Twenty years of field studies reveal that as the Earth has gotten warmer, plants and microbes in the soil have given off more carbon dioxide. So-called soil respiration has increased about one-tenth of 1 percent per year since 1989, according to an analysis of past studies in today's issue of Nature.
The scientists also calculated the total amount of carbon dioxide flowing from soils, which is about 10-15 percent higher than previous measurements. That number -- about 98 petagrams of carbon a year (or 98 billion metric tons) -- will help scientists build a better overall model of how carbon in its many forms cycles throughout the Earth. Understanding soil respiration is central to understanding how the global carbon cycle affects climate.
"There's a big pulse of carbon dioxide coming off of the surface of the soil everywhere in the world," said ecologist Ben Bond-Lamberty of the Department of Energy's Pacific Northwest National Laboratory. "We weren't sure if we'd be able to measure it going into this analysis, but we did find a response to temperature."
The increase in carbon dioxide given off by soils -- about 0.1 petagram (100 million metric tons) per year since 1989 -- won't contribute to the greenhouse effect unless it comes from carbon that had been locked away out of the system for a long time, such as in Arctic tundra. This analysis could not distinguish whether the carbon was coming from old stores or from vegetation growing faster due to a warmer climate. But other lines of evidence suggest warming is unlocking old carbon, said Bond-Lamberty, so it will be important to determine the sources of extra carbon.
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