As the sun reaches this years peak of its 11-year activity cycle, it is emitting less radiation than during the same stage of last two solar maximums, a scientist said.

Before the current sunspot cycle started, the previous two cycles occurred during the periods 1974-1985 and 1985-1996. Those two cycles "turned out to have two of the three highest peaks of activity ever recorded," the IAU said.

"Building up a detailed picture of how the suns output varies over timewill help determine how much (if any) of the global warming presently affectingthe Earth is caused by long-term changes to the sun."


Lean cautioned that sensitive spaceborne instruments called radiometers do not all agree on solar radiation levels, with the Solar and Heliospheric Observer (SOHO) spacecraft measuring increases greater than 0.06 percent so far this cycle.

The reduction in peak solar radiation levels is consistent with a peak in sunspot activity that also is expected to be lower than the last two cycles.

"As far as sunspot numbers, we are expecting a decrease of about 15 percent from the last two cycles," said U.S. Air Force Capt. Chris Smithtro, a forecaster at the governments Space Environment Center in Boulder, Colorado.

Lean said high-precision instruments on various spacecraft monitor the sun for the slightest changes.

"Building up a detailed picture of how the suns output varies over time will help determine how much (if any) of the global warming presently affecting the Earth is caused by long-term changes to the sun," the IAU said in a news release.

During the past few decades, Earths surface temperature has increased a few tenths of a degree while there has been little or no long-term trend in 11-year cycles of solar radiation. That suggests recent climate change is due to factors other than the sun.

Nevertheless, Lean indicated long-term changes in solar radiation likely contributed to global warming early in the 20th century and could contribute in the future.

"To understand what future change might be, we need to continue with high-precision monitoring of the suns radiation," she said.

Smithtro said the current solar cycle now is expected to peak in December.

Meanwhile, two more Cluster spacecraft are scheduled for launch from Baikonour, Kazakhstan, at 7:13 a.m. Eastern Daylight Time (11:13 GMT) Wednesday August 9. They will rendezvous in mid August with two other Cluster satellites, completing a fleet that will study Earths magnetic field and how it is affected by solar activity.

The two new craft, named Rumba and Tango, will fly in formation with Samba and Salsa, which were launched July 16 and reached their parking orbit July 21.