Charles Camp and Ka Kit Tung of the University of Washington's department of applied mathematics said that to accurately assess effects from human sources on the planet's climate, scientists must first be able to quantify the contribution of natural variation in solar irradiance to temperature changes.

Camp and Tung said that while the existence of a long-term trend in solar output is controversial, its periodic change within an 11-year cycle has been measured by satellites.

To assess how that oscillatory forcing affects climate on Earth, Camp and Tung compared the Earth's surface temperature measurements between years of solar maximum and years of solar minimum.

They determined that times of high solar activity are on average 0.2 degrees Celsius warmer than times of low solar activity, and that there is a polar amplification of the warming.

That finding is believed the first to document a statistically significant globally coherent temperature response to the solar cycle, the authors note.

The research is reported in the current issue of the journal Geophysical Research Letters.