Query: Most glaciers are retreating, posing a serious problem for millions who rely on glaciers for water.

Relevant passages:
Water scarcity <=SEP=> Water scarcity is the lack of sufficient available water resources to meet water needs within a region . It affects every continent and around 2.8 billion people around the world at least one month out of every year . More than 1.2 billion people lack access to clean drinking water .   Water scarcity involves water shortage , water stress or deficits , and water crisis . The relatively new concept of water stress is difficulty in obtaining sources of fresh water for use during a period of time ; it may result in further depletion and deterioration of available water resources . Water shortages may be caused by climate change , such as altered weather-patterns ( including droughts or floods ) , increased pollution , and increased human demand and overuse of water . The term water crisis labels a situation where the available potable , unpolluted water within a region is less than that region 's demand . Two converging phenomena drive water scarcity : growing freshwater use and depletion of usable freshwater resources .   Water scarcity can result from two mechanisms :   physical ( absolute ) water scarcity  economic water scarcity   Physical water scarcity results from inadequate natural water resources to supply a region 's demand , and economic water scarcity results from poor management of the sufficient available water resources . According to the United Nations Development Programme , the latter is found more often to be the cause of countries or regions experiencing water scarcity , as most countries or regions have enough water to meet household , industrial , agricultural , and environmental needs , but lack the means to provide it in an accessible manner .   Many countries and governments aim to reduce water scarcity . The UN recognizes the importance of reducing the number of people without sustainable access to clean water and sanitation . The Millennium Development Goals within the United Nations Millennium Declaration aimed by 2015 to `` halve the proportion of people who are unable to reach or to afford safe drinking water . ''

Glacier <=SEP=> A glacier ( -LSB- pronˈɡleɪʃər -RSB- or -LSB- ˈɡlæsiə -RSB- ) is a persistent body of dense ice that is constantly moving under its own weight ; it forms where the accumulation of snow exceeds its ablation ( melting and sublimation ) over many years , often centuries . Glaciers slowly deform and flow due to stresses induced by their weight , creating crevasses , seracs , and other distinguishing features . They also abrade rock and debris from their substrate to create landforms such as cirques and moraines . Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water .   On Earth , 99 % of glacial ice is contained within vast ice sheets in the polar regions , but glaciers may be found in mountain ranges on every continent except Australia , and on a few high-latitude oceanic islands . Between 35 ° N and 35 ° S , glaciers occur only in the Himalayas , Andes , Rocky Mountains , a few high mountains in East Africa , Mexico , New Guinea and on Zard Kuh in Iran . Glaciers cover about 10 percent of Earth 's land surface . Continental glaciers cover nearly 5 e6sqmi or about 98 percent of Antarctica 's 5.1 e6sqmi , with an average thickness of 7,000 ft. Greenland and Patagonia also have huge expanses of continental glaciers .   Glacial ice is the largest reservoir of fresh water on Earth . Many glaciers from temperate , alpine and seasonal polar climates store water as ice during the colder seasons and release it later in the form of meltwater as warmer summer temperatures cause the glacier to melt , creating a water source that is especially important for plants , animals and human uses when other sources may be scant . Within high altitude and Antarctic environments , the seasonal temperature difference is often not sufficient to release meltwater .   Because glacial mass is affected by long-term climatic changes , e.g. , precipitation , mean temperature , and cloud cover , glacial mass changes are considered among the most sensitive indicators of climate change and are a major source of variations in sea level .   A large piece of compressed ice , or a glacier , appears blue as large quantities of water appear blue . This is because water molecules absorb other colors more efficiently than blue . The other reason for the blue color of glaciers is the lack of air bubbles . Air bubbles , which give a white color to ice , are squeezed out by pressure increasing the density of the created ice .

Effects of global warming on human health <=SEP=> The effects of global warming include effects on human health . The observed and projected increased frequency and severity of climate related impacts will further exacerbate the effects on human health . This article describes some of those effects on individuals and populations .

Retreat of glaciers since 1850 <=SEP=> The retreat of glaciers since 1850 affects the availability of fresh water for irrigation and domestic use , mountain recreation , animals and plants that depend on glacier-melt , and , in the longer term , the level of the oceans . Studied by glaciologists , the temporal coincidence of glacier retreat with the measured increase of atmospheric greenhouse gases is often cited as an evidentiary underpinning of global warming . Mid-latitude mountain ranges such as the Himalayas , Alps , Rocky Mountains , Cascade Range , and the southern Andes , as well as isolated tropical summits such as Mount Kilimanjaro in Africa , are showing some of the largest proportionate glacial losses .   Glacier mass balance is the key determinant of the health of a glacier . If the amount of frozen precipitation in the accumulation zone exceeds the quantity of glacial ice lost due to melting or in the ablation zone a glacier will advance ; if the accumulation is less than the ablation , the glacier will retreat . Glaciers in retreat will have negative mass balances , and if they do not find an equilibrium between accumulation and ablation , will eventually disappear .   The Little Ice Age was a period from about 1550 to 1850 when the world experienced relatively cooler temperatures compared to the present . Subsequently , until about 1940 , glaciers around the world retreated as the climate warmed substantially . Glacial retreat slowed and even reversed temporarily , in many cases , between 1950 and 1980 as global temperatures cooled slightly . Since 1980 , a significant global warming has led to glacier retreat becoming increasingly rapid and ubiquitous , so much so that some glaciers have disappeared altogether , and the existences of many of the remaining glaciers are threatened . In locations such as the Andes of South America and Himalayas in Asia , the demise of glaciers in these regions has the potential to affect water supplies in those areas .   The retreat of mountain glaciers , notably in western North America , Asia , the Alps and tropical and subtropical regions of South America , Africa and Indonesia , provide evidence for the rise in global temperatures since the late 19th century . The acceleration of the rate of retreat since 1995 of key outlet glaciers of the Greenland and West Antarctic ice sheets may foreshadow a rise in sea level , which would affect coastal regions .