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You can tell age of a tree by counting its rings. But these records of a tree’s life really say a lot more. Scientists are using tree rings to learn what’s been happening on the sun’s surface for the last ten thousand years. Each ring represents a year of growth. As the tree grows, it adds a layer to its trunk, taking up chemical elements from the air. By looking at elements in the rings from a certain year, scientists can what elements were in the air that year. Dr. Stevenson is analyzing one element, carbon-14, in rings from both living and dead trees. Some rings go back almost ten thousand years to the end of the Ice Age. When Stevenson followed the carbon-14 track back in time, he found carbon-14 levels changed with the intensity of solar buring. You see, the sun has cycles. Sometimes it burns fiercely, and at other times it’s relatively calm. During the sun’s violent periods, it throws off charged formation of carbon-14 on earth. When there is more solar wind activity, less carbon-14 is produced. Ten thousand years of tree rings show that the carbon-14 level rises and falls about every 420 years. The scientists concluded that solar wind activity must follow the same cycle.

What effects the amount of carbon-14 on earth（）

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