On thinning ice: are the world's glaciers in mortal danger?
Glaciers affect more than just the scenery. They're the sources of many rivers that provide water for drinking, irrigation, and hydroelectric power for millions of people. What's more, in the layers of snow from which they formed, glaciers chronicle climate characteristics, such as local temperatures and rates of precipitation. Once the ice melts, that's the end of data that sometimes span tens of thousands of years. So, scientists are now racing the heat to collect ice samples that might outlast their glacial sources.
GROW WITH THE SNOW Glaciers can appear wherever temperatures are cold enough and precipitation is sufficient to form a snowfield that survives the summer and therefore accumulates from year to year. Fresh layers of snow compress the older ones beneath them. When the stack is high enough, lower layers of snow transform into ice, and pressure from the overlying weight makes that ice begin to move. In steep mountainous settings, gravity assists the flow.
The fluctuation in a glacier's volume depends on the difference between the amount of new snowfall piling on top of the glacier and how much of the glacier's ice melts, says Richard B. Alley, a glaciologist at Pennsylvania State University in State College. Although the two largest influences on a glacier's volume are temperature and precipitation, other factors can play important roles. These include the cloud cover over the glacier, the wind blowing across its surface, the humidity of the air surrounding it, and the dust and rock in and on the glacier's ice.
The effect of any one factor is difficult to predict. For example, the moisture from humid air that condenses on a glacier may add some ice. However, the condensation also gives up heat, thereby warming the ice. That can lead to melting and an overall reduction of volume. "It's a wonderfully complex system,' says Alley.
Average global warming may actually spell good times for glaciers, according to some scientists, including Hezi Gildor of Lamont-Doherty Earth Observatory in Palisades, N.Y. In general, these scientists argue that, the higher temperatures increase evaporation from the oceans and provide more moisture and precipitation to Earth's higher latitudes, home to many glaciers.
Alley strongly disagrees with this view. Although a 1[degrees]C rise in a specific region's temperature might bring a glacier there 10 percent more precipitation, in most cases, the ice mass would actually need a 40 percent boost in snowfall just to keep up with the increased melting that the added warmth would fuel, Alley says. He outlines the probable effects of global warming on glaciers in the Aug. 19 Eos.
A small proportion of the world's glaciers, including some in Norway, are growing despite a warming climate. Alley suspects that those growing ice masses are receiving bonus snowfalls from storms that would have dumped their precipitation elsewhere in the absence of global warming. In other words, Norse glaciers are expanding at the expense of other European ice masses. As a general rule, Alley says, glaciers have grown when Earth chilled and they've shrunk when the planet warmed.
LOSING A HOT RACE Today, ice covers about 10 percent of Earth's continental area. Most of that ice--more than 32 million cubic kilometers of it--shrouds Antarctica and Greenland, but around 100,000 [km.sup.3] of ice are locked in glaciers. That's enough to raise global sea levels by half a meter, if the glaciers melted, says Roger G. Barry, director of the National Snow and Ice Data Center at the University of Colorado at Boulder.
Most glaciers have been waning since the end of a global cool spell dubbed the Little Ice Age, which lasted from the mid-1300s to the mid-1800s. During particularly frigid intervals of that period, Swiss glaciers advanced downslope to consume farms and villages, England's Thames River often froze in the winter, and sea ice surrounded Iceland for miles in every direction.
Since the mid-1900s, rates of glacial melting have skyrocketed, says Barry. Between 1961 and 1976, the world's glaciers posted an average annual ice loss of 56 [km.sup.3]. Since then, the melt rate has almost tripled. Water entering the oceans from melting glaciers is now boosting sea levels by about 0.4 millimeter per year to provide 15 to 20 percent of the current annual rise. Most of the remaining increase stems from the thermal expansion of seawater in response to the globe's rising average temperatures.
Glaciers in central Asia and the Coast Range of Alaska have tallied inordinate melting rates, says Barry. Although ice masses in those areas make up only 30 percent of the world's glaciers, they contribute 70 percent of the total glacial meltwater.
Barry and his colleagues have focused on the Ak-shirak mountain range of Kyrgyzstan. This central Asian range boasts more than 170 glaciers that together swathe about 436 square kilometers. Comparisons of aerial photos taken in 1943 and 1977 and a satellite photo taken in 2001 show accelerated rates of melting in recent decades. For example, the Davydov Glacier, which covered 12.7 [km.sup.2] in 1943, had lost only 0.5 [km.sup.2] in area between 1943 and 1977, but it had melted back another 4.8 [km.sup.2] by 2001.
Similarly, the Sary-Tor Glacier lost just 0.1 [km.sup.2] between 1943 and 1977, but then dropped 0.9 [km.sup.2] over the subsequent 24 years.
The team recently analyzed nearly 6 decades of glacier surveys of the Ak-shirak mountain range and 7 decades of data from the nearest weather station. The area gets about 300 mm of precipitation each year, most of it in the summer months, and temperatures at the lower ends of the Ak-shirak glaciers typically range from -11[degrees]C in the winter to 4[degrees]C in the summer.
Overall, Barry and his team estimate that the area covered by glaciers in the Ak-shirak range shrank by 3.4 percent between 1943 and 1977 and more than 20 percent between 1977 and 2001. Increased summer temperatures and decreased summer precipitation in the region contributed to the accelerated melting of recent decades, says Barry. The last 2 decades of the 1900s in this area were, on average, about 0.6[degrees]C warmer than the period from 1951 to 1980, he notes. The scientists report their analyses in the Aug. 15 Geophysical Research Letters.
Other scientists have found that the glaciers of another mountain range just to the west of the Ak-shiraks are melting at comparable rates. Those glaciers lost 29 percent of their area to melting between 1955 and 1990, says Barry.
DATA MELTDOWN Glacial advances during the Little Ice Age and recent warming-fueled retreats are just two examples of the climate-driven fluctuations in ice volume that have occurred since the end of the last major ice age about 12,000 years ago. Scientists learned of a particularly swift fluctuation when they applied carbon dating to a well-preserved plant that had been exposed last year by the melting margin of the Quelccaya ice cap high in the Peruvian Andes. They had thought the ice cap was only 1,500 years old, says Lonnie G. Thompson of the Ohio State University in Columbus. To their surprise, they found that the plant had lived about 5,200 years ago.
The remarkably good condition of the plant suggests that the climate at the site suddenly changed and snows quickly entombed the plant. Thompson described his team's findings in July at a meeting of the International Union for Quaternary Research in Reno, Nev. That big chill in Peru 5,200 years ago coincides with a major drought whose signs are recorded, among other places, in six long tubes of ice that Thompson and another team of researchers drilled from the glaciers atop Tanzania's Mount Kilimanjaro. Other extended dry spells chronicled in those cores of ancient African ice occurred about 8,300 years ago and around 4,000 years ago. The oldest of those droughts happened about the same time as an abrupt shift in worldwide climate associated with the sudden drainage of a large Canadian lake into the North Atlantic Ocean (SN: 11/2/02, p. 283).
Analyses of variations in the proportions of oxygen isotopes in the ancient layers of precipitation suggest that compared with the present, equatorial Africa was significantly colder from about 1270 to 1850, an interval that roughly corresponds with the Little Ice Age. The results of recent surveys of the glaciers atop Kilimanjaro don't bode well for the data on ancient climates that the ice harbors. Between 1962 and 2000, the ice thinned by about 0.5 m each year. The area covered by Kilimanjaro's glaciers decreased from 12 [km.sup.2] in 1912 to just 2.6 [km.sup.2] in 2000. At these rates, says Thompson, the peak's ice--and its storehouse of climate clues--will be gone by 2020.
Around the same time, Montana's Glacier National Park may lose the last of its namesake masses of ice. Although the glaciers there were always small by global standards, they once were plentiful. About a century ago, the park hosted 150 glaciers; today, there are 26. None of the thin, fragmented remnants of the park's ancient ice masses covers more than 1.5 [km.sup.2], and at least two of the glaciers have shrunk so much they've stopped flowing.
Scientists see a similar fate for many other ice masses just across the border in the Canadian Rockies. They, too, are predicted to last no more than a few decades at their current rate of melting. In many regions of what seems to be an ever-warming world, glaciers have become the geological equivalent of an endangered species.
Nature Water Towers
Many river systems start in the ice
Glaciers store precipitation that falls in winter months and discharge it gradually during the summer, a season when rainfall may be lacking and demand for water can be high. For example, about 15 percent of the Himalayas is swathed by glaciers. An additional 35 percent of the region is covered in snow at the height of winter. Each year, as many as 800 cubic kilometers of meltwater from these sources nourish the Himalayan streams that eventually feed into major Asian rivers such as the Ganges, Indus, Hawang Ho, and Yangtze.
Most of the main rivers that cross Canada's western plains originate at glaciers high in the Rockies, says David W. Schindler, an ecologist at the university of Alberta in Edmonton. The rivers' flow volumes typically surge in the spring as the seasonal snowpack melts, but for the rest of year, the rivers are fed only by glaciers and groundwater. These sources are particularly important because the rivers drain watersheds that don't receive much summer rainfall and where rates of evaporation are relatively high.
Despite increased glacier melting in recent decades, the Canadian rivers' flow volumes now measure only about 60 percent of those gauged a century ago, says Schindler. That reduction is affecting water quality in Lake Winnipeg because the river water entering the lake now holds higher concentrations of algae-nourishing chemicals than it did in the early 1900s. The problem is compounded by increased fertilizer use by farmers in the watersheds, he notes.
The freezing waters that tumble from the feet of melting Canadian ice masses bear a burden of eroded and dissolved minerals. They also carry pollutants--including pesticide residues and fallout from atomic bomb tests--that were lofted to the glaciers on and in snowflakes that fell decades ago, says Schindler.
Field studies suggest that DDT and some other pesticides found in modern meltwater were deposited on the ice masses in snowfall as many as 50 years ago. Analyses of ice layers from intact portions of the glacier indicate that the concentrations of DDT and other pesticides actually peaked about a decade after use of these substances had been banned. That finding provides scientists with insight about how such chemicals cycle through the environment.--S.P.
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|Date:||Oct 4, 2003|
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