Glaciers and Climate:  Space

This web page was written to inform folks about what drives glaciation globally by looking at what influences the growth and preservation of glaciers.  To check out at a specific topic, click on the following table.  Otherwise, scroll down to read our hypertext and explore the links!

World distribution of modern glacial ice

World distribution of glacial ice today. Glacial ice currently covers 10 percent (16 million km²) of the earth's surface. The majority of glacial ice is tied up in the Greenland (2 million km²) and the Antarctic (13.5 million km²) ice sheets as shown in the above figure. The remaining 500,000 km² of glacial ice is found in sub polar to polar regions, in high elevation mountains near the coastline, and in the mid-continental mountains (Benn & Evans, 1998).  To grow a glacier, annual snow accumulation must be greater than the annual summer melt.  Low precipitation at the poles does not hinder glaciation as much as the cold climate maintains it.  High precipitation along coastlines due to orographic uplift of water-saturated air masses brings lots of snow to the higher elevations, and the cooler air temperatures and higher albedo at these altitudes preserves the snow and ice. Note that most glaciers outside polar regions occur in mountains resulting from collisions between tectonic plates.

For a more comprehensive look at global glacier distribution, check out the World Glacier Inventory from the University of Colorado, or the Satellite Images of Glaciers around the World from the U.S. Geological Survey.

The Sun's rays and the Earth

Rays striking Earth at low angles (modified from McKnight, 1996, p. 70).  Presently, the tilt of the earth's axis is 23.5^o from due north as shown in the above figure. As short wave solar radiation penetrates the earth's atmosphere, the rays can be diffused, reflected, scattered or absorbed. Near the equator (latitude 0^o), where the sun's rays hit the Earth's surface at approximately 90^o, the sun's rays have less atmosphere to penetrate, which results in more energy directly reaching the surface. In the polar regions (latitudes greater than 60^o), the sun's rays strike at a much lower angle.  Here, the low angle of the sun's rays forces the energy to pass through a longer transect through the atmosphere, which diffuses the radiation and allows less solar radiation to reach the surface of the earth.  At high elevation there is little atmosphere for the sun's rays to penetrate, therefore short wave radiation is intense and daytime temperatures can be high. However, the thin atmosphere also allows for night-time loss of long wave radiation, thus cold nights.

Glaciers and Latitude

The relationship between glaciers and latitude, shown along a line from Alaska to the tip of South America (modified from Ives and Barry, 1974).  The snowline is any indicator of glaciation, such as the equilibrium line altitude. Glaciers may exist at any latitude, if sufficiently high precipitation and low temperatures allow ice to exist year-round.  Snowlines vary with latitude, and exhibit a wide range in elevation close to the equator, and a narrow range at the poles.  Near the poles, low summer temperatures help to retain winter snowfall, even near sea level.. At about 30ºN and S latitude, subtropical high pressure yields low precipitation, thus glaciers occur only on the highest (coldest) mountains.    Glaciers at or near the equator are typically affected by tropical low pressure systems where the air converges, rises, cools, and dumps large amounts of precipitation. This high precipitation offsets high summer temperatures near the equator.

Glaciers and Altitude

Glaciers with Altitude (as discussed by Barry, 1992).

1. It is common knowledge that air temperature usually decreases with elevation - the average rate ("lapse rate") is about 6ºC per vertical kilometer.  Because heat transfer from warm air can be a significant component of melting, high mountains are obviously a favorable environment for glaciers.  
2. It is often thought that precipitation increases with elevation.  While this is commonly true in mid-latitudes, precipitation may decrease with elevation in other environments.  For example, international astronomical observatories in Hawai'i and Peru take advantage of the thin atmosphere and dry conditions at mountain tops.
3. Finally, mountains provide shade.  Although mountain ice caps can form on plateaus or grow to cover the mountains, most mountainous regions harbor cirque and valley glaciers on the flank hidden from the afternoon sun (NE, in the Northern Hemisphere).  The higher the mountains, the deeper (potentially) the valley, and the better the shading from direct short wave radiation.

For a more detailed description of glacier flow, mass budgets, and albedo, check out Glacial Systems in our hypertext.

Global Precipitation
(major glaciated areas are noted in red)

Average annual precipitation of the world (from NOAA's Global Precipitation Climatology Project).  For a glacier to form, a sufficient amount of precipitation must fall in the form of snow and be preserved. This map displays one major control on the spatial distribution of glaciers. The greatest precipitation is in the equatorial regions. This is due to the "inter-tropical convergence zone" where air masses from the north and the south converge, rise, and cool.  When these air masses reach 100% humidity, they drop their excess moisture, resulting in precipitation. In the high polar latitudes (60-90^o latitude) precipitation levels are low. Cold air masses (which hold little moisture to begin with) diverge and sink in polar regions, thus little precipitation occurs. Higher elevations defy the normal latitudinal variations in precipitation due to orographic uplift. The heavy local precipitation typical of some mountainous regions cannot be shown at the scale of this figure.

Orographic Uplift

Orographic Uplift occurs in where a moist maritime air mass encounters a mountain front. The air mass is then forced to rise. As the air mass rises it cools, reaches 100% humidity, and releases moisture as precipitation. Precipitation falls on the upwind side of mountains and little precipitation falls on the downwind (or lee) side of mountains. Areas that are located far inland from large water bodies have less precipitation because much of the moisture in the atmosphere has already been dropped on intervening mountainous regions. At a given latitude, the elevation at which glaciers occur rises inland.  Thus continentality, elevation, and latitude all influence the spatial distribution of glaciers.

Global Temperatures
(major glaciated areas are noted in white)

World pattern of average annual temperature (modified from Noa Climate Diagnostics Center).  Temperature is another important variable in controlling the distribution of glaciers thoughout the world. If the summer temperatures in an area are too high, they will melt all of the snow that has fallen during the previous winter, and a glacier cannot form. Latitude has a large influence on temperature differences. Temperatures decrease from the ground surface to the stratosphere due to the "adiabatic lapse rate" of approximately 6^oC per 1000 meters. This relationship is demonstrated by cooler temperatures at higher elevations thoughout the globe, which only begins to be evident at this scale.

Glaciers of the western United States

Modern distribution of glaciers in the western conterminous United States (C.R.E.E.L. publication, U.S. Army, 1975).  Alpine glaciers form in high elevation areas throughout the world. High elevations receive more moisture due to orographic uplift, thus they have a high potential for building glaciers. In the troposphere, temperatures decrease as elevation increases.  In midlatitudes, the combined effects of increased precipitation and decreased temperature with elevation ensure that major mountain ranges (throughout the western United States) support small glaciers.

Coastal mountains (like the Cascades and Mount Rainier) typically get lots of snow each winter due to the saturated air masses that come in from the ocean.  Typical snow near a coast is heavy and wet, and sometimes even slushy.  Because there is a lot of water mass in this snow, which may turn to ice, this is a good place to grow a glacier.  In the Rocky Mountains (the Wind River Range, for example), storms have a long way to come from the coast and become less saturated in water as they travel inland.  So, continental mountains have a tendency to be drier environments.  Often in these areas of dry, high plateaus, average winter temperatures are significantly less than those at or near the coastline.  Snowfall known as "powder" or "cold smoke" which graces these mountains and high valleys contains little water mass, so it takes a lot more snow of this sort to grow a glacier.  However, the low winter temperatures at inland high elevations are better at keeping the snow around.

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