In 1611, Galileo Galilei and other astronomers noticed there were fewer sunspots on the sun, and the coldest part of what?s known as the Little Ice Age began shortly after. Now NASA scientists have learned how that reduced solar activity chilled much of the Earth by changing the atmospheric circulation of the Northern Hemisphere from the 1400s into the early 18th century.

During the Little Ice Age, access to Greenland was cut off by ice, ending the Viking colonization there, and canals in Holland routinely froze solid. Londoners ice-skated on the Thames and glaciers advanced down the Alps.

A team of NASA scientists led by Drew Shindell at the Goddard Institute for Space Studies and the Center for Climate Systems Research at Columbia University in New York used a computer model to figure out that a dimmer sun reduces westerly winds, cooling the continents during winters.

Based on temperature readings taken from tree rings and a limited number of recorded temperature readings, researchers believe that between the mid-1600s and the early 1700s, winter surface temperatures in the Northern Hemisphere were 2 to 3 degrees lower than average. These were the coldest times of the past thousand years.

?This period of low solar activity in the Middle Ages led to atmospheric changes that seem to have brought on the Little Ice Age,? says Shindell. ?However, we need to keep in mind that variations in solar output have had far less impact on Earth?s recent climate than human actions.? Richard Willson, of Columbia University, has the opposite theory and believes that increased brightening of the sun in recent decades has been a major cause of global warming.

Until we traveled into space, the only way scientists could detect solar activity was by counting the number and size of sunspots, which indicate solar flares and a more energetic sun. Galileo made drawings of sunspot activity beginning in 1611, and records from other early astronomers describe an episode called the Maunder Minimum, when there were only about 50 sunspots over 30 years, when typically there would be 40,000 to 50,000 sunspots over three decades.

During periods of lower solar activity, levels of the sun?s ultraviolet radiation decrease, causing ozone to build up in the upper atmosphere and block more of the energy received from the sun, further cooling things down. ?The changes in the upper atmosphere then feed down to the surface climate,? says Shindell. His researchers found that the effects of a dimmer sun are concentrated regionally rather than globally, meaning they affect some parts of the Earth more than others. They particularly seem to affect the seasonal jet-stream patterns that keep the coldest Arctic air out of North America and Europe and moderates the winters there by pumping warmer ocean air over the continents.

Solar flares usually rise and fall over an 11-year cycle, but the sun also has longer patterns of brightening and dimming that can run for hundreds or even thousands of years. British solar researcher Joanna Haigh says, ?Recent evidence from satellite data has shown that the term ?solar constant? is a misnomer.?

To learn more about this, read ?The 23rd Cycle? by Sten Odenwald,click here.

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