The albedo
Earth’s brightness has a significant influence on its temperature and climate. This applies not only to Earth’s surface, but also to clouds, gases and aerosols in the atmosphere. The reason is that light-coloured surfaces reflect more sunlight, deflecting energy away from the planet. This keeps Earth cooler. Dark surfaces absorb more sunlight and thus heat up.
Reflection and absorption of sunlight by various water-containing surfaces
The brighter a surface, the more it reflects sunlight and the less it heats up. The effect of clouds on the climate is complex: they can reflect solar rays or allow them to pass through, but they can also absorb heat radiation emitted by Earth’s surface and re-emit it into the atmosphere (greenhouse effect). For the sake of simplicity, this illustration only shows their reflection and whether they have a cooling effect (bluish shading) or a warming effect (reddish shading).
Yellow sunbeams strike different water‑containing surfaces. The ocean reflects only a small share (about 5–20%) and absorbs the rest. Sea ice reflects much more (50–75%), and snow the most (75–95%). Above, clouds are shown: stratus clouds reflect 30–60% and cool the surface by shading; cirrus clouds are high and let more sunlight through but warm the climate by trapping outgoing heat (greenhouse effect). A legend indicates incident, reflected, and absorbed sunlight.
We are familiar with this from everyday life: a black T-shirt makes us feel warmer in the sun than a white one. This reflectivity depending on the brightness of a surface is called albedo.
DARKER AND WARMER DUE TO THE MELTING OF ICE
The melting of snow and ice (glaciers, the Greenland and Antarctic ice sheets, Arctic sea ice) due to climate change not only causes Earth to lose large, bright reflective surfaces, but it usually also exposes the dark land or ocean water beneath. This further darkens Earth’s surface, causing it to heat up even more. The illustration shows how important ice and snow are in balancing out the many dark surfaces on Earth due to their high albedo values.
Albedo values for various surfaces and clouds
A horizontal scale from 0 to 100% albedo (reflectivity) runs across the image, shaded dark on the left (no reflection, maximum warming) to bright on the right (total reflection). Labeled photo bars mark typical ranges: asphalt 5–25, oceans 5–20, forests 5–18, dark roofs 8–18, Earth average 30, deserts 36, bright roofs 40, stratus clouds (low/dark) 30–60, sea ice 50–75, cirrus clouds (high/bright) 10–30, cropland 16–26, concrete 10–40, grass 22–28, and snow 75–95. The chart shows that bright surfaces and clouds reflect more sunlight, while dark surfaces absorb it.
RAISING THE ALBEDO THROUGH URBAN DEVELOPMENT
In settlement areas, the albedo can be raised and heating reduced through suitable measures in building and construction. Roofs, facades and road surfaces, for example, can be made in lighter colours.