The Global Carbon Cycle
USGCRP Recent Accomplishments

The Global Carbon Cycle

Overview

Recent Accomplishments

Near-Term Plans

Archived News Postings (June 2000 - July 2005)

Related Sites

Calls for Proposals

 

For long term plans, see Carbon Cycle Chapter of the draft Strategic Plan posted on web site of US Climate Change Science Program

Carbon Cycle Science Home Page

Additional Past Accomplishments:

Fiscal Year 2007

Fiscal Year 2006

Fiscal Year 2004-5

Fiscal Year 2003

Fiscal Year 2002

Fiscal Year 2001

Fiscal Year 2000

These accomplishments span carbon cycle issues related to climate forcing factors, terrestrial and oceanic sinks and sources, the atmospheric reservoir, global carbon analysis, carbon management, and other relevant biogeochemical exchanges between the major Earth reservoirs that link to climate. (as reported in the fiscal year 2009 edition of the annual report, Our Changing Planet).

Climate Forcing

Carbon dioxide (CO2) and methane (CH4) are significant forcing agents of climate, and their atmospheric concentrations have been increasing over the past 2 centuries, attributed primarily to human activities. Approximately 85 to 90% of present-day anthropogenic emissions are attributed to fossil fuel combustion, with land-use change accounting for most of the rest. Future concentrations of CO2 and CH4 in the atmosphere will depend on the long-term trends in terrestrial and oceanic reservoirs, the rate of exchange between Earth reservoirs, the variability in natural and anthropogenic emissions, and the capacity of natural and managed sinks.

Factors Affecting Fossil Fuel Emissions.1 An analysis was completed of factors that influence the magnitude, regional patterns, and temporal trends of global emissions of CO2 from fossil fuels, which dominate climate change forcing. The analysis included demographic, economic, and technological drivers of fossil fuel emissions by using annual time-series data on national emissions, population, energy consumption, and gross domestic product (GDP). Fossil fuel CO2 emissions can be represented as the product of four driving factors: global population, per capita world GDP, energy intensity of world GDP, and carbon intensity of energy. Results show that growth of global CO2 emissions since 2000 was driven by a cessation or reversal of earlier declining trends in the energy intensity of GDP and the carbon intensity of energy, coupled with continuing increases in population and per capita GDP. Nearly constant or slightly increasing trends in the carbon intensity of energy have recently been observed in both developed and developing regions.

Terrestrial Carbon Cycle

The terrestrial carbon and water cycles comprise a complex set of interactive biogeochemical 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 processes in the carbon cycle reduces uncertainty about the factors influencing greenhouse gas increases and provides a stronger foundation for climate change decision support, in particular for carbon management to mitigate CO2 and CH4 increases in the atmosphere.

Northern Hemisphere Terrestrial Carbon Sink Analysis.2,3 Temperate and boreal forests in the Northern Hemisphere act as a substantial carbon sink of 0.6 to 0.7 GtC yr-1, yet recent results from the AmeriFlux research network show that forest disturbance from harvest and fire are responsible for much of the overall variability in forest carbon sequestration. Forests are a carbon source to the atmosphere for as many as 20 years after these events, followed by a long period of carbon sequestration. Using results from observation networks in the United States and Europe, a recently completed Northern Hemisphere synthesis of net ecosystem exchange of CO2 from differently aged forests found that forest age, as a function of stand-replacing disturbances, accounted for approximately 90% of the total variability in net carbon sequestration. The average net carbon uptake over the harvest cycle of the forests was 56% of the maximum observed in mature forests. After accounting for age and disturbance effects (wildfires, harvesting, infestations, etc.), low continuous levels of nitrogen deposition (up to 10 kg N ha-1 yr-1 wet deposition), largely the result of anthropogenic activities, appear to overwhelmingly account for additional carbon sequestration by these forests (see Figure 12).

Figure 12: Distribution of AmeriFlux Clusters and Wet Nitrogen Deposition. Distribution of AmeriFlux sites (clusters) and wet nitrogen deposition across the United States used in the Northern Hemisphere analysis of the effects of nitrogen deposition on carbon sequestration by temperate and boreal forests. Credit: F. Magnani, University of Bologna, (reproduced from Nature with permission).

Carbon Distribution between Forest Root and Shoot Systems.4 One of the largest uncertainties in estimating changes in carbon stocks and understanding the effects of global change on forest carbon sequestration involves carbon allocation to coarse tree roots. Using an approach incorporating successional dynamics in plant communities and whole-tree harvesting including root excavation, a dynamic pattern of root-to-shoot ratios was revealed. Ratios varied from 0.17 for sites with trees less than 5 years old, to 0.80 for a site with 8-year old trees, to 0.29 for a site with 55-year old trees. Determining the causes of this variability in the root-to-shoot biomass allocation through forest maturation requires further research on how these patterns change as functions of growth, environment, and management. Carbon allocation in forest systems has important implications for projecting below ground net primary production responses to global change in studies of regional and continental carbon fluxes.

Carbon and Nitrogen Cycles of Terrestrial Ecosystems.5 One of the most central processes in the global carbon cycle is the breakdown of plant litter—dead leaves, branches, roots—that releases CO2 to the atmosphere and provides nitrogen in various forms that can support new plant growth. Understanding what controls the rate of breakdown and nitrogen release in different environments is critical for predicting the effects of climate change on the carbon balance of terrestrial ecosystems. The results of a 10-year experiment conducted at 21 sites showed that in all ecosystems except dry grasslands the amount of nitrogen released is controlled by the initial concentration of nitrogen in the litter and the mass remaining. The results were used to produce simple equations to predict nitrogen release. Because nitrogen is intimately involved in controlling both plant growth and litter breakdown, these equations will contribute to better models of the global carbon cycle.

Oceanic Carbon Cycle

The global ocean is a large and important carbon reservoir that regulates the uptake, storage, and release of CO2, CH4, and other climate-relevant chemical species to the atmosphere. The future biogeochemical behavior of this reservoir is uncertain because of potential anthropogenic impacts on many ocean processes, in particular the impact of ocean circulation on carbon exchange and the impact of ocean acidification on the physiology, function, and structure of the complex and diverse ocean ecosystem.

Atmospheric Impact on the Ocean Carbon Reservoir.6,7 The absorption of anthropogenic CO2 and the deposition of acid rain from fossil fuel and agriculture emissions can both contribute to the acidification of the global ocean, altering surface seawater acidity, and inorganic carbon storage. Researchers have compared these inputs and concluded that (1) acid rain contributes a minor amount (2%) of acidity compared to the ocean uptake of anthropogenic CO2, although this value likely represents an upper limit, and (2) the decrease in surface alkalinity from acid rain drives a net air-sea release of CO2, reducing surface dissolved inorganic carbon (DIC). Total alkalinity and DIC changes mostly offset each other, resulting in a small increase in surface acidity. Additional impacts arise from atmospheric nitrogen deposition, leading to elevated primary production and biological drawdown of DIC that in some places reverses the sign of the surface acidity and air-sea CO2 exchange. On a global scale, the alterations in surface water chemistry from anthropogenic deposition are a few percent of the acidification, although the impacts are more substantial in coastal waters, where the ecosystem responses to ocean acidification could have the most severe implications for humans.

High-Latitude Systems

High-latitude systems are becoming increasingly important sources of CO2 and CH4 to the atmosphere as regional warming changes ecosystem 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 reservoirs, ecosystems, land cover, hydrology, and climate variability.

Significance of Marginal Ice Zones.8 Within the Southern Ocean lie regions where biological dynamics are low, called High Nutrient, Low Chlorophyll (HNLC) regimes. When adjacent to seasonal sea-ice retreat, these marginal ice zones produce relatively high chlorophyll concentrations, indicative of phytoplankton production, extending thousands of kilometers to sea. These high chlorophyll anomalies are extremely variable temporally and spatially because the size and location of the marginal ice zone, defined as areas of recent ice melting and retreat, are highly variable between seasons and years. The production of phytoplankton biomass within this zone is an important food source for higher trophic levels and significantly affects carbon cycling in and across the region. The magnitude and distribution of these regimes with greatly elevated production were unknown prior to the satellite era. Since then the elevated chlorophyll and production associated with these regions have been documented with Sea-Viewing Wide Field-of- View Sensor (SeaWiFS) satellite data for the Southern Ocean, particularly within the marginal ice zone of the Southern Ocean where melting ice stabilizes the water column leading to shallower mixed layers and the release of critical elements, such as iron, to the water column. Both processes lead to production of phytoplankton, which have an unambiguous impact on carbon cycling and eventual export to the deep sea, where it remains sequestered for a very long time.

Permafrost Thaw Releases Additional Carbon and Water to Arctic Streams.9 Pursuing the hypothesis that permafrost thaw and increased infiltration could potentially increase terrestrial respiration of dissolved organic carbon (DOC) and decrease DOC export, researchers investigated historical stream flow records from the Yukon River Basin in Alaska and Yukon Territory with the goal of isolating and quantifying permafrost thaw and/or glacial and perennial snowpack melt effects on the basin water cycle. The analysis quantified a basin-wide increase in groundwater contribution to streamflow of 0.7 to 0.9% per year, but did not find any compelling evidence for a change in total annual water discharge by the basin’s rivers. The Yukon River annually discharges approximately 50 km3 of groundwater-derived flow to the Bering Sea. The increased groundwater contribution is consistent with the increased infiltration and DOC consumption hypothesis and supports load calculations that indicate a downward trend in the relationship between water and carbon export by the Yukon River during summer and autumn (see Figure 13).

Figure 13: Observed Trends in Groundwater Input and Annual Flow • Yukon River. Observed trends in groundwater input (denoted by left side of symbol) and annual flow (denoted by right side of symbol) at Yukon River Basin streamflow stations. Circles and squares indicate flow records longer and shorter than 35 years, respectively. Symbol color scheme indicates statistical significance of Mann-Kendall trend analysis: red = very highly significant (P<0.01) increasing trend, orange = highly significant (0.01<P<0.05) increasing trend, yellow = moderately significant (0.05<P<0.1) increasing trend, light blue = moderately significant (0.05<P<0.1) decreasing trend. Credit: M.A. Walvoord and R.G. Striegl, USGS (reproduced from Geophysical Research Letters with permission from the American Geophysical Union).

Wildfire Disturbances in High-Latitude Terrestrial Ecosystems.10,11,12 Wildfire is a common occurrence in northern high-latitude ecosystems, and the ecosystem changes have consequences for carbon feedback to the climate system. Researchers, using a processbased terrestrial ecosystem model, assessed the influence of increases in atmospheric CO2, climate variability and change, and change in fire disturbance on the exchange of CO2 and CH4 in high-latitude terrestrial ecosystems. Using historical fire records through 2002, model analysis indicates that fire played a central role in interannualand decadal-scale variation of carbon source and sink relationships of northern ecosystems and also suggests that increases in atmospheric CO2 may be important to consider in addition to changes in climate and fire disturbance. Model projections for northern terrestrial ecosystems indicate that these ecosystems could lose up to 50 GtC over the next 100 years and that net CH4 emissions could double by the end of the 21st century. These studies suggest that carbon storage in northern terrestrial ecosystems is vulnerable to projected changes in climate and fire disturbance.

Global Carbon Analysis

Ocean phytoplankton and land plants are presently absorbing about half the carbon emissions that humans produce. However, recent global carbon analyses indicate that Earth’s reservoirs are becoming less efficient at absorbing fossil fuel emissions and losing their ability to take up additional CO2.

Carbon Cycle Feedbacks to Climate.13 How carbon cycle dynamics will change with climate and feed back to atmospheric CO2 concentration is not fully understood. Increases in plant growing season length are hypothesized as contributing factors to the current observed terrestrial carbon sink. An analysis of growing season variation of CO2 exchange for a range of vegetation sites, from evergreen and deciduous forests to crop to grasslands and including both cool-season and warm-season vegetation types, found that while the growing season length affected how much CO2 could be potentially assimilated by a plant community over the course of a growing season, other factors such as nutrient and water availability were also important at this scale. This implies that the climate warming-induced increase in growing season length may have a limited enhancement effect on terrestrial carbon uptake.

Amazon Forests May be More Resilient than Predicted.14 Coupled carbon-climate models predict substantial carbon loss from tropical ecosystems and drought-induced collapse of the Amazon forest. These models include a physiological feedback mechanism whereby transpiration is reduced in response to initial drought, which in turn exacerbates the drought by interrupting the supply of transpired water that would otherwise contribute to “recycled” precipitation that supports forest growth. Satellite observations of intact Amazon forests subjected to a widespread and severe drought in 2005 show that the response of the forest is actually to green up, which is indicative of increased transpiration and carbon uptake. Apparently, these deep-rooted forests are responding to the increased availability of sunlight and not to water limitations. These observations suggest that intact Amazon forests may be more resilient than some current models assume, at least to short-term climatic anomalies. Future studies are needed to address forest responses to longer term drought in order to better understand the conditions under which water limitations may actually trigger reductions in carbon uptake (see Figure 14).

Figure 14: 2005 Drought in the Amazon. During the 2005 drought in the Amazon, intact primary forest showed an increase in photosynthetic activity (right image) despite below-average rainfall (left image). Data from NASA's Terra satellite (right) showed areas of higher (green) and lower (red) growth during the peak of the drought (July-September). Data from the Tropical Rainfall Measuring Mission satellite (left) showed areas of severe rainfall reduction due to the drought (yellow to red) and few areas with above normal rainfall (green to blue). Credit: K. Didan, University of Arizona.

Carbon Management and Decision Support

Carbon cycle research provides scientific information to policy and resource decisions about carbon management and mitigation of climate change. The research supported by the carbon cycle program is informing agricultural and resource managers on sequestration, alternative fuels, and inventories, as described in the highlights below. The impact of this research on management strategies is expected to increase over the course of this program.

Soil Carbon Sequestration in Agricultural Lands.15,16 Based on in situ soil carbon concentrations, crop growth characteristics, tillage practices, land-use classification using satellite imagery, and climate variables, a geographic information system (GIS)-based Environmental Policy Integrated Climate (EPIC) model projected the amounts of soil carbon sequestered for an agricultural region in sub-Saharan Africa (Mali). Under contemporary ridgetillage management practices, year-to-year crop variations were attributed primarily to rainfall, the amount of plant-available water, and the amount of fertilizer applied. Under conventional cultivation, with minimal fertilization and no crop residue management, topsoil was continuously lost due to erosion. The model projections suggest that soil erosion is controlled and soil carbon sequestration is enhanced with a ridge-tillage system because of increased water infiltration. The combination of modeling with landuse classification was used to calculate that about 54 kg C ha-1 yr-1 (5.4 g C m2 yr-1) may be sequestered in the study area with ridge tillage, increased application of fertilizers, and residue management. The EPIC model is now incorporated in a web-based decision-support system for soil carbon management.

Biofuels from Prairie Grasses.17 The search for alternatives to fossil fuels has attracted attention to biofuels (fuels derived from plants) and especially to corn as a source of ethanol. A study in Minnesota showed that mixtures of native prairie plants, grown on degraded land, may be a better source of biofuel than corn ethanol or soybean biodiesel. Prairie vegetation yields 51% more energy per hectare than is obtained in ethanol from corn grown on fertile land. The higher net energy gain is due in part to much lower inputs such as cultivation, herbicides, irrigation, and fertilizer, as well as the use of the entire above-ground plant rather than just the seed. The prairie vegetation also stores more carbon over time in soils than do systems growing annual crops, thus removing CO2 from the atmosphere. Low-input, high-diversity prairie vegetation grown on degraded lands can serve as an efficient source of energy and does not compete for fertile soils needed for food production.

An Assessment of the North American Carbon Budget.18 An evaluation of North American carbon sources and sinks was generated as part of CCSP’s Synthesis and Assessment Product 2.2, The First State of the Carbon Cycle Report (SOCCR): North American Carbon Budget and Implications for the Global Carbon Cycle. The report quantifies North America’s fossil fuel emissions for 2003 as 1,856 million metric tons of carbon ±10% (27% of global emissions) and its land sink, primarily in U.S. forests, as 500 million metric tons of carbon ±50% (approximately 30% of North America’s emissions). The net release to the atmosphere is 1,350 million metric tons of carbon per year ±25% (see Figure 15). A key finding of the report is that this difference between the sources and sinks is expected to become larger, and that actions to address the imbalance will likely require options that include both emissions reduction and sink enhancement.

Figure 15: North American Carbon Sources and Sinks. North America is currently a net carbon source (1,336 ± 334 MtC yr-1). A net terrestrial sink of 520 ± 260 MtC yr-1 is equivalent to about 30% of fossil fuel emissions in 2003. Credit: CCSP Synthesis and Assessment Product 2.2.

North American Carbon Exchange.19 A CO2 reanalysis system, CarbonTracker, was used to determine global surface sources and sinks of CO2. The reanalysis extends from 2000 to 2006 (see <carbontracker.noaa.gov>). Weekly estimates of surface fluxes were produced for 221 land and ocean regions. This advanced data assimilation scheme is focused on relatively well-observed regions, and uses a two-way, nested transport model over North America to simulate circulation at relatively high resolution. The system is capable of handling large amounts of data, and in the most recent analysis over 28,000 individual atmospheric CO2 observations from the U.S. and Canadian Meteorological Services were assimilated. North American land regions were determined to be a net sink of CO2, with a mean annual uptake of 0.6 ± 0.4 GtC yr-1 and interannual variations of about 0.3 GtC yr-1. This offsets almost one-third of the approximately 1.8 GtC yr-1 emissions from fossil fuel burning across North America during the same period.

Additional Past Accomplishments:

• Fiscal Year 2007
• Fiscal Year 2006
• Fiscal Year 2004-2005
• Fiscal Year 2003
• Fiscal Year 2002
• Fiscal Year 2001
• Fiscal Year 2000

GLOBAL CARBON CYCLE CHAPTER REFERENCES

1)  Raupach, M.R., G. Marland, P. Ciais, C. Le Quere, J.G. Canadell, G. Klepper, and C.B. Field, 2007: Global and regional drivers of accelerating CO2 emissions. Proceedings of the National Academy of Sciences, 104, 10288-10293.

2)  Magnani, F., M. Mencuccini, M. Borghetti, P. Berbigier, F. Berninger, S. Delzon, A. Grelle, P. Hari, P.G. Jarvis, P. Kolari, A.S. Kowalski, H. Lankreijer, B.E. Law, A. Lindroth, D. Loustau, J. Manca, J. Moncrieff, M. Rayment, V. Tedeschi, R. Valentini, and J. Grace, 2007: The human footprint in the carbon cycle of established temperate and boreal forests. Nature, 447, 848-850.

3)  Irvine, J., B.E. Law, and K. Hibbard, 2007: Post-fire carbon pools and fluxes in semi-arid ponderosa pine in Central Oregon. Global Change Biology, 13, 1-13.

4)  King, J.S., C.P. Giardina, K.S. Pregitzer, and A.L. Friend, 2007: Biomass partitioning in red pine (Pinus resinosa) along a chronosequence in the Upper Peninsula of Michigan. Canadian Journal of Forest Research, 37, 93-102.

5)  Parton, W., W.L. Silver, I.C. Burke, L. Grassens, M.E. Harmon, W.S. Currie, J.Y. King, E.C. Adair, L.A. Brandt, S.C. Hart, and B. Fasth, 2007: Global-scale similarities in nitrogen release patterns during long-term decomposition. Science, 315, 361-364.

6)  Bates, N.R. and A.J. Peters, 2007: The contribution of acid deposition to ocean acidification in the subtropical North Atlantic Ocean. Marine Chemistry, 107, 547-558.

7)  Doney, S.C., N. Mahowald, I. Lima, R.A. Feely, F.T. Mackenzie, J.-F. Lamarque, and P.J. Rasch, 2007: The impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system. Proceeding of the National Academy of Sciences, 104, 14580-14585, doi:10.1073/pnas.0702218104.

8)  Fitch, D.T. and J.K. Moore, 2007: Wind speed influence on phytoplankton bloom dynamics in the Southern Ocean Marginal Ice Zone. Journal of Geophysical Research, 112, C08006, doi:10.1029/2006JC004061.

9)  Walvoord, M.A. and R.G. Striegl, 2007: Increased groundwater to stream discharge from permafrost thawing in the Yukon River basin: Potential impacts on lateral export of carbon and nitrogen. Geophysical Research Letters, 34, L12402, doi:10.1029/2007GL030216.

10)  Zhuang, Q., J.M. Melillo, M.C. Sarofim, D.W. Kicklighter, A.D. McGuire, B.S. Felzer, A. Sokolov, R.G. Prinn, P.A. Steudler, and S. Hu, 2006: CO2 and CH4 exchanges between land ecosystems and the atmosphere in northern high latitudes over the 21st century. Geophysical Research Letters, 33, L17403, doi:10.1029/2006GL026972.

11)  Zhuang, Q., J.M. Melillo, A.D. McGuire, D.W. Kicklighter, R.G. Prinn, P.A. Steudler, B.S. Felzer, and S. Hu, 2007: Net emissions of CH4 and CO2 in Alaska: Implications for the region&rsquo;s greenhouse gas budget. Ecological Applications, 17, 203-212.

12)  Balshi, M.S., A.D. McGuire, Q. Zhuang, J.M. Melillo, D.W. Kicklighter, E.S. Kasischke, C. Wirth, M. Flannigan, J. Harden, J.S. Clein, T.J. Burnside, J. McAllister, W.A. Kurz, M. Apps, and A. Shvidenko, 2007: The role of fire disturbance in the carbon dynamics of the pan-boreal region: A process-based analysis. Journal of Geophysical Research, 112, G02029, doi:10.1029/2006JG000380.

13)  Gu, L., W.M. Post, D. Baldocchi, T.A. Black, A.E. Suyker, S.B. Verma, T. Vesala, and J.W. Munger, 2008: Characterizing the seasonal dynamics of plant community photosynthesis across a range of vegetation types. In: Phenology of Ecosystem Processes [Noormets, A. and L. Gu (eds.)]. Springer Science + Business Media, New York, NY, USA, in press.

14)  Saleska, S.R., K. Didan, A.R. Huete, and H.R. da Rocha, 2007: Amazon forests green-up during 2005 drought. Science, 318, 612, doi:10.1126/science.1146663.

15)  Daughtry, C.S.T., P.C. Doraiswamy, E.R. Hunt Jr., A.J. Stern, J.E. McMurtrey III, and J.H. Prueger, 2006: Assessing crop residue cover and soil tillage intensity. Journal of Soil and Tillage Research, 91, 101-108.

16)  Doraiswamy, P.C., G.M.McCarty, E.R. Hunt, R.Yost, M. Doumbia, and A.J. Franzluebbers, 2007: Modeling of soil carbon sequestration in agricultural lands of Mali. Agricultural Systems, 94(1), 63-74.

17)  Tilman, D., J. Hill, and C. Leyman, 2006: Carbon-negative biofuels from low-input high-diversity grassland biomass. Science, 314, 1598-1600.

18)  CCSP, 2007: The First State of the Carbon Cycle Report (SOCCR): 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 [King, A.W., L. Dilling, G.P. Zimmerman, D.M. Fairman, R.A. Houghton, G. Marland, A.Z. Rose, and T.J. Wilbanks (eds.)]. National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC, USA, 242 pp.

19)  Peters, W., A.R. Jacobson, C. Sweeney, A.E. Andrews, T.J. Conway, K. Masarie, J.B. Miller, L.M.P. Bruhwiler, G. P&eacute;tron, A.I. Hirsch, D.E.J. Worthy, G. van der Werf, J.T. Randerson, P.O. Wennberg, M.C. Krol, and P.P. Tans, 2007: An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker. Proceedings of the National Academy of Sciences, 104, 18925-18930.