Web Extra  Friday, June 21

Solar links to climate change

If the ultimate goal in climate studies is to unravel the myriad factors that affect Earth's climate so we can predict and prepare for future climatic trends, Dartmouth College Professor Mukul Sharma may be one step closer.

If substantiated, Sharma's new hypothesis will prove that solar activity cycles have to be considered as a powerful parameter affecting Earth's climate.

Earth's climate is affected by its orbit, ocean and atmospheric circulations, volcanic and orogenic activity, solar radiation, galactic dust, global warming, and a slew of other factors. Researchers have long hypothesized that Earth's climate is affected also by solar cycles: sunspot cycles, magnetic cycles, polarity cycles, orbital cycles and others. Recent research such as Sharma's bolsters such theories but also begins to illuminate just how dominant those cycles are. His research illustrates that the longest cycles of solar magnetic activity yet discovered have been the most dominant affecter of Earth's climate over the past one million years.

Over the last one million years, Earth's climate record has revealed a 100,000-year cycle oscillating between cooler and warmer conditions. Earth and the Sun also operate in shorter cycles, including 41,000-year cycles and 20,000-year glacial cycles (Milankovitch Theory), 1,500-year cycles (Bond Cycle), 90-year sunspot cycles (Gleissberg Cycle), 22-year solar magnetic polarity cycles (Hale Cycle) and 11-year sunspot cycles (Schwabe Cycle).

"These cycles run together to simultaneously affect Earth's average surface temperature," Sharma says. He examined existing sets of geophysical data and noticed that the Sun's magnetic activity has been varying in 100,000-year cycles, and that this activity appears to cause the 100,000-year climate cycles on Earth.

Published in the June 10 issue of Earth and Planetary Science Letters, Sharma's study used data on the varying production rates of beryllium -10, an isotope found on Earth that is produced when high-energy galactic cosmic rays bombard Earth's atmosphere. He combined the beryllium-10 rates with data on past variations in Earth's magnetic field intensity. With this information, Sharma calculated variations in solar magnetic activity going back 200,000 years, and he noticed the pattern.

His calculations suggest that when the Sun is magnetically more active, Earth experiences a warmer climate, and vice versa -- when the Sun is magnetically less active, Earth experiences a glacial period.

Sharma went one step further to make a connection to Earth's history of ice ages.

He explains, "I compared the estimated past variations in the solar activity with those of the oxygen isotopes in the ocean. There is a strong relationship between solar activity and oxygen isotopic variations," which are a measure of historic global surface temperature.

Right now, Earth is in an interglacial period, meaning global temperatures have been increasing since the last glacial maximum 11,000 years ago. Not coincidentally, the Sun's magnetic activity has increased over this period. If Sharma's hypothesis is correct, the Sun will remain magnetically active for the next several thousand years as well, which translates into higher temperatures on Earth.

This observation suggests that solar activity is variable over several tens of thousands of years and may be exerting a significant control on Earth's climate, Sharma says.

Sharma points out that his findings are preliminary. "I've only looked at 200,000 years," he notes. "My calculations need to be verified for a million years, two million years. The hypothesis in my paper needs to be substantiated using another independent set of data."

And that's precisely what he is working on now - using deep-marine, iron-manganese crusts to estimate solar activity and unearth one more clue about the past and future climate of Earth.

Meg Rudolph
Geotimes contributing writer

Dartmouth College
Earth and Planetary Science Letters

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