SunEarth’s closest star, our sun, provides the key energy input that drives global weather patterns on our home planet. Scientists studying global climate change now believe that small, sustained changes in the energy coming from the sun (called total solar irradiance) could play a role in climate change on time scales of many decades. By measuring the total amount of energy that the sun delivers to the Earth, scientists will be able to build better scientific models of the Earth’s climate system, providing a vital piece of the global climate change puzzle.

How sunlight drives the Earth system
The sun has been powering the Earth system for the past 4.6 billion years. Since most of the sun’s heat is deposited into the tropics of the Earth, the polar latitudes receive on average much less solar heating than the equator. As a result, the average difference in solar heating between the equator and the poles is large. This situation drives a strong “heat engine,” or circulation of the atmosphere, steering weather systems around the globe. In addition, the sun is a major player in the hydrologic cycle, as a steady influx of solar energy induces continual phase changes in water. As incoming solar energy heats the surface layers of the Earth’s oceans, lakes, and rivers, some of the water molecules break away to form water vapor in the atmosphere. This water vapor rises high into the atmosphere where it condenses into clouds, and eventually precipitation, and falls back to the surface where the cycle begins again. These precipitation processes lead to the weathering of rocks and erosion processes, which shape Earth’s landscapes. Through photosynthesis, sunlight is used by green plants to reduce carbon dioxide in the atmosphere and convert it to carbohydrates and oxygen, an essential process for the sustainability of life on our planet.


by Steve Graham
December 21, 1999


Variations in Total Solar Irradiance
Total Solar Irradiance Data in Climate Studies
Total Solar Irradiance Data in Solar Studies

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ACRIM Instruments

top left: Image of the sun from the Solar and Heliospheric Observatory’s (SOHO) Michelson Doppler Imager. (Image courtesy NASA Goddard Space Flight Center)


Scientists will acquire new solar data from NASA’s ACRIMSAT spacecraft, a five-year mission launched on December 20, 1999 that will measure the sun’s total solar irradiance with the Earth Observing System’s (EOS) Active Cavity Radiometer Irradiance Monitor (ACRIM) III instrument. This instrument, the third in a series of long-term solar-monitoring tools built for NASA by the Jet Propulsion Laboratory (JPL), will extend the database first created by ACRIM I, which was launched in 1980 on the Solar Maximum Mission spacecraft, and ACRIM II, which is still flying on the Upper Atmosphere Research Satellite (UARS) launched in 1991. The ACRIM III design is based on the same sensor technology design used in ACRIM I and ACRIM II.

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  left: The ACRIM satellite will monitor the total energy emitted by the sun for five years, beginning in December, 1999. An animation (1.5 MB Quicktime) shows the satellite deploying its solar panels. (Image courtesy ACRIM Project, JPL)
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