'Sun has maximum impact when the Earth is cooler'

Giving fresh insight into the role of the Sun in climate change, a new study shows its impact is not constant over time but has greater significance when the Earth is cooler.

During the last 4,000 years, there appears to have been a close correlation between solar activity and the sea surface temperature in summer in the North Atlantic region.

This correlation is not seen in the preceding period.

"We know that the Sun is very important for our climate but the impact is not clear. Climate change appears to be either strengthened or weakened by solar activity," said Marit-Solveig Seidenkrantz, professor at the Aarhus University in Denmark.

"The extent of the Sun's influence over time is thus not constant, but we can now conclude that the climate system is more receptive to the impact of the Sun during cold periods - at least in the North Atlantic region," Seidenkrantz added.

Researchers looked at the sea surface temperatures in summer in the northern part of the North Atlantic during the last 9,300 years.

Direct measurements of the temperature are only found for the last 140 years, when they were recorded by ships.

By examining studies of marine algae found in sediments deposited on the North Atlantic sea bed, it is possible to use the species distribution of these organisms to reconstruct fluctuations in sea surface temperatures much further back in time.

The detailed study makes it possible to draw comparisons with records of fluctuations of solar energy bursts in the same period.

"The new knowledge is a small but important piece of the overall picture as regards our understanding of how the entire climate system works," said Seidenkrantz.

The study appeared in the journal Geology.