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Updated 9 November 2005
OUR CHANGING PLANET
A Report by the Climate Change Science Program (CCSP) and the Subcommittee
on Global Change Research
PDF version of this section of the report
The hardcopy version of this report is available free of charge from the GCRIO Online Catalog
Global Carbon Cycle
CCSP research on the global carbon cycle in FY 2006 will continue to address questions of how large and variable the dynamic reservoirs and fluxes of carbon within the Earth system are, and how carbon cycling might change and be managed in future years, decades, and centuries. This research is needed to help human societies evaluate their options for managing carbon sources and sinks to achieve an appropriate balance of risk, cost, and benefit.
Over the past 2 centuries, fossil-fuel emissions, land-use change, and other human activities increased atmospheric carbon dioxide (CO2) by 30% and methane (CH4) by 150% to concentrations unprecedented over the past 420,000 years (the time span of the longest fully documented ice core record). Future atmospheric concentrations of these greenhouse gases will depend on trends and variability in natural and human-caused emissions and the capacity of terrestrial and marine sinks to absorb and retain carbon. Options available to societies for stabilizing or mitigating concentrations of greenhouse gases in the atmosphere through management of carbon in the environment involve: (1) reduction of carbon emissions at their source and/or (2) enhanced sequestration of carbon through biospheric storage or engineered approaches.
To address these and related research questions, the United States is focusing its carbon cycle science on targeted research areas that are ripe for scientific progress and that are most relevant to societal concerns. The agencies responsible for CCSP carbon cycle research have organized a coordinated, interagency, and multidisciplinary research strategy to bring together the broad range of infrastructure, resources, and expertise essential for providing this information. This approach involves investigators with atmospheric, oceanic, terrestrial, and human dimensions research expertise and observational, experimental, and modeling skills. These agencies also are establishing an ongoing dialog with stakeholders, including resource managers, policymakers, and other decisionmakers to ensure that information is provided in a useful form.
The North American Carbon Program (NACP), designed to address strategic research question 7.1 (see chapter banner), will continue to be a major priority. The NACP will quantify the magnitudes and distributions of terrestrial, freshwater, oceanic, and atmospheric carbon sources and sinks for North America and adjacent oceans; improve understanding of the processes controlling source and sink dynamics; and produce consistent analyses of North America's carbon budget that explain regional and sectoral contributions and year-to-year variability. The NACP is committed to reducing uncertainties related to the buildup of CO2 and CH4 in the atmosphere and the amount of carbon, including the fraction of fossil-fuel carbon, being taken up by North America's ecosystems and adjacent oceans.
The first studies to address the objectives of CCSP's new Ocean Carbon and Climate Change (OCCC) program plan, designed to address strategic research question 7.2, will be initiated in FY 2006. The OCCC program is an integrated effort for oceanic monitoring and research aimed at determining how much CO2 is being taken up by the ocean at the present time and how climate change will affect the future behavior of the oceanic carbon sink. NACP and OCCC program interests and objectives converge in addressing carbon dynamics in the coastal oceans adjacent to North America and in the land-sea margins – diverse and variable areas where impacts of changes in the terrestrial environment and climate greatly complicate carbon cycle processes. New investigations of carbon dynamics in the coastal oceans adjacent to North America will address both OCCC and NACP objectives.
Other FY 2006 priorities, focused on reducing scientific uncertainties regarding carbon sources and sinks, are: (1) conducting and reporting on experimental studies to investigate the processes regulating carbon balance in terrestrial ecosystems, and (2) developing and improving global carbon cycle models and coupled carbon-climate models to aid in improving projections of atmospheric CO2 concentrations and climate.
Highlights of Recent Research
AmeriFlux Network Measures Terrestrial Carbon Sinks and Sources and Identifies Biological Controls [7, 10, 18]
The AmeriFlux network of research sites measures terrestrial carbon sinks and sources and biological processes that regulate the net exchange of CO2 between terrestrial ecosystems and the atmosphere [referred to as "net ecosystem exchange" (NEE)]. The measurements show that mature forests are important sinks for atmospheric CO2 (Hollinger et al ., 2004). Disturbances that replace or remove forests can result in the land being a net source of CO2 for a few years in mild climates or up to a decade in harsh climates while the forests are recovering (Law et al ., 2004), after which they may gradually revert to being a sink until the next disturbance. Moist tropical forests are not necessarily strong sinks because some of them have experienced recent mortality events that leave large amounts of decaying plant material that can release more CO2 to the atmosphere than the forests remove [net source of about 1 tonne per hectare per year (Saleska et al ., 2003). Thus, the range of observed annual NEE of CO2 ranges from a source of about 1 tonne per hectare to a net sink of 2 to 4 tonnes per hectare for forests and about 1 tonne or less per hectare for agricultural crops and grasslands. These results are expected to dramatically improve process depictions in carbon and climate models.
AmeriFlux Measurements Reveal Direct Influence of Volcanic Eruptions on Terrestrial Carbon Cycle. [1, 5]
The growth rate of atmospheric CO2 concentration decreased after the explosive eruption of Mt. Pinatubo in the Philippines in June 1991. Scientists have been debating the cause of this atmospheric CO2 decrease for many years. In a recent study by Gu et al . (2003), a group of AmeriFlux scientists found that aerosols (tiny particles suspended in the air) formed in the aftermath of the eruption altered the quantity and quality of solar radiation for vegetation, and this alteration significantly enhanced photosynthesis under cloudless conditions in a deciduous forest in the United States. These measurements indicate that increased CO2 uptake by terrestrial vegetation caused by the effects of aerosols from the Pinatubo eruption on incident solar radiation were at least partly responsible for the sudden decline in the growth rate of atmospheric CO2 concentration after the eruption. The study also indicates that anthropogenic aerosols, which can similarly change the quantity and quality of solar radiation for vegetation, may affect carbon cycling dynamics. A modeling study by Angert et al . (2004) suggests an alternate mechanism for the enhanced CO2 sink in 1992-1993, due to a unique combination of an enhanced ocean CO2 sink, reduced respiration driven by cooling and drying of the upper layers of the soil thereby reducing heterotrophic respiration, and reduced biomass burning. These studies illustrate the complexity of the global carbon cycle and underscore the crucial need for continued observations to resolve carbon cycle uncertainties.
Impact of Fires on Interannual Variability in Global Atmospheric Trace Gases 
Year-to-year changes in the atmospheric concentration of CO2 and CH4 are linked to fire activity associated with the El Niño/La Niña cycle. Information on vegetation, precipitation, surface air temperatures, and changes in Earth's radiation, derived from the Terra, Aqua, Tropical Rainfall Measuring Mission, and Active Cavity Radiometer Irradiance Monitor satellites, was used in modeling analyses of fire effects for 1997 to 2001. Researchers quantified the amount of CO2 and CH4 emitted by fires by combining the satellite data with in situ measurements of atmospheric gases. Emissions of greenhouse gases from fires increased across multiple continental regions in the El Niño year (1997-1998), including Southeast Asia (60% of the global increase), Central and South America (30%), and boreal forests of North America and Eurasia (10%). Vast areas of the tropics dry out and become vulnerable to fire during El Niño events, thus enabling humans to use fire more effectively as a tool for clearing land. Increases in fires and associated emissions of greenhouse gases are expected if El Niño events increase in frequency and/or intensity in the future.
Regionalization of Methane Emissions from the Amazon Basin 
The largest single source of CH4 emissions to the atmosphere is natural wetlands, which account for between 20 and 40% of annual emissions; 60% of this is estimated to come from tropical wetlands, although there is large uncertainty regarding the magnitude and variability of that source. The Amazon Basin contains one of the largest areas of seasonally inundated tropical wetlands. Microwave remote-sensing techniques were used to measure wetland extent and seasonal variability, and the resulting data were combined with results from existing field studies of CH4 flux in a model to extrapolate CH4 flux across the entire Amazon Basin. Extrapolation to the Central Amazon Basin (1.77 million km2 area) produces an estimated 6.8 TgC yr -1 , and extrapolation to the full Amazon River Basin below 500-m elevation (5.2 million km2 , of which 17% is wetland) produces an estimated 22 TgC yr -1 from CH4 emissions. This estimate is lower than those previously reported for the region. The satellite data analyses indicate that past studies overestimated the length of time seasonal wetlands were flooded and therefore overestimated annual methane emissions.
Variability in Low Productivity Ocean Gyres 
The mid-ocean gyres are huge circulation cells within the major ocean basins. They are vast expanses of very low primary production and biomass. Their low productivity results from the physical dynamics of the circulation within the gyre, which depresses the depths at which significant nutrient concentrations (e.g., nitrate, phosphate, and silicate) required for photosynthesis occur. Global surface chlorophyll concentrations measured over seven years by the U.S. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Japanese Ocean Color and Thermal Scanner (OCTS) satellite sensors were used to estimate temporal changes in the size of each gyre. The analysis shows that these areas have a distinct seasonal cycle and that the overall area of the gyres in the North Pacific and North Atlantic expanded during 1996 to 2003, while there was little change in the area of the South Pacific, South Atlantic, and southern Indian Ocean gyres. The study did not attempt to explain why the Northern Hemisphere gyres are expanding and was too short to determine whether the trends will continue. This first-ever quantification of year-to-year variability across the global oceans has implications for carbon dynamics, and raises a concern about whether continued expansion of the low-productivity gyres would reduce the size of the ocean carbon sink. Continuing systematic satellite observations will be available to monitor future trends and address this concern.
The Repeat Hydrography CO2 / Tracer Program is a systematic and global re-measurement of select cross-sections of the ocean to quantify changes in storage and transport of heat, freshwater, CO2, chlorofluorocarbon tracers, and related properties. For cruises in the North Pacific, difference plots for the 2004 Repeat Hydrography compared with the 1994 World Ocean Circulation Experiment quantitatively document significant changes. Increases in dissolved inorganic carbon (DIC) of up to 35 µmol kg -1 were observed in surface waters and in intermediate depths ranging from 200 to 1000 m. On average, mixed layer DIC increases of 1.5 ± 0.2 µmol kg -1 yr -1 were observed in the subtropical waters of the North Pacific, indicating that over the past decade the oceanic uptake of CO2 in this part of the global ocean has been faster than the rate of growth of CO2 in the atmosphere. These results indicate that the rate of uptake of CO2 may differ significantly in different regions of the global ocean.
Common data sets were assembled from four Free-Air CO2 Enrichment (FACE) experiments being conducted in forest ecosystems. The ongoing, multi-year experiments were in established stands in North Carolina (loblolly pine) and Tennessee (sweetgum), and in young stands in Wisconsin (aspen-birch-maple) and Italy (poplar). Soil respiration increased in response to CO2 enrichment, and the relative response was larger in the young stands. The temperature sensitivity of soil respiration was unaffected by CO2 enrichment. No effects on soil nitrogen pools or processes were observed in the three forest FACE experiments in the United States, indicating that nitrogen limitations are unlikely to constrain increases in forest productivity at similar sites in the initial years following CO2 enrichment. Continued study of these effects over longer time periods will be critical for our ability to predict long-term changes in soil nitrogen availability and the potential for sustained increases in productivity in a CO2-enriched atmosphere.
Analysis of a nearly continuous 6-year record of fine root production and mortality in an experimental sweetgum forest revealed a large increase in root production and a significant change in the depth distribution of roots in forest plots exposed to a CO2-enriched atmosphere. These responses, which were measured in a FACE experiment, have important implications for carbon sequestration and nitrogen and water uptake in this and other forest ecosystems. Allocation of carbon to fine roots reduces the potential for carbon sequestration in plant biomass, but increases the potential for carbon storage in soil. Comparison of the responses of the deciduous forest with those in a similar experiment in a pine forest suggests that root system dynamics can explain differences among ecosystems in their response to elevated atmospheric CO2. These results indicate that accurate assessments of carbon flux and storage in forests must account for the responses of root systems.
Long-Term Forest Management Studies Inform Carbon Management Options 
The Long-Term Soil Productivity (LTSP) study seeks to understand how anthropogenic disturbances affect the land's capacity to store carbon through the conduct of a series of 62 long-term field experiments in major forest types of the United States and Canada. This unique study has been in place for more than a decade and is the largest and most extensive experiment of its type in the world. Results show differences in carbon and nutrient dynamics by soil type and climate regime and provide critical information for developing management systems that maintain and enhance productivity and carbon sequestration options. Findings from studies of alternative fuel treatments for reducing fire risk indicate no significant differences in CO2 efflux following treatment and provide a basis for local communities to reduce fire risk and improve the health and vigor of forests without affecting greenhouse gas emissions.
Land Resource Management and Reclamation Activities have Significant Potential to Sequester Carbon [ 2, 3, 9, 16, 19]
As noted previously, changes in soil carbon can potentially change the concentrations of greenhouse gases in the atmosphere. If CO2 from the atmosphere, captured through photosynthesis, is ultimately stored in the soil to a greater degree, the resulting soil carbon sequestration may help slow the rise of atmospheric CO2. Studies of soil carbon dynamics and storage in a variety of managed systems in the United States and elsewhere have identified and quantified significant carbon sequestration potentials:
These results are examples of findings that contribute new scientific knowledge about relationships of terrestrial ecosystem processes and management practices on carbon sequestration. Land and resource managers, agricultural consultants, and environmental organizations are using this new information to foster the development of agricultural and land management systems that mitigate greenhouse gas emissions, enhance soil fertility, and improve soil and water quality.
Climate-Induced Thawing of Permafrost and Implications for Soil Carbon Stocks 
The high-latitude regions of North America, including interior Alaska, are critical areas for research because thawing of permafrost within the soil zone can cause a release of carbon to the atmosphere. In the past 40 years, boreal North America has warmed by at least 2°C and has experienced pervasive drying of lakes and water tables. Landscapes with near-surface permafrost store greater than 60% of their carbon stocks in organic soils. While long-term records indicate net carbon storage on land, fires in recent decades have burned these landscapes in unexpected proportions. Trace gas studies at the Bonanza Creek Long-Term Ecological Research Station have revealed complex responses of permafrost landscapes to fire, with post-burn hydrological conditions and plant ecological processes determining the net balance between CH4 and CO2 emissions and carbon sequestration. Drought and fire induce complex responses (e.g., water table fluctuation, revegetation patterns) due in part to the discontinuous nature of the permafrost. These findings emphasize the importance of surface and subsurface hydrology for understanding the carbon dynamics of boreal and arctic regions and emphasize the need for baseline studies of carbon cycling on a variety of temporal and spatial scales.
Highlights of FY 2006 Plans
In FY 2006, continuing observations, field campaigns and experiments, and model development will be a priority under the NACP. Significant effort will be devoted to continuing the development, expansion, and optimization of observations and monitoring networks for North American, coastal, and global carbon sources and sinks. The Mid-Continent NACP Intensive Campaign will be completing its initial field phase. New ocean carbon research will be underway in support of the OCCC program, with early emphasis on studies of carbon dynamics in the coastal oceans adjacent to North America that also support NACP objectives. Data management plans and capabilities for NACP and OCCC will be implemented. New experimental studies will be conducted and the results of previous experimental studies will be synthesized to explain the processes regulating carbon dynamics in terrestrial ecosystems, including carbon sequestration. Modeling investigations will focus on improved coupling of carbon-climate models, carbon data assimilation, and better projections of CO2 emissions and ecosystem responses for climate models. The first State of the Carbon Cycle Report is one of the CCSP synthesis and assessment products.
NACP: Measurements and Monitoring
In FY 2006, NACP will continue to assign high priority to global observations by enhancing observational capabilities and monitoring networks for carbon fluxes and stocks in North America and adjacent oceans. The following activities represent major commitments:
NACP: Field Studies
Intensive field campaigns and experimental studies under NACP will feature the following activities in FY 2006:
Ocean Carbon and Climate Change Research
New studies will be initiated to advance the goals of the OCCC science implementation strategy:
Study of Terrestrial Processes Regulating Carbon Balance
Long-term field experiments and major campaigns are identifying processes that are critical to reducing uncertainties in carbon budgets and improving model-based predictions of future terrestrial carbon dynamics. A variety of experimental studies will be continued and several scientific syntheses are anticipated in FY 2006; new studies to better understand climate-related ecosystem effects on carbon balance will be conducted:
Development of New Measurement and Analysis Methods
In FY 2006, the following work will be underway to develop new measurement and data analysis methodologies for carbon cycle science:
Improved Modeling for Projections of Carbon Dioxide and Climate
Continuing and new investments will be made in modeling studies to support NACP, the Mid-Continent NACP Intensive Campaign, the OCCC program, and activities on global carbon dynamics and climate change assessment.