In the January 2009 issue of Physics Today, Philip Duffy, Benjamin Santer, and Tom Wigley attempted (page 48) to rebut our argument that there is significant climate response to solar variability (Physics Today, March 2008, page 50). We find their arguments unconvincing.
The composite curve in their figure 1 is the PMOD composite of satellite data for total solar irradiance (TSI), which has no upward trend for the period 1980–2000. However, the second well-known composite, ACRIM, does show a significant upward trend during that period. 1 We find it curious that Duffy and coauthors cite the PMOD composite as the only one of consequence.
For the period before 1995, any TSI composite is constructed with data from ACRIM1, NIMBUS7, and ACRIM2 satellite experiments. The ACRIM composite uses these data as they are published by the experimental teams, while the PMOD composite is constructed by altering the published data on the basis of a TSI proxy model and the low-quality ERBS (Earth Radiation Budget Satellite) record. The ACRIM and NIMBUS7 experimental teams have rejected the PMOD alterations as arbitrary. 2 , 3
Recent work 3 that uses measurements of solar magnetic fluxes at Earth’s surface establishes that a significant degradation of the TSI record from ERBS occurred during the gap in the ACRIM records (1989–92), as the ACRIM team has always claimed. That degradation invalidates the trust placed in the PMOD composite and its downward alterations of the NIMBUS7 record. Thus one is forced to select the ACRIM composite, which shows a TSI increase between 1980 and 2002, as we discussed in our Opinion piece.
Duffy and coauthors’ choice of preferring an arbitrary TSI composite that shows no upward trend from 1980 to 2000 clearly undercuts their first major claim, that the Sun could not contribute to the warming observed since 1980, and consequently everything they deduced from it.
The second claim by Duffy and coauthors is that climate sensitivity to solar variability is low. To support that conclusion, they cite a 2004 study 4 by Gerald North and coworkers that summarizes findings obtained from simple energy-balance models. However, Duffy and coauthors omitted that study’s major finding: that the empirical solar signature exceeds the energy-balance model predictions by a factor of two on average, implying that the climate is much more sensitive to solar changes than what climate models predict. Also, they do not realize that using a 10-year running average in their figure 2 suppresses the solar cycle’s 11-year signature on climate.
The authors also ignore three other important points. First, our findings are consistent with secular paleoclimate temperature reconstructions that were recently made and confirmed. 5 Second, the glacial epochs were induced by small changes in the redistribution of sunlight due to the Milankovitch astronomical cycles—variations in the eccentricity, obliquity, and precession of Earth’s orbit; that fact suggests significant climate sensitivity to changes in TSI inputs. And third, the oscillations of greenhouse gases observed between the glacial epochs were not induced by human activity but were a complex climate-dynamics response to the small redistribution of sunlight produced by Milankovitch cycles; that fact contradicts the assumption implicit in all climate models adopted in the Intergovernmental Panel on Climate Change 2007 report, that only humans can modify greenhouse gas concentrations.
Finally, the assumption underlying the piece by Duffy and coworkers is that the anthropogenic global warming theory is settled, those who claim otherwise are in error, and their studies should be dismissed. Yet an international team of scientists has published a comprehensive research review 6 disproving that claim by summarizing and organizing the findings of thousands of scientific papers; their review contradicts several conclusions of the IPCC 2007 report, which ignored many of the papers reviewed in Climate Change Reconsidered. 6 The review also lists more than 30,000 US scientists who have signed a petition stating that there is no convincing evidence to support the anthropogenic global warming theory. We remind readers about the dangers of dogma replacing science.
