A study of the sun's recent decline in activity suggests that it may have actually caused the Earth to become warmer.

The research, published this week in the journal Nature, says that while the overall amount of radiation from the sun decreased from 2004-07, its energy at visible wavelengths increased.

This increase in energy in visible light caused a warming effect, even during the declining part of the sun's 11-year activity cycle.

"These results are challenging what we thought we knew about the sun's effect on our climate," said Joanna Haigh, lead author of the study, in a statement.

Researchers at Imperial College London and the University of Colorado used the Spectral Irradiance Monitor on NASA's SORCE satellite, launched in 2003, to monitor radiation from the sun from 2004-07.

They found that during the declining phase of the solar cycle, the decline in ultraviolet radiation was four to six times larger than they were expecting.

This was accompanied by an increase in radiation in the visible part of the light spectrum.

The researchers said it is possible that as the sun's activity increases, its warming effect on the Earth's climate actually decreases.

"Our findings raise the possibility that the effects of solar variability on temperature throughout the atmosphere may be contrary to current expectations," the researchers wrote.

Haigh, who is head of the physics department at Imperial College London, cautioned that the results only show a snapshot of the sun's effect on the Earth's climate over one three-year period.

"We cannot jump to any conclusions based on what we have found during this comparatively short period and we need to carry out further studies," she said.

"However, if further studies find the same pattern over a longer period of time, this could suggest that we may have overestimated the sun's role in warming the planet, rather than underestimating it," said Haigh.