In addition to the natural cycling of carbon through the living and nonliving worlds, human activities, in particular deforestation and the burning of fossil fuels, are releasing increasing amounts of carbon dioxide into the atmosphere and undermining the natural processes for removing it. When we extract coal and oil from Earth's crust and then burn these fossil fuels to provide energy for transportation, heating, cooking, electricity, and manufacturing, we are adding carbon to the atmosphere more rapidly than it is naturally removed through sedimentation and photosynthesis. Because of this, atmospheric carbon dioxide concentrations are higher today than they have ever been over the last half-million years or more.

The carbon dioxide emitted by human activities does not all remain in the atmosphere. The oceans absorb some of it because, as the amount of carbon dioxide in the atmosphere increases, the rate at which carbon dioxide diffuses into the oceans also increases. However, when scientists have tried to account for all the sources of carbon dioxide and all the places where it is absorbed, they've discovered some mysteries. Burning fossil fuels releases roughly 5.5 billion metric tons of carbon (GtC [G = giga = billion]) per year into the atmosphere, and land-use changes such as deforestation contribute roughly 1.6 GtC per year, for a total of 7.1 GtC released per year by human activities. Measurements of atmospheric carbon dioxide levels suggest that approximately 3.2 GtC of this 7.1 GtC remains in the atmosphere and another 2 GtC diffuses into the oceans. This leaves 1.9 GtC unaccounted for.

What's happening to the extra 1.9 GtC? Scientists do not know for sure, but evidence suggests that it is being absorbed by the land surface. In addition, scientists do not agree on which environmental processes are responsible for the missing carbon or where the carbon is going. A variety of different scenarios could cause the land to take up more carbon dioxide each year than it releases. 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, and the changing climate could also contribute to greater uptake than release. Regardless, if we can better understand the many processes that control carbon dioxide emission and uptake, we will also be better able to predict their behavior in the future.