London: The impact of the Sun on Earth's climate has greater significance when our planet is cooler, a new study has found.
An international team of researchers shows that, 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.
Since the end of the Last Ice Age about 12,000 years ago, the Earth has generally experienced a warm climate. However, the climate has not been stable during this period, when temperatures have varied for long periods.
The Earth has generally had a slightly cooler climate during the last 4,000 years, and the ocean currents in the North Atlantic have been weaker.
"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 Professor Marit-Solveig Seidenkrantz from 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," said Seidenkrantz.
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 taken from ships.
However, by examining studies of marine algae - diatoms - 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, and the results show a clear correlation between climate change in the North Atlantic and variations in solar activity during the last 4,000 years, both on a large time scale over periods of hundreds of years and right down to fluctuations over periods of 10-20 years.
The study was published in the journal Geology.
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