"Earth's oceans and land cover are doing us a favor. As people burn fossil fuels and clear forests, only half of the carbon dioxide released stays in the atmosphere, warming and altering Earth's climate. The other half is removed from the air by the planet's vegetation ecosystems and oceans. As carbon dioxide levels in the atmosphere continue their rapid, human-made rise past levels not seen for hundreds of thousands of years, NASA scientists and others are confronted with an important question for the future of our planet: How long can this balancing act continue? And if forests, other vegetation and the ocean cannot continue to absorb as much or more of our carbon emissions, what does that mean for the pace of climate change in the coming century? "Today and for the past 50 to 100 years, the oceans and land biosphere have consistently taken up about half of human emissions," said Dave Schimel of NASA's Jet Propulsion Laboratory, Pasadena, California. "If that were to change, the effect of fossil emissions on climate would also change. We don't understand that number, and we don't know how it will change in the future.""