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.


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

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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