What has happened to atmospheric carbon dioxide levels over the past 800,000 years?
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What must occur in order for carbon dioxide levels to reach 550 parts per million (ppm) by the year 2100?
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Why do carbon dioxide projections fall between 550 and 900 parts per million (ppm) by the year 2100?
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If the atmosphere contains 900 parts per million (ppm) of carbon dioxide by the year 2100, what can we conclude about the twenty-first century?
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