Australia: The Land Where Time Began

A biography of the Australian continent 

The Cryosphere - Albedo of Snow and Ice

Clouds are the only natural surface that contributes more to the Earth's albedo than snow and ice, the albedo referring to the amount of solar radiation that is reflected back into space. The albedo thereby determines the degree to which the received solar ration, insolation, warms the Earth and lowers the mean annual temperature of the planet. Variations in regional and seasonal variability in the amount of absorbed solar radiation that is available.

Cooling and atmospheric circulation are the main aspects of regional and local climate that are impacted by the albedo of the cryosphere. Strong ablation increases as the melt season progresses result from the seasonal evolution of snow albedo, and the transition from seasonal snow to ice cover on sea ice and glaciers. A 3- to 4-fold increase in the net shortwave radiation available for melt is caused by darkening of the surface. This feedback drives higher rates of late summer melt, that contributes to the lag between peak insolation and the annual sea-ice minimum, occurring in September in the Arctic and March in Antarctica. The runoff from glaciers and ice sheets peak in late summer is caused by this same lag.

The most variable feature of the land surface of the Earth is the cover of seasonal snow, with the solar-driven seasonality at mid-latitudes and high latitudes being amplified by albedo feedbacks. Insolation is weak or absent at high latitudes during the winter months, which mutes the effects of snow cover; therefore the local energy budget is influenced by snow and ice most strongly at mid-latitudes. A number of assumptions can be used when estimating the impact on the energy budget of the snow and ice cover. An adaptation of the linearised, zonally averaged surface energy balance, that was explored in the 1960s  (Budyko & Sellers, 1960), makes possible a simple representation of global energy balance. The author1 considers this by discretising the Earth into of latitude bands of 5o, then calculating the zonal mean topography, land fraction, and surface properties of land and ocean. There is a distinct area, albedo and total heat capacity.

The incoming solar radiation can be estimated from the potential direct radiation and the temperature can be solved for each band. Surface albedo is assigned for ice-free terrain from representative values for the ocean (0.1), forest (0.2), tundra (0.25) and desert (0.3). Albedo values of 0.8 are assigned for snow and 0.6 for ice. A simple temperature rule is the basis for the transition from open water/bare land to sea ice and snow cover, and as temperature decreases from a mean annual value of 0oC to -10oC, with a snow/ice cover that increases linearly. A composite of ice-free and ice-covered values over this transition is used to calculate the surface albedo. This simplistic model is purely illustrative.

According to the author1 a study with boundary conditions for the LGM gave a global cooling of 3oC which was accompanied by an planetary albedo increase of 0.31-0.33. When the same model is applied to an ice-free world, the results show a surface albedo reduction, though this is offset by increased evaporation and cloudiness, which results in a warming of 1.3oC, with no net planetary albedo change. The sensitivity of the climate system to feedbacks from both the cryosphere and the hydrological cycle is demonstrated by these results. At high latitudes there are also direct connections, with more open water being exposed by sea ice loss, resulting in increases of local and regional cloud cover. Planetary inferred albedo changes are massive in terms of the planetary energy balance. The average incoming radiation at the top of the atmosphere at the present is Qso = 341 W/m2, of which 102 W/m2 is reflected back into space (αp=  0.30), and the energy (Qs = 239 W/m2) remaining available at the surface of the Earth and atmosphere. This gives ∂Qs / ∂αp = -3.4 W/m2/%. An example is the snow and ice loss that causes planetary albedo to be decreased from 0.3-0.28, representing a surplus of energy of 6.8 W/m2 which compares with an estimated (about 2006) radiative forcing of 2.6 W/m2 associated with the increasing levels of atmospheric greenhouse gases. Albedo feedbacks can have a magnitude that is similar to or greater than the greenhouse gas forcing that is driving climate change at the present.

Sources & Further reading

  1. Marshall, Shawn J., 2012, The Cryosphere, Princeton University Press.

Author: M. H. Monroe
Last updated 22/04/2013

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