Polar Albedo: The Earth’s White Caps Help Keep the Climate in Balance

31 Jan 2006 - Articles, Ice & Snow, Arctic, Antarctic

Ice and water have different albedos - Copyright: IPF

Albedo lowers the planet's average temperature. Albedo is a technical term (from the Latin alba for "white") but a very simple concept: the extent to which a surface reflects the Sun's rays. So in the case of the Earth as a whole, the darker its surface on average the more heat it absorbs. In recent climate history the Earth, and its atmosphere, has reflected around 30 per cent of the sun's rays. Cloud cover reflects much of this but, at the surface, the whiteness of polar environments is critical to key climate systems. In the Arctic, all that may be about to change ...

The planet's albedo (or level of reflectance) has been important to supporting global mean temperatures of around 15 degrees Celsius over the period of human civilisation. How does this happen? Around half the Earth's surface is generally covered by clouds, which reflect or absorb over 20 per cent and a significant portion is also either reflected or absorbed by the air itself. Of the energy that does reach the surface, nearly ten per cent is reflected.

Calculating and measuring differing levels of albedo

Three things can happen when solar radiation hits an object: it can be reflected, transmitted or absorbed. While a mirror reflects almost all the radiation reaching it, transparent objects (like water or glass) transmit light and dark objects absorb it, transforming it into heat in the process.

Albedo is calculated as the ratio of radiation reflected from a surface compared with the total amount it receives. In addition, the calculation takes into account the frequency of the radiation in question and the angle of incidence of the radiation (ie whether the radiation is striking the surface from overhead or more obliquely).

The planet's overall albedo can be measured in two ways: first, using a range of satellite measurements; second, more ingeniously, by measuring the amount of sunlight reflected from the Earth (called 'earthshine' or 'ashen light') onto the surface of the moon facing away from the Sun (and which is, in turn, reflected back to Earth).

Each of the Earth's different natural environments has its own particular albedo. Open water generally has a low albedo of 10% to 15%. Swamps, forests, urban areas and dark soils also have low albedos ranging from 5% to 40%. Snow covered ice sheets and sea ice, on the other hand, have very high albedos of 80% to 90%. Clouds can vary enormously, from less than 10% to more than 70%, depending on water content and consistency, thickness and the angle at which the sun's rays strike them (the sun's "zenith angle").

Overall albedo balance

Global annual averaged albedo is a key climate variable. Taken together with the "solar constant" (the level of the sun's output reaching the Earth), it determines the radiative energy input to the planet's climate system.

Whilst albedo varies markedly across the planet and over time, the global annual averaged albedo has been consistently around 30% in recent climate history. Conversely, the Earth absorbs around 70% of the sun's rays as heat.

Cloud formation plays an important role in balancing global albedo. The Northern Hemisphere has more continental land than the Southern Hemisphere and yet the annual average albedo of the two hemispheres is nearly the same - in large part balanced out by distribution of cloud cover.

While much work still needs to be done in regard to clouds' complexity and effects, it is clear they play a central role in regulating global average albedo and climate. According to some scientists, cloud formation could even be affected by human induced or natural variations in regional albedos - for example forest fires, forest clearing and farming.

Sea ice reflecting solar radiation

Sea ice and climatic feedback mechanisms

Sea ice (with its high albedo of more than 80%), serves as a screen preventing solar radiation from being transmitted into the open water beneath (with its weak albedo of 10% to 15%) and warming it up.

In the past few decades, atmospheric temperatures have risen most quickly in the Arctic, potentially triggering a vicious circle: each year less sea ice has been forming, exposing more open water and causing more solar radiation to be absorbed. This further warms the ocean and the atmosphere, which in turn brings about further sea ice retreat.

The dwindling extent of Arctic sea ice is a cause of great concern for many scientists. It has the potential to become one of the Earth's most devastating positive feedback mechanisms - by which human induced global warming will likely be exacerbated by natural self-reinforcing responses.

Antarctic sea ice is currently less exposed to this vicious circle. The Southern Ocean's circumpolar current has to date kept temperatures in the Antarctic from rising to the same extent as in the Arctic.

Falling albedo, accelerated global warming and other key climate mechanisms

Most researchers now estimate that Arctic summer sea-ice will disappear entirely by the end of the century and possibly much sooner. If/when that happens then the Arctic region will become considerably warmer, removing a vital "release valve" which has to date contributed to keeping global temperatures in balance. Moreover, if Arctic temperatures continue to rise significantly then other critical climate mechanisms which rely on cold Arctic temperatures will likely also be affected - see Sciencepoles articles on thermohaline circulation and permafrost.

By: Jean de Pomereu