Increased solar brightness and warming of the Earth since 1979
The sun may have increased in brightness over the last couple of decades as summarized in the following press release:
Researcher Finds Solar Trend That Can Warm Climate
Ends debate over whether sun can play a role in climate change
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.
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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For more information about ACRIM, please go to: http://www.acrim.com
The key quote is “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).” Such as an increase is sufficient to explain most of the observed warming.
Even if this result is not correct, there are reasons to believe the sun varies in brightness over decades and centuries. The evidence includes:
1. Variations in sunspot structure and the Earth’s temperature closely followed each other from 1874 to 1976. Sunspot structure provides a measure of the strength of small scale turbulence in the sun and hence indicates long-term changes in solar luminosity.
2. Changes in solar cycle length closely follow changes in sunspot structure and the Earth’s temperature. The changes in cycle length are probably caused by changes in the large scale turbulence of the sun as reflected in meridional flows and hence provide more evidence for solar luminosity changes.
3. Sunspots are particularly long lived during the Maunder Minimum (1645-1715). This indicates reduced turbulence in the sun and reduced luminosity and it is reflected by a cool Earth.
4. Numerous solar and Earth climate proxies are correlated indicating the sun is a major driver of climate change.
References:
Clough, H. W., 1943. The long period variations in the length of the 11-year solar period, and on current variations in terrestrial phenomena. Bull. AMS, 24, 154-163.
Friis-Christensen, E., and K. Lassen, 1991. Length of the solar cycle: An indicator of solar activity closely associated with climate. Science, 254, 698-700.
Hathaway, D. H., D. Nandy, R. M. Wilson, and E. J. Reichmann, 2003. Evidence that a deep meridional flow sets the sunspot cycle period. Ap. J., 589, 665-670.
Hoyt, D. V., 1979. Variations in sunspot structure and climate . Climatic Change, 2, 79-92.
Hoyt, D. V., and K. H. Schatten, 1997. The Role of the Sun in Climate Change , Oxford University Press, 279 pp.
Hoyt, D. V., and K. H. Schatten, 1998. Group sunspot numbers: A new solar activity reconstruction. Part 2. Solar Physics, 181, 491-512.