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The Global Carbon CycleRecent Accomplishments Archived News Postings (June 2000 - July 2005)
Carbon Cycle Science Home Page
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The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2008 edition of the annual report, Our Changing Planet). The highlights are derived from carbon cycle science programs spanning terrestrial, oceanic, and atmospheric reservoirs and the complex interactions that link these reservoirs within the global carbon cycle. Climate Forcing Atmospheric carbon dioxide (CO2) and methane (CH4) are significant forcing agents of Earth's climate and have been increasing over the past 2 centuries as a result of human activities. Approximately 85 to 90% of today's anthropogenic emissions are attributed to fossil-fuel combustion with land-use change accounting for most of the rest. Future concentrations of carbon dioxide and methane in the atmosphere will depend on the long-term trends and variability in natural and anthropogenic emissions and the capacity of natural and managed sinks in the carbon cycle.
Distinguishing Fossil Fuel Emissions from Biological CO2 Cycling.1Measurements of atmospheric 14CO2, carbon monoxide (CO), and sulfur hexafluoride (SF6) as tracers of fossil-fuel CO2 emissions show that 14CO2 provides accurate quantification of fossil-fuel CO2. Issues remain concerning the distributions of SF6 and CO emissions relative to CO2 that may bias regional estimates. This research focused mainly on winter and spring data from two different regions, Colorado and New England, to test the approach under widely different conditions. The effort as a whole, however, extends to much of the atmospheric research network and is anticipated to be enhanced to produce viable regional estimates of fossil-fuel contributions to atmospheric CO2 and CH4 concentrations, thus allowing more accurate estimates of biological contributions to the atmospheric carbon budget.Forest Fire Impacts on Climate Change.2Contemporary wisdom supports the concept that increases in temperature would lengthen the growing season in boreal ecosystems and increase the probability of fire, leading to a positive feedback between warming, fires, carbon loss, and future climate change. However, a new multi-factor analysis of the long-term effects of a well-characterized boreal forest fire indicates that the net radiative forcing may be negative. During the first year after the fire, all factors combined to increase radiative forcing (34 ± 31 Wm-2 of burned area), but when averaged over an 80-year fire cycle the net effect was to decrease radiative forcing (-2.3 ± 2.2 Wm-2). The reason is that multidecadal increases in surface reflectance had a larger impact than the fire-emitted greenhouse gases. This study illustrates the importance of interdisciplinary, multi-factor analysis and the need to examine effects over decades to centuries in order to fully understand the impacts of disturbance and the feedbacks between Earth's physical and biogeochemical systems.Changes in Forest and Soil Carbon.3,4A 13-year study of carbon flux measurements at the Harvard Forest clearly shows that the uptake of atmospheric CO2 has been increasing in this ecosystem; mean carbon gain for the 1992 to 2004 period was 2.4 metric tons of carbon ha-1 yr-1, and carbon uptake has been increasing at the rate of 0.15 metric ton of carbon ha-1 yr-1. This magnitude of forest carbon gain is typical of other findings from the AmeriFlux observation network. In the top 20-cm soil layer in preserved old-growth forests in southern China, a study shows that soils accumulated atmospheric carbon at an unexpectedly high rate from 1979 to 2003. These long-term observations show that the rate of photosynthetic carbon assimilation can exceed ecosystem carbon loss by respiration. Such studies demonstrate the significant potential for terrestrial sources and sinks to affect atmospheric CO2 increases.Terrestrial Carbon Cycle The terrestrial carbon cycle is composed of a complex set of interactive biological, chemical, and physical processes that transfer carbon between land, oceans, and the atmosphere. Collectively, these processes influence atmospheric CO2 and CH4 concentrations. Improving the scientific understanding of the role of these reservoirs and links in the carbon cycle reduces the uncertainty about the factors influencing greenhouse gas increases and provides a stronger basis for climate change decision support, in particular for carbon management to mitigate CO2 and CH4 increases.
Carbon Storage in Forested Ecosystems.5,6,7 Forested ecosystems in the United States have a major impact on regional and global sources of CO2–taking up 25 to 50% of CO2 emitted annually from fossil-fuel combustion in the United States, a significant amount since the United States accounts for about 20% of global emissions. The CO2 Boundary-Layer and Regional Airborne (COBRA) study developed model-data fusion methods to determine accurate regional- and continental-scale carbon budgets and to attribute carbon fluxes to specific causes and processes. Model predictions for the 1990s, with and without
contributions from CO2 fertilization, are within the range of uncertainties; but, in the absence of CO2 fertilization, the carbon sink in forests will approach zero by 2050, as the effects of forest harvesting offset those of agricultural land abandonment. Long-term measurements, such as those undertaken by AmeriFlux and tall-tower networks, are critical for detecting the signatures of processes that give rise to long-term uptake of atmospheric CO2 by major ecosystems of North America.
Seasonality of Amazonian Forests.8Measurements of vegetation "greenness" from the Moderate Resolution Imaging Spectrometer (MODIS) on the Terra and Aqua satellites revealed an increase in greenness across Amazonian forests during the dry season, a pattern opposite to what had been previously thought and portrayed in models (see Figure 8). Researchers explained that the deep-rooted forests are able to access deep soil water and take advantage of the increased sunlight in the dry season. In contrast, areas converted to pastures, with more shallowly rooted plants, show the expected decline in greenness during the dry season. With this knowledge, scientists can refine models used to represent the tropical biosphere and how it exchanges carbon with the atmosphere, thus improving long-term carbon cycle and climate models.
Carbon Storage in Forest Soils.9A novel approach was developed to monitor carbon sequestration in aggregates of small particles that has profound implications for carbon protection in soil. The approach is based on ultra-small angle X-ray scattering of synchrotron X-rays that indicates the tendency of soil containing organic carbon to become more stable as a greater proportion of the soil micropores become filled. This mechanism suggests that the potential for organic matter storage in soil is larger than generally thought because it is neither limited by the surface area of minerals nor dependent on strong sorption by these surfaces. Another critical implication is that agricultural management and land use, particularly as they affect the stability of a microporous soil structure, can significantly alter the extent of carbon sequestration via both positive and negative feedbacks. Oceanic Carbon CycleThe global ocean carbon sink is an important component of the climate system that regulates the uptake, storage, and release of CO2 and other climate-relevant chemical species to the atmosphere. The future behavior of this carbon sink is quite uncertain because of the potential impacts of climate change on many ocean processes.
Ocean Carbon Dynamics.10
Coastal Carbon Storage.11,12Recent research on carbon storage in intertidal and subtidal estuarine ecosystems in the northeastern United States shows that these systems store more organic carbon to a depth of 1 m than forest soils in the adjacent uplands. The reason for this observation is that subtidal and intertidal soils are generally located on stable landforms and are accumulating organic carbon annually as sea level rises. Contrary to the common assumption that the carbon being stored in these ecosystems is from terrestrial vegetation, the majority of the organic carbon found was fixed by benthic algae. These results indicate the importance of including subtidal and intertidal estuarine ecosystems in global carbon models.High-Latitude Systems High-latitude systems are becoming increasingly important sources of CO2 and CH4 in the atmosphere as regional warming changes carbon dynamics in the cold regions. Understanding carbon dynamics in high-latitude systems and the factors that may lead to changes in those dynamics are crucial elements of global carbon modeling and essential for understanding the linkages and feedbacks between carbon, ecosystems and land cover, hydrology, and climate variability and change.
Arctic Ocean Sensitivity to Climate Change.13The Arctic Ocean and adjacent continental shelf seas are particularly sensitive to long-term changes and low-frequency modes of atmosphere-ocean-sea ice forcing arising from climate change. The cold, low-salinity surface waters of the Canadian Basin of the Arctic Ocean are undersaturated with respect to CO2 in the atmosphere and thus the region has the potential to take up atmospheric CO2, although presently covered over by sea ice. Undersaturated CO2 conditions in the Arctic Ocean are maintained by export of water with low dissolved inorganic carbon content and modified by intense seasonal shelf primary production. Sea ice extent and volume in the Arctic Ocean have decreased over the last few decades, and researchers have estimated that the Arctic Ocean sink for CO2 has nearly tripled over the last three decades (24 to 66 million metric tons of carbon per year) due to sea ice retreat with future sea ice melting projected to enhance air-to-sea CO2 flux by about 28% per decade.Carbon Export from Permafrost Ecosystems.14,15 The Yukon River discharges almost 7.8 million metric tons of carbon to the Bering Sea annually, of which about 30% is organic. With permafrost thawing throughout the Yukon basin, scientists are concerned about a potentially large release of bio-available dissolved and particulate organic carbon to the Arctic Ocean. Radiocarbon analysis of the discharged carbon indicates that most of the dissolved organic carbon is modern, but that particulate carbon has a large component that is thousands of years old. This is consistent with most particulate organic carbon originating from erosion of riverbanks and most dissolved organic carbon originating from surface runoff–not from thawed permafrost. Earlier work suggests that this pattern may continue if newly thawed dissolved organic carbon is respired rather than exported downstream.
Methane Release from Thawed Permafrost Lowlands.16Global warming is causing permafrost to thaw in areas of Alaska, Canada, and Siberia. When permafrost thaws, localized areas in a landscape may collapse and flood, forming wetlands where there were once forests. According to recent research, these wetlands release large amounts of CH4 to the atmosphere compared to the surrounding forests. Wetlands in interior Alaska formed from permafrost thaw released up to 35 times more CH4 per unit area than the surrounding forest soils over 1 year. If widespread wetland formation occurs across areas of thawing permafrost, the increase in CH4 release could have significant impacts on global climate change because it is a strong greenhouse gas.Fire Disturbance and Permafrost Degradation in Boreal Regions.17,18Fire regimes and permafrost degradation, which are closely linked through organic soil (ground fuel) in boreal regions, have the potential to shift in response to changes in climate. Over the past few decades, fire disturbance maintained the heterogeneous patterns of both permafrost and vegetation in boreal forests, mainly through the regulation of soil temperature by organic soil thickness. More severe burns such as occurred in 2004 and 2005, however, may promote a vastly different landscape depending in part on how much post-fire organic soil is retained. In all boreal landscapes, whether burned or not, ecosystem response to climate as well as associated carbon and energy exchanges depend largely on whether thaw-water is pooled on or drained from the landscape. Over the next several years, investigations will focus on detecting and studying the fate of both surface water and ecosystem carbon under various thaw regimes in the Alaskan interior.Carbon Management and Decision Support Carbon cycle research provides scientific input to policy and resource management decisions for carbon management and mitigation of climate change. The results of research supported by the carbon cycle research element of the CCSP are informing carbon management, as described in the research highlights below, and their impact is expected to increase over the course of this program.
The First State of the Carbon Cycle Report: CCSP Synthesis and Assessment Product 2.2.19This report finds that in 2003, North American terrestrial carbon sinks removed approximately 520 million metric tons of carbon per year (±50% with 95% confidence) from the atmosphere, which is equivalent to approximately 30% of North American fossil-fuel emissions in 2003. Approximately 50% of the sink is due to the regrowth of forests in the United States on former agricultural land and on forested land recovering from harvest. This sink is expected to decline. As forests mature they grow more slowly and take up less carbon from the atmosphere. The current source to sink imbalance of more than 3 to 1 (ratio of fossil-fuel emissions to net terrestrial carbon uptake) and the potential trend of increasing sources and decreasing sinks suggest that addressing imbalances in the North American carbon budget will likely require actions focused on reducing fossil-fuel emissions. Options focused on enhancing carbon sinks in soils and vegetation can contribute as well, but their potential is far from sufficient to offset current fossil-fuel emissions.Carbon Sequestration in Low-Production Lands.20Sequestering carbon through afforestation on low-production cropland and rangeland may be a means of reducing net carbon emissions from the United States and a process worth monitoring for carbon management. Information on the potential for such sequestration was derived through application of an ecosystem carbon model using a "greenness" product obtained from the Advanced Very High-Resolution Radiometer (AVHRR)–a space-borne instrument. The model results, at 8-km resolution, show the spatial variability in monthly net primary production (NPP) and accumulation rates of biomass (i.e., carbon storage) in low-production cropland and rangelands (see Figure 10). The model predictions indicate potential carbon sequestration rates of 300 million metric tons per year–the equivalent of 20% of the current U.S. carbon emissions from fossil-fuel combustion.
Forest Management.21Healthy, productive forests constitute an important terrestrial buffer against rising atmospheric CO2, a major driver of climate change. A team of American and Brazilian scientists, using extensive high-resolution multi-satellite analyses, concluded that forest harvests in the Brazilian Amazon are dominated by practices that leave forests more susceptible to drought and fire and threaten their long-term health and productivity. At the same time, they found that recently logged forests have a high probability of being cleared for farming and settlements, further reducing the potential of this land for carbon storage. Recently, the Brazilian Government enacted legislation to regulate forest lands and the timber industry, and this new policy has the potential to maintain forests under long-term timber management.Landowners' Greenhouse Gas Reporting Tools.22,23,24A new set of tools have been developed for the agricultural and forestry sectors to enable farmers and landowners to estimate carbon sequestration and greenhouse gas emissions. These sectors can reduce atmospheric concentrations of greenhouse gases by increasing carbon sequestration in biomass and soils, by reducing fossil-fuel emissions through use of climate-neutral fuels, and by substituting agricultural and forestry products that require less energy than other materials to produce. The tools have been adopted by DOE for use in the National Voluntary Greenhouse Gas Reporting Registry. The tools could also be adopted for use in state or regional registries and for use by voluntary greenhouse gas markets, opening new opportunities to reward landowners for reducing atmospheric greenhouse gases, enabling industry to meet global environmental goals at lower costs, and strengthening rural economies while protecting the environment.Societally Useful Measure of Greenhouse Gas Forcing.25The challenge of effectively informing society led scientists to develop a simple way to express greenhouse gas forcing for decisionmakers and the public. The perturbation to radiative climate forcing that has the largest magnitude and smallest scientific uncertainty is the forcing related to changes in long-lived and well-mixed greenhouse gases–in particular CO2, CH4, nitrous oxide (N2O) and halocarbons. The change in annual total radiative forcing by these gases since the pre-industrial era (1750) is used to define the Annual Greenhouse Gas Index (AGGI), which is normalized to radiative forcing in 1990. The AGGI shows that between 1990 and 2006 there was a 23% increase in radiative forcing due to long-lived greenhouse gases. This index is designed to help bridge the technical gap between scientists and decisionmakers and is now used and disseminated by the World Meteorological Organization.Additional Past Accomplishments:
Chapter References1) Turnbull, J.C., J.B. Miller, S.J. Lehman, P.P. Tans, R.J. Sparks, and J. Southon, 2006: Comparison of 14CO2, CO, and SF6 as tracers for recently added fossil fuel CO2 in the atmosphere and implications for biological CO2 exchange. Geophysical Research Letters, 33, L01817, doi:10.1029/2005GL024213.2) Randerson, J.T., H. Liu, M.G. Flanner, S.D. Chambers, Y. Jin, P.G. Hess, G. Pfister, M.C. Mack, K.K. Treseder, L.R. Welp, F.S. Chapin, J.W. Harden, M.L. Goulden, E. Lyons, J.C. Neff, E.A.G. Schuur, and C.S. Zender, 2006: The impact of boreal forest fire on climate warming. Science, 314, 1130-1132. 3) Urbanski, S., C. Barford, S. Wofsy, C. Kucharik, E. Pyle, J. Budney, K. McKain, D. Fitzjarrald, M. Czikowsky, and J.W. Munger, 2006: Factors controlling CO2 exchange on time scales from hourly to decadal at Harvard Forest. Journal of Geophysical Research, 112, G02020, doi:10.1029/2006JG000293. 4) Zhou, G., S. Liu, Z. Li, D. Zhang, X. Tang, C. Zhou, J. Yan, and J. Mo, 2006: Old-growth forests can accumulate carbon in soils. Science, 314, 1417, doi:10.1126/science.1130168. 5) Albani, M., P.R. Moorcroft, G.C. Hurtt, and S.W. Pacala, 2006: The contributions of land-use change, CO2 fertilization and climate variability to the carbon sink in the Eastern United States. Global Change Biology, 12, 2370-2390. 6) Matross, D.M., A.E. Andrews, P. Mahadevan, C. Gerbig, J.C. Lin, S.C. Wofsy, B.C. Daube, E.W. Gottlieb, V.Y. Chow, J.T. Lee, C. Zhao, P.S. Bakwin, J.W. Munger, and D. Hollinger, 2006: Estimating regional carbon exchange in New England and Quebec by combining atmospheric, ground-based, and satellite data. Tellus, 58B, 344-358. 7) Pathmathevan M., S.C. Wofsy, D.M. Matross, X. Xiao, J.C. Lin, C. Gerbig, J.W. Munger, V.Y. Chow, and E. Gottlieb, 2007: A satellite-based biosphere parameterization for net ecosystem CO2 exchange: Vegetation Photosynthesis and Respiration Model (VPRM). Global Biogeochemical Cycles (in press). 8) Huete, A.R., K. Didan, Y.E. Shimabukuro, P. Ratana, S.R. Saleska, L.R. Hutyra, W. Yang, R.R. Nemani, and R. Myneni, 2006: Amazon rainforests green-up with sunlight in dry season. Geophysical Research Letters, 33, L06405, doi:10.1029/2005GL025583. 9) McCarthy, J.F., J. Ilavsky, L.M. Mayer, J. J. Dastrow, E. Perfect, and J. Zhuang, 2007: Protection of organic carbon in soil microaggregates occurs via restructuring of aggregate porosity and filling of pores with accumulating organic matter. Geochimica et Cosmochimica Acta (in press). 10) Sabine, C.L. and N. Gruber, 2006: Introduction to special section on North Pacific Carbon Cycle Variability and Climate Change. Journal of Geophysical Research, 111, C07S01, doi:10.1029/2006JC003532. 11) Jespersen, J.L. and L.J. Osher, 2007: Carbon storage in the soils of a mesotidal Gulf of Maine estuary. Soil Science Society of America Journal (in press). 12) Osher, L.J. and C.T. Flannagan, 2007: Soil/landscape relationships in a mesotidal Maine estuary. Soil Science Society of America Journal (in press). 13) Bates, N.R., S.B. Moran, D.A. Hansell, and J.T. Mathis, 2006: An increasing CO2 sink in the Arctic Ocean due to sea-ice loss. Geophysical Research Letters, 33, L23609, doi:10.1029/2006GL027028. 14) Striegl, R.G., G.R. Aiken, M.M. Dornblaser, P.A. Raymond, and K.P. Wickland, 2005: A decrease in discharge-normalized DOC export by the Yukon River during summer through autumn. Geophysical Research Letters, 32, L21413, doi:10.1029/2005GL024413. 15) Striegl, R.G., M.M. Dornblaser, G.R. Aiken, K.P. Wickland, and P.A. Raymond, 2006: Carbon export and cycling by the Yukon, Tanana, and Porcupine Rivers, Alaska, 2001-2005. Water Resources Research, 43, doi:10.1029/2006WR005201. 16) Wickland, K.P., R.G. Striegl, J.C. Neff, and T. Sachs, 2006: Effects of permafrost melting on CO2 and CH4 exchange of a poorly drained black spruce lowland. Journal of Geophysical Research, 111, G02011, doi:10.1029/2005JG000099. 17) Harden, J.W., K.L. Manies, J.C. Neff, and M.R. Turetsky, 2006: Effects of wildfire and permafrost on soil organic matter and soil climate in interior Alaska. Global Change Biology, 12, 1-13, doi:10.1111 /j.1365-2486.2006.01255.x. 18) Carrasco, J.J., J.C. Neff, and J.W. Harden, 2006: Modeling physical and biogeochemical controls over carbon accumulation in a boreal forest soil. Journal of Geophysical Research, 111, G02004, doi:10.1029/2005JG000087. 19) CCSP, 2007: The North American Carbon Budget and Implications for the Global Carbon Cycle. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Dilling, L. and A. King (eds.)]. National Oceanic and Atmospheric Administration, Boulder, CO, USA, 260 pp. 20) Potter, C., S. Klooster, S. Hiatt, M. Fladeland, V. Genoveses, and P. Gross, 2007: Satellite-derived estimates of potential carbon sequestration through afforestation of agricultural lands in the United States. Climatic Change, 80, 323-336, doi: 10.1007/s10584-006-9109-3. 21) Asner, G.P., E.N. Broadbent, P.J.C. Oliveira, M. Keller, D.E. Knapp, and J.N.M. Silva, 2006: Condition and fate of logged forests in the Brazilian Amazon. Proceedings of the National Academy of Sciences, 103, 12947-12950. 22) Birdsey, R.A, 2006: Carbon accounting rules and guidelines for the United States forest sector. Journal of Environmental Quality, 35, 1518-1524. 23) Smith, J., E.L.S. Heath, K.E. Skog, and R.A. Birdsey, 2006: Methods for Calculating Forest Ecosystem and Harvested Carbon with Standard Estimates for Forest Types of the United States. Gen. Tech. Rep. NE-343. USDA Forest Service, Northeastern Research Station, Newtown Square, PA, 216 pp. 24) Reporting guidelines are available at the Energy Information Administration web site. 25) Hofmann, D.J., J.H. Butler, T.J. Conway, E.J. Dlugokencky, J.W. Elkins, K.A. Masarie, S.A. Montzka, R.C. Schnell, and P. Tans, 2006: Tracking climate forcing: The Annual Greenhouse Gas Index. EOS, Transactions of the American Geophysical Union, 87(46), 509-511.
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The air-sea exchange and storage of CO2 in the tropical and extra-tropical North Pacific Ocean play a key role in the global carbon cycle. Upwelling in the equatorial Pacific brings high carbon content water to the surface, generating the ocean's largest natural source of CO2 to the atmosphere. The tropical Pacific outgassing is well known to respond very sensitively to climatic fluctuations, particularly the El Niño Southern Oscillation (ENSO), with outgassing nearly vanishing during a fully developed El Niño. Modeling studies have suggested that fluctuations in the tropical Pacific dominate the global air-sea CO2 flux variability on interannual time scales. The most important results on surface ocean CO2 partial pressure (pCO2) and air-sea CO2 flux variability in the North Pacific to date are summarized in Figure 9. The small plots in each of the regions illustrate the observed trends in surface ocean pCO2 over the last 2 decades. Most regions show a steady increase at rates roughly the same as that of atmospheric pCO2 over the same time period. The two notable
exceptions are the far western subarctic region where surface water pCO2 has decreased with time, and the equatorial Pacific which shows a rate of pCO2 increase akin to that of atmospheric CO2 only after 1990, while oceanic pCO2 remained nearly constant before. The reasons behind these different trends are still under investigation, but they highlight the need for continuing observations of the oceanic carbon system. 
