The Earth’s climate is an incredibly complicated system impacted by a large number of factors. Likewise, the amount of radiation that hits the Earth from the Sun can be impacted by a number of things. The weather on the Sun, including solar flares and other activity and a number of factors on Earth such as cloud cover, ozone and apparently climate.

For an article in the journal Geology, researchers from Denmark studied marine algae found in sediments in the North Atlantic sea bed. They found that solar radiation appears to have less of an impact when the Earth is cooler.

While there is no doubt that current climate change is predominantly due to human activity, there has been little discussion about the role of solar activity during past periods of climate change.

The researchers behind this most recent study found that, during the last 4,000 years, there appears to have been a strong correlation between solar activity and sea surface temperatures which is not seen during the preceding period at least in the North Atlantic.

Since the last ice age ended 12,000 years ago the Earth’s climate has generally been warm. However, there have been protracted temperature variations during that time. For the last 4,000 years the climate has generally been in a cool phase and North Atlantic ocean currents have been weaker than in the past.

“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. 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,” says Marit-Solveig Seidenkrantz in a statement.

Seidenkrantz is a professor at Aarhus University, and one of the researchers behind the study.

Accurate sea surface temperatures, taken by ships, are only available for the last 140 years. However diatoms, a form of marine algae, are strongly influenced by changes in sea temperatures. By taking sediments from the floor of the North Atlantic researchers were able to use distributions of the diatoms to reconstruct fluctuations in surface temperatures for the last 9,300 years.

The researchers were able to use this information to draw comparisons with fluctuations in solar energy during the same period. The results, they say, show a strong correlation between climate change in the North Atlantic and solar activity during the last 4,000 years. The results show correlations down to time periods as short as 10-20 years.

Professor Seidenkrantz and her team believe that the information is a small but important piece of the overall climate system.

“Our climate is enormously complex. By gathering knowledge piece by piece about the way the individual elements work together and influence each other to either strengthen an effect or mitigate or compensate for an impact, we can gradually get an overall picture of the mechanisms. This is also important for understanding how human-induced climate change can affect and be affected in this interaction,” she says.