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A Melting Planet Major Points Glacier formation and dynamics Glacier distribution Retreat of mountain glaciers Ice sheets of Antarctica and Greenland Effects of ice loss Glacier Formation A glacier must form under three conditions: the regional climate must be cold enough that snow will not melt away entirely during the summer, there must be sufficient snowfall for accumulation to occur, and the slope must be gradual enough so that the snow pack does not slide away. As snowfall accumulates in the zone of accumulation, it exerts pressure on the snow beneath it. Over time, this pressure becomes strong enough that the underlying layers of snow turn to glacial ice called firn. Gravity will pull the ice to lower altitudes, which, in warmer regions, will cause the toe of the glacier to cross the equilibrium line into the zone of ablation, where sublimation (the evaporation of ice into water vapor) causes glacial mass to be lost. Glacier Dynamics Glacial Dynamics If the rate at which snow accumulates in the zone of accumulation exceeds the rate of sublimation in the zone of ablation, then the toe of the glacier will move forward into previously unglaciated terrain. This is referred to as glacial advance. Conversely, if the rate of sublimation in the zone of ablation exceeds the rate of accumulation, then the toe of the glacier will move backward. This is called glaical retreat. Advance and Retreat Glacial Distribution The vast majority of ice on earth lies in the ice sheets of Antarctica (91.8%) and Greenland (7.9%). The remaining .3% is distributed in alpine and piedmont glaciers around the world. In terms of the area of ice coverage, the Antarctic ice sheets account for 85.7%, Greenland 10.9%, and the rest of the world’s glaciers account for 3.4%. Of this 3.4%, two-thirds are found in ice caps and ice fields, and one-third alpine glaciers. Glacial Distribution Effects of Global Warming It is accepted that the earth is warming due to human activities Since many glaciers are in lower latitudes than the polar ice sheets, they are more susceptible to climate change. As the line of equilibrium rises in altitude due to warming temperatures, the zone of accumulation shrinks while the zone of ablation grows. In this way glaciers around the world are retreating. Glacier National Park Due to the park’s relatively low latitude and the low elevation of its glaciers, it especially susceptible to warming temperatures. Of the 83 observed glaciers in the park that are greater than 0.1 sq km, 34 are named as well as three that are not the minimum area. All of these 37 glaciers have had considerable loss of mass and have retreated dramatically since the mid 1800’s. The reduction in area is between 46% and 77% on varying glaciers in the park. A recent computer model indicates that at present rates of warming all glaciers in the park will disappear by 2030. Glacier Disappearance Franz Josef Glacier The Franz Josef valley is steep, narrow, and long-characteristics that accentuate the glacier's very quick response to changing local conditions. Like other glaciers, Franz Josef's advance and retreat patterns reflect changes of precipitation delivery to its accumulation zone in relation to the temperatures within its ablation zone-it just reflects the relationship more quickly than other glaciers. According to New Zealand's National Institute of Water and Atmospheric Research (NIWA), a long lasting "shift" in New Zealand's climate started around 1977. This shift coincided with an eastward movement in the longitude of the South Pacific Convergence Zone, and more frequent El Nino events in the recent record. FJ Glacier Continued The shift meant more persistent westerlies over central New Zealand, resulting in the west of South Island becoming 10% wetter and 5% cloudier with more damaging floods. Since 1977 temperatures have continued to rise, resulting in warmer night time temperatures and fewer frosts nationwide, and an increase in very hot days in eastern areas in recent decades. The increase in precipitation in the west of South Island could explain Franz Josef's most recent advance and the reversal, along with the continuing increase in temperatures, may explain its current trend to retreat, which will dominate over the long term. As of May of 2002, "The 48 glaciers of the Southern Alps monitored annually by NIWA continue to lose ice mass...This is the fourth year in five in which the glaciers have lost ice mass, an ongoing trend which began in 1998. The loss over the past year was among the most pronounced recorded." Franz Josef Glacier FJ Glacier Movements Ice Sheets The ice sheets of Antarctica and Greenland represent 97% of the world’s total glaciated area. They also contain 99.7% of all glaciated ice in the world. Recent analysis estimates that nearly 80% of the world’s freshwater resides in these ice sheets. Unlike most alpine and piedmont glaciers, the ice sheets of Antarctica and Greenland are in a relatively stable climate. Even with a rise in global temperatures, the regional climate of the ice sheets would remain sufficiently cold. The Greenland ice sheet covers nearly 2 million square km., and is in a slightly warmer climate than the Antarctic sheet due to its lower latitude. Antarctica Antarctica is composed of two major, geologically distinct parts bridged by a vast ice sheet. East Antarctica, the larger of the two, is roughly the size of the United States and is composed of continental crust covered by an ice sheet that averages 1.6 miles in thickness. West Antarctica, the smaller portion, is comprised of small blocks of continental crust covered by the West Antarctic Ice Sheet and an Andean-like mountain chain forming the Antarctic Peninsula. Most of the West Antarctic Ice Sheet is grounded below sea level, in places over 1.5 miles below sea level. These two ice sheets cover all but 2% of Antarctica's 14 million square kilometers. At its thickest point the ice sheet is almost 14,000 feet deep. Effects of Ice Loss Increased glacial melt could cause increased flooding on a regional scale. Regions which rely on melt water from glaciers for irrigation or hydroelectric power could face water shortages in the future when glaciers shrink. Loss of glaciers in national parks could result in a decrease in tourism to areas that rely on tourism for their economy. Unforeseen effects of a decrease of ocean salinity. Change in sea level… Sea Level Change The average rate of sea level rise in the past century was between .3mm/yr and 3mm/yr. This corresponds with the rise in global temperatures. With an expected rise in temperatures between 1.5 and 4.5 degrees C over the next century, sea level is also expected to rise. There are three main factors that contribute to the change in sea level: thermal expansion, ice sheet mass change, and glacier mass change. Thermal expansion is the increase of the volume of water as the temperature rises, and it can have a significant effect on sea level change. It is estimated that thermal expansion alone contributed between 3-6cm over the past century. Future projections estimate that at current warming rates thermal expansion alone could contribute .1-.5mm/yr to future sea levels. Ice Sheet Mass Change Due to the relative stability of the polar climates, the polar ice sheets have not contributed much to the rise in sea level. It is expected that as global temperatures increase, precipitation rates around the world will also increase, including in the dry polar regions. The increased accumulation rate will outweigh any increase of sublimation caused by increased temperatures for the Antarctic ice sheet. This will result in a decrease in sea level by .5 mm/yr per degree C from the Antarctic sheet. However the stability of the West Antarctic ice sheet is uncertain. Greenland, however, will be slightly more affected by an increase in temperatures due to its low latitude, so it is estimated to add .5 mm/yr per degree C. Therefore the polar ice sheets actually cancel each other out. West Antarctic Ice Sheet The West Antarctic Ice Sheet, containing more than 3 million cubic kilometers of ice, is the last ice sheet on Earth resting in a deep marine basin and is the most likely player in any future, rapid sea level rise. It is comprised of a grounded ice sheet, a marine ice sheet (grounded below sea-level) and an extensive floating ice shelf. Glacial geologic studies have shown that this type of ice sheet is inherently unstable and vulnerable to rapid collapse. Nearly 90% of the ice flowing across West Antarctica converges into ice streams that are the most dynamic and unstable components of the ice sheet. Some of these ice streams may be responding to climatic and sea-level changes of the recent past, which could lead to more rapid retreat and global sea-level rise in the future. A few active volcanoes may also affect the ice sheet's behavior. The sea level equivalent of the WAIS is about 5 m. Glacier Mass Change Mountain glaciers, although small in volume, are much more affected by warming temperatures, so their impact on the future sea levels is significant. It is estimated that glaciers accounted for 4 cm of sea level rise in the past century. If all the glaciers in the world melted, it would result in a .5 m rise in sea level. If the polar ice sheets melted it would result in a 70 m rise in sea level, however this is impossible within the foreseeable future, whereas it is possible for most of the world’s glaciers to melt. At a 4 degrees C increase in global temperature over the next century, it is estimated that glaciers would contribute 20 cm to sea level rise. Summary The increase in global temperatures is resulting in a rapid decrease of the volume of mountain glaciers around the world, and an increase in sea level. The East Antarctic ice sheet is actually growing due to increased precipitation rates, thereby reducing sea-level. The West Antarctic ice sheet is increasing in volume, but it is also becoming less stable and could partially collapse in the distant future. The Greenland ice sheet is becoming more sensitive to the changing climate, but is considerably less affected than mountain glaciers.
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