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Carbon Cycle Science Breakthroughs. . . . at NOAA and Beyond

Carbon is the backbone of life on earth, from the food that sustains us to the energy that fuels the world economies. In the form of carbon dioxide and methane in the atmosphere, it is also an important regulator of the earth's climate as a major contributor to the planetary greenhouse effect. A small active fraction of the Earth's large carbon reservoirs exchanges naturally between the ocean, atmosphere and land (Figure 1). While the climate is directly affected by atmospheric concentrations of greenhouse gases, we must also understand how carbon cycles among these active reservoirs if we hope to be able to predict future atmospheric greenhouse gas concentrations accurately.

Carbon dioxide (CO2) and methane (CH4) concentrations have been increasing in the atmosphere since 1850 and are now higher than they have been for more than 400,000 years, primarily as a result of human use of fossil fuels and land clearing. Of the total emissions to the atmosphere, only about 50% remains in the atmosphere. The land and oceans absorb the other 50% by processes known as "sinks". Quantifying and reconciling these sinks with the remaining concentrations in the atmosphere has been and remains a major challenge, although recent advances have been made understanding the continental U.S. land sink through NOAA-supported research (see below**).

A complete understanding of all the processes involved in the carbon cycle is still lacking, especially on land. Natural sources and sinks in the land and ocean are highly variable (Figure 2). As we do not yet fully understand the direct causes of this variability it is difficult to produce reliable forecasts of how they will change in the future. The coupled dynamics of carbon in the land, ocean, and atmosphere could have a dramatic impact on future atmospheric CO2, and therefore climate.

Recognizing the integrated nature of the carbon cycle, 6 federal agencies (DOE, NASA, NSF, NOAA, USDA, and USGS) are coordinating their individual carbon cycle research programs to address common research goals outlined in A U.S. Carbon Cycle Science Plan. See http://www.carboncyclescience.gov for more information.

The main objective for NOAA's Global Carbon Cycle research program is to improve our ability to predict the fate of anthropogenic CO2 and future atmospheric CO2 concentrations using a combination of global observations, process-oriented field studies, and modeling. There are three major goals:

1. Quantifying spatial patterns and variability of carbon sources and sinks at global to regional scales;
2. Documenting the fate of anthropogenic CO2 in the atmosphere and oceans; and
3. Improving future climate predictions by incorporating a dynamic understanding of the carbon cycle into models.

Highlights of currently funded research:
The Cooperative Air Sampling Network (Figure 3) and a global climatology based on ocean data (Figure 4) are key tools to monitor CO2 concentrations in the global atmosphere and document sources and sinks for carbon. Several modeling groups use these data to calculate global sources and sinks at the broad continental scale.

Figure 3 (L). The NOAA CMDL Cooperative Air Sampling Network. (Tans) Figure 4 (R). Mean annual air-sea fluxes of CO2. "Warm" colors indicate release of CO2 from the ocean to the atmosphere; "cool" colors indicate uptake, or "sink" regions. (Takahashi et al. 1999)

Research suggests that land masses in the Northern Hemisphere (North America, Eurasia) are a net sink for excess CO2 in the atmosphere. However, quantifying this sink over continents is a major challenge because of the complex covarying signals of CO2 exchange over the land. A major program combining field observations, modeling, and data assimilation seeks to test and build the next generation of observational tools to quantify sources and sinks at regional scales over North America and adjacent ocean basins. This program is the first effort to be collaboratively managed by the Carbon Cycle Interagency Working Group. For more see: www.carboncyclescience.gov. NOAA hopes to support this effort through atmospheric observations over the land and ocean, surface carbon measurements in adjacent ocean basins, and innovative data assimilation and modeling techniques.

NOAA also funds process research to improve the quality of our observations and to be able to use data more accurately at global to regional scales. One example of this research is the GCC's focus on air-sea exchange, currently a major source of uncertainity in quantifying ocean uptake at regional scales. NOAA and NSF are supporting a coordinated field campaign (Gas Ex 2001) studying the complex physical, chemical, and biological forcing functions of CO2 air-sea exchange in the equatorial Pacific (Figure 5).

Once the CO2 enters the surface ocean, its fate is further dependent upon physics, chemistry, and biology in the ocean interior. Quantifying ocean interior inventories of total dissolved CO2 and the anthropogenic CO2 is very difficult on a global scale. NOAA and NSF are coordinating a repeat ocean survey of some of the major survey lines sampled as part of the Joint Global Ocean Flux Study and the World Ocean Circulation Experiment (Figure 6). These interdisciplinary surveys will be jointly sponsored by the CLIVAR program (Climate Variability). Details at www.aoml.noaa.gov/ocd/repeathydro

Figure 6. Proposed ocean interior survey lines. Black lines are the proposed U.S. lines, black and white lines are committed international lines, and gray lines are additional lines proposed for CLIVAR. (Feely and Sabine)

Finally, model development is required to project future concentrations of CO2 in the atmosphere, to aid in planning optimal sampling networks, and to narrow uncertainty in model disagreements, including the role of oceanic CO2 uptake (Figure 7).

Figure 7. Scenarios for future atmospheric CO2 concentrations depend on accurate model representations of future ocean uptake. Models show wide variations in their representations of the future. (OCMIP, courtesy Caldeira)

NOAA will endeavor to reduce these uncertainties through the development of coupled models that evaluate carbon fluxes, identify key processes and predict future atmospheric CO2 concentrations. Such innovative research can assist in designing operational observational systems, and bring dynamic understanding of the carbon cycle to be incorporated into improved climate forecasting products. NOAA now has a call for proposals in the Global Carbon Cycle program for FY2002. For more details, see: http://www.ogp.noaa.gov/mpe/gcc/index.htm.

**Research results in the carbon cycle were touted as a runner-up "2001 Breakthrough of the Year" by Science magazine. "Carbon Consensus. Researchers who had been puzzling over how much carbon dioxide is absorbed by U.S. forests and fields have finally reconciled their conflicting results. The outcome will help hone estimates of how much the planet may warm in future years..." (to read entire article see http://www.sciencemag.org or Science, 294:2447.). The lead article that is mentioned was initiated by a workshop funded by NOAA/OGP (and led by a P.I. at the NOAA/OGP-funded Carbon Modeling Consortium-- Steve Pacala).

[1/14/02]

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