Science

The OCO-2 Project science objectives are to collect the space-based measurements needed to quantify variations in the column averaged atmospheric carbon dioxide (CO₂) dry air mole fraction, XCO₂, with the precision, resolution, and coverage needed to improve our understanding of surface CO₂ sources and sinks (fluxes) on regional scales (≥1000km) and the processes controlling their variability over the seasonal cycle. This mission validates a space-based measurement approach and analysis concept that could be used for future systematic CO₂ monitoring missions.

What does this all mean? Carbon Dioxide or CO₂ is one of the primary greenhouse gases on Earth. Greenhouse gases are those which can trap thermal radiation or heat that the Earth would otherwise emit to space.

CO₂ is an important gas for life on the planet and integral to maintaining the protective blanket that is our atmosphere. However, sharp increases or decreases may affect the delicate atmospheric balance, and increases in atmospheric CO₂ concentration may adversely alter the global climate. Although we know and understand the human activity impacts on CO₂ concentrations in the atmosphere, several questions remain unanswered. Our ability to answer those questions will provide a more complete understanding that those impacts have and will have on the global climate.

The Earth system maintains a check and balance on CO₂ through the carbon cycle and what we call sources and sinks. A source is any process where CO₂ is released into the atmosphere such as plant and animal decay, deforestation, when we breathe out, or the burning of fossil fuels such as coal or gas. A sink is a reservoir that removes some of the CO₂ from the atmosphere, such as when vegetation and trees take up CO₂ for photosynthesis. The oceans remove some of the CO₂ from the atmosphere as well.

The nature and the locations of the sinks that absorb about half of the human produced 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? By better understanding the nature, locations, and processes that make these natural sinks, we can better predict the rate of buildup of CO₂ in the atmosphere and its impact on our climate.

Today, fossil fuel combustion and other human activities are currently contributing about 39 billion tons or 39 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 same 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 CO₂2 "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.

Carbon Cycle

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 2013, the atmospheric CO₂ concentration had increased by over 25% to values at or near 400 ppmv. At this rate and over a sufficient period of time, this efficient greenhouse gas will impact the Earth's climate.

OCO-2 measurements provides the global coverage, spatial resolution, and accuracy to provide a basis to characterize and monitor the geographic distribution of CO₂ sources and sinks and quantify their variability. Based on these measurements, scientists 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 allow more reliable forecasts of the atmospheric CO₂ abundance and its impact on the Earth's climate.

The OCO-2 mission contributes 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