only happen if the air temperature is colder than the water under the ice, and the water itself is at the sea water freezing point. Heat is transferred from the warmer water through the ice by conduction to the colder air above. The rate of thickening is proportional to the difference in temperature between air and water, but inversely proportional to the ice thickness. This means that when there is a greater temperature difference between air and water, the ice will grow faster. Since Arctic Ocean surface water changes very little throughout the year, this usually means colder air temperatures are required for continuing ice growth. Similarly, the rate of sea ice thickening slows down as the ice grows thicker. Snow that falls on sea ice promotes and preserves sea ice. What in particular does snow do to achieve this? A, It acts as insulation blanketing the sea ice. B, It decreases reflectivity, promoting more absorption of solar radiation. C, It increases albedo. Or D, It promotes less absorption but more reflection of incoming solar radiation. More than one answer may be correct so check all that apply. Answers A, C and D are correct. Snow acts as insulation for the ice from the cold atmospheric temperatures because it has a lot of air between snow crystals. Snow also protects sea ice from absorbing more solar radiation by increasing the surface albedo. And reflecting a large portion of incoming energy. After young ice forms a continuous crust, sea ice is typically classified according to its age. Young ice thicker than thirty centimeters but thinner than two meters is called first year ice. First year ice is relatively rich in salts that form a honeycomb like channel system inside the ice. These unfrozen brine channels serve as a living space for various microbes, algae and small crustaceans that fish and marine mammals feed on. Sea ice that survives a summer season to regrow the next winter becomes Second Year Ice. If sea ice has survived at least two summer seasons, it is called Multi-Year Ice. This is typically between three and seven meters thick. During summer melt, the brine contained in first year ice drains away, leaving only pure ice. This makes second and multi-year ice relatively salt free and composed essentially of fresh water. Multi-year ice cannot sustain life. But when it is melted, it is completely drinkable. In fact, many Arctic expeditions have relied on multi-year sea ice for their drinking water supply. In the Arctic, multi-year sea ice is typically concentrated on the North American side. Piled up against the Canadian Arctic Islands and Northern Greenland. Why do you think this is? When considering your response, think back to lesson two. A, Due to winds blowing off the surrounding land masses. B, Due to the oceanographic circulation in the Arctic Ocean. C, Due to the Beaufort Gyre and Transpolar Drift. Or D, Due to the Gulf Stream? More than one answer may be correct, so check all that apply. Both B and C are correct. The multi-year ice found against the Canadian Arctic islands and northern Greenland, Is due to the prevailing Arctic Ocean circulation of the Beaufort Gyre and Transpolar Drift. In addition to this, the islands stand in the path of a direct through flow from the Arctic Ocean to the Atlantic. Sea ice is further classified by whether or not it moves. Sea ice physically attached or frozen to the shore line is Fast Ice. This anchored sea ice can extend as much as several hundreds of kilometers from shore. Beyond the limits of Fast Ice, sea ice can drift with winds and ocean currents. This is Drift or Pack Ice. This ice is formed over the central Arctic and the North Pole. But also at the southern reaches of sea ice extent. Pack ice is composed of individual floes, which are pieces of ice that range from 20 meters across and can grow to more than 10 kilometers. Within regions of pack ice, ice floes may cover anywhere from less than 10% to as much as 100% of the ocean surface. A special example of thick multi-year sea ice is that of sea ice shelves. Unrelated to glaciers, these are found only in northern most Canada on the north coast of Ellesmere Island. Formed over as many as 5,500 years by basal freezing under landfast sea ice, these ice shelves grew up to 40 meters thick and covered over 9,000 square kilometers. As recently as the late nineteenth century. These ice shelves existed for at least three thousand years. Notably, in the last one hundred years, nearly all of the original Elsmere Island ice shelf has broken away as ice islands as a result of rapid climate warming. Less than 1,000 square kilometers of the shelf has been left tenuously intact. This remainder is expected to break up and drift away in the next several years. If you were to walk from Greenland to the North Pole, what do you think the surface you would be walking on would consist of? A, strictly land. B, large forms of flat ice allowing for easy passage. C, various forms of ice, and lack of it, that would become more difficult as you progress. Or, D, patchy areas of ice that would be difficult to circumnavigate. The answer is C, as we will see in our next section. Imagine, we are part of an expedition walking to the North Pole, starting from northernmost Canada or Greenland. Each of us is pulling a sled loaded with over 100 kilograms of food and equipment, over roughly 800 kilometers for several weeks. We will encounter many different types of ice with different travel characteristics. Our first leg across the ice shelves and fast ice would be fairly smooth and relatively relaxed. But as we move farther north, the edge of the fast ice would quickly approach. The ice floes, drifting with the ocean currents, collide with the edge of the fast ice, creating vast piles of crushed ice. Between the mobile and the fixed ice would be stretches of patchy open water called a flaw lead. The most important flaw lead is the circumpolar flaw lead, as it stretches almost continuously from northern Greenland all the way around the Arctic Ocean to Svalbard. To continue our journey, we would have to cross the flaw lead, pulling the sleds across broken ice would be hard and exhausting. If we were to find a place to cross from fast ice to pack ice, without having to travel across water, we would be fortunate. Sometimes, we would easily cross from floe to floe. Other times, we would walk several kilometers around a great gap between floes known as a lead, formed as gaps between floes or in areas where ice floes diverge due to wind and current patterns. Leads are often many kilometers long and can be impractical to travel around. We might use floatable sleds and emergent suits to float across these but partially frozen leads would be particularly challenging, as the ice may not be able to support our heavy loads. Frozen leads however may be a blessing, as they can provide a smooth highway across the pack ice. In our travels, we would also come across great masses of broken, jumbled ice. These pressure ridges form when winds and currents converge, pushing ice flows into one another. Sails, up to 12 meters in height, and sub-sea keels as deep as 45 meters, can form as a result of this action, and the ice flows can be tens of kilometers long. Sails and keels interact with wind and currents to move the pack ice in various directions, pressure ridges evolve over time ultimately stabilizing, sealing, and losing their surface prominence, especially in multi-year ice. A special type of pressure ridge, called stamukhi, is formed when the pressure ridge impacts the sea floor, typically at the junction of fast ice and pack ice. When one floe is forced over another, this is rafting rather than ridging. It results in less depredation but still thickens the ice. Having faced the perils of drifting ice, leads and pressure ridges, eventually, we might reach the North Pole. If we planned our trip right, and the ice is sufficiently thick and stable, an aircraft may be able to fly in to pick us up. Otherwise, it's a long walk back with all the same challenges on the return journey. Besides leads, it is impossible that on a journey across the ice we may encounter regions that remain unfrozen throughout the year. These could be, A, regions of persistent ice free conditions. B, caused by persistent winds. C, sitting over undersea volcanoes. Or, D, associated with exceedingly fresh surface water. More than one answer may be correct, so check all that you think apply. Both A and B are correct answers. These open water areas are called polynyas from the Russian word meaning natural holes in the ice. Polynyas are formed where winds persistently blow ice away from a coastline or away from the fast ice edge. This open water causes sea ice to form continuously. Extremely salty, very dense and very cold brines are formed this way. These can sink into the ocean and sometimes to the ocean floor as deep water. Polynyas are critical for marine life as they enable year round primary productivity by phytoplankton. This allows marine animals, including fish, seals, walrus, and whales, to remain in the Arctic throughout the winter without the need to migrate south. Polynyas are especially important to marine mammals because they serve as breathing holes. Which is why when they freeze, it leads to mass deaths. Polynyas are important feeding areas for polar bears because of the abundance of their favorite food, seals. In fact, Polar bears have greatly benefited from polynyas freezing over, allowing them increased access to their prey, which are stuck on the ice and unable to escape. The largest Arctic polynya is the North Open Water, which is located between northwest Greenland and Ellesmere Island at the north end of Baffin Bay. This polynya usually occupies the same location year after year. When summer temperatures increase to between -5 and 0 degrees Celsius, the snow on the surface of sea ice begins to melt. These form interconnected pools on the ice surface. Water has substantially lower albedo and absorbs more energy. This leads to increased melt. Eventually, the water will drain into the ocean through existing cracks. The deepest melt pool may break through as a thaw hole. Because the melt pools absorb more heat, the underside of the ice begins to bend, forming something of a mirror image of the surface. As melt progresses, water begins to percolate through the brine filled pore spaces, eventually removing nearly all of the brine from the ice. This is the process that allows ice that survives the summer melt season to become multi-year ice. Since 1979, satellite observations have demonstrated a rapid decrease in the area covered by Arctic sea ice in both winter and summer. The average thickness of sea ice has decreased and the proportion of multi-year ice has diminished. This can be seen in the disintegration of the Ellesmere Island ice shelves that had previously existed for over 3,000 years. These trends suggest that the Arctic Ocean will become ice free in summer before the year 2100. Perhaps even as early as 2050. Scientists have determined that the observed sea ice reductions have resulted from human induced climate warming. In the next lesson, we will talk about some of the implications of such sea ice reductions for people and economic activities in the Arctic.
