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Without the Sun, the Earth would be no more than a frozen rock stranded in space. The Sun warms the Earth and makes life possible. Its energy generates clouds, cleanses our water, produces plants, keeps animals and humans warm, and drives ocean currents and thunderstorms. Despite the Suns importance, scientists have only begun to study it with high precision in recent decades. Prior to 1979, in fact, astronomers and Earth scientists did not even have accurate data on the total amount of energy from the Sun that reaches the Earths outermost atmosphere. Variable absorption of sunlight by clouds and aerosols prevented researchers from accurately measuring solar radiation before it strikes the Earths atmosphere.
The launch of the Nimbus-7 satellite in 1978 changed all that. It enabled us for the first time to detect sunlight without interference from the atmosphere. The Earth Radiation Budget (ERB) instrument on the satellite measured levels of solar radiation just before it strikes the Earths atmsophere. Through subsequent satellite missions, scientists have gathered a wealth of information on the Sun and the solar energy that drives our worlds climate system.
Today researchers know that roughly 1,368 watts per square meter
(W/m2) of solar energy on average illuminates the outermost atmosphere of the
Earth. They know that the Earth absorbs about only 70 percent of this
total solar irradiance (TSI), and the rest is reflected into space.
Perhaps most intriguing, researchers have affirmed that the TSI doesnt
stay constant, but varies slightly with sunspots and solar weather
activity. In particular, by analyzing satellite data, scientists have
observed a correlation between the Suns output of energy and the
11-year sunspot cycle, which physicists have known of since Galileos
time. These data show that TSI varies just as regularly as the sunspot
activity over this 11-year period, rising and falling 1.4 W/m2 through
the course of the cycle (0.1 percent of the TSI). There are also
longer-term trends in solar weather activity that last anywhere from
years to centuries to millennia and may have an impact on global
Due to technological barriers and a limited amount of data, however, scientists understanding of the Sun-Earth system continues to be incomplete. They are unable to predict fluctuations in TSI due to 11-year and long-term solar cycles, and scientists do not yet have accurate enough measurements to determine the trend from one cycle to the next with sufficient precision. In fact, the TSI is currently known to within an accuracy of a few Watts per square meter, which is greater than the entire fluctuation of the TSI over one 11-year solar cycle. Additionally, scientists havent pinned down what proportion of solar energy is absorbed by the land or atmosphere. They also do not have complete measurements of the energy variation for the distinct wavelengths of incoming solar radiation. These different wavelengths affect the various components of the Earths atmosphere, land, and ocean in different ways.
In 2003, Earth scientists will move a step closer to a full understanding of the Suns energy output with the launch of the Solar Radiation and Climate Experiment (SORCE) satellite. SORCE will be equipped with four instruments that will measure variations in solar radiation much more accurately than anything now in use and observe some of the spectral properties of solar radiation for the first time. Robert Cahalan of NASA Goddard Space Flight Center serves as SORCE Project Scientist, and the four instruments are being built at the University of Colorado under the direction of Gary Rottman, SORCE Principal Investigator, with participation by an international team of scientists. SORCE will be launched in January 2003 from Kennedy Space Center on a Pegasus XL launch vehicle provided by Orbital Sciences Corporation. With data from NASAs SORCE mission, researchers should be able to follow how the Sun affects our climate now and in the future.
next: Earths Energy Balance