The original title to the paper as published in GRLs is: "Estimated
Solar Contribution to the global surface warming using the ACRIM TSI
The following concluding statement is found at the beginning of the
article: "We estimate that the ACRIM upward trend (solar variation)
might have minimally contributed ~10-30% of the global surface
temperature warming over the period 1980-2002."
Note the qualifying language: "might have minimimally contributed".
Sun's Direct Role in Global Warming May Be Underestimated, Duke
Study does not discount the suspected contributions of 'greenhouse
gases' in elevating surface temperatures
Friday, September 30, 2005
Durham, N.C. -- At least 10 to 30 percent of global warming measured
during the past two decades may be due to increased solar output
rather than factors such as increased heat-absorbing carbon dioxide
gas released by various human activities, two Duke University
The physicists said that their findings indicate that climate models
of global warming need to be corrected for the effects of changes in
solar activity. However, they emphasized that their findings do not
argue against the basic theory that significant global warming is
occurring because of carbon dioxide and other "greenhouse" gases.
Nicola Scafetta, an associate research scientistworking at Duke's
physics department, and Bruce West, a Duke adjunct physics professor,
published their findings online Sept. 28, 2005, in the research
journal Geophysical Research Letters.
West is also chief scientist in the mathematical and information
sciences directorate of the Army Research Office in Research Triangle
Scafetta's and West's study follows a Columbia University
researcher's report of previous errors in the interpretation of data
on solar brightnesscollected by sun-observing satellites.
The Duke physicists also introduce new statistical methods that they
assert more accurately describe the atmosphere's delayed response to
solar heating. In addition, these new methods filter out temperature-
changing effects not tied to global warming, they write in their
According to Scafetta, records of sunspot activity suggest that solar
output has been rising slightly for about 100 years. However, only
measurements of what is known as total solar irradiance gathered by
satellites orbiting since 1978 are considered scientifically
reliable, he said.
But observations over those years were flawed by the space shuttle
Challenger disaster, which prevented the launching of a new solar
output detecting satellite called ACRIM 2 to replace a previous one
called ACRIM 1.
That resulted in a two-year data gap that scientists had to rely on
other satellites to try to bridge. "But those data were not as
precise as those from ACRIM 1 and ACRIM 2," Scafetta said in an
Nevertheless, several research groups used the combined satellite
data to conclude that that there was no increased heating from the
Sun to contribute to the global surface warming observed between 1980
and 2002, the authors wrote in their paper.
Lacking a standardized, uninterrupted data stream measuring any
rising solar influence, those groups thus surmised that all global
temperature increases measured during those years had to be caused by
solar heat-trapping "greenhouse" gases such as carbon dioxide,
introduced into Earth's atmosphere by human activities, their paper
But a 2003 study by a group headed by Columbia's Richard Willson,
principal investigator of the ACRIM experiments, challenged the
previous satellite interpretations of solar output. Willson and his
colleagues concluded, rather that their analysis revealed a
significant upward trend in average solar luminosity during the
Using the Columbia findings as the starting point for their study,
Scafetta and West then statistically analyzed how Earth's atmosphere
would respond to slightly stronger solar heating. Importantly, they
used an analytical method that could detect the subtle, complex
relationships between solar output and terrestrial temperature
The Duke analyses examined solar changes over a period twice as long -
- 22 versus 11years -- as was previously covered by another group
employinga different statistical approach.
"The problem is that Earth's atmosphere is not in thermodynamic
equilibrium with the sun," Scafetta said. "The longer the time period
the stronger the effect will be on the atmosphere, because it takes
time to adapt."
Using a longer 22 year interval also allowed the Duke physicists to
filter out shorter range effects that can influence surface
temperatures but are not related to global warming, their paper said.
Examples include volcanic eruptions, which can temporarily cool the
climate, and ocean current changes such as el Nino that affect global
Applying their analytical method to the solar output estimates by the
Columbia group, Scafetta's and West's paper concludes that "the sun
may have minimally contributed about 10 to 30 percent of the 1980-
2002 global surface warming."
This study does not discount that human-linked greenhouse gases
contribute to global warming, they stressed. "Those gases would still
give a contribution, but not so strong as was thought," Scafetta said.
"We don't know what the Sun will do in the future," Scafetta
added. "For now, if our analysis is correct, I think it is important
to correct the climate models so that they include reliable
sensitivity to solar activity.
"Once that is done, then it will be possible to better understand
what has happened during the past hundred years."
Title in GRL, Vol. 32, L18713, doi:10.1029/2005GL023849,2005: