by Claire Hernon ’14

Glaciers, which fall within the category of land ice, are defined as persistent bodies of ice which consist mostly of recrystallized snow and show evidence of gradual downslope or outward movement due to gravity. There are several different types of glaciers, including ice caps and ice sheets. Ice caps cover mountain highlands or low-lying land at high latitudes and typically flow radially outward from their center. Ice caps cover an area of less than 50,000 km2, while a glacial mass covering an area greater than 50,000 km2 is called an ice sheet. Enormous, continent-sized ice sheets include Antarctica and the Greenland ice sheet; these huge ice sheets override the vast majority of the land surface within their margins, often with only a few mountain peaks sticking up from the ice. In fact, the Greenland ice sheet covers approximately 80% of Greenland’s surface. Large ice sheets account for roughly 95% of the glacial ice on earth, and tend to reach thicknesses of over 3,000 m. There are also several smaller ice sheets within the Canadian Arctic islands.

Glacial mass is continuously changing both season to season as weather varies, and on longer time frames, due to changes in worldwide climate. The snow that is added to the glacier is termed accumulation, while the snow and ice lost is called ablation. Ablation can occur through various different processes, including melting, sublimation, evaporation, and calving. Calving is the progressive breaking off of icebergs from the front of a glacier that terminates in deep water. Once begun, calving continues quickly and irreversibly until the glacier’s front retreats into water too shallow for significant calving to occur. The relative amounts of accumulation and ablation is a measure of a glacier’s mass balance. Periods in which the average temperature of Earth’s surface decreases and remains low enough to support the growth (positive mass balance) of existing ice sheets and formation of new ones are known as glaciations. The time in between glaciations, when ice sheets retreat (negative mass balance) and sea levels rise, is called an interglacial stage. We are currently experiencing an interglacial stage.

The existence of glaciers relies on many parts of the Earth System working together. Tectonic forces have produced high mountains; adjacent oceans provide ample sources of moisture, and the atmosphere delivers this moisture to the land in the form of snow. On the other hand, changes in the cryosphere have broader implications, affecting the other spheres of the Earth System including the “anthroposhere,” or human society.  About 30% of Earth’s surface that is not currently covered by glaciers has been shaped by glaciers in the recent past. One way glaciers alter land surface is by scraping up weathered rocks and soil and removing blocks of bedrock. Glaciers also grind away solid bedrock, and carry acquired sediment and rock debris along their paths of motion. Due to its high viscosity, glacial ice can carry coarse loads at its sides and on its surface, unlike water and wind: thus, glacial ice transports and deposits sediment directly, without sorting or stratifying it. As they erode, ice sheets sometimes mold smooth, parallel ridges of sediment termed drumlins, which are extended along the direction of ice flow.  The Boston Harbor Island are mostly drumlins that have been reshaped by ocean erosion since their creation during the most recent glaciation (~20,000yrs ago).

Increased emissions of radiatively active gases (greenhouse gases) by humans into the atmosphere are tied to changes in global climate. How these changes will affect the cryosphere, and specifically ice sheets, is of great scientific interest. Previous studies have suggested that global climate change and increased temperatures have a strong correlation with the accelerating melting of glaciers and ice caps. Greenland is currently losing ice at roughly five times the rate recorded during the early 1990s.[1] Large-scale, rapid melting of glaciers has significant implications for the Earth System, causing effects such as lower global albedo, a measurement of the Earth’s reflectivity. The ice-albedo effect is a positive feedback cycle, in which warming decreases ice cover, which decreases the amount of reflective surfaces, which increases heat absorption, creating more melting and thus feeding the cycle. Additionally, increasing melting of ice sheets, ice caps, and other glaciers is causing sea levels to rise.[2] A recent study estimating the mass balance of Earth’s polar ice sheets found that, between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by approximately -142, +14, -65, and -20 gigatonnes per year, respectively.[3] Scientists believe meltwater from above and seawater from below is seeping under the ice sheets of  Greenland and West Antarctica, lubricating ice streams and causing them to move more quickly into the sea.[4] Since 1992, ablation of polar ice sheets has added an average of 0.59 millimeters annually to the rate of global sea-level rise.[5] The melting of ice sheets in Greenland and Antarctica specifically have contributed one-fifth of the total global sea-level rise.[6] Rises in sea-level put thousands of coastal cities and even whole islands at risk.[7] Even small increases in sea level can have detrimental effects on coastal habitats, such as causing destructive erosion, flooding wetlands, contaminating aquifers, and eliminating habitat for plants and animals. Additionally, higher sea levels would make big storms more powerful upon hitting land.[8] Major uncertainties remain as to how the melting of ice sheets will change over time, and how these changes will impact the Earth System as a whole, thus highlighting the need for ongoing research of these phenomena.

 

[1] Olive, Heffernan. “Grim picture of polar ice-sheet loss.”Nature. (2012). http://www.nature.com/

news/grim-picture-of-polar-ice-sheet-loss-1.11921 (accessed March 11, 2014).

[2] National Geographic, “Sea Level Rise: Ocean Levels Are Getting Higher—Can We Do

Anything About It?.” Accessed March 11, 2014. <http://ocean.nationalgeographic. com/ocean/critical-issues-sea-level-rise/.>

[3] Shepherd, Andrew. “A Reconciled Estimate of Ice-Sheet Mass Balance.” Science. no.

6111 (2012): 1183-1189. http://www.sciencemag.org/content/338/6111/1183 (accessed March 11, 2014).

[4] National Geographic, “Sea Level Rise: Ocean Levels Are Getting Higher—Can We Do

Anything About It?.” Accessed March 11, 2014. <http://ocean.nationalgeographic. com/ocean/critical-issues-sea-level-rise/.>

[5] Shepherd, Andrew. “A Reconciled Estimate of Ice-Sheet Mass Balance.” Science. no.

6111 (2012): 1183-1189. http://www.sciencemag.org/content/338/6111/1183 (accessed March 11, 2014).

[6] Olive, Heffernan. “Grim picture of polar ice-sheet loss.”Nature. (2012). http://www.nature.com/

news/grim-picture-of-polar-ice-sheet-loss-1.11921 (accessed March 11, 2014).

[7] National Geographic, “Sea Level Rise: Ocean Levels Are Getting Higher—Can We Do

Anything About It?.” Accessed March 11, 2014. <http://ocean.nationalgeographic. com/ocean/critical-issues-sea-level-rise/.>

[8] Ibid.