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Increasing the brightness of the sun might paradoxically lead to cooler temperatures on Earth, and vice versa, new findings suggest.

The impact of the sun on the Earth's climate has proved controversial. For instance, the so-called Maunder Minimum when extraordinarily few sunspots were seen between 1645 to 1715 partially coincided with Europe and North America plunging into the Little Ice Age, leading to debates over whether the sun was the cause of that past climatic shift, as well as the current one the world is undergoing.

To learn more about what effects changes in solar brightness might have on climate, scientists first analyzed a rod of sediment about 50 feet (15 meters) long from 1,770 feet (530 meters) below the surface of the sea off Baja California Sur, Mexico.

Ancient records

They focused on a specimen of the plankton species Globigerina bulloides. By analyzing magnesium levels in the shells of these organisms, which rise as temperatures go up, the researchers could reconstruct surface temperatures in the tropical Pacific during the Holocene, the epoch from approximately 12,000 years ago up to the present.

To deduce solar radiation levels during that time, the researchers next looked at levels of carbon-14 in tree rings and beryllium-10 in polar ice; cosmic rays from outside the solar system would have generated these isotopes (varieties of an element that have a differing number of neutrons). When solar radiation is high, it strengthens the interplanetary magnetic field that protects Earth from these high-energy rays, so fewer of those isotopes will be present in tree rings and ice that formed at a time when solar radiation was high.

By comparing the solar radiation and temperature records, the researchers found that as solar output rose in the early and middle Holocene, ocean temperatures in the region actually declined in a pattern resembling La Niña events, when the equatorial Pacific experiences cooler-than-normal water temperatures. When solar output decreased, ocean temperatures would rise as occurs during El Niño events, which are marked by warmer water in the Pacific off the coast of the Americas. The cold La Niña-like conditions might have counteracted the tendency for a brighter sun to warm the Earth's surface, while the warmer El Niño-like conditions might have ameliorated cooling from a dimmer sun.

Counterintuitive climate change

The findings may be counterintuitive, but there are climate models that can explain what might have occurred, said researcher Thomas Marchitto, a paleoclimatologist at the University of Colorado at Boulder. Solar radiation is apparently better at warming the atmosphere over the western equatorial Pacific than the eastern there is more convergence of winds over the equator in the west, leading to a greater volume of air to absorb heat from the sun. This warm air enhances trade winds blowing from the east toward the west. These in turn push at surface waters, leading colder waters from the deep ocean to well upward. The resulting cooling of the ocean enhances trade winds even more, compounding the cooling effect.

These new findings might raise questions regarding the causes of current climate change. "Changes in the sun's brightness over the recent decade have been extremely small, a tenth of 1 percent, and that amount of change in radiation pales in comparison to the effect that greenhouse gases have," Marchitto said. "Another question people ask is whether greenhouse gases might lead to a more La Niña-like state just as a brighter sun does. We don't think so, because greenhouse gases affect circulation patterns in quite different ways."

El Niño and La Niña are part of a climate pattern known as the El Niño-Southern Oscillation, or ENSO. "ENSO is immensely important for year-to-year climate variability it's why southern California might see floods and mudslides one year and droughts and wildfires the next," Marchitto told OurAmazingPlanet. "While different computer models generally agree on many aspects of climate change, there's less agreement on the future of ENSO. The hope with our work is that we can learn more about the future by studying the past."

The scientists detailed their findings in the Dec. 3 issue of the journal Science.