Activists concerned about climate change have planned more than 4,000 events around the world this weekend to draw attention to global warming. The group's organizers argue that we need rapid and radical changes in the world's energy sources and forestry practices. So why 350? According to the group's website, scientists say that 350 is the highest amount of carbon dioxide (measured in parts per million) that can safely exist in our atmosphere. Their goal is to reduce the atmosphere's carbon dioxide back to that level.
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