News Archive

posted 03/20/03

Researcher Finds Solar Trend That Can Warm Climate
Ends debate over whether sun can play a role in climate change

The sun on 6/28/2000 during the current solar cycle 23 activity maximum. This photo was taken in a far ultraviolet wavelength (the He II line at 30.4 nanometers) and emphasizes the enhanced radiative activity of magnetically active regions. Image from the MDI experiments archive.

New data indicate that the sun may contribute to global climate change, according to a new study by Richard Willson, a Columbia-affiliated researcher.

Since the late 1970s, the amount of solar radiation the sun emits during times of quiet sunspot activity has increased by nearly .05 percent per decade, according to the study. “This trend is important because, if sustained over many decades, it could cause significant climate change,” said Willson, a researcher affiliated with NASA Goddard Institute for Space Studies and the Earth Institute at Columbia University, and lead author of the study recently published in Geophysical Research Letters.

“Historical records of solar activity indicate that solar radiation has been increasing since the late 19th century,” says Willson. “If a trend comparable the one found in this study persisted during the 20th century it would have provided a significant component of the global warming that the Intergovernmental Panel on Climate Change report claims to have occurred over the last 100 years.”

Willson found errors in previous satellite data that had obscured the trend. The new analysis, Willson says, should put an end to a debate in the field over whether solar irradiance variability can play a significant role in climate change.

The solar cycle occurs approximately every 11 years when the sun undergoes a period of increased magnetic and sunspot activity called the "solar maximum," followed by a quiet period called the "solar minimum." A trend in the average solar radiation level over many solar magnetic cycles would contribute to climate change in a major way. Satellite observations of total solar irradiance have now obtained a long enough record (over 24 years) to begin looking for this effect.

The computer graphic animation shows the ACRIMSAT/ACRIM3 satellite/experiment deploying its solar panels and spinning-up following launch. The solar panels provide power to operate the dedicated ACRIMSAT satellite and its ACRIM3 experiment. The cylindrical ACRIM3 instrument is seen, with its three sensor apertures, in the center of the satellite. The satellite is precisely pointed at the sun using the spin stabilization shown in the animation. The ACRIM3 experiment began monitoring the total solar irradiance in April 2000 and is designed for a minimum 5 year mission. Its 700 km Earth orbit will provide measurement opportunities lasting much longer. The actual lifetime of the experiment will depend on the reliability of its electronic systems.

Total Solar Irradiance (TSI) is the radiant energy received by the Earth from the sun over all wavelengths outside the Earth's atmosphere. Its interaction with the Earth’s atmosphere, oceans and land masses is the biggest factor determining the Earth’s climate. To put it into perspective, decreases in TSI of 0.2 percent occur during the week-long passage of large sunspot groups across our side of the sun. These changes are relatively insignificant compared to the sun’s total output of energy, but are equivalent to all the energy that mankind uses in a year. According to Willson, small variations like the one found in this study, if sustained over many decades, could have significant climate effects.

In order to investigate the possibility of a solar trend, Willson needed to put together a long-term dataset of the Sun’s total output. Six overlapping satellite experiments have monitored TSI since late 1978.The first record came from the National Oceanic and Atmospheric Administration’s (NOAA) Nimbus7 Earth Radiation Budget (ERB) experiment (1978-1993). Other records came from NASA’s Active Cavity Radiometer Irradiance Monitors: ACRIM1 on the Solar Maximum Mission (1980-1989), ACRIM2 on the Upper Atmosphere Research Satellite (1991-2001) and ACRIM3 on the ACRIMSAT satellite (2000 to present). Also, NASA launched its own Earth Radiation Budget Experiment on its Earth Radiation Budget Satellite (ERBS) in 1984. And, the European Space Agency’s (ESA) SOHO/VIRGO experiment also provided an independent data set during 1996-1998.

In this study, Willson, who is also Principal Investigator of the ACRIM experiments, compiled a TSI record of over 24 years by carefully piecing together the overlapping records. In order to construct a long-term dataset, Willson needed to bridge a two-year gap (1989-1991) between ACRIM1 and ACRIM2. Both the Nimbus7/ERB and ERBS measurements overlapped the ACRIM ‘gap.’ Using Nimbus7/ERB results produced a 0.05 percent per decade upward trend between solar minima, while ERBS results produced no trend. Until this study, the cause of this difference, and hence the validity of the TSI trend, was uncertain. Now, Willson has identified specific errors in the ERBS data responsible for the difference. The accurate long-term dataset therefore shows a significant positive trend (.05 percent per decade) in TSI between the solar minima of solar cycles 21 to 23 (1978 to present).

The ACRIMSAT/ACRIM3 experiment began in 2000 and will carry out long-term solar observations for at least five more years. The instrumentation for the ACRIMSAT/ACRIM3 experiment was the latest in a series of ACRIM’s developed for satellite experiments by Willson and the Jet Propulsion Laboratory (JPL) of the California Institute of Technology. JPL operates the ACRIMSAT/ACRIM3 experiment for Willson using their tracking station at the Table Mountain Observatory in California. One of the missions of NASA’s Earth Science Enterprise, which funded this research, is to study the primary causes of climate variability, including trends in solar radiation that may be a factor in global climate change.
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The Earth Institute at Columbia University is among the world’s leading academic centers for the integrated study of Earth, its environment, and society. The Earth Institute builds upon excellence in the core disciplines—earth sciences, biological sciences, engineering sciences, social sciences and health sciences—and stresses cross-disciplinary approaches to complex problems. Through its research, training and global partnerships, it mobilizes science and technology to advance sustainable development, while placing special emphasis on the needs of the world’s poor.