Can Digging into the Details Settle this Debate?


Each scientist points out the problems associated with the other’s approach. “Adjusting other people’s published results is a questionable practice,” says Willson. “You can’t make any modifications to the data sets without a thorough understanding of the instrument and its calibration history. The best time-proven approach for getting the most out of a set of data is the peer-reviewed publication process. The original science teams for the experiments have more information about the experiments than any other individual or group can ever acquire.”

He also draws attention to the crucial issue that between mid-1989 and late 1991 (the ‘ACRIM gap’), ERBS recorded generally decreasing values of total solar irradiance while the Nimbus7/ERB results were increasing. “One of the most significant and universally accepted findings from satellite observations to date is that TSI is directly proportional to solar activity levels, ”says Willson. “The ‘ACRIM gap’ was a period of rapidly increasing solar activity levels, and TSI should have been rapidly increasing as well. The Nimbus7/ERB results are consistent with this finding whereas the ERBS results are not.” Using ERBE data to confirm that the correction made to the NIMBUS 7 data is accurate only makes sense if you are sure the ERBE data themselves are accurate—and Willson says we can’t be sure they are.

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Graph Comparing Nimbus 7/ERB and ERBS/ERBE Data from 1989 to 2001


ERBE mission scientist Robert B. Lee, III dismisses that claim. “You have to look at the data set as a whole, not just bits and pieces of it. The ERBS/ERBE Solar Monitor is still making precise TSI measurements after more than 19 years of operations. During this period, the ERBS/ERBE solar monitor experienced no instrument malfunctions which affected the resulting TSI measurements,” he says. According to Lee, the high level of variability in the sensor’s measurements between 1989-1992 reflected real variability in solar irradiance during that time, not instrument problems. Lee offers what seems to be a reasonable approach. “The only way this will be resolved is for all of us to turn over our data to an independent source like the National Institute of Standards and Technology and let them investigate and make their own conclusion,” concludes Lee.

For her part, Lean says that the original results from NIMBUS 7 couldn’t be corrected because the technology and expertise to do so weren’t available at that time. As evidence that their combined data set is the correct way to assemble the satellite data, Lean compares the results of the data set Fröhlich and she put together to a model of solar activity that uses observable features of the Sun, namely the sunspots and faculae that are the primary cause of total solar irradiance variation, as substitutes (proxies) for total solar irradiance. These proxies are necessary because the atmosphere absorbs so much of the Sun’s total output that we can’t measure it accurately from the ground. These models also help us extrapolate what the total solar irradiance might have been like in the past, since sunspot records go back to the 1700s. Lean points out how well their composite compares to the solar models, which give no suggestion of an increase in total solar irradiance between the solar minima in 1986 and 1996.

  Although the trends measured by different satellite total solar irradiance instruments usually agree, there are exceptions. During the critical period between the death of ACRIM 1 and launch of ACRIM 2, measurements from the Nimbus-7 satellite (green line) showed an increase in solar irradiance, while the Earth Radiation Budget Satellite (blue line) showed a decrease in total solar irradiance from July 1989 through December 1991. Sorting out these differences will likely resolve questions about the sun’s energy output. (Graph by Robert Simmon, based on data from the National Geophysical Data Center)

Graph Comparing Modelled Total Solar Irradiance Data with Willson and Lean's Results


The deviation of his data set from the solar models doesn’t trouble Willson, who says that while the solar proxy models are useful for trying to describe solar activity back in time before scientists had actual observations of total solar irradiance , “the models are not competitive in accuracy or precision with even the worst total solar irradiance satellite observations.”

Joe Gurman, US Project Scientist for SOHO, isn’t so quick to dismiss the models. “These models have done a very good job in simulating the solar activity over recent solar cycles as well as more pronounced, long-term changes in solar output that can be linked to historically documented changes in climate,” he says. “If you accept Willson’s conclusion that total solar irradiance has increased over the last two solar cycles, then not only do you have to explain why the models don’t show it, but you also have to explain why no other single instrument shows a similar increase, and why none of our other solar indicators, like total magnetic flux or ultraviolet light output, has shown a similar increase.” Although there has been a lot of evidence suggesting that magnetic activity is the primary cause of observable solar variability, Gurman says that there is still a lot we don’t know about the Sun. It is always possible there is some physical process that influences solar irradiance that we don’t know about yet.

That possibility doesn’t seem like such a stretch to astrophysicist Willie Soon of the Harvard-Smithsonian Center for Astrophysics. Soon says that, over the years, his study of stars similar to the Sun suggests that there is no physical reason why changes in total solar irradiance couldn’t be real. Other Sun-like stars have been observed to vary over longer cycles. He is excited about the possibility that Willson’s results suggest a longer term solar variation that is yet to be explained. Even this is controversial, however; other physicists think that the Sun is not as much like other stars as we once thought and that there is no evidence to date that anything other than the magnetic activity that we already know about is influencing solar irradiance.

Hugh Hudson, a solar physicist with University of California in Berkeley, agrees the door should be kept open to the possibility of solar variability such as that claimed by Willson, but says he’s not sure that the measurements collected to date are precise enough to support those claims. “Still,” he says, “models and theory will only take you so far, and at some point, you may have to suspend your disbelief if the data require it.” Like several others in the relatively small solar irradiance research community, Hudson seems to think it’s just too soon to tell.


Computer models based on sunspots and faculae on the sun’s surface (black line) suggest that there was no increase in solar irradiance from 1986 to 1996. These models are useful in the study of historical solar energy output, but they may not be as reliable as more direct satellite measurements. (Graph Courtesy Judith Lean, Naval Research Laboratory)

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