Atmospheric Absorption of solar Radiation

solar radiation is radiant energy emitted by the Sun. The process begins at the Sun's core, where hydrogen atoms are fused to helium atoms via nuclear fusion. For each second of nuclear fusion, the Sun converts 700 million tons of hydrogen into 695 million tons of helium, with 5 million tons of electromagnetic energy radiating out into space.

Some of this energy travels the approximately 92.89 million mi. (149.5 million km.) across the solar system to Earth. The importance of the Sun to Earth is hard to overstate: quite simply, it makes life here possible. It provides us with heat and light, controlling everything from daily weather patterns to the placement of climate zones, while our trips around it provide us with our annual seasons.

Not all incoming solar radiation makes it to the Earth's surface; in fact, almost half of it is deflected back into space by the outer layers of our atmosphere. Inside the atmosphere, it scatters and interacts with gasses and particles in different ways, depending on the size and composition of its wavelengths. For example, gamma rays, ultraviolet light, and x-rays of 200 nanometers in wavelength are absorbed by ozone and nitrogen is converted into heat energy. Ultraviolet rays of 200-300 nanometers in wavelength are absorbed by ozone in the stratosphere, while infrared rays of 700 nanometers in wavelength are partially absorbed by ozone, carbon dioxide, and water vapor in the lower atmosphere.

In all, an estimated 6 percent of incoming solar radiation is reflected back into space by the atmosphere, 51 percent is absorbed at the Earth's surface (where 4 percent of it is reflected back), while 39 percent is either reflected or absorbed by clouds. Solar radiation that has passed beyond these forces of scattering or absorption and reaches the Earth's surface is called diffused solar radiation, while that which reaches the ground without stopping is direct solar radiation.

The Sun's intensity changes in a number of long and short cycles, and those who do not believe in anthropogenic (human-caused) global warming often point to the Sun as the true culprit. While there is no denying the Sun's influence on past cycles of heating and cooling, the majority of scientists believe that it is a combination of natural and human-caused processes that are driving observable climate changes today.

Changes in albedo are an important part of the global warming picture. Albedo, a word derived from the Latin word for white, is the surface reflectivity of solar radiation. A dark surface will absorb more solar radiation than a light surface. The overall albedo of the Earth is 30 percent, and it plays a crucial role in keeping the planet's temperature habitable.

Albedo varies depending on the color of the surface. Open water has an albedo of 8 percent, meaning it reflects only 8 percent of the solar radiation that strikes it, absorbing the remaining 92 percent as heat. Dry desert sands have an albedo of 35-45 percent, coniferous forests between 10-20 percent, and deciduous forest between 5-10 percent. It is the polar regions that do the most to reflect solar radiation, with fresh snow carrying an albedo of 95 percent, snow-covered sea ice about 70 percent, and melting snow about 50 percent.

The interplay of anthropogenic and solar impact is best illustrated by recent events in the Arctic, where each year has seen the shrinking of summer sea ice at the North Pole and in Greenland. Something—perhaps the burning of fossil fuels—has allowed more heat to become trapped in the atmosphere and has warmed the environment. This causes some snow and ice to melt, which drops the region's surface albedo, which in turn allows an increased amount of solar radiation to reach the surface. The radiation produces more heat, which causes more snow and ice to melt, and soon the process becomes unstoppable. The loss of the polar ice caps could have tremendous repercussions around the world, from destabilizing the world's weather patterns, to rising global sea levels from the melting of the Greenland ice sheet.

SEE ALSO: Albedo; Arctic Ocean; Radiation, Absorption; Sunlight; Weather.

BIBLIOGRAPHY. Robert Roy Britt, "Sun's Output Increasing in Possible Trend Fueling Global Warming," (cited November 2007); Edward Bryant, Climate Process and Change (Cambridge University Press, 1997; John Theodore Houghton, Climate Change 1994: Radiative Forcing of Climate Change and an Evaluation (Cambridge University Press, 1995); Douglas V. Hoyt and Kenneth H. Schatten, The Role of the Sun in Climate Change (Oxford University Press, 1997); Intergovernmental Panel on Climate Change, "Radiative Forcing in Climate Change 2001: The Scientific Basis," (cited November 2007).

Heather K. Michon Independent Scholar

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