The United States is in many ways a creation of a frontier experience. The push westward remains part of recent American history, and many still live who remember the days of early settlement, of the often heroic struggle with the land.The American frontier is largely gone today. Although humans presumably have the technology to live anywhere on the earth's surface, those areas of the United States that can be occupied with moderate physical and economic effort are already staked out.
Extending as far south as the northern Great Lakes states and including the interior to the Canadian border, as well as parts of Alaska, the North Shield remains sparsely settled. The inhospitable nature of the physical environment plus the consequent thinness of settlement give the North Shield its special character.
The Arctic harbors one of the world’s least disturbed marine ecosystems, plays a crucial role in moderating the planet’s climate and is home to spectacular wildlife, fish and marine mammal populations. The biological riches of the Arctic provide physical and spiritual sustenance for indigenous peoples who have vibrant communities in the north.
The Arctic and its people now face enormous challenges. Climate change is melting polar ice that contributes to climate stability, regulates ocean currents and is the foundation of the Arctic’s biological richness. The warming ocean is altering the Arctic marine ecosystem in ways never before witnessed by humans. At the same time, the disappearance of ice is beginning to provide unprecedented access for industrial development.
Tundra is a region that is covered with low shrubs and vines because the summer temperatures are not warm enough to support forests. Over the last 25 years, the tundra has changed, with some regions having an increase in the amount and type of shrubs, and other regions being converted to wetland. Satellite observations of tundra area (NDVI) can be calibrated by comparison with surface measurements such as those shown on the map below. This method indicates an approximate decrease in tundra area of 18% over the past twenty years. Tundra area can also be roughly estimated as the region with monthly summer temperatures less than 10 degrees Centigrade.
Permafrost is permanently frozen ground. For example, in Fairbanks, Alaska, the soil is frozen just some 30 to 40 centimeters below the surface, and in fact, has been frozen for the last several thousand years and maybe even longer. Only the upper 30 to 100 cm of soil (called the active layer) thaws every summer and then completely refreezes during the winter. Typical thickness of permafrost around Fairbanks is about 50 meters, but varies between a few meters and 150 m and more. Permafrost gets colder and thicker northward. On the Alaskan Arctic Plain, permafrost could be as cold as -9 to -11°C cold and up to 650 meters thick.
When permafrost degrades (melts), there are impacts on drainage, ground water, river runoffs, ecological systems (such as plants and ponds), release of carbon that has been sequestered in the frozen soil, and infrastructure (such as houses, roads, airports, pipelines, and other facilities based on permafrost).
Average temperature during the year is the most important factor for permafrost existence. Permafrost temperatures at 1 m below ground in central Alaska have been warming since the 1960s and were reaching near to the melting point in the mid-1990s. There has been a retreat to colder temperatures (less than -1°C) in the last few years.
The Arctic Oscillation (AO) is an important Arctic climate index with positive and negative phases, which represents the state of atmospheric circulation over the Arctic. The positive phase brings lower-than-normal pressure over the polar region, steering ocean storms northward, bringing wetter weather to Scotland and Scandinavia, and drier conditions to areas such as Spain and the Middle East. While the value of the AO index was strongly positive in the early 1990's compared to the previous forty years, the value of the AO has been low and variable for the last nine years. The year to year persistence of positive or negative values and the rapid transition from one to the other is often referred to as "regime-like".
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