Earth’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.
 

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

Related Data Sets
Outgoing Longwave Radiation

Related Sites
ACRIM Instruments
ACRIMSAT

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.

next: Variations in Total Solar Irradiance

Variations in Total Solar Irradiance
The ACRIM I instrument was the first to clearly demonstrate that the total radiant energy emanating from the sun was not a constant, and varied in proportion to solar magnetic activity. However, the sun’s output changes so slowly and solar variability is so slight (less than 0.00425% of the total energy per year on time scales of days), that continuous monitoring by state-of-the-art instrumentation is necessary to detect changes with climate significance. Scientists theorize that as much as 25% of the 20th century anticipated global warming of the Earth may be due to changes in the sun’s energy output. Systematic changes in irradiance as little as 0.25% per century can cause the complete range of climate variations that have occurred in the past, ranging from ice ages to global tropical conditions. For example, scientists believe the "Little Ice Age" that occured in Europe in the late 17th century could have been related to the minimum in sunspot activity (and a correlated minimum in total solar irradiance) that occured during the same period.
 

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

Related Data Sets
Outgoing Longwave Radiation

Related Sites
ACRIM Instruments
ACRIMSAT

left: Total solar irradiance measured by ACRIM I and ACRIM II. The overall trend of decreasing then increasing energy is caused by the solar sunspot cycle. (Graph courtesy of Dr. Richard C. Willson, ACRIM III Principal Investigator, Center for Climate Research, Columbia University)

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Total Solar Irradiance Data in Climate Studies
Consistent measurements of total solar irradiance provide a long- term data set for climate modelers to use in creating global climate models.

Most of the energy emanating from the sun to the Earth is primarily transmitted in the optical wavelengths of visible (~85%), ultraviolet (~10%), and infrared (~5%) energy. Climate modelers want to know how much of this energy reaches the Earth, so it is necessary to make two primary measurements. The first measurement is the total irradiance delivered by the sun to the Earth. This is the measurement ACRIM makes that include the ultraviolet, visible, and near-infrared wavelengths. The second measurement is to determine how much of the sunlight is reflected back into space so that it does not become part of the Earth’s energy system. These observations are made by another series of NASA satellites as part of the Earth Radiation Budget Experiments. Subtracting the amount of reflected light from the total delivered results in the amount of energy that becomes part of the Earth system. It is this energy that creates the winds, heats the land, and helps to drive the ocean currents. Climate scientists take the total input energy along with complementary measurements of ocean currents, winds, and surface temperatures and make models that they hope will predict the climate behavior of the Earth in the future.

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

Related Data Sets
Outgoing Longwave Radiation

Related Sites
ACRIM Instruments
ACRIMSAT

left: Net radiation absorbed or emitted by the Earth, measured by the Earth Radiation Budget Experiment (ERBE.) ERBE’s descendent—Clouds and the Earth’s Radiant Energy System (CERES) measures the energy captured by the Earth, while ACRIM measures the total energy emitted by the sun. (Image courtesy Dennis Hartmann, University of Washington)

Total Solar Irradiance Data in Solar Studies
Total solar irradiance measurements made by ACRIM will also provide solar physicists with information about the interior of the sun. Imagine that the sun is a ball of fluid (like JELLO) hanging in space. Now imagine the ball of JELLO wiggling, or oscillating. Physicists describe these oscillations as two different types - pressure mode (caused by pressure forces in the sun’s outer atmosphere) and gravity mode (caused by gravitational forces outside of the sun). These oscillations, or waves, travel through the sun and can provide information about its interior. Using the periods (repetitive cycles) of these oscillations, solar physicists hope to gain information about the underlying layers of the sun that cannot be detected by other methods.

The ACRIMSAT mission is funded by NASA through the Earth Science Programs Office at Goddard Space Flight Center. The ACRIM III experiment is under the direction of Dr. Richard Willson and his science team at Columbia University. The ACRIMSAT Project Office at the Jet Propulsion Laboratory (Pasadena, CA) manages the design, fabrication, and test of the ACRIM III instrument and manages the subcontract for the ACRIMSAT spacecraft being built by Orbital Sciences Corporation. The ACRIM III data products are available through the Langley Distributed Active Archive Center and the ACRIM III Science Team website http://acrim.com.

Additional information on the ACRIMSAT mission can be found at http://acrim.jpl.nasa.gov/ and http://www.acrim.com/

back: Total Solar Irradiance Data in Climate Studies
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ACRIMSAT
Introduction
Variations in Total Solar Irradiance
Total Solar Irradiance Data in Climate Studies
Total Solar Irradiance Data in Solar Studies

Related Data Sets
Outgoing Longwave Radiation

Related Sites
ACRIM Instruments
ACRIMSAT

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

Related Data Sets
Outgoing Longwave Radiation

left: ACRIMSAT deploying its solar panels (Animation courtesy ACRIM Project, JPL)

return to: Introduction