[MUSIC] Welcome back to the Arctic. Up to now, we've learned a great deal about the Arctic environment. But this environment is rapidly changing, largely due to industrial human activity throughout the world. You'll remember the natural greenhouse effect that we discussed in lesson one. Carbon dioxide and water vapor are among the natural occurring greenhouse gases in the atmosphere. They trap outgoing long wave radiation raising the average temperature of the Earth from a bone chilling minus 18 degrees Celsius, to a relatively balmy plus 15 degrees Celsius. Human activity has caused major environmental change throughout history. Some examples of this are the clearing of forests for agricultural land, to the hunting of species to extinction. Since the start of industrial activity in the late 1700s, human-caused climate warming has begun to strongly affect the entire planet. These impacts include locations far from intensive human industrial activity, such as the Arctic. In particular, fossil fuels such as coal, oil, gasoline, natural gas, and kerosene have been extensively burned to support human industrial activity. This has led to the release of staggering amounts of carbon dioxide, methane, and nitrous oxide into the atmosphere. The most abundant of the human produced greenhouse gases is carbon dioxide. It's concentration has increased from 280 parts per million in preindustrial times to greater than 400 parts per million in 2015. The last time CO2 was as high as today's 400 parts per million was more than 3.6 million years ago. When the Earth was a very different place and modern humans did not yet exist. As we discussed in lesson two, the Intergovernmental Panel on Climate Change, or IPCC, is a United Nations established organization. That examines and evaluates climate research carried out around the world. It produces information that is relevant to ongoing and future climate change and of particular interest to decision makers. It is an internationally acclaimed and accepted authority on climate change. Having won the Nobel Peace Prize in 2007 jointly with Al Gore. In their most recent assessment in 2013, the IPCC has called climate warming unequivocal, unprecedented, and directly linked to human activities. In addition, 97.1% of scientific studies published between 1991 and 2011 that expressed a position on human induced climate change, agreed that it is occurring and is a result of human activities. Nowhere on Earth is this more evident than in the Arctic. One of the most apparent changes in the Arctic in recent decades has been the decrease of sea ice. As we learned in lessons two and three, sea ice is instrumental in global ocean circulation and climate, and a fundamental component of the circumpolar environments. It keeps the underlying ocean cool, reflects incoming solar radiation due to it's high albedo, and prevents mixing of water masses. It also plays a role in the formation of deep water, and the overturning of the ocean conveyor belt. In short, sea ice is instrumental in global ocean circulation and climate. Sea ice also plays a key role in polar ecosystems. For example, it provides living space for microorganisms, such as algae. Algae are at the lower levels of the food web on which higher organisms, such as fish, and marine mammals rely. Sea ice melting during the summer provides much needed nutrients for algae in the surrounding ocean water. This kick starts biological productivity in the summer season. Sea ice is intimately linked with the lifestyles of several key Arctic species. For example, walrus rely on sea ice for breeding and resting. They also use it as a platform from which to dive to the seabed to forage for their preferred food, shellfish. Similarly, polar bears use sea ice for hunting seals and as a breeding platform. Loss of sea ice, therefore has fundamental implications for the whole of the Arctic ecosystem, including humans. Its loss means, on the one hand, opening up of economic opportunities through new shipping routes and access to gas and oil reserves as well as new fishery grounds. On the other hand, its loss will also mean substantial changes to cultural ways of life, such as subsistence hunting of marine mammals Direct observations of the Arctic, from satellites circling high above the Earth's surface, have highlighted the stark decline in sea ice since the 1970s. Given figures showing the minimum summer sea ice extent from different years. Which do you think is the most recent? Both sea ice extent and thickness have decreased substantially during the last three decades. The years 2007 and 2012 marked particular minimum years. As local weather conditions, interacting with the long-term decreasing trend, led to significant ice melt and export. So, D is the correct answer. Since the start of direct sea ice observations via satellite in 1979, the area covered by Arctic multi-year sea ice has decreased dramatically. This decrease has been by about 0.7 to 0.98 million square kilometers per decade, leading to an increased proportion of first year ice. The decrease in multi-year ice is the result of a longer melt season and locally enhanced by the deposition of industrial black carbon soot in snow. This has occurred in tandem with an increased rate of ice export out of the Arctic Ocean. Along with this trend, the total average annual sea ice extent is decreasing by 3.8% per decade. Together, satellite measurements and direct submarine measurements indicate that Arctic sea ice has also been thinning since 1979 or earlier. The thinning has been, on average, by 1.3 to 2.3m. Thus the volume of sea ice is also decreasing. Put into the context of a longer record, Arctic summer sea ice extent has decreased by 40% since the year 1900. Ice, whether in the form of frozen seawater on the ocean, or as glacial ice on land, is not just a passive component of the Earth's climate system. Losses of ice amplify and exaggerate regional warming due to ice albedo feedbacks. The albedo of dark, open water is vastly less than that of ice and snow. Dark, open water reflects about 10% of the incoming shortwave radiation. Compared to up to 90% for ice and snow. As a result, open water absorbs nearly ten times more heat than snow covered sea ice. The additional heat in the surface waters, along with warmer air temperatures, leads to an earlier melt season and a later freeze up and overall a longer open water season. With less ice each year, more heat is absorbed, which melts more ice. This is an example of a positive feedback loop linked to the reduction of ice albedo. Some areas that were historically ice covered year round now experience near ice free periods. Good examples of this are the fabled Northwest Passage, as well as Russia's Northern Sea Route. Why do you think that the opening of the Northwest Passage and the Northern Sea Route might be economically important? A, They could provide shipping companies shorter routes to get products to market. B, They could provide easier access to new oil and gas and mineral resources. C, Re-supplying Arctic communities by ship may be cheaper and easier. D, Sea ice scares away potential Arctic tourists. More than one answer might be correct, so check all that you think apply. The opening of these passages allows for increased access. As a result, A, B, and C are all true. We will now discuss this in more detail.