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 Sun’s 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 Earth’s outermost atmosphere. Variable absorption of sunlight by clouds and aerosols prevented researchers from accurately measuring solar radiation before it strikes the Earth’s atmosphere.

Energy from the Sun makes life on Earth possible. Solar energy also drives the Earth’s climate, and slight variations in solar radiance could offset (or increase) global warming. (Photograph courtesy Philip Greenspun)

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 Earth’s atmsophere. Through subsequent satellite missions, scientists have gathered a wealth of information on the Sun and the solar energy that drives our world’s 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 doesn’t stay constant, but varies slightly with sunspots and solar weather activity. In particular, by analyzing satellite data, scientists have observed a correlation between the Sun’s output of energy and the 11-year sunspot cycle, which physicists have known of since Galileo’s 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 warming.
 

by John Weier & Robert Cahalan
January 21, 2003

Solar Radiation and Climate Experiment (SORCE)
Introduction
Earth’s Energy Balance
Solar Variability
The Sun and Global Warming
Uncertainties in Solar Measurements

The SORCE Satellite
Total Irradiance Monitor (TIM)
Spectral Irradiance Monitor (SIM)
Solar Stellar Comparison Experiment (SOLSTICE)
Extreme Ultraviolet Photometer System (XPS)

Related Articles
SOLSTICE
Watching the Sun
ACRIMSAT
Sunspots and the Solar Max
Clouds and Radiation
Why isn’t Earth Hot as an Oven?

Related Datasets
Reflected Solar Radiation
Outgoing Heat Radiation

The total energy emitted by the Sun varies on an 11-year cycle. Even greater variation occurs at shorter time scales as sunspot groups form and dissipate. At longer time scales the Sun may undergo other cycles. By monitoring these changes in the Sun, scientists hope to better understand its role in changing the Earth’s climate. (Graph adapted from Goddard Institute for Space Studies Data and Images: Solar Irradiance)

Due to technological barriers and a limited amount of data, however, scientist’s 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 haven’t 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 Earth’s atmosphere, land, and ocean in different ways.

In 2003, Earth scientists will move a step closer to a full understanding of the Sun’s 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 NASA’s SORCE mission, researchers should be able to follow how the Sun affects our climate now and in the future.

next: Earth’s Energy Balance