In addition to the natural fluxes of carbon through the Earth system, anthropogenic (human) activities, particularly fossil fuel burning and deforestation, are also releasing carbon dioxide into the atmosphere. When we mine coal and extract oil from the Earth's crust, and then burn these fossil fuels for transportation, heating, cooking, electricity, and manufacturing, we are effectively moving carbon more rapidly into the atmosphere than is being removed naturally through the sedimentation of carbon, ultimately causing atmospheric carbon dioxide concentrations to increase. Also, by clearing forests to support agriculture, we are transferring carbon from living biomass into the atmosphere (dry wood is about 50 percent carbon). The result is that humans are adding ever-increasing amounts of extra carbon dioxide into the atmosphere. Because of this, atmospheric carbon dioxide concentrations are higher today than they have been over the last half-million years or longer.

Not all of the carbon dioxide that has been emitted by human activities remains in the atmosphere. The oceans have absorbed some of it because as the carbon dioxide in the atmosphere increases it drives diffusion of carbon dioxide into the oceans. However, when we try to account for sources and sinks for carbon dioxide in the atmosphere we uncover some mysteries. For example, fossil fuel burning releases roughly 5.5 gigatons of carbon (GtC [giga=1 billion]) per year into the atmosphere and land-use changes such as deforestation contribute roughly 1.6 GtC per year. Measurements of atmospheric carbon dioxide levels (going on since 1957) suggest that of the approximate total amount of 7.1 GtC released per year by human activities, approximately 3.2 GtC remain in the atmosphere, resulting in an increase in atmospheric carbon dioxide. In addition, approximately 2 GtC diffuses into the world's oceans, thus leaving 1.9 GtC unaccounted for.

What happens to the leftover 1.9 GtC? Scientists don't know for sure, but evidence points to the land surface. However, at this time, scientists do not agree on which processes dominate, or in what regions of the Earth this missing carbon flux occurs. Several scenarios could cause the land to take up more carbon dioxide than is released each year. For example, re-growth of forests since the massive deforestation in the Northern Hemisphere over the last century could account for some of the missing carbon while changing climate could also contribute to greater uptake than release. The missing carbon problem illustrates the complexity of biogeochemical cycles, especially those in which living organisms play an important role. It is critically important that we understand the processes that control these sources and sinks so that we can predict their behavior in the future. Will these sinks continue to help soak up the carbon dioxide that we are producing? Or will they stop or even reverse and aggravate the atmospheric increases? With the use of satellites and field studies, NASA scientists will help to obtain crucial information on the carbon cycle.