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A big share of the carbon dioxide that pours out of our tailpipes and smokestacks is taken right out of the air by green plants on land. But where exactly it goes from there is a bit of a mystery. And it's an important mystery because that affects how fast carbon dioxide is building up in the atmosphere. Well, a new study offers a major clue by showing how plants inhale and exhale vast quantities of carbon dioxide over the course of the seasons. NPR's Richard Harris explains.
RICHARD HARRIS, BYLINE: If you look at the graph of carbon dioxide in the atmosphere, you can't help notice that it has climbed sharply over the past 50 years. But Heather Graven at the Scripps Institution of Oceanography says the upward trend of that line isn't the whole story.
DR. HEATHER GRAVEN: In addition to this steady increase caused by fossil fuel combustion and other human activities, there's a regular seasonal cycle of CO2 concentration.
HARRIS: The graph actually looks like the edge of a saw. The annual zigzag follows the seasons. There is more carbon dioxide in the winter and a bit less in the summer. That's the collective breathing of all the plants in the Northern Hemisphere.
GRAVEN: Plants are accumulating carbon in the spring and summer when they're active, and they're releasing carbon back to the air in the fall and winter.
HARRIS: And that up-and-down pattern contains important clues about the plants that grab that carbon dioxide and use it to grow. Graven, a postdoctoral researcher, took part in an experiment in which an airplane sampled air over the Pacific Ocean from one pole to the other to look for small differences in carbon dioxide. And as the group reports in Science magazine, the teeth in that sawtooth pattern have grown bigger over the past 50 years.
GRAVEN: So the vegetation is taking deeper breaths, if you will.
HARRIS: That's happening especially in the far northern parts of the planet, the boreal forest and the Arctic tundra. And Graven says you can actually see this effect from space and in aerial photographs.
GRAVEN: The area covered by forest in these northern latitudes has grown over the past few decades, so there's more forest. We've also seen that some species and ecosystems have been migrating pole-ward.
STEVE OBERBAUER: The really big visual difference is the amount of shrubs, which is increasing greatly.
HARRIS: Steve Oberbauer at Florida International University studies plant life in the far north of Alaska, so he's witnessed these big changes firsthand. He says it's not so simple, though, to say that plants are taking up more carbon dioxide. They're also changing the ecosystem up north. For example, shrubs hold onto snow that would otherwise blow away.
OBERBAUER: And then that snow makes an insulation blanket, basically, over the soil, which allows the soil to be warmer in the winter.
HARRIS: And warmer soil is more likely to release carbon dioxide that's stored there. That could be very worrisome.
OBERBAUER: The kicker for the Arctic is there's a huge amount of carbon stored in the peat there, the organic soils. And if that were to be released to the atmosphere, it would increase the atmospheric CO2 concentrations enormously.
HARRIS: So plants are take up carbon dioxide and soil puts it back into the air. And Oberbauer can't say which effect is bigger. But we should be rooting for the plants because this has repercussions far from the Arctic.
OBERBAUER: Oh, it's important because I live in Florida, and the sea level is right there.
HARRIS: Oberbauer was not involved in the study that shows how plant life on our planet has been taking deeper breaths. But he says the study should help the many scientists who've been trying to figure out the fate of all that carbon dioxide. Richard Harris, NPR News.