Distribution of the mean annual sea-air pCO2 flux (partial pressure of carbon dioxide, moles CO/m2/yr) over the global oceans estimated for a reference year 1995. These data show the global pattern of surface CO2 uptake and release by the ocean. Note that the major sink regions are the North Atlantic and Southern Oceans, and that the Equatorial Pacific is a large source region in a typical year. This map does not reflect the variability due to El Niño/Southern Oscillation (ENSO) cycles, for example, which can alter the size of the ocean sink on an interannual basis. This map has been constructed based on about 2 million measurements of sea-air pCO2 difference made over the past 25 years. These values have been corrected to a reference year of 1995 for the increase in pCO2 of the atmosphere and surface ocean water that has occurred since the measurements were made, and the measurements made during El Niño years in the equatorial Pacific have been excluded. Thus, the map represents a climatological mean for non-El Niño conditions. The net CO2 flux across the sea surface has been computed using the effect of wind speed on the CO2 gas transfer coefficient formulation by Wanninkhof (Equation 1, 1992) and the mean monthly wind speed of Esbensen and Kushnir (1981). The numerical method used for the construction of these maps has been described in Takahashi et al.(1997) and Takahashi et al.(in press). The map yields an annual CO2 flux for the oceans of 2.2 PgC/yr, in which the North Atlantic (N of 14°N) and the Southern Ocean (S of 50°S) are major CO2 sink areas taking up 0.8 and 0.6 PgC/yr respectively. et al. 1994, Feely et al.1996), and the ongoing feasibility studies to instrument the ATLAS moorings of the TropicalAtmosphere Ocean (TAO) array with CO2 sensors are an important development (Friederich et al. 1995). There are also two time-series measurements near Bermuda and Hawaii (e.g.,Bates et al. 1996, Winn et al.1994). However, most of the ocean is unknown with regard to the temporal variability of CO2 fluxes. Analyses of stable carbon isotopes in atmospheric CO, suggest that sinks in both the ocean and the terrestrial biosphere vary by large amounts from year to year (Keeling et al.1989, Keeling et al.1995a, Ciais et al. 1995b, Francey et al. 1995). No oceanic mecha nism has been developed to explain such conspicuous variability. A related need is for the calculation of temporal changes in oceanic carbon uptake on the global scale. The global oceanic uptake rate of CO2 is currently not known to better than about ± 40 percent. This knowledge is not sufficient to determine whether the ocean carbon uptake rate has increased or decreased over the past few decades. Moreover, uncertainties of this magnitude limit the ability to constrain the historical global CO2 budget using the record of atmospheric CO2 concentra tions over the last 200 years. However, the problem of understanding the ocean should not be viewed as simply a comparison of current and future DIC "snapshots." It is also important to identify and understand the mechanisms that might cause future changes in the ocean carbon sink. One of the most important issues to address is which mechanisms of ocean circulation that significantly shape anthropogenic CO2 uptake are likely to be affected by a changing climate? Changes in ocean circulation resulting from climate change will immediately affect the way anthropogenic A US Carbon Cycle Science Plan CO2 is exchanged between the atmosphere and the ocean. The behavior of the present and past ocean, including the response to temporal variability, provides the strongest clues to the links between ocean circulation and atmospheric CO2. An additional question is how is the biological pump affected by the thermohaline circulation and changing climate? And is there any evidence that the C/N and C/P ratios of marine production are changing,or that "preformed" nutrients (the nutrient concentration of water that has cooled and sunk to depth) are changing? CO2 exchange between the ocean and atmosphere naturally occurs at the ocean surface. Photosynthesis by organisms in the upper, sunlit layer of the ocean keeps the CO2 concentration of the surface waters substantially lower than that of deep waters. It does so by producing organic carbon that is exported to the deep ocean where it is converted to DIC. The excess deep ocean DIC that thus results is analogous to the organic carbon stored in soils, in that it is isolated from the atmosphere. Without photosynthesis in the ocean, and assuming no other sur face changes due to organisms (such as calcification), the excess deep ocean DIC would escape to the atmosphere and atmospheric CO2 concentration would be between 900 and 1,000 ppm (the current value, again, is 365 ppm). If, on the other hand, photosynthesis continued everywhere until all of the plant nutrients were fully depleted in all surface waters,atmospheric CO2 would be between 110 and 140 ppm. (The actual pre-industrial atmospheric CO2 concentration was 280 ppm.) These figures indicate the great power of the oceanic biological pump. Long-term surface ocean time series that include detailed biogeochemical and CO2 measurements are improving the mechanistic understanding of processes that affect ocean atmosphere carbon uptake and parti tioning. Time-series data sets from Bermuda and Hawaii are being used to develop model parameterizations of ocean biogeochemical processes affecting the ocean carbon cycle (Doney et al. 1996, Fasham 1995). Such time series are also increasingly being used as test beds for novel high-resolution measurement instruments. Notable findings from the time-series sites include an increased awareness of the complexity of the ocean's nitrogen cycle (Karl et al. 1997). This finding has the potential to alter the view of the sensitivity of atmospheric CO2 concentrations to biological processes in the ocean (Falkowski 1997). On longer time scales, ocean biology can have an impact on the atmosphere (see the following chapter). The time-series sites are among the few locations where seasonal variations in upper ocean 13C/12C ratios are measured (Bacastow and Keeling 1973). Such time-resolved information is critical for correctly interpreting the large "snapshot" data sets collected along ocean transects. To summarize, our current understanding is that, as atmospheric CO2 levels increase through time, the ocean responds by dissolving more CO2 in the surface mixed layer, and by mixing the CO2-enriched surface waters downward through exchange with deeper waters. The possibility also exists that changes in the biological pump may affect the future air-sea balance of CO2. If our pres ent understanding of these processes is correct,the ocean should have the capacity to absorb large quantities of anthropogenic CO2 over time scales of decades to millennia. Most model simulations of future atmospheric CO2 levels assume that present-day factors controlling plant physiology, air-sea exchange, and ocean mixing will remain constant into the future (Schimel et al. 1996). However, as previously noted, these assumptions have been questioned, especially because of the potential for significant responses to climate change. Air-sea gas exchange, ocean circulation, and marine photosynthesis are susceptible to changes in air temperatures, wind velocities, sea surface roughness, and wind and precipita tion patterns. The most powerful tool for understanding the mechanisms underlying potential changes is in studying long-term trends and shorter-term fluctuations. Given all these considerations, a major CCSP initiative in ocean carbon cycle research is proposed to achieve the following: One of the main driving factors in determining the Northern Land Sink may be land use, both past and present. For example,there has been widespread reforestation since 1900 in the eastern United States following the movement of the center of agricultural production toward the Midwest. Also,less agricultural land is needed today than during the first half of this century;the productivity of agriculture has improved so much that double the output can be produced on half as much land. The heavy use of fertilizer, together with improved tilling practices, may also lead to increased stores of organic matter in soils. The carbon balance in the tropics also affects estimates of the magnitude of the Northern Land Sink. As pointed out before, the major constraint on the estimated magnitude of the global terrestrial carbon sink comes from two numbers. The first is the estimate of a global net terrestrial uptake of 0.2 ± 0.9 Gt C/yr for the period from 1980 to 1989. This number is obtained from calculating fossil fuel Top panel: Managed forests in the Coast Range of Oregon. Conversion of old-growth forests to managed plantations in the Pacific Northwest has reduced the store of carbon to less than 25-35% of the maximum value in the last century. This has resulted in a substantial loss of carbon to the atmosphere. Altering management by increasing the interval between harvests and/or removing less carbon each harvest would "re-store" much of this carbon over the next century. Bottom panel: Deforestation in tropical regions has released a substantial amount of carbon in the last 50 years. Here the moist tropical forest of Los Tuxlas in Veracruz State, Mexico has been converted to maize and pasture agriculture. This conversion has reduced carbon stores on these sites at least 5-fold. 2 2 a A U.S. Carbon Cycle Science Plan emissions minus atmospheric growth and ocean uptake. The second number is the estimate of 1.6 ± 1,0 Gt C/yr released to the atmosphere from tropical deforestation. The difference between these numbers gives a required terrestrial uptake of 1.8 ± 1.4 Gt C/yr (Schimel et al. 1996), much of which appears to be in the Northern Hemisphere. If the net carbon flux from changes in tropl cal land use were at the lower limit of 0.6 Gt C/yr, the Northern Land Sink would drop to 0.8 Gt C/yr. Maintaining the observed north-south gradient of CO2 in the atmosphere would require that the Southern Ocean would have to be a smaller sink than currently estimated. The recent land use estimate of Houghton et al. (1998) yields 2.0 ± 0.8 Gt C/yr for the tropical land use source. This value would require a compensatory terrestrial carbon sink of 2.2 Gt C/yr as well as a larger Southern Ocean carbon sink. Gaining more certainty in the land use numbers will reduce uncertainty in other contributing compo nents of the Northern Land Sink, such as the fertilizing effects of rising atmospheric CO2 and N deposition. The uncertainty of net carbon flux from land use stems largely from incomplete and often incompatible databases used to compile land use changes, and from the lack of knowledge of carbon fluxes associated with specific activities. These points are particularly true of tropical areas and for land uses involving economically nonproductive ecosystems (i.e..nonproductive in a direct sense), such as wetlands, riparian forests, and natural grasslands. For example,Houghton et al.(1998) notes that no reliable data exist for Latin America on that portion of the loss of agricultural land to degraded land that is not recovering to forest. This situation has forced the omission of such land from the calculation of net carbon flux. The same is true for land subjected to shifting cultivation and the harvest of wood in Sub-Saharan Africa. Tests of the Northern Land Sink hypothesis will thus require the further development and refinement of data sets on historical land use changes,carbon stores per unit area,and models that can use this information. The goal should be to acquire and analyze accurate global invento ries of highly fractionated land use change. Doing so, however, requires a concerted effort to develop high-quali ty, spatially explicit,long-time-series data sets on land use. These data sets should be constructed from a variety of sources,including high-resolution, remotely sensed imagery, explicit data on land use going back many decades, government publications, research data,and reconstruction using proxy information. Side-by-side comparison of population change and measured rates of defor estation, where the data coexist,lends confidence to the reconstruction of rates of deforestation using relatively well-defined changes in population as the predictor. A major new CCSP initiative is therefore proposed to meet the following goal: Chapter 3: Predicting the Future Carbon Cycle The previous chapter addressed the first of the two overarching questions that the Carbon Cycle Science Plan (CCSP) must attempt to answer. In this chapter, we turn to the second of the two fundamental issues for carbon cycle research: What will be the future atmospheric carbon The research program outlined in this report will ultimately be measured by its ability to provide reliable estimates of future atmospheric CO2 concentrations under dif ferent conditions. Only with such knowledge will it be possible to assess alternative scenarios of future emissions from fossil fuels, effects of human land use, sequestration by carbon sinks, and responses of carbon cycling to potential climate change. Thus, the foremost reason for additional research is to develop the ability to predict responses of the global carbon cycle to various types of change. The CCSP must be integrated in the form of a rigorous and comprehensive effort to build and test models of carbon cycle change, to evaluate and communicate uncertainties in alternative model simulations, and to make these simula tions available for public scrutiny and application. The models must also be capable of evaluating alternative scenarios for management of the carbon cycle. There are grounds for optimism that in coming years the fate of CO2 in the ocean can be ascertained with reasonable accuracy. The global mass balance among emissions,atmosphere,ocean,and terrestrial biosphere ensures that a better quantitative estimate of ocean uptake also improves the estimate for changes in terrestri al storage,if only in very coarse geographical detail. Direct measurements of terrestrial inventories and fluxes, in conjunction with atmospheric measurements and models, will help to refine the geographical details. Unfortunately, improved knowledge of the environmen tal fate of historical CO2 emissions cannot by itself give us confident predictions of future atmospheric CO2. The CO2 concentration trajectories calculated by the Intergovernmental Panel on Climate Change (IPCC) for scenarios of fossil fuel burning assume that the future carbon cycle will continue operating exactly as it is thought to have operated in the past. This assumption is not likely to be correct. There is a fundamental difference between the ocean and the terrestrial biosphere for policy decisions relating to greenhouse gases. The ocean remains the biggest longterm player in the carbon cycle.and any research program that neglects the ocean is doomed to be nearly irrelevant for policy. However, direct human interventions in the ocean carbon cycle have thus far been minimal. Furthermore, any future human interventions such as direct injection of CO2 in the deep ocean or enhance ment of the biological flux of carbon by fertilization, will likely be dwarfed by the magnitude of the ongoing pas sive uptake. On the other hand.humanity is already manipulating the terrestrial biosphere on a global scale, and its influence on atmospheric CO2 is substantial. The effect on the carbon cycle of ecological interventions on land has been mostly inadvertent to date. Most of the interventions have resulted in decreasing the amount of carbon in various terrestrial carbon reservoirs. In particu lar, woody biomass and active soil organic matter, because of their intermediate turnover times (30 to 100 years), are the largest terrestrial pools affected by land use conversion and agricultural establishment. This past reduction in terrestrial carbon storage,however, suggests the opportu nity to increase carbon in terrestrial systems through intentional management. The realization that sequestra tion of carbon in wood and soil may play a significant role in offsetting CO2 from fossil fuel burning is already evident in international negotiations. Concerning how high atmospheric CO2 might go in the future,two general questions must be answered: • What will be the partitioning of carbon among the mobile reservoirs, and how will climate change affect this partitioning? • How can the future growth of atmospheric CO2 be managed? Each of these points is examined,and thereafter, a major new research initiative is proposed to address the most compelling scientific issues,an initiative that is also scientifically feasible and cost-effective. Projecting Future Atmospheric CO2 The IPCC has provided some scientific basis for inter national policy decisions by extrapolating the historical behavior of the terrestrial biosphere and ocean into the future. Again, the assumption is that carbon uptake by the terrestrial biosphere will continue to occur through the same mechanisms as at present,and that ocean circulation and biology will remain constant through time (Houghton et al. 1996). However, from coupled atmosphere ocean simulations with time-dependent radiative forcing |