Temperature proxies link solar activity with local climate

Given that the solar system’s primary energy source is the Sun, our local star is undoubtedly the ultimate driver of the Earth’s climate. But the Earth is a complex, dynamic system, and we have to ask exactly how solar activity exerts its influence on climate, over what timescales it does so, and whether the changes are local or global. So far the evidence points to the Sun having only a minor effect on global climate, but many of the precise details remain uncertain.

Solar variations are likely to have a strong influence on regional climate change. We could cite examples such as the Maunder Minimum/Little Ice Age of the late 17th century, and possibly the Mediaeval Warm period at the turn of the first millennium.

Lund University climatologist Raimund Muscheler has been studying solar activity and regional climate change, and with his colleagues shown how water temperature in the tropical parts of the eastern Pacific varied with the Sun’s output during the early Holocene, some 7,000-11,000 years ago. Using measurements of magnesium and calcium isotopes, the researchers were able to reconstruct prehistoric water temperatures, and they suggest that solar activity has influenced temperature by changing local ocean circulation patterns. This is related to the regional weather phenomena known as El Niño and El Niña…

“We know that El Niño brings a warmer climate, while El Niña brings a cooler climate in the eastern part of the Pacific Ocean,” says Muscheler. “If we presume that this connection existed during the early Holocene, this means that there could be a link between solar activity and El Niño/El Niña on long time scales.”

This is an important study, as it helps to join the dots between detailed processes in physical oceanography and larger-scale reconstructions of the Earth’s climate system in times long past.

The ‘cosmogenic isotopes’ used by Muscheler are formed in the atmosphere as a result of chemical interactions with cosmic radiation from interstellar space, which is modulated by solar activity via electromagnetic forces. For example, when solar activity is high, the flux of galactic cosmic rays impacting on the Earth’s atmosphere is reduced, and this is reflected in the isotope record. Cosmogenic isotopes in the ocean are proxies for temperature, in much the same way as tree-ring growth can be used to reconstruct prehistoric climate on land. As Muscheler says, this is the most reliable method we have to reconstruct solar activity, and thus past climate.

Further reading

Marchitto et al., “Dynamical response of the tropical Pacific Ocean to solar forcing during the early Holocene”, Science 330, 1378 (2010)

M Lockwood, “What do cosmogenic isotopes tell us about past solar forcing of climate?”, Space Sci. Rev. 125, 95 (2006)