Long-term growth of North America's vegetation soaks up millions of tons of carbon dioxide from the atmosphere each year. Though impressive, that rate doesn't keep pace with the prodigious emissions of the planet-warming gas due to human activity.
Scientists have several ways of monitoring the movement of carbon through the world's ecosystems. So-called bottom-up approaches entail comprehensive and repeated inventories of the amount of carbon in trees, soil, water, minerals, and other natural reservoirs. Such analyses hint that about half of human-generated emissions of carbon dioxide are sequestered in vegetation or soaked up by the ocean, says Andrew R. Ja