Page - 100 - in Water, Energy, and Environment - A Primer

parts in 10 billion of the total energy radiated by the Sun. Nevertheless, this very small fraction is what maintains life on Earth, and on an annual basis is approximately 10,000 times larger than all the energy currently used by the Earth’s human inhabitants (which is approximately 600quads). What happens to the 6 million quads of solar energy that annually reach the Earth’s atmosphere? While the amount of energy radiated by the Sun does vary slightly due to sunspot activity, this variation is negligibly small compared to the energy released by the Sun’s basic radiative process.As a result the amount of energy received at the outer boundary of the Earth’s atmosphere is called the Solar Constant because it varies so little. This number, averaged over the Earth’s orbit around theSun, is 1367W/m2on a surface perpendicular to the sun’s rays. In fact, the Earth’s orbit around the sun is not circular but elliptical, and the ‘Solar Constant’ varies by about 3% during the year. In the northern hemisphere the highest value is in thewinter and the lowest in the summer. Aboutaquarterof the radiation incidenton theEarth is lostby reflectionbackintospacefromthetopof theatmosphereandtops Figure8.1 RadiationspectrumoftheSun(Source:GreenRhinoEnergy). Water,Energy, andEnvironment–APrimer100