Volume 285, Number 5427, Issue of 23 Jul 1999, pp. 544-545.
Copyright © 1999 by The American Association for the Advancement of Science.

BIOGEOCHEMICAL CYCLES:
The Not-So-Big U.S. Carbon Sink

Less than half of the carbon emitted to the atmosphere [HN1] through deforestation, fossil fuel combustion, and cement manufacture [HN2] remains in the atmosphere. The remainder of the carbon emitted through these human activities is stored, at least temporarily, in carbon sinks in the oceans and in terrestrial ecosystems [HN3]. Quantifying these sinks and understanding the underlying mechanisms are top priorities for understanding Earth's major biogeochemical cycles [HN4] and for establishing how changes in their magnitude could affect the future trajectory of atmospheric CO₂ concentrations [HN5].

Measured atmospheric CO₂, ¹³C, and O₂/N₂ distributions indicate that during the past two decades, a substantial fraction of the carbon sink has been on land, in the temperate and boreal latitudes of the Northern Hemisphere [HN6] (1). However, the mechanisms and the detailed spatial pattern of this Northern Hemisphere terrestrial sink remain elusive. On page 574 of this issue, Houghton et al. [HN7] (2) synthesize the information on a leading candidate--historical changes in land use--for the United States [HN8]. They conclude that during the 1980s, U.S. ecosystems [HN9] accumulated carbon at a rate of 0.15 to 0.35 Pg/year [petagrams (10¹⁵ grams) per year], equivalent to about 10 to 30% of U.S. fossil fuel emissions. This conclusion is consistent with atmospheric analyses (1), which indicate that there is a sink for carbon of about 2 Pg/year north of approximately 30°N (3), although these analyses were unable to constrain the longitudinal distribution of the sink. However, it contrasts sharply with the result of Fan et al. [HN10] (4), who suggest on the basis of atmospheric and oceanic data and modeling that the Northern Hemisphere carbon sink is predominantly North American, south of 51ºN, with a magnitude about that of U.S. fossil fuel emissions. The apparent contrast between the conclusions of these two studies highlights the differences between and uncertainties associated with atmospheric "top-down" and terrestrial ecosystem "bottom-up" approaches.

From the perspective of terrestrial processes, the list of candidate mechanisms for explaining the terrestrial sink is becoming longer but better quantified. Strictly biogeochemical mechanisms, such as increased plant growth in response to rising temperatures, atmospheric CO₂ concentrations, and nitrogen deposition [HN11], are clearly not the exclusive drivers. Elevated CO₂ concentrations and nitrogen deposition stimulate plant growth in many ecosystems (5). Measured responses to elevated CO₂ are generally not large enough, however, to explain the recent terrestrial sink, especially if this sink is concentrated in a small fraction of the land area (6). Recent experimental evidence indicates that elevated nitrogen deposition is also not likely to be a major contributor to the Northern Hemisphere carbon sink (7). Changes in ecosystems in response to past alterations in land management--including changes in timber harvesting, abandonment of agricultural land, and wildfire suppression--are emerging as additional explanations for the terrestrial U.S. carbon sink.

Changes in land management in the Northern Hemisphere are pervasive, with the impact of past changes often persisting for many decades. Until about 1960, cumulative carbon emissions from land-use modification were greater than those from fossil fuel combustion (8). Houghton et al. (2) have synthesized historical data on land use with a carbon cycle model (see the figure). According to their analysis, a substantial fraction of the current carbon sink in the Northern Hemisphere appears to be a result of past land management.

  Quantifying carbon sinks. Schematic diagram of three approaches to quantifying terrestrial carbon sinks that result, at least in part, from changes in land use and land cover.

CREDIT: M. MILLER/VISUAL UNLIMITED

The spatial resolution of these conclusions and the attribution of the sink to distinct processes are necessarily coarse. Other independent data on terrestrial sinks can come from national forest inventories [HN12]. Especially in economically developed regions, these inventories contain many thousands of samplings of nationwide forest resources, collected over several decades. These data, initially collected to quantify harvestable timber, can be analyzed to yield information about changes in forest carbon stocks (see the figure) (9). Data from experimental plots [HN13], especially long-term experimental plots in a range of managed and unmanaged ecosystems, are indispensable for improving carbon estimates from both land use and inventory approaches.

Atmospheric data such as the concentrations of CO₂ and O₂ are becoming increasingly useful for inferring terrestrial processes. Large-scale atmospheric measurements generally cannot distinguish between terrestrial mechanisms but rather allow estimates of the overall fluxes of the atmospheric compounds with limited spatial resolution (see the figure). The global network of monitoring stations provides reasonable resolution for interpretation of large latitudinal zones. Quantifying continental sources and sinks is much more technically challenging, a consequence of the small longitudinal gradients, the sparse monitoring network, and the limited capabilities of current atmospheric models. Fan et al.'s error analysis (4) clearly reveals this limit. Their central estimate assigns about 75% of the Northern Hemisphere sink to North America, but one standard deviation around this mean includes both 100% of the sink in North America and a 50-50 balance between North America and Eurasia. An increase in the number and accuracy of the stations that monitor the three-dimensional distribution of atmospheric CO₂, including vertical profiles and column totals, will be critical for assessments seeking greater spatial resolution. Accurate measurements of other atmospheric constituents, including ¹³C, ¹⁴C, O₂, and CO, will be increasingly important for interpreting measurements of CO₂ concentrations and validating hypotheses about carbon sinks.

The growing appreciation of the role of past changes in land management for terrestrial sources and sinks of carbon should stimulate a rethinking of the terrestrial carbon cycle. A substantial carbon source from forest clearing and a subsequent sink from land management have broad implications. Future research on the terrestrial sink may need to focus as sharply on the history of past management as on ecosystem responses to future changes in climate and atmospheric composition. In experimental studies on the carbon cycle, the interaction of global changes with land management should receive the same priority as the responses of natural ecosystems.

Sinks associated with recovery from past land management eventually saturate. The time until saturation is determined largely by the date at which forests started growing. In the United States, a substantial component of the current sink appears to result from long-term consequences of processes begun in the early decades of this century, which may be well on their way to saturation. This also means they are explicitly eliminated from consideration in the Kyoto Protocol [HN14] as sinks for carbon offsets, because the protocol counts only deliberate actions to reforest, establish new forest, or slow deforestation since 1990.

References and Notes

1. P. P. Tans, I. Y. Fung, T. Takahashi, Science 247, 1431 (1990); P. Ciais et al., ibid. 269, 1098 (1995); R. F. Keeling et al., Nature 381, 218 (1996) [GEOREF].
2. R. A. Houghton, J. L. Hackler, K. T. Lawrence, Science 285, 574 (1999).
3. The area of the United States is about 15% of the land from 30º to 70ºN.
4. S. M. Fan et al., Science 282, 442 (1998).
5. H. A. Mooney et al., in The Terrestrial Biosphere and Global Change: Implications for Natural and Managed Ecosystems, B. H. Walker, W. L. Steffen, J. Canadell, J. S. I. Ingram, Eds. (Cambridge Univ. Press, Cambridge, 1999), pp. 141-189 .
6. M. V. Thompson, J. T. Randerson, C. M. Malmström, C. B. Field, Global Biogeochem. Cycles 10, 711 (1996); P. Friedlingstein et al., Global Biogeochem. Cycles 9, 541 (1995).
7. K. J. Nadelhofer et al., Nature 398, 145 (1999).
8. R. A. Houghton et al., Ecol. Monogr. 53, 235 (1983); R. J. Andres, G. Marland, I. Fung, E. Matthews, Global Biogeochem. Cycles 10, 419 (1996).
9. P. E. Kauppi, K. Mielikäinen, K. Kuusela, Science 256, 70 (1992).

HyperNotes
Related Resources on the World Wide Web

General Hypernotes

The Carbon Dioxide Information Analysis Center (CDIAC), maintained by the Environmental Sciences Division of Oak Ridge National Laboratory, is the primary global-change data and information analysis center of the U.S. Department of Energy. A FAQ is provided.

The Carbon Cycle Research Program of the U.S. Geological Survey (USGS) is an interagency partnership that has the objective of developing a whole-system predictive capability for the global carbon system.

The Global Change Master Directory, provided by NASA Goddard Space Flight Center, provides links to Internet resources related to global change, Earth science, and environmental data and information.

The U.S. Global Change Research Program (USGCRP) focuses on the scientific study of the Earth system and its components. Our Changing Planet is a report issued annually about USGCRP. "Carbon cycle science: An FY 2000 initiative" is a section of the FY2000 report, which also includes an overview of the biology and biogeochemistry of ecosystems. A collection of links to global change-related Web sites is provided.

R. Ford, International Studies program, Westminster College, Salt Lake City, UT, offers Important WWW Sites for Earth System Science Education. A section on land use change is included.

E. Takle, Department of Geological and Atmospheric Sciences, Iowa State University, offers lecture notes and Web links for an Internet course on global change.

The Woods Hole Research Center offers a presentation on the global carbon cycle.

The 10 July 1998 issue of Science had a Perspective by P. P. Tans and J. W. C. White about the global carbon cycle titled "In balance, with a little help from the plants."

The executive summary of the 1998 report Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1996, made available by the Global Warming Web site of the U.S. Environmental Protection Agency (EPA), includes a section on carbon dioxide emissions. The full report is available in Adobe Acrobat format, as is the draft of the 1999 report.

The International Geosphere-Biosphere Programme (IGBP) is an interdisciplinary scientific activity established by the International Council for Science (ICSU) to study the scientific aspects of global change. Several IGBP publications are available online in Adobe Acrobat format.

The EUROFLEX Project, sponsored by the European Commission's Environment and Climate Program, is a study of the long-term carbon dioxide and water vapor fluxes of European forests and their interactions with the climate system.

AmeriFlux, a cooperative effort funded by U.S. federal agencies, is the carbon dioxide flux measurement network of North, South, and Central America.

The USGS Global Change Research Program provides a fact sheet titled "Can the global carbon budget be balanced?" about the research conducted by the USGS Mississippi Basin Carbon Project, which is designed to examine interactions between the global carbon cycle and human-induced changes to the land surface.

The NASA Goddard Institute for Space Studies offers an article by I. Fung titled "Oh where oh where does the CO₂ go?"

Numbered Hypernotes

1. R. Hellström, Atmospheric Sciences Program of the Department of Geography, Ohio State University, provides lecture notes on the composition of the atmosphere for a course on the atmospheric environment. The Global Climate Change Information Programme, offered by the Atmospheric Research and Information Centre, Manchester Metropolitan University, UK, provides a fact sheet titled "Greenhouse gases - sources, sinks and concentrations." E. Takle provides lecture notes on carbon dioxide in the atmosphere. The Carbon Cycle-Greenhouse Gases group of the NOAA Climate Monitoring and Diagnostics Laboratory provides a typical vertical profile of the three major carbon cycle trace gases taken over a Colorado site. CDIAC provides a table of current greenhouse gas concentrations in the atmosphere. The Intergovernmental Panel on Climate Change presents information about the greenhouse gas concentrations in its " Summary for policymakers: The science of climate change." The NASA Goddard Institute for Space Studies offers a presentation on tracking carbon dioxide emissions from fossil fuel burning.
2. The Information Unit on Conventions of the United Nations Environmental Programme provides a fact sheet about why cement-making produces carbon dioxide. The Greenhouse Gas R&D Programme of the International Energy Agency presents a paper titled "Emission reduction of greenhouse gases from the cement industry."
3. The United Nations Environmental Programme provides a fact sheet on the oceans and the carbon cycle. The contribution by J. Toggweiler titled "Anthropogenic CO₂: The natural carbon cycle reclaims center stage" in the U.S. National Report to IUGG, 1991-1994 reviewed research on the oceanic uptake of carbon. The Joint Global Ocean Flux Study (JGOFS) was established as an international interdisciplinary effort to plan and execute research on the ocean carbon cycle and its effect on climate; an introduction to JGOFS titled "Oceans, carbon, and climate change" appeared in Oceanography in the Next Decade, Building New Partnerships, a 1992 publication of the U.S. National Academy of Sciences. An information and data access page for the U.S. DOE Global Survey of CO₂ in the Oceans is maintained by CDIAC. An article by P. Falkowski, R. Barber, and V. Smetacek titled "Biogeochemical controls and feedbacks on ocean primary production," appeared in the 10 July 1998 issue of Science, which was a special issue on the chemistry and biology of the oceans. J. Adams, Environmental Sciences Division, Oak Ridge National Laboratory, TN, discusses carbon storage and presents an inventory of data on carbon storage in terrestrial ecosystems. A section on the terrestrial carbon cycles is included in an article by B. Walker and W. Steffen titled "An overview of the implications of global change for natural and managed terrestrial ecosystems" that appeared in Conservation Ecology (vol. 1, no.2, 1997). "Forests as carbon sinks, stores and sources" is a section of a report for the WWF-World Wide Fund For Nature by N. Dudley and A. Markham titled "Global Warming: Impacts on Forests."
4. W. Reeburgh, Earth Systems Science, University of California, Irvine, provides a collection of figures summarizing the global cycles of carbon and other biologically active elements. D. McShaffrey, Biology Department, Marietta College, OH, includes an introduction to biogeochemical cycles within ecosystems for a course on environmental biology. K. Paterson, Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, offers lecture notes on global biogeochemical cycles for a course on atmospheric physics and chemistry. The World Resources Institute provides an introduction to the global carbon cycle. D. McConnell, Department of Geology, University of Akron, OH, provides lecture notes on the global carbon cycle and greenhouse gases for a geology course. For a course on air chemistry and pollution, I. Sokolik, Atmospheric and Oceanic Sciences Department, University of Colorado, provides lecture notes on the global biogeochemical cycle of carbon.
5. B. Shakhashiri, Department of Chemistry, University of Wisconsin, provides an introduction to carbon dioxide. The About.com Guide to Chemistry offers a feature on carbon dioxide. The NASA Goddard Institute for Space Studies makes available an article by Q. Ma titled "Greenhouse gases: Refining the role of carbon dioxide." The Atmosphere-Biosphere Interactions project, a NASA Earth Observing System interdisciplinary effort, has as its goal the construction of a set of linked models that will model changes in the physical climate system and carbon cycle over the next 50 to 100 years in response to changes in atmospheric composition and land use. The EPA Global Warming Web site has a presentation about possible future trends in atmospheric change that includes a discussion of carbon dioxide.
6. An Introduction to Biomes by S. Woodward, Department of Geography, Radford University, VA, prepared for the Virtual Geography Department Project, provides information on boreal forests, temperate forests, and temperate grasslands. Carbon Storage and Accumulation in United States Forest Ecosystems, a 1992 report from the U.S. Department of Agriculture Forest Service, is available from the Renewable Resource Data Center in the Biomass Resource Information Clearinghouse. The Boreal Ecosystem-Atmosphere Study (BOREAS) was a large-scale international interdisciplinary experiment in the northern boreal forests of Canada to study interactions with the atmosphere, the storage of carbon dioxide, and the effects of climate change; additional information on BOREAS and the data collected are available from the Distributed Active Archive Center for Biogeochemical Dynamics maintained by Oak Ridge National Laboratory. The Woods Hole Research Center offers presentations on carbon and the changes in land use and the missing carbon sink.
7. R. A. Houghton and colleagues are at the Woods Hole Research Center, Woods Hole, MA.
8. Human Impacts on Land Use/Land Cover is an educational module from the Virtual Geography Department program; a collection of Web links on land use cover change and human dimensions of global change is provided. Consequences (vol. 1, no. 1, Spring 1995), available from the U.S. Global Change Research Information Office, had an article by W. Meyer titled "Past and present land use and land cover in the USA." Land Use History of North America, a project of the USGS Biological Resources Division, provides introductions to research on land use changes in the United States; an essay by S. Picket titled "Land use as an ecological variable" is included. The State of the Land Web site from Natural Resources Conservation Service of the U.S. Department of Agriculture provides an overview of land use in the United States and an index to available maps, facts, and figures on land use; a section on private forest land provides an index of maps, facts, and figures on private forest land use. Emissions of Greenhouse Gases in the United States 1997, a report from the U.S. Energy Information Administration, has a section on land use issues; the 1996 report is also available and includes a discussion on forests and the carbon budget in the chapter on land use issues.
9. The Commission for Environmental Cooperation offers a presentation on the ecological regions of North America.
10. S. Fan is in the Atmospheric and Oceanic Sciences Program at Princeton University; the Carbon Modeling Consortium offers a page with links to further information about this research. In the 16 October 1998 issue of Science, Joceyln Kaiser reported on Fan et al.'s research (4) in a news article titled "Possibly vast greenhouse gas sponge ignites controversy."
11. For a course on global change, E. Takle provides lecture notes on the responses of vegetation to a changing environment. B. Walker and W. Steffen discuss ecosystem responses to climate change in an article in Conservation Ecology titled "An overview of the implications of global change for natural and managed terrestrial ecosystems." A chapter on the responses of biota to climate change is included in the report titled "Climate change and the maintenance of conservation values in terrestrial ecosystems," which is available from the Environment Australia Web site. The Vegetation/Ecosystem Modeling and Analysis Project is an international effort addressing the response of biogeography and biogeochemistry to environmental variations in climate and other factors. The Ecological Society of America makes available a report titled "Human alteration of the global nitrogen cycle: Causes and consequences" by P. Vitousek et al.
12. The Forest Inventory and Analysis division of the U.S. Forest Service provides background information about the program; the data available are organized by regions. The Canadian Forest Service provides information about Canada's National Forest Inventory.
13. Research at the Harvard Forest, a department of the Faculty of Arts and Sciences of Harvard University, is aimed at understanding the historical and modern changes in the forests of central New England resulting from human and natural disturbance processes; the Environmental Measurement Site monitors the concentration of carbon dioxide and other trace gases. The H. J. Andrews Experimental Forest, located in the western Cascade Range of Oregon, provides a description of its research on carbon and nutrient dynamics.
14. An information sheet about the Kyoto Protocol and the text of the protocol (in Adobe Acrobat format) are provided by the United Nations Framework Convention on Climate Change. The Washington Post provides a copy of the full text of the Kyoto Protocol in a special report on climate change. A report from the World Resources Institute titled Climate, Biodiversity, and Forests: Issues and Opportunities Emerging from the Kyoto Protocol examines the role of forests and land-use change under the Kyoto Protocol and how forests and land-use change can both exacerbate and mitigate climate change. A policy article titled "The terrestrial carbon cycle: Implications for the Kyoto Protocol" by the IGBP Terrestrial Carbon Working Group appeared in the 29 May 1998 issue of Science; the Terrestrial Carbon Working Group provides additional information on the Web.
15. C. B. Field is in the Department of Plant Biology, Carnegie Institution of Washington. The Field Lab has a Web page.
16. I. Y. Fung is at the Center for Atmospheric Sciences, University of California, Berkeley.