Levels of carbon dioxide in the atmosphere rise and fall each year as plants, through photosynthesis and respiration, take up the gas in spring and summer, and release it in fall and winter. Now the range of that cycle is expanding as more carbon dioxide is emitted from burning fossil fuels and other human activities.<img src="http://feeds.feedburner.com/~r/sciencedaily/~4/2xkunTqXi20" height="1" width="1"/>
