Updated 12 October, 2003
in Carbon Sources and Sinks: The Outlook for Climate Change and Managing
Carbon in the Future
USGCRP Seminar, 8 December 1998
Dr. Pieter Tans
Dr. Jorge L. Sarmiento
Dr. William H. Schlesinger
Natural Variations of the Global Carbon Cycle and Human Influence
The unrestrained burning of the Earth's conventional fossil fuel resources during the next few hundred years has the potential to increase the atmospheric CO2 concentration to more than ten times the pre-industrial concentration of CO2. The main difference between a rapid burning of fossil fuels and a scenario in which CO2 emissions would be capped at 1990 rates is that the latter affords more time to develop alternative sources of energy before the concentration of atmospheric CO2 can reach such high levels. In terms of climate change, such an atmospheric loading would produce a human-induced climate forcing of more than 12 watts per square meter (W/m2), roughly five times the present forcing of climate (warming) due to the human-induced enhancement of greenhouse gas concentrations. This estimate assumes that the rest of the climate system (water vapor, clouds, snow and ice cover, etc.) remains unchanged. Climate feedbacks would probably amplify or possibly reduce this figure to some extent, thus the role and magnitude of various feedbacks are the subjects of on-going inquiry and discussion. Currently, CO2 alone is responsible for a climate forcing (warming) of 1.4 W/m2, while the combination of all long-lived greenhouse gases account for a climate forcing of about 2.3–2.5 W/m2, and this level is growing. For comparison, the estimated possible changes in solar output during the last few centuries are up to 0.5 W/m2. Increased CO2 has not only atmospheric effects, but such elevated levels of atmospheric CO2 will also significantly alter the chemistry of sea water, rendering it more acidic for example.
The fact remains that the concentration of atmospheric CO2 will continue to rise even if society elects to keep global CO2 emissions constant indefinitely. Nature redistributes carbon among the three reservoirs, and it is very likely that the concentration of atmospheric CO2 will remain elevated for hundreds of years and probably thousands of years. In addition, the climate itself will influence how carbon is partitioned among the three reservoirs. In a warmer world, for example, Arctic permafrost soils may thaw, releasing large quantities of CO2 to the atmosphere.
A number of independent lines of evidence support the idea of the existence today of large terrestrial sinks of CO2. The mechanisms by which these sinks operate are not yet sufficiently understood, thus there is uncertainty as to how such sinks will operate in the future under an altered climate. There is uncertainty as well as to the permanency of carbon sinks. For example, present and future sinks for CO2 might become sources of CO2 if the climate were to change significantly. However, the existence of such large sinks for CO2 also suggests that there might be ways to manage natural systems to store additional carbon, therefore mitigating climate warming to an as yet unknown extent.
Of the carbon not taken up by terrestrial ecosystems, the ocean will be the eventual repository for about 85% of the rest of the carbon emitted to the atmosphere by human activities. This estimate of uptake capacity is based on well-understood physical and chemical processes. However, this uptake occurs quite slowly. For example, the ocean is presently taking up only 40% (with an uncertainty of +/-16%) of the annual anthropogenic carbon emissions not removed by terrestrial processes. Because of the slow rate of mixing of the ocean, it would take many centuries for the ocean to realize most of its uptake capacity, even if anthropogenic emissions were to stop today. An additional 5-10% of the anthropogenic atmospheric carbon emitted today will be taken up by the ocean by the reaction of excess oceanic CO2 with limestone in ocean sediments, but this occurs on a time scale of millenia.
Calculations of the rate of uptake of anthropogenic carbon by the oceans require the use of carefully formulated ocean circulation models. A major contribution to our confidence in such models is our ability to calibrate them using observations of a wide variety of tracers of ocean circulation, as well as direct observations of ocean carbon inventory changes. However, future predictions must take into account the warming of ocean waters (which reduces the oceans capacity to absorb CO2 and other gases), reductions in ocean mixing (which reduces the rate at which the oceans can absorb CO2 and other gases), and changes in biology that will likely take place as a result of global warming. A number of recent studies with coupled atmosphere/ocean models of climate suggest that the impact of global warming will likely reduce the oceans capacity to absorb CO2 and other gases by about 10-30% by the middle of the next century, but further research is needed. A major concern raised by such models is the possibility that environmental changes will also have a substantial impact on ocean ecology.
Models of the ocean carbon system are also required in order to estimate the possible contribution of the ocean to absorb atmospheric carbon and thereby moderate future atmospheric growth rates. Such models indicate that the efficiency of permanent sequestration in reducing the atmospheric increase in CO2 between now and the middle of the next century would be about 75% if only the ocean is considered, but may drop to only about 60% using one particular model of the terrestrial biosphere. The ocean's efficiency to absorb atmospheric CO2 is also sensitive to the model scenario utilized to project the ocean's capacity to sequester carbon; the scenario used in this analysis is a gradually increasing rate of sequestration over the time period considered. If, for example, sequestration is confined to the ocean alone, some of the sequestered CO2 will eventually escape to the atmosphere, reducing the efficiency of the ocean to absorb CO2 even more, although this effect can be minimized by sequestering the CO2 in waters that are relatively isolated from mixing.
A major focus of recent ocean carbon research has been aimed at determining the spatial and temporal distribution of exchanges of CO2 between the atmosphere and the ocean. Such research is essential for understanding the basic mechanisms that determine the cycling of carbon within the ocean, and also for combining with atmospheric CO2 observations to constrain the location of the terrestrial carbon sink. One recent estimate of the terrestrial carbon sink suggests that there was a surprisingly large uptake of CO2 in North America during the period from 1988 to 1992. This study also identified additional measurements that would help to improve the confidence in such estimates and improve their spatial resolution.
At the present time, two processes may lead to a net storage of carbon in vegetation and soils. The first process is the regrowth of vegetation on land that is abandoned from agriculture. The second is the likely positive response of plants to fertilization because of the rising CO2 concentration and the deposition of atmospheric nitrogen. Because CO2 is one of the primary reactants for photosynthesis, a rising CO2 concentration may stimulate the rate of photosynthesis and plant growth wherever vegetation occurs. An important sink for carbon in North America derives from the regrowth of plants on abandoned agricultural land. Despite hundreds of studies in greenhouses, however, much less is known about the potential sink for carbon due to the direct plant response to CO2 in natural ecosystems. And, there is good reason to suspect that plant responses in simulated greenhouse conditions may differ from those found in nature, where soil, water, and nutrients may be in short supply, thus limiting growth. Understanding the response of vegetation to rising CO2 is a critical component of global change research, because the sink for carbon on abandoned land is finite; it will cease when the vegetation has matured on these lands.
A recent experiment was undertaken to study the response of vegetation to rising atmospheric CO2 concentrations using a technique known as Free Air CO2 Enrichment (FACE), developed at Brookhaven National Laboratory. FACE experiments allow one to expose large plots of forest, desert, grassland, or other vegetation to constant, high levels of CO2 with minimal disruption of light, microclimates, and soil conditions that often determine the growth of plants. In the Duke Forest in central North Carolina, a FACE experiment was initiated in late 1996 to examine the growth of 15-year-old loblolly pine trees when subjected to a CO2 concentration of 560 ppm—the concentration that is anticipated globally, as early as the middle of the next century. Loblolly pine was selected because it is one of the fastest growing tree species and should, therefore, exhibit a significant growth response under conditions of elevated CO2.
The first year (1997) of this experiment witnessed a 12% increase in the growth of these trees above ground. Preliminary results for the second year (1998) of this experiment show a lesser increase in growth rate, but the trees subjected to high CO2 remain larger than the trees in the control plots. The proportional response below ground was somewhat larger, although the absolute amount of plant tissue below ground is rather small. By July 1998, investigators found 68.9 gC/m2 in the production of live and dead roots in control plots and 109.2 gC/m2 in experimental plots. These results suggest that it is the plant tissues above ground, rather than soils, that are more likely to act as a sink for atmospheric CO2.
If this kind of response were to apply to forested land globally, calculations indicate that about 20% of the fossil fuel use expected in the year 2050 might be stored in forests and their soils. While helpful to the problem, this storage (about 3 GtC/yr) is much smaller than many have speculated; thus, a large fraction of the CO2 released from fossil fuel combustion may well remain to accumulate in the atmosphere. Moreover, it is likely that the response of vegetation will decline with time, as soil nutrients become depleted. For example, the growth of trees near some CO2-emitting springs in Italy is stimulated only during the first few years of their life.
In considering potential risks of CO2 build-up, decisionmakers might well anticipate other large changes in the terrestrial biosphere as the climate changes. For example, should rising CO2 concentrations lead to a significant global warming, particularly in northern ecosystems, soils may become a large additional source of CO2 to the atmosphere due to more rapid rates of decomposition in warmer soils. Recent observations of spruce trees in Manitoba, Canada, show a much greater emission of CO2 from soils in warmer years, such that the forest is a net source of CO2 to the atmosphere adding to the concentration of greenhouse gases already present.
Dr. Pieter Tans is Chief Scientist at the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration in Boulder, Colorado. He has researched the global carbon cycle for several decades, starting with his Ph.D. dissertation research in the Netherlands, and has published about 100 scientific papers on the subject. His group maintains the world's largest global monitoring network of atmospheric greenhouse gas concentrations. Isotopic ratios of several of the greenhouse gases are also measured. From these data temporal trends and large-scale spatial patterns are derived of the sources and sinks of greenhouse gases such as carbon dioxide and methane. The latter effort requires the use of atmospheric and chemical circulation models.
Dr. Tans has served on the Committee on Oceanic Carbon and the Panel on Climate Variability on Decade to Century Time Scales, of the National Research Council. He is also a member of the interagency Carbon and Climate Working Group, associate editor of the Journal of Climate, and a member of the editorial board of Tellus.
Dr. Jorge L. Sarmiento is a Professor of Geological and Geophysical Sciences at Princeton University, Princeton, New Jersey. Dr. Sarmiento's primary research interests are focused on oceanic cycles of climatically important chemicals such as carbon dioxide, and on the use of chemical tracers to study ocean circulation. He has published widely on ocean tracers and the ocean carbon cycle, its history, its ongoing and potential future perturbations by mankind, and its relationship to climate change.
Ongoing research includes the use of ocean general circulation models (GCMs) to estimate uptake of anthropogenic CO2, and the use of atmospheric general circulation models constrained with atmospheric CO2 observations to estimate transport of CO2 in the atmosphere and carbon sinks in the terrestrial biosphere. Dr. Sarmiento is working in conjunction with ocean biologists to develop ecosystem models for predicting photosynthetic uptake of carbon in the surface ocean, as well as remineralization of organic matter in the deep ocean. He is also using coupled atmosphere-ocean models of climate warming to study the impact of anthropogenic climate warming on the global carbon cycle.
Dr. Sarmiento has participated in the scientific planning and execution of many of the large-scale, multi-institutional, and international oceanographic, biogeochemical, and tracer programs of the last two decades. He has served on the Climate Research Committee and Committee on Oceanic Carbon of the National Research Council, as well as on the Advisory Committee of Ocean Sciences of the National Science Foundation. He is the Project Coordinator of the Carbon Modeling Consortium, a multi-institutional program to study the anthropogenic carbon transient; he is co-chair of the Synthesis and Modeling Project of the Joint Global Ocean Flux Study; and he is co-chair of the Carbon and Climate Working Group that is presently putting together a plan for U.S. carbon cycle research.
Dr. William H. Schlesinger is James B. Duke Professor in the Department of Botany at Duke University, and he holds a joint appointment in the Division of Earth and Ocean Sciences of the Nicholas School of the Environment. Upon completing his A.B. degree at Dartmouth (l972) and his Ph.D. at Cornell (l976), he joined the faculty at Duke University in 1980. He is the author or co-author of over 120 scientific papers and the widely adopted textbook "Biogeochemistry: An Analysis of Global Change" (Academic Press, 2nd ed. l997). He was also elected as a member of the American Academy of Arts and Sciences in 1995.
Currently, Dr. Schlesinger's research is focused on the role of soils in the global carbon cycle. He is the principal investigator for the Free Air CO2 Enrichment (FACE) Experiment in the Blackwood Division of the Duke Forest—a project that aims to understand how an entire forest ecosystem (vegetation and soils) may likely respond to elevated CO2 concentrations. He has also worked extensively in desert ecosystems and their response to global change, and is currently serving as Principal Investigator for the NSF-sponsored program of Long Term Ecological Research (LTER) at the Jornada Experimental Range in southern New Mexico.
Dr. Schlesinger has testified before U.S. House and Senate Committees on a variety of environmental issues.