While researching heat, the topic of heat reflection on the Earth came up. The reflectance of sunlight reflected by the atmosphere and the Earth’s surface is called albedo, and the global average is about 0.3, which is about 30%. The percentage of sunlight reflected is 90% on pure white snowy fields, surprisingly 6% in the ocean, 9-15% in forests, and 40% in deserts.Snow fields reflect most of the sunlight, while oceans and forests absorb it. In the ocean, phytoplankton performs photosynthesis, while in the forests, plants perform photosynthesis, so it seems that a lot of sunlight is absorbed.
The reflectance of snow fields comes from their whiteness, and when dirt or algae get on the snow or ice and it turns black, the albedo goes down. It seems that the importance of snow fields is not just the temperature, but also the whiteness. When the balance between the absorption and reflection of heat from the sun is maintained, the temperature seems to be stable.If there is a lot of absorption, the temperature rises, and if there is a lot of reflection, the temperature falls, so the albedo of 0.3 is probably just right. After absorbing sunlight, the Earth releases heat into space by emitting infrared rays. The energy of this solar radiation and the Earth’s radiation are in balance, and this is a state called radiative equilibrium.
According to one calculation, if the albedo is 0.3, the Earth would freeze if the amount of solar radiation was about 80%. The Sun billions of years ago was about 80% of its current value, but no evidence has been found that the Earth ever completely froze over. How the Earth avoided freezing over is one of the mysteries known as the “faint sun paradox.”To begin with, the albedo of 0.3 is exquisite, and it seems to be a mystery how it is maintained. The albedo seems to be greatly affected by clouds, and it seems very strange that an average albedo can be maintained for a long period of time by irregular clouds that appear and disappear within a few hours.
Since there is no particular control over the movement of clouds, it is amazing that a stable albedo is created by the Earth’s overall circulation system, including the changing seasons and weather, and the proportion of snow fields, deserts, and oceans. Albedo is amazing. The Earth is amazing.
