• About Glaciers
• Glacial retreat world-wide
  • Why are glaciers retreating?
  • Some glaciers are still getting bigger
  • Anthropogenic or Natural Climate Changes?
• Consequences
• Further Information and References

Figure 2. "On the left is a photograph of Muir Glacier taken on August 13, 1941, by glaciologist William O. Field; on the right, a photograph taken from the same vantage on August 31, 2004, by geologist Bruce F. Molnia of the United States Geological Survey (USGS)." Image credit: National Snow and Ice Data Center, W. O. Field, B. F. Molnia.

Over the last century, mountain glaciers worldwide have, on average, been decreasing in length and volume (see Figure 2 and links). Some have been disappearing at such a rate that they may completely disappear soon. In North America's Glacier/Waterton National Park, for example, there were about 150 glaciers when the park was established in 1910. Today the number of moving glaciers is below thirty. Dan Fagre, a U.S. Geological Survey ecologist working in the Park, says, "The last one will probably disappear by the year 2030, tops" (Chadwick, 2007). There have been significant changes in the ice sheets and their tidewater glaciers as well, which are discussed in our articles on Antarctica and Greenland.

The glaciers in the famed North American park are not alone. A 2005 study of 173 glaciers across the world found that since 1970, 83% of surveyed glaciers were thinning at an average loss of 0.31 m/yr (see Figure 3) (Dyurgerov and Meier, 2005). According to the 2007 IPCC report, glaciers and icecaps have lost increasing amounts of mass since the middle of the last century. In the period between 1961 to 2004, glaciers and icecaps were losing 0.50 ± 0.18 mm yr^.1 in sea level equivalent (SLE) in mass. Between 1991 and 2004, however, the rate was actually above the average, at 0.77 ± 0.22 mm yr^.1 SLE (IPCC, 2007). The numbers differ regionally, with the strongest mass losses per unit area in Patagonia, Alaska, and northwest USA and southwest Canada. For example:

• According to a 2007 study of glaciers in southeast Alaska and western Canada using aerial and spatial data, glacier surface elevations have dropped in over 95% of the area analyzed from 1948 to 2000. Some of the measured glaciers had thinned by as much as 640 meters (Larsen et al., 2007).
• A 2006 study of the European Alps found that Alpine glaciers lost 35% of their total area from 850 until the 1970s, and almost 50% by 2000. Total glacier volume around 1850 is estimated at some 200 km3 and is now close to one-third of this value. (Zemp et al., 2006).

"Glaciers and ice caps provide among the most visible indications of the effects of climate change…" the 2007 IPCC report explains. Glaciers respond quickly to changes in climate because they generally lose more mass on an annual basis compared to their total mass. This is because the mass balance of a glacier is determined by the hydrological cycle, which is in turn determined by the climate. Variability in climate in glacier-producing regions creates variation in the size of a glacier. For example, at high and mid-latitudes such as Alaska and Scandinavia, accumulation tends to occur in the winter while ablation occurs in the summer months. In these areas, the hydrological cycle is controlled in large part by the annual cycles of air temperature (IPCC, 2007). Across the Himalayas, however, both accumulation and ablation primarily take place during the summer (Fujita and Ageta, 2000). On tropical glaciers, ablation takes place throughout the year, but accumulation is dependent upon seasonal precipitation (Kaser and Osmaston, 2002). Therefore, according to Kaser et al. (2004), the response of tropical glaciers to changes in climate lag by only a few years, compared to a response time described by Paterson (2004) of up to several centuries for the largest, coldest glaciers on the smallest inclines.

Figure 3. "Cumulative mass balances of selected glacier systems compiled from individual time series showing differing changes in time up to the last decade of twentieth century." The map on the left shows the information portrayed in the graph on the right of average annual rate of thinning since 1970 for the 173 glaciers measured in Dyurgerov and Meier (2005). This sample includes mountain glaciers only, not ice sheets in Antarctica and Greenland or calving glaciers. (Larger changes are represented with larger circles in the map.) Image credit: (Map) Global Warming Art compiled from the findings of Dyurgerov and Meier (2005). (Graph) Dyurgerov and Meier (2005).

In general, glacial retreat has been tied to two main climatic changes. These are increased temperatures and changes in precipitation and atmospheric moisture. Other factors, such as solar radiation, also play a role in glacial retreat.

Air temperature is considered to be one of the most important factors governing glacial fluctuations (Houghton et al., 2001). While there is regional variation, the global average temperature has increased by about 0.74°C between 1906 and 2005 (IPCC, 2007). In many areas, warmer seasons are hotter and longer lasting than they were previously, while colder season temperatures have also increased. In Alaska, annual air temperatures have increased over the past 50 years — with winter increases double those experienced during the summer (Stafford et al., 2000). These increasing temperatures have been identified as the main driver of glacial retreat and melting. In fact, Rasmussen and Conway (2003) point out that the summer temperature increases alone could explain the losses experienced in Alaska and northwestern Canada. One reason for this is that these glaciers cover large areas at low elevations. "The greatest changes occurred at lower elevations but large changes are also apparent at higher elevations," state Larsen et al. in a 2007 study.

Increasing levels of precipitation are not enough to offset the effects of warmer temperatures in some areas, such as the Tarim River Basin in Northwestern China. Despite an increase of 23% in average annual precipitation from the period of 1956–1986 to 1986–2000, warmer temperatures have caused significant decreases in glacial mass and area (Liu et al., 2006). The Alps face a similar predicament. Model experiments in the European Alps show that a 3°C warming of summer air temperature "would reduce the currently existing Alpine glacier cover by some 80%" whereas a 5°C temperature increase would render the Alps almost completely ice-free (Zemp et al., 2006). According to the study's authors, "Annual precipitation changes of ±20% would modify such estimated percentages of remaining ice by a factor of less than two," reinforcing the significance of the role of increasing temperatures (Zemp et al., 2006).

Kilimanjaro and its rapidly dwindling glaciers are often pointed to as evidence of climate change. Retreat in tropical glaciers is due to what a recent study of Kilimanjaro's glaciers describes as a "complex combination of changes in air temperature, air humidity, precipitation, cloudiness, and incoming shortwave radiation" (Kaser et al., 2004). Melting due to an increase in temperatures, however, is not the primary underlying cause of loss of glacial mass on Kilimanjaro. The climate in the region has been getting drier since the end of the 19^th century, which is likely the factor forcing glacier retreat on Kilimanjaro. A drop in precipitation and air moisture inhibits the glaciers' ability to add new snow. Additionally, glacial mass on Kilimanjaro is lost primarily through sublimation (conversion of snow and ice directly to water vapor). A 2007 study found that "glacier mass balance is 2–4 times more sensitive to a 20% precipitation change than to a 1°K air temperature change… The main cause of this sensitivity characteristic is the strong albedo feedback, which is significantly stronger than on mid-latitude glaciers. Results suggest that precipitation availability is crucial to glacier retention on Africa's highest mountain" (Mölg et al., 2007). The authors of the 2004 study report that, under present conditions, Kilimanjaro's glaciers will continue to retreat, and the mountain may lose its famous glaciers by mid-century, making it the first time Kilimanjaro glacier-free for the first time in over 11,000 years (Kaser et al., 2004).

Many locations are losing glacial mass due to a combination of increasing temperatures and changing levels of precipitation and atmospheric moisture. Connections between solar activity and glacier melting processes are also being explored. In a 2006 study, Hormes et al. measured long-term glacier length variations and found there was a significant correlation with the total solar irradiance. The IPCC (2007) also points to dynamic thinning as a culprit behind the disappearance of glaciers. Dynamic thinning has increased the velocity of a number of glaciers, leading to enhanced melting and calving rates as well as reductions in overall mass balance. Additionally, ice reflects sunlight, but the increase of dark, heat-retaining rock and soil left uncovered by retreating and melting glaciers in turn heats up the ground and causes more reduction in glacial mass.

Despite the worldwide trend toward retreat, a number of glaciers are growing. Dyurgerov and Meier's 2005 study found that Scandanavian glaciers were gaining mass balance. Likewise, certain glaciers in areas general retreat, including Alaska, are exhibiting evidence of expansion. A 2007 study showed that in 5% of the study area in Alaska and Canada, glaciers, such as the Taku Glacier, were experiencing thickening (Larsen et al., 2007). Fealy and Sweeny (2005) find "an increased moisture flux over the North Atlantic" as being behind glacier advances in Scandinavia. Other glaciers, such as the Taku, are getting larger mainly due to the fact they are tidewater glaciers in the late stage of their cycles. As such, they are losing mass primarily due to calving . which means they are losing mass in their ablation zones. With smaller ablation zones, the glacier tries to restore its mass balance, resulting in growth in the accumulation region and glacial advance (Larsen et al., 2007).

According to Hormes, et al., "…glaciers show a response to changing climate, but cannot give any answer to the question about whether the forcing is natural or not" (Hormes et al., 2006). Glacial retreat, after all, is not something new. In the Swiss Alps, for example, glaciers were much larger during the Mini Ice Age than they are now. However, they have been both smaller in volume and shorter in length than they are currently at a number of times throughout the past 320 to 2500 years (Hormes et al., 2006). Additionally, there is now evidence that during the last interglacial period 125,000 years ago, some Alpine glaciers were smaller than they are now or even non-existent (Joerin et al., 2006).

Glaciers are particularly sensitive to changes in climate, and that is why they are pointed to as an indicator of climate change in general and of anthropogenic climate change in specific. The IPCC directly links current rates of glacial retreat to anthropogenic climate change in its 2007 report by saying, "The late 20th-century glacier wastage likely has been a response to post-1970 warming" (IPCC, 2007). (In the summary of the IPCC report it is also mentioned that "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.") Around 1970, mean glacier mass balances were close to zero regionally and globally, leading scientists to believe they were close to equilibrium. Since that time, however, the global mean has tended toward a negative mass balance, indicating .glacier wastage in the late 20th century is essentially a response to post-1970 global warming. (IPCC, 2007).