More on glaciers Pt 1- Intro to glaciology

Roughly 10% of the land area on Earth is covered by glaciers– most of this number comes from the Antarctic and Greenland ice sheets. Other areas of permanent ice are scattered around in places like the Rockies, Alps, Andes, Himalayas, etc. Ice also covers roughly 7% of the oceans in the annual mean, though both hemispheres experience sea ice loss in their respective summers, and regrowth in their winters. Around 75% of the freshwater on Earth is stored in glaciers, and they provide water for millions of people worldwide. Glaciers form when more snow falls each year than can melt or evaporate. The snow piles up, is squeezed into ice under the weight of more snow (with an intermediate form called ‘firn’), and begins to flow under gravity. Such conditions are generally a function of both temperature and precipitation and are dominant at low latitudes and high altitudes, or high latitudes.

Although it may seem strange that ice can move, glaciers do in fact flow. One process by which ice flows is when the weight of the ice above deforms the ice below and the glacier begins to move much like a thick fluid. Another method of flow is by basal sliding, or when water is on the surface below the glacier allowing for a lubricated base. When glaciers flow, they transfer mass toward their margins where ice is lost due to melt, or calving into icebergs in marginal seas or lakes. This process is not unlike pouring pancake batter into a pan and watching it spread. The compaction of ice underneath explains why older (deeper) layers of ice are more “squished” together. 10 meters of ice core from a mile below the surface will represent a much larger amount of time than 10 meters of core from the surface. This also shows why young earth creationists have a misplaced argument when they claim that World War 2 planes that were buried deep into the ice show that the ice sheet is younger than we think.

Some glacio- anatomy-physiology: All glaciers have an accumulation zone and an ablation zone. The zone of ablation (or zone of wastage) is the part of the glacier where there is net melt and continuation of the glacier cannot be sustained. This occurs at the edges. The base of the glacier is called the terminus. The glacier mass balance is a competition between accumulation (from precipitation) and ablation. If there is net accumulation, the glacier will grow. If there is more ablation that accumulation, the glacier will shrink. Changes in climate can affect the glacier mass balance significantly, along with regional factors such as land use and cloud cover. A glacier forming at high altitudes will advance downward where it will extend into a region of net melt until the ice supplied from the high-altitudes is removed by melt in the lower altitudes. The equilibrium line of a glacier is a boundary between the zone of accumulation and zone of ablation. On a mountain glacier for instance, if there is a climate change and the equilibrium line moves to the mountain top, the glacier will disappear.

Sea ice is formed by the freezing of seawater in the polar oceans. Sea ice regulates exchanges of heat, moisture, and salinity in the polar oceans. This serves to insulate the oceans below, from the cold atmosphere above. Sea ice covers around 15 million square kilometers in late winter in the Arctic, and around 8 million square kilometers in the arctic summer. Antarctica has a much larger seasonal contrast, with about 17 to 20 million square kilometers in the Antarctic’s Southern Ocean, and only about 3 to 4 million square kilometers at the end of summer (remember Arctic and Antarctica summer occurs at opposite times of the year) (Armstrong et al. “State of the Cryosphere: Response of the Cryosphere to Global Warming” ).

Ice Shelves are thick forms of ice that are connected to the land and float on ocean. They lose mass through iceberg calving and melting, and gain mass from glacial and ice sheet flow. Ice shelves respond very rapidly to climate changes.

Permafrost is soil that is permanently covered in ice over many years. Nearly one fifth of the land surface on Earth is covered by permafrost. It can be found as far north into the Arctic as around 86 N, and as far south in the northern hemisphere as 26 N, around the Himalayas. The good majority of NH permafrost is covered by evergreen boreal forest and acts as a carbon source and sink.

Ice and Climate

Ice on Earth effects climate, and climate effects ice. Ice reflects most incoming solar radiation (i.e., it has a high albedo) and so changing ice cover can have implications for the Earth’s radiative budget. Sea ice also influences the movement of ocean waters. When sea ice forms, most of the salt is pushed into the ocean water below the ice. Because salt water is denser than fresh water, it sinks, and so presence of sea ice contributes to meridional circulation (i.e., the “conveyor-belt”). After the oceans, the cyrosphere is the largest component of the climate systems in terms of heat capacity.

Sea levels are directly related to ice sheet/glacier conditions. If both ice sheets completely melted, sea levels would rise roughly 70m (or over 200 feet). Sea ice melt does not directly contribute to sea level rise, but it can affect ocean circulation patterns, as well as its influence on albedo (i.e., albedo declines with increased solar absorption).

The existence of glaciers in low latitudes is possible because of the lapse rate (i.e., the temperature decreases with height). Thus, tropical glaciers are only found at high altitudes. The seasonal contrast (i.e., winter to summer) in low latitudes is not very large. In fact, day to night variations are larger than summer to winter variations as one moves into the tropics. The incident solar radiation does not change much at the equator between seasons, but rather the seasonal contrast becomes greater as one moves up in latitude due to the tilt of the Earth. The seasonal cycle of precipitation is strong in the tropics however, and is generally dominant over temperature as a factor in changes over tropical glacial mass balance. Seasonality in tropical climate is mainly caused by the annual cycle in moisture-related parameters (e.g. precipitation and clouds). Temperatures have risen in the tropics however, and because horizontal variations in temperature are not large in the tropics, a uniform retreat of glaciers in the tropics may be a good indication of temperature-related glacier retreat.

Ice changes as a function of temperature becomes more dominant at higher latitudes, and northern hemisphere extra-tropical ice is highly responsive to temperature variations. The majority of Antarctica seems to be well shielded against temperature variations, and one would need quite large changes for significant effects there (though this does not appear to reflect conditions in the Antarctic Peninsula which is receding very much now).

Glaciers represent important indicators of climate change. Different aspects of ice are affected on various timescales, but changes in climate show up rather quick in terms of ice retreat, though lag times exist such that if all things were held constant, there would still be recession for some time as glaciers equilibrate to the new surrounding temperature.

It is not the “mean annual temperature” but the seasonal energy budgets, and snowfall amounts that determine what glaciers do. The energy available for ablation of a glacier is determined by the energy budget of the glacier surface. Albedo depends on precipitation amount and frequency, and directly controls net shortwave radiation. Fresh snow has a higher albedo than older snow, and this has profound impacts for the surface energy budget on an ice mass. From the work of Mölg and Hardy, 2004 on Kilimanjaro, they found that precipitation contributes to net accumulation more through the albedo affect (by reducing ablation), than by means of the mass added directly by snowfall accumulation. Longwave energy also is involved, as surfaces emit radiation proportional to T(g)^4. T(a)^4 (from the atmosphere) is also involved as back-radiation incident on the glacier surface. Moisture in the atmosphere is important, because moist air can radiate more efficiently than dry air. Since infrared rays can travel at long distances, the IR emitted from the upper layers of the atmosphere is important for the surface budget of the glacier, not just the low levels of the atmosphere. Because of the Clausius-Clapeyron relationship between water vapor content and temperature, the upper layers become moister as the atmosphere warms up.

The effects latent and sensible heat is also significant, and the fluxes are in the direction of the gradients in temperature and humidity. The surface budget of the glacier will be given by
F_surface flux = “S”(1-a) + L_in + L_out + S + H + C

where L_in and L_out are the incoming and outgoing infrared radiation, S and H are the Sensible and Latent heat fluxes, and C represents heat moving in or moving out of the glacier via conduction or convection. “S” will be the solar radiation available on the surface.

Taking into account shading from mountains, slope, etc are other important aspects of glacier micro-climate.

Ice surfaces also lose energy by sublimation (when ice is converted directly to vapor and is carried away by winds). The moisture in the air above the glacier surface is in equilibrium with the moisture source provided by the ice. The specific humidity of this overlying air will then depend upon the temperature of the glacier surface (equal to the saturation specific humidity). Since Clausius-Clapeyron dictates that the saturation vapor pressure increases quasi-exponentially with temperature, the specific humidity of the surface air will increase with higher temperatures, and so the specific humidity gradient will increase between the surface air and boundary layer. Since latent heat goes from areas of high specific humidity, to areas of low specific humidity, the latent heat term will increase as the gradient between the surface and boundary layer increases– this may result in ablation by means of sublimation. Because of this, the sublimation factor approaches zero as one moves to very cold conditions. Note– the sublimation factor will not outweigh other terms on very small temperature variation scales, but can become important as ΔT becomes large.

The effects of clouds on shortwave and longwave radiation concerning the glacier surface mass balance are opposite in sign. This depends on the clouds themselves (e.g. their transmissivity) as well as albedo. For a low glacier surface albedo, the solar effects dominates, whereas the net change in longwave radiation is greater than the net change in shortwave radiation if you were to increase clouds.

When a glacier surface reaches 0 C (glacier ice is near its melting point in general), it cannot rise anymore as long as there is any ice left. When the glacier is pinned at 0 C, and is receiving more energy than getting rid of, energy goes to melting (phase change) rather than due surface temperature increase. One may need to look at energy fluxes and conditions aloft for a melting surface to get an idea of “trends.” If melting sets in, and evaporation and sublimation are occurring, the latter terms do not cease but rather continue at a rate set by the wind and the moisture gradient.

Pt. 2 will include recent trends, and what is going on for future scenarios

……..

Mölg T, Hardy DR. 2004. Ablation and associated energy balance of a horizontal glacier surface on Kilimanjaro. Journal of Geophysical Research. 109, D16104, doi:10.1029/2003JD004338.

2 responses to “More on glaciers Pt 1- Intro to glaciology

  1. Interesting material chris!
    as usual i learned a lot from you post.
    thanx!

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