Fossil fuel combustion and other human activities are currently emitting about 30 billion tons or 30 Gigatons of CO₂ into the atmosphere each year. So for each ton of carbon that is put in, we get 3.7 tons of CO₂. This is only a small fraction (4%) of the 770 Gigatons of CO₂ emitted into the atmosphere each year by natural processes in the ocean and on land. However, unlike the human activities, which only emit CO₂ into the atmosphere, these natural processes absorb as well as emit CO₂. In fact, atmospheric CO₂ measurements collected by a global network of surface stations indicate that these natural CO₂ “sinks” not only absorb almost all of the CO₂ emitted by natural processes, they also absorb almost half of the CO₂ that is being emitted by human activities as well.

The concentration of CO₂ in the atmosphere is measured in “parts per million by volume” (ppmv). For example, in 1970, atmospheric measurements indicated that about 330 out of every million air molecules was a CO₂ molecule, yielding a concentration of 330 ppmv. Over the past 40 years, human emissions have increased the CO₂ concentration by 1 to 2 ppmv per year, such that by 2011, the atmospheric CO₂ concentration had increased by over 18% to values over 390 ppmv. If these CO₂ increases continue over a sufficient period of time, this efficient greenhouse gas could reduce the Earth’s ability to thermal radiation to space, altering its climate.

Larger View of Carbon Cycle (JPG, 175 KB)

The nature and the geographic distribution of the sinks that absorb about half of the human generated CO₂ are currently not well known and present important, yet unanswered questions. For example, if the efficiency of these sinks decreases in the future, will the rate of buildup of atmospheric CO₂ increase? If so, how much? Can some of these natural sinks be exploited to further reduce the rate of CO₂ buildup? An improved understanding of nature, location, and processes governing the efficiency of these natural sinks is therefore essential to predict the rate of buildup of CO₂ in the atmosphere, and its impact on our climate.

The global coverage, spatial resolution, and accuracy of OCO-2 measurements will provide a basis to characterize and monitor the geographic distribution of CO₂ sources and sinks and quantify their variability. Based on these measurements, scientists will map the natural and man-made processes that regulate the exchange of CO₂ between the Earth's surface and the atmosphere on both regional to continental scales. These measurements will enable more reliable forecasts of the atmospheric CO₂ abundance and its impact on the Earth's climate.

The OCO-2 mission will contribute to a large number of additional scientific investigations that are related to the global carbon cycle. Among these studies are:

• the dynamics of ocean carbon exchange
• the seasonal dynamics of northern hemisphere terrestrial ecosystems in Eurasia and North America
• the exchange of carbon between the atmosphere and tropical ecosystems due to plant growth, respiration, and fires
• the movement of fossil fuel plumes across North America, Europe, and Asia
• the effect of weather fronts, storms, and hurricanes on the exchange of CO₂ between different geographic and ecological regions
• the mixing of atmospheric gases across hemispheres