The increase would only be significant to Earth's climate if it has been going on for a century or more, said study leader Richard Willson, a Columbia University researcher also affiliated with NASA Goddard Institute for Space Studies.
The Sun's increasing output has only been monitored with precision since satellite technology allowed necessary observations. Willson is not sure if the trend extends further back in time, but other studies suggest it does.
"This trend is important because, if sustained over many decades, it could cause significant climate change," Willson said.
In a NASA-funded study recently published in Geophysical Research Letters, Willson and his colleagues speculate on the possible history of the trend based on data collected in the pre-satellite era.
"Solar activity has apparently been going upward for a century or more," Willson told SPACE.com today.
Further satellite observations may eventually show the trend to be short-term. But if the change has indeed persisted at the present rate through the 20th Century, "it would have provided a significant component of the global warming the Intergovernmental Panel on Climate Change reports to have occurred over the past 100 years," he said.
That does not mean industrial pollution has not been a significant factor, Willson cautioned.
Scientists, industry leaders and environmentalists have argued for years whether humans have contributed to global warming, and to what extent. The average surface temperature around the globe has risen by about 1 degree Fahrenheit since 1880. Some scientists say the increase could be part of natural climate cycles. Others argue that greenhouse gases produced by automobiles and industry are largely to blame.
Willson said the Sun's possible influence has been largely ignored because it is so difficult to quantify over long periods.
Confounding efforts to determine the Sun's role is the fact that its energy output waxes and wanes every 11 years. This solar cycle, as it is called, reached maximum in the middle of 2000 and achieved a second peak in 2002. It is now ramping down toward a solar minimum that will arrive in about three years.
Changes in the solar cycle -- and solar output -- are known to cause short-term climate change on Earth. At solar max, Earth's thin upper atmosphere can see a doubling of temperature. It swells, and denser air can puff up to the region of space where the International Space Station orbits, causing increased drag on the ship and forcing more frequent boosts from space shuttles.
Sun Cams: See the Sun Now
Long-term: A previous study showed that changes in the Sun's output appear to be related to temperatures on Earth, based on studies of tree rings, sunspots and other data. Learn More
Solar max has also been tied to a 2 percent increase in clouds over much of the United States.
It might seem logical to assume tie climate to solar output, but firm connections are few. Other studies looking further back in time have suggested a connection between longer variations in solar activity and temperatures on Earth.
Examinations of ancient tree rings and other data show temperatures declined starting in the 13th Century, bottomed out at 2 degrees below the long-term average during the 17th Century, and did not climb back to previous levels until the late 19th Century. Separate records of sunspots, auroral activity (the Northern Lights) and terrestrial deposits of certain substances generated in atmospheric reactions triggered by solar output, suggest the Sun was persistently active prior to the onset of this Little Ice Age, as scientists call the event.
Solar activity was lowest during the 17th Century, when Earth was most frigid.
Large-scale ocean and climate variations on Earth can also mask long-term trends and can make it difficult to sort out what is normal, what is unusual, and which effects might or might not result from shifts in solar radiation.
To get above all this, scientists rely on measurements of total solar energy, at all wavelengths, outside Earth's atmosphere. The figure they derive is called Total Solar Irradiance (TSI).
The new study shows that the TSI has increased by about 0.1 percent over 24 years. That is not enough to cause notable climate change, Willson and his colleagues say, unless the rate of change were maintained for a century or more.
On time scales as short as several days, the TSI can vary by 0.2 percent due to the number and size of sunspots crossing the face of the Sun. That shift, said to be insignificant to weather, is however equal to the total amount of energy used by humans, globally, for a year, the researchers estimate.
The study analyzed data from six satellites orbiting Earth at different times over the 24 years. Willson ferreted out errors in one of the datasets that had prevented previous studies from discovering the trend.
A separate recent study of Sun-induced magnetic activity near Earth, going back to 1868, provides compelling evidence that the Sun's current increase in output goes back more than a century, Willson said.
He said firm conclusions about whether the present changes involve a long-term trend or a relatively brief aberration should come with continued monitoring into the next solar minimum, expected around 2006.
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