Watching the Sun


Pop Quiz:

1) What proportion of the girls born today (in the developed world) will live through the entire 21st century?

a) one-quarter
b) one-half
c) three-quarters
d) all

2) In studies of the Bighorn Basin, Wyoming, ecologists are finding that the largest environmental changes may be caused by climate changes spanning how many years?

a) 100
b) 500
c) 5000
d) 10,000
e) all of the above

3) Changes in the total solar irradiance of +/- what percent over a century could explain every type of climate change that the Earth has seen within the last one million years?

a) +/- 50
b) +/- 5.0
c) +/- .50
d) +/- .05

What on Earth, or really what in space, do these apparently unrelated questions have to do with each other?

In the late 1960s and the decade of the 1970s, scientists at the Jet Propulsion Laboratory (JPL), working to perfect an instrument capable of measuring the total energy delivered to Earth, were not worrying about such global issues. Just being able to measure solar variability was a dream in those years—a dream which came to fruition with the flight of the Active Cavity Radiometer Irradiance Monitor I (ACRIM I) instrument on the Solar Maximum Mission spacecraft launched in 1980. Data returned from the ACRIM I instrument demonstrated that the energy output of the sun was variable on the scale of seconds to years. In fact, ACRIM I could measure the sun’s energy in fine enough detail to demonstrate that sun spots cause temporary decreases in the total irradiance output of the sun when sunspots are visible from Earth.


Photograph of the Sun
Virtually all life on Earth depends on the sun for energy. Although it appears constant, the total amount of energy radiated by the sun is constantly changing, on scales of seconds, to years, to centuries. By measuring the variability in solar output, scientists are beginning to decipher the sun’s role in global climate change. (Image copyright Corel)

Graph of Total Solar Irradiance

The measurement of the total solar irradiance is key for climatologists who need to perform the energy “accounting” for the Earth’s climate system. Total solar irradiance is the light energy coming to the Earth, most being in the wavelengths from 200 to 2000 nanometers. To calculate the sun’s heating of Earth’s climate system, the first thing a researcher needs to know is the amount of energy that reaches the Earth from the sun. Separate instruments measure the amount of light that is reflected from the Earth. The difference between the incoming and the reflected total solar irradiance is the amount of energy that is absorbed by the Earth—in the air, in the clouds, in the ocean, and on the land. This absorbed energy determines the Earth’s average temperature. The differences in the absorption at equatorial versus polar latitudes, and at the surface versus in the clouds, are responsible for driving the fluid motions that control weather and climate.

  Continuous measurements of solar irradiance go back just over 20 years. Much longer-term monitoring is necessary to fully understand the effect of solar variability on climate. Several instruments collected the data used in the graph at left—the Hickey-Frieden radiometer aboard Nimbus 7 (HF), ACRIM I, ACRIM II, and Variability of Solar Irradiance and Gravity Oscillations (Virgo). (Graph by the World Radiation Center)

An important consideration in the solar irradiance measurement is that it must be made for a very long time. Climate models show that changes as small as 0.5 percent of full scale over a century could produce equivalents of the most extreme periods of warm or cold climate that the Earth has experienced within the period humans have lived here. In real terms, this means total irradiance data gathered over a few decades is just beginning to scratch the surface of what we can understand about the sun’s behavior and its impact on the Earth’s climate.

The Recent Launch of ACRIM III
The excitement in the near term is that these measurements are ongoing. ACRIM I flew on Solar Max for nine and a half years. Its follow-on instrument, ACRIM II, is presently flying on the Upper Atmosphere Research Satellite (UARS) and is in its eighth year of operation. Twice in 1992 and once in 1994, the Active Cavity Radiometer (ACR) instrument flew on the Space Shuttle as part of the Atmospheric Laboratory for Applications and Science (ATLAS) payload. Recently (December 20, 1999), the ACRIM III instrument launched on an instrument-dedicated spacecraft called ACRIMSAT. Based on the anticipated 5-year lifetime of the ACRIMSAT mission, the Total Solar Irradiance database will be extended to nearly a quarter of a century. Subsequent launches being planned for 2002 and 2005, the "SORCE" missions, will extend the record and also provide the first measurements of visible and near-infrared wavelengths of sunlight that penetrate through the ozone layer to heat the Earth’s surface.

Given that girls born this year stand a good chance of living to the 22nd century, one can only imagine that they, their brothers, and eventually their sons and daughters will be the scientists who will continue the long-term database to help unravel the secrets of the sun’s contribution to the Earth’s climate. And this understanding will provide the basis for predicting the Earth’s climate as part of humankind’s stewardship of the Earth for the decades and centuries to come.

Answers to the questions at the beginning:

“Half of the girls born today [in the developed world] will live through the entire 21st century.”—James Vaupel, executive director of the Max Planck Institute for Demographic Research in Rostock, Germany.

In studies of the Bighorn Basin, Wyoming, ecologists are finding that the largest environmental changes may be caused by climate changes spanning 5000 years (Kloor 2000).

Changes in the total solar irradiance of +/- 0.5 percent over a century could explain every type of climate change that the Earth has seen within the last one million years (R.C. Willson, et al. 1986).


Kloor, Keith, 2000: "Returning America's Forests to Their ‘Natural’ Roots," Science, Volume 287, p. 573-75.

Willson, R.C., H.S. Hudson, C. Frohlick, and R.W. Brusa, 1986: "Long-Term Downward Trend in Total Solar Irradiance," Science, Volume 234, p. 1114.

For more information:

Sunspots & the Solar Max
ACRIM Science Team page (Columbia University)

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

  Juniper encroachment in Wyoming
Climate change is at least partially responsible for the encroachment of Utah junipers into the Bighorn Basin in Wyoming. This pair of photographs were taken from the same spot 76 years apart. Junipers appear in the foreground of the later image, and are packed more densely on the slopes of the mountain in the background. (Photographs courtesy Utah Juniper Project, USGS)