Earth's land ecosystems absorb a large portion of all the carbon dioxide emissions produced by human activities, helping to slow global warming. On average for a given year, plants and soil take up, or fix, about 30 percent of human emissions. But from one year to the next, that number can be as high as 40 percent or as low as 20 percent. Climate scientists aim to pin down exactly what produces this variability so they can account for it and create the most accurate models for predicting future climate.