Research > Carbon Cycle Science

Carbon Cycle Science

Global climate projections and regional climate forecasts depend on understanding the path of carbon through our environment.

Projecting climate into the future and forecasting regional impacts depends on our understanding of the exchange of carbon dioxide between the atmosphere, oceans and land ecosystems. NOAA is charged to provide the atmospheric measurements and analyses required to track the fate of carbon dioxide emissions caused by the burning of fossil fuels and biomass, and to reduce uncertainties in how the exchange of carbon responds to the variations and trends of climate and land use.

What is the Carbon Cycle?

Carbon is exchanged, or "cycled" among Earth's oceans, atmosphere, ecosystem, and geosphere. All living organisms are built of carbon compounds. It is the fundamental building block of life and an important component of many chemical processes. It is present in the atmosphere primarily as carbon dioxide (CO2), but also as other less abundant but climatically significant gases, such as methane (CH4).

Diagram of carbon cycle sources and sinks.

Sources and Sinks

Because life processes are fueled by carbon compounds which are oxidized to CO2, the latter is exhaled by all animals and plants. Conversely, CO2 is assimilated by plants during photosynthesis to build new carbon compounds. CO2 is produced by the burning of fossil fuels, which derive from the preserved products of ancient photosynthesis. The atmophere exchanges CO2 continuously with the oceans. Regions or processes that predominately produce CO2 are called sources of atmospheric CO2, while those that absorb CO2 are called sinks.

Why is the Carbon Cycle important?

While CO2 is only a very small part of the atmosphere (0.04%), it plays a large role in the energy balance of the planet.

CO2 in the atmosphere acts like a blanket over the planet by trapping longwave radiation, which would otherwise radiate heat away from the planet. As the amount of CO2 increases, so will its warming effect. CO2 is the largest contributor (currently 63%) to this effect by long-lived gases and its role increases each year. The additional burden of CO2 in the atmosphere will remain for a very long time, of the order of thousands of years, if we have to rely on the natural mechanisms of erosion and sedimentation to process the added CO2.

What do we know about the Carbon Cycle?

Owing primarily to the burning of fossil fuels and secondarily to changes in land-use, the amount of CO2 in the atmosphere has been increasing globally since the onset of the industrial revolution. Based on 50 years of direct observations of the atmosphere, it is clear that this trend continues and is accelerating. From observatories and cooperative sampling sites around the world, NOAA measures global greenhouse gases and works with partners to improve the accuracy and reliability of these measurements in order to improve our understanding of the sources, sinks, and trends of this important gas and to improve our predictive capability. This continuing record is critical to understanding the potential evolution of global climate as well as aiding or verifying international management strategies.

Greenhouse gas index

Radiative forcing of all the long-lived greenhouse gases, relative to 1750, and the NOAA Annual Greenhouse Gas Index (AGGI) on the right axis, which is indexed to 1 on January 1, 1990.

What don't we know about the Carbon Cycle?

Needed Developments

Although we have a good sense of what is happening with CO2 on a global basis, and have a sound system for following large-scale trends, regional information is needed if society is ever to manage or verify carbon emissions. We must understand regional variations in the sources and sinks of CO2 because they help identify possible sequestration or emission management options. Ideally, these regional evaluations would be done on a global basis. Our first and perhaps most important step is to focus on the North American continent.

Regional Attribution

North American CO2 source and sink model.

Global measurements establish a baseline for understanding CO2, but they do not show the smaller details needed to manage CO2 regionally or to mitigate regional impacts. The U.S. has initiated new programs in these areas to improve our understanding of regional source and sinks in which NOAA plays a vital role:

Analysis of North American Production and Absorption of CO2.

Two of the primary aims of the North American Carbon Program (NACP), a multi-agency effort, are

  • to measure the uptake and loss of CO2 by monitoring ecosystem emissions and
  • to verify estimates of human and natural emissions or uptake.

NOAA's responsibility is for the latter of these efforts.

CO2 measuring tower.

Vertical sampling of the atmosphere

In support of this effort, NOAA is developing a vertical sampling network over North America, including routine samples from aircraft and nearly continuous measurements from very tall towers, to determine the distribution of CO2 throughout the height of the atmosphere. Without this effort, credible accurate estimates of regional emissions over time based on atmospheric measurements are not possible.

Impacts of increasing CO2 on other systems

Continued emission of carbon dioxide to the atmosphere will affect climate and ocean chemistry, subsequently influencing both marine and terrestrial ecosystems. The warming effects of increasing CO2 and other greenhouse gases impinge on agriculture, natural systems, and a host of environmental variables. Increasing CO2 in the atmosphere also directly translates to increasing acidity of the oceans. Carbon dioxide dissolves in water to form carbonic acid, which is corrosive to the shells and skeletal material of many marine organisms. Subsequent impacts on ecosystems are largely not understood.

Improved modeling and monitoring

Understanding regional influences on atmospheric CO2 requires a carefully designed sampling and analysis network. Our approach involves making use of analysis modeling, where data are used to optimize models so that they can best explain observed spatial and temporal variations in atmospheric CO2. Analysis modeling also helps us identify where new sites are needed or where old sites could be repositioned to obtain better results. We are increasing the suite of natural and human emitted gases to improved our ability to attribute the observed changes in atmospheric CO2 to specific sources and sinks. This interactive approach for observation, analysis, and modeling will lead to the development of a stronger, more precise predictive capability for NOAA and its partners and thus a more reliable source of information for society.

What is NOAA's role?

NOAA is responsible for acquiring and maintaining the global, regional, and local record of CO2 and other greenhouse gases. NOAA's network is the backbone of the global system embraced by World Meteorological Organization (WMO) that not only provides accurate and timely information on CO2 and other gases, but also fosters international cooperation and collaboration on an issue that is of global importance. Maintaining such a network requires a dedication to accuracy, precision, cooperation, and complete transparency. NOAA's scientists not only are involved in maintaining a large portion of the world network, but also provide the calibration necessary for an integrated network and serve on several advisory groups and expert committees for assuring quality control, improving understanding, and identifying future needs. Only through careful management and interpretation of accurate, high-resolution measurements can we manage our environmental resources efficiently and effectively.

Map of monitoring network

The NOAA global cooperative air sampling network used to determine the AGGI. Red dots are weekly flask sampling sites and blue circles are continuous measurement sites.

What will we need to know in the future?

Anthropogenic emissions, emissions limitations, sequestration, and ocean chemistry will likely play leading roles in the future atmospheric CO2 burden. Coupled models will be required for long term projections. Answers to key questions such as the following will require careful observation and skilled modeling, all of which we aim to achieve, working together with our partners. We will also need an early warning system for potentially large, but hard to predict, changes in the carbon cycle, such as massive emissions of CO2 from frozen carbon compounds in Arctic permafrost as it warms up.

  • How can we gain enough confidence in these models for them to aid in decision making?
  • Which features can be validated?
  • Can we estimate the length of time that a particular sequestration option is secure?
  • What are biophysical limits of biological sequestration?
  • How much CO2 can be stored in geological reservoirs, how much in the oceans?
  • What are likely environmental impacts of different strategies?
  • How does the effectiveness of sequestration compare to decreasing the "carbon intensity" of our activities?

How does society benefit from this knowledge?

If society is to manage or reduce carbon emissions in the future, reliable and accurate information will be needed on local, regional, and global scales. Atmospheric measurements of the spatial and temporal trends of CO2 in the atmosphere are essential for reaching quantitative understanding the sources and sinks of this gas. Without accurate measurements, the effectiveness of mitigation or verification of emission reduction become very uncertain. So is evaluation of new energy strategies. The current world-wide network operated by NOAA provides a global foundation for monitoring and understanding these trends. Our developing North American network will increasingly provide the kind of regional information and source characterization necessary for rational management of carbon emissions. Society will be increasingly prepared to deal with the shifts of regional and global climate. In the future, coupling CO2 models with other environmental models will improve predictions and long-term stewardship strategies.

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