"Thinning of glaciers since the mid-19th century has been obvious and pervasive in many parts of the world." Intergovernmental Panel on Climate Change, 1996 "... so far the length reduction of mountain glaciers still remains the most detectable, unequivocal proof from cold regions that fast and worldwide secular climatic change is taking place." Professor Wilfreid Haeberli, Swiss glaciologist, 1990

Summary

As the climate has changed over the last century, the world's land ice cover has responded. Nowhere is this more noticeable than in mountain glaciers. Global temperatures have increased by 0.3-0.6oC since late last century and the United Nation's Intergovernmental Panel on Climate Change (IPCC) has concluded that "the balance of evidence suggests a discernible human influence on global climate". In response to increasing temperatures over the past 100 years, mountain glaciers have generally thinned, lost mass and retreated. This glacial retreat is consistent with an observed alpine warming of 0.6oC - 1.0oC.

Mountain glaciers are in retreat in Argentina, Peru, Alaska, Iceland, Norway, the European Alps, Kenya, the Himalayas, New Zealand, Antarctica and Indonesia (See Table 1). In the European Alps, glacial cover has fallen to levels unseen in the last 5,000 years. The Bering Glacier - North America's largest glacier - has lost 130 km2 of its frontal area and has thinned dramatically over the past century. In the past 50 years, parts of the glacier have thinned by as much as 180 metres.

Scientists project pronounced reductions in glacial mass in the future. Up to a quarter of global mountain glacier mass could disappear by 2050 and up to a half could be lost by 2100.

Over the last century, global sea level has risen by between 10-25cm and the loss of mountain glaciers has been estimated to have contributed between 2-5cm to this change. Scientists estimate that for the mid-range sea level rise scenarios mountain glaciers and small ice caps could contribute 16cm of the 50cm projected by 2100.

In addition to affecting global sea level, glacial discharges are an important contributor to some regional water resources. As glaciers melt some areas will experience increases in water supply and flood risk while others would see the opposite. Alpine tourism is also likely to be affected by the retreat of glaciers as skiing fields disappear and the visual appeal of mountain regions diminishes.

The worldwide twentieth century retreat of glaciers supports the conclusion that the Earth is warming. The speed at which this is occurring also strengthens findings that the Earth is entering a period of climate change unprecedented in the last 10,000 years.

Table 1. Summary of changes in the Earth's glaciers

Region	Observations
Circum-Arctic	Glaciers have generally lost mass over the last 30 years. In East and West Greenland glaciers are retreating rapidly while in North Greenland the situation is unclear. Northern Swedish and Spitsbergen glaciers are loosing mass
European Alps	Since the Little Ice Age - surface area: 30-40% reduction; ice volume: 50% reduction. Late 20th century extent is the smallest in the past 5,000 years.
Central Asia	From the 1950s to the 1980s, 73% of glaciers were retreating, 15% were advancing and 12% were stable.
Tropical mountains	The retreat of glaciers has been documented in the Ecuadorian Andes, New Guinea, East Africa (Mount Kenya's glacier has receded markedly since the late 1800s and since the 1960s its mass has decreased by 40%), Venezuela and Peru.
New Zealand	Most glaciers have retreated during the 20th century. The Tasman Glacier has thinned by more than 100m. Since 1983, the recession of western glaciers has reversed and these glaciers are growing (e.g. the Franz Josef glacier).
Southern and central South America	The Upsala glacier has retreating about 60m per year over the last 60 years and this rate appears to be accelerating. In the last 40 years, the area of the South Patagonian Ice Field has diminished by about 500km2. The Soler and Tyndall Glaciers also appear to have lost mass, while the Pio XI glacier in Patagonia is larger now than at any time in the last 6,000 years.
Antarctica and sub-Antarctic Islands	The alpine glaciers of the Dry Valleys in Antarctic has fluctuated with no apparent trends. Numerous glaciers along the Antarctic Peninsula are in retreat. Glaciers in many sub-Antarctic islands are also in retreat, for example, some small glaciers in Heard Island have decreased by up to 65%.

Introduction

During the Earth's history the climate has varied between periods of relative warmth and cold. Within these variations the amount of the planet's surface covered by ice has also changed dramatically. At the height of the last ice age 25,000 years ago, massive sheets of ice covered much of North America, northern Eurasia and Antarctica. One such ice sheet covered most of what is now Canada and Alaska. As the Earth warmed at the end of the ice age the majority of this ice disappeared and now only Antarctica and Greenland are covered by ice sheets. The vast majority of the world's land ice is found in Antarctica and Greenland, with the rest found in permafrost, glaciers and small ice caps. As the climate has changed over the last century, the world's land ice cover has responded and nowhere is this more noticeable than in mountain glaciers.

Global Glacial Loss

Simply put, changes in the mass of a glacier are generally expressed by adding ice accumulation in the form of snow fall and internal meltwater freezing, to ice loss, in the form of meltwater and iceberg calving. For example, if a glacier is in a region of increasing snowfall, the glacier will grow unless ice loss from the glacier also increases. Conversely, if a glacier is in a region with rising temperatures, greater snowfall would be required if it is not to lose mass. The sum of accumulation and loss is called the "mass balance" of the glacier, with positive figures representing growth and negative figures showing decline.

Mountain glaciers are in retreat in Argentina, Peru, Alaska, Iceland, Norway, the European Alps, Kenya, the Himalayas, and Indonesia (See Table 1). Many summit glaciers may soon disappear entirely. In the European Alps alpine ice has been reduced to levels unseen in the last 5,000 years. Overall, glaciers in the Alps are losing mass at a rate an order of magnitude faster than at the end of the last ice age and this rate is broadly consistent with what might be expected in response to the human-enhanced greenhouse effect. In addition to this, the rate of glacial wastage in the European Alps appears to be accelerating.

While it is clear that overall mountain glaciers are losing mass, some of the Earth's glaciers are growing. However, this is not necessarily inconsistent with projections of human-induced climate change because precipitation is expected to increase in the high northern and southern latitudes in response to warming temperatures. High latitude precipitation has increased over recent decades; in some cases this could have counteracted the loss of glacial mass due to warming.

The Bering Glacier: An Example of a Global Phenomenon

The Bering Glacier, North America's largest glacier, is an example of a major glacier in retreat. Originating some 25 kilometres east of the US border in Canada, the Bering Glacier flows 191 kilometres to its terminus in south central Alaska. The areal extent of the glacier is over 5170 km2 and in some places the glacier is over 800 metres thick. During the last 10,000 years the size of the glacier has varied in response to climate changes, including significant and rapid retreat over the last 100 years.

An important feature of changes in the Bering Glacier's mass has been its "surging" behaviour. Glacial surging is defined as short and periodic rapid displacements and movements of large quantities of ice within the glacier. Surges are frequently accompanied by a significant advance of the glacial front. The Bering Glacier surged around 1900 and 1920 and in 1938-40, 1957-60, 1965-67 and 1993-94. After each surge the glacier generally stagnated, continued thinning and then resumed retreat. As US Geological Survey scientists have noted, "The interplay of retreat and surging has resulted in rapid and very dramatic changes throughout the entire glacier." It is unclear whether climatic factors influence glacial surging.

Since the beginning of this century the glacier's frontal region has declined 130 km2 in area. As the glacier has retreated, several large lakes have formed, the greatest being the 70 km2 Vitus Lake. Over recent decades the rate of glacial retreat has been increasing with maximum rates of surge since 1990. Parts of the glacier retreated by over one kilometre a year before the 1993-94 surge. Glacial retreat is generally measured in metres to tens of metres annually.

In addition to the overall retreat of the glacial front the Bering Glacier has thinned dramatically over the past century. Aerial photography shows that parts of the glacier have thinned by as much as 180 metres in the last 50 years and in some places the glacier has lost between 20-25% of its thickness.

Glacial Loss and Global Climate Change

Measurements taken from glaciers around the world complement the global instrumental meteorological record and provide an indirect measure of climate change. In many regions, measurements of glacier length extend back into the 19th century and in Europe some records date back to 1600 AD. Indirect measurements of glacial extent such as sediment deposits left by advancing and retreating glaciers can also be used estimate glacial extent.

Global temperatures have increased by 0.3-0.6oC since late last century and the United Nation's Intergovernmental Panel on Climate Change (IPCC) has concluded that "the balance of evidence suggests a discernible human influence on global climate". In response to increasing temperatures, mountain glaciers have generally thinned, lost mass and retreated over the last 100 years. In addition to this some glaciers are also loosing mass at an accelerating pace. The 20th century glacier retreat is consistent with a alpine warming of 0.6oC - 1.0oC.

Future Changes in the World's Mountain Glaciers

Scientists project pronounced reductions in glacial mass in the future with the predicted disappearance of up to a quarter of global mountain glacier mass by 2050 and up to a half of global mass by 2100. Even in areas where precipitation is expected to increase, changes in temperatures are expected to dominate and glaciers are expected to shrink even if some regions become wetter. One recent study concluded that, "It ... appears that even a significant increase in precipitation cannot compensate for melting, and further shrinkage of glaciers and small ice caps must occur in a warmer climate." One study assessed by the IPCC suggests that if current warming trends continue the glacial mass of the European Alps could be reduced to few percent in decades. While many glaciers are likely to disappear altogether, the largest mountain ones such as those found in the Gulf of Alaska, Patagonia and the Himalayas should continue to exist into the 22nd century.

The Impacts of Glacial Mass Loss

Global sea level has risen by between 10-25cm over the last century. A number of factors such as the expansion of the oceans in response to higher temperatures (called thermal expansion) and changes in the mass balance of glaciers and the Antarctic and Greenland ice sheets are thought to have contributed to a greater or lesser extent to this change.

The biggest known contributor to recent sea level rise has been thermal expansion but the loss of mountain glaciers has been estimated to have contributed between 2-5cm to 20th century sea level rise. The IPCC estimate that for mid-range sea level rise scenarios, glaciers and small ice caps would contribute 16cm to 50 cm projected by 2100. Projected changes in sea level are expected to have major impacts on many coastal and small island communities.

In addition to affecting global sea level, glacial discharges are an important contributor to the water resources of some regions. In some semi-arid places near high mountains such as central Asia and Argentina, increases in future glacial runoff may increase the amount of available water and is likely to increase the risk of river flooding and landslides. Summer water resources in other regions are likely to diminish as glaciers disappear, for example, the Quelcccaya glacier in Peru is crucial for water supply of Lima, a city of 10 million people and is retreating rapidly.