Showing papers by "Martin Heimann published in 2001"

Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems

Abstract: Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

The Carbon Cycle and Atmospheric Carbon Dioxide

Abstract: Contributing Authors D. Archer, M.R. Ashmore, O. Aumont, D. Baker, M. Battle, M. Bender, L.P. Bopp, P. Bousquet, K. Caldeira, P. Ciais, P.M. Cox, W. Cramer, F. Dentener, I.G. Enting, C.B. Field, P. Friedlingstein, E.A. Holland, R.A. Houghton, J.I. House, A. Ishida, A.K. Jain, I.A. Janssens, F. Joos, T. Kaminski, C.D. Keeling, R.F. Keeling, D.W. Kicklighter, K.E. Kohfeld, W. Knorr, R. Law, T. Lenton, K. Lindsay, E. Maier-Reimer, A.C. Manning, R.J. Matear, A.D. McGuire, J.M. Melillo, R. Meyer, M. Mund, J.C. Orr, S. Piper, K. Plattner, P.J. Rayner, S. Sitch, R. Slater, S. Taguchi, P.P. Tans, H.Q. Tian, M.F. Weirig, T. Whorf, A. Yool

Carbon balance of the terrestrial biosphere in the Twentieth Century: Analyses of CO2, climate and land use effects with four process‐based ecosystem models

Abstract: The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term (1920-1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr(-1), which is within the uncertainty of analysis based on CO2 and O-2 budgets. Three of the four models indicated tin accordance with O-2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Nino/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system.

Global Biogeochemical Cycles in the Climate System

Abstract: 1 Uncertainties of Global Biogeochemical Predictions 2 Uncertainties of Global Climate Predictions 3 Uncertainties in the Atmospheric Chemical System 4 Inferring Biogeochemical Sources and Sinks from Atmospheric Concentrations: General Consideration and Applications in Vegetarian Canopies 5 Biogeophysical Feedbacks and the Dynamics of Climate 6 Land-Ocean-Atmosphere Interactions and Monsoon Climate Change: A Paleo-Perspective 7 Paleobiogeochemistry 8 Should Phosphorus Availability Be Constraining Moist Tropical Forest Responses to Increasing CO2 Concentrations? 9 Trees in Grasslands: Biogeochemical Consequences of Woody Plant Expansion 10 Biogeochemistry in the Arctic: Patterns, Processes, and Controls 11 Evaporation in the Boreal Zone During Summer--Physics and Vegetation 12 Past and Future Forest Response to Rapid Climate Change 13 Biogeochemical Models: Implicit vs. Explicit Microbiology 14 The Global Soil Organic Carbon Pool 15 Plant Compounds and Their Turnover and Stability as Soil Organic Matter 16 Input/Output Balances and Nitrogen Limitation in Terrestrial Ecosystems 17 Interactions Between Hillslope Hydrochemistry, Nitrogen Dynamics and Plants in Fennoscandian Boreal Forest 18 The Cycle of Atmospheric Molecular Oxygen and its Isotopes 19 Constraining the Global Carbon Budget from Global to Regional Scales -- the Measurement Challenge 20 Carbon Isotope Discrimination of Terrestrial Ecosystems -- How Well do Observed and Modeled Results Match? 21 Photosynthetic Pathways and Climate 22 Biological Diversity, Evolution and Biogeochemistry 23 Atmospheric Perspectives on the Ocean Carbon Cycle 24 International Instruments for the Protection of the Climate and Their National Implementation 25 A New Tool to Characterizing and Managing Risks 26 Contrasting Approaches: The Ozone Layer, Climate Change and Resolving the Kyoto Dilemma 27 Optimizing Long-Term Climate Management Subject Index

Modeling Modern Methane Emissions from Natural Wetlands. 1; Model Description and Results

Abstract: Methane is an important greenhouse gas which contributes about 22% to the present greenhouse effect. Natural wetlands currently constitute the biggest methane source and were the major source in preindustrial times. Wetland emissions depend highly on the climate, i.e., on soil temperature and water table. To investigate the response of methane emissions from natural wetlands to climate variations, a process-based model that derives methane emissions from natural wetlands as a function of soil temperature, water table, and net primary productivity is used. For its application on the global scale, global data sets for all model parameters are generated. In addition, a simple hydrologic model is developed in order to simulate the position of the water table in wetlands. The hydrologic model is tested against data from different wetland sites, and the sensitivity of the hydrologic model to changes in precipitation is examined. The global methane- hydrology model constitutes a tool to study temporal and spatial variations in methane emissions from natural wetlands. The model is applied using high-frequency atmospheric forcing fields from ECMWF reanalyses of the period from 1982 to 1993. We calculate global annual methane emissions from wetlands to be 260 Tg yr -. Twenty-five percent of these methane emissions originate from wetlands north of 30oN. Only 60% of the produced methane is emitted, while the rest is reoxidized. A comparison of zonal integrals of simulated global wetland emissions and results obtained by an inverse modeling approach shows good agreement. In a test with data from two wetlands the seasonality of simulated and observed methane emissions agrees well.

Exchanges of Atmospheric CO2 and 13CO2 with the Terrestrial Biosphere and Oceans from 1978 to 2000. I. Global Aspects

Abstract: Author(s): Keeling, Charles D; Piper, Stephen C; Bacastow, Robert B; Wahlen, Martin; Whorf, Timothy P; Heimann, Martin; Meijer, Harro A | Abstract: From 1978 through 1999 the global average concentration of atmospheric carbon dioxide increased from 335 ppm to 368 ppm according to measurements of air samples collected at an array of ten stations extending from the Arctic to the South Pole. The global average rate of increase varied widely, however, with highest rates occurring in 1980, 1983, 1987, 1990, 1994, and 1998, all but the first of these calendar years near times of El Nin˜o events. The 13C/12C isotopic ratio of carbon dioxide, measured on the same air samples, varied in a similarly irregular manner, suggesting that exchange of atmospheric CO2 with terrestrial plants and soil is the dominant cause of both signals. Quantitative analysis of the data by a procedure called a "double deconvolution" supports this hypothesis but also suggests a variable exchange with the oceans, opposite in phase to the terrestrial exchange. This result may be in error, however, because it depends on an assumption that the global average isotopic discrimination of terrestrial plants has been constant. Allowing for a variation in discrimination of only about 1°/°° would eliminate the opposing fluctuations in oceanic flux, if its phasing has been opposite to that of the observed fluctuations in rate of change of CO2 concentration. In three companion articles that follow, we further deduce regional exchanges of CO2, making use of latitudinal gradients computed from the same atmospheric carbon dioxide data used in this global study.

On aggregation errors in atmospheric transport inversions

Abstract: This paper explores the consequences of resolution of surface fluxes on synthesis inversions of carbon dioxide. Synthesis inversion divides the Earth's surface into a set of regions and solves for the magnitudes of fluxes from these regions. The regions are generally quite large. By considering an inversion performed at the resolution of the underlying transport model we show that the aggregation to large regions can cause significant differences in the final results, with errors of the same order of magnitude as the fluxes themselves. Using a simple model, we derive an algorithm to reduce this error. This algorithm accounts for the extra data uncertainty that is caused by uncertainty in the small-scale flux components. In the spatial synthesis inversion this extra data uncertainty reaches a maximum value of 3.5 ppmv. Accounting for it can halve the aggregation error. We provide suggestions for dealing with this problem when high-resolution inversions are not feasible.

Isotopic composition and origin of polar precipitation in present and glacial climate simulations

Abstract: The Hamburg atmospheric general circulation model (AGCM) ECHAM-4 is used to identify the main source regions of precipitation falling on Greenland and Antarctica. Both water isotopes H 2 18 O and HDO are explicitly built into the water cycle of the AGCM, and in addition the capability to trace water from different source regions was added to the model. Present and LGM climate simulations show that water from the most important source regions has an isotopic signature similar to the mean isotope values of the total precipitation amount. But water from other source regions (with very different isotopic signatures) contributes an additional, non-negligible part of the total precipitation amount on both Greenland and Antarctica. Analyses of the temperature-isotope-relations for both polar regions reveal a solely bias of the glacial isotope signal on Greenland, which is caused by a strong change in the seasonal deposition of precipitation originating from nearby polar seas and the northern Atlantic. Although the performed simulations under LGM boundary conditions show a decrease of the δ 18 O values in precipitation in agreement with ice core measurements, the AGCM fails to reproduce the observed simultaneous decrease of the deuterium excess signal. DOI: 10.1034/j.1600-0889.2001.01154.x

Uncertainties in global terrestrial biosphere modeling: 1. A comprehensive sensitivity analysis with a new photosynthesis and energy balance scheme

Abstract: Modeling the terrestrial biosphere's carbon exchanges constitutes a key tool for investigation of the global carbon cycle, which has lead to the recent development of numerous terrestrial biosphere models. However, as demonstrated by recent intercomparison studies, results of plant carbon uptake, expressed as net primary productivity (NPP), still diverge to a large degree. Here, we address the question of uncertainty by conducting a series of sensitivity tests with a single, process-based model, the Biosphere Energy-Transfer Hydrology (BETHY) scheme. We calculate NPP globally for a standard model setup and various alternative model setups representing either changes in modeling strategy or approximate uncertainties of the most important model parameters. The results show that estimated uncertainties of many process parameters are still too large for reliable predictions of global NPP. The largest uncertainties come from plant respiration, photosynthesis and soil water storage. The surface radiation balance and day-to-day variations in weather, often not included into terrestrial vegetation models, are also found to contribute significantly to overall uncertainties, while stomatal behavior, the aerodynamic coupling of vegetation and atmosphere, and the choice of the vegetation map turn out to be relatively unimportant. A further comparison with field measurements of NPP suggests that such data are too unreliable for validating biosphere model predictions. We conclude that the inherent uncertainties in process-oriented biosphere modeling are able to explain the discrepancies that have occurred when comparing the results of different models.

Modeling Modern Methane Emissions from Natural Wetlands

Abstract: A global run of a process-based methane model [Walter et al., this issue] is performed using high-frequency atmospheric forcing fields from ECMWF reanalyses of the period from 1982 to 1993. We calculate global annual methane emissions to be 260 Tg/ yr. 25% of methane emissions originate from wetlands north of 30 deg. N. Only 60% of the produced methane is emitted, while the rest is re-oxidized. A comparison of zonal integrals of simulated global wetland emissions and results obtained by an inverse modeling approach shows good agreement. In a test with data from two wetlands, the seasonality of simulated and observed methane emissions agrees well. The effects of sub-grid scale variations in model parameters and input data are examined. Modeled methane emissions show high regional, seasonal and interannual variability. Seasonal cycles of methane emissions are dominated by temperature in high latitude wetlands, and by changes in the water table in tropical wetlands. Sensitivity tests show that +/- 1 C changes in temperature lead to +/- 20 % changes in methane emissions from wetlands. Uniform changes of +/- 20% in precipitation alter methane emissions by about +/- 18%. Limitations in the model are analyzed. Simulated interannual variations in methane emissions from wetlands are compared to observed atmospheric growth rate anomalies. Our model simulation results suggest that contributions from other sources than wetlands and/or the sinks are more important in the tropics than north-of 30 deg. N. In higher northern latitudes, it seems that a large part, of the observed interannual variations can be explained by variations in wetland emissions. Our results also suggest that reduced wetland emissions played an important role in the observed negative methane growth rate anomaly in 1992.

Inverse modeling of atmospheric carbon dioxide fluxes.

Abstract: Bousquet et al. ([1][1]) and Fan et al. ([2][2]) presented atmospheric transport inversions, the conclusions of which were based on the consistency of the models with observed atmospheric concentrations at a global monitoring network of 60 to 120 sites ([3][3]). This consistency, however, is not

Uncertainties in global terrestrial biosphere modeling, Part II: Global constraints for a process‐based vegetation model

Abstract: The terrestrial biosphere is one of several key components of the global carbon cycle. Because the mechanisms by which climate determines terrestrial biosphere carbon fluxes are not well understood, significant uncertainties concerning model results exist even for the current state of the system, with important consequences for our ability to predict changes under future climate change scenarios. We assess how far this uncertainty can be reduced by constraining a global mechanistic model of vegetation activity, either with global satellite-derived vegetation index data or with measurements of the seasonal CO2 cycle in the atmosphere. We first show how constraining the model with satellite data from the National Oceanic and Atmospheric Administration advanced very high resolution radiometer reduces the sensitivity to estimated uncertainties in model parameters, and thus the estimated error range of net primary productivity. Regionally, the satellite data deliver the largest constraint for vegetation activity in boreal and arctic as well as in tropical water-limited environments. In a second analysis through an atmospheric tracer transport model, we check the consistency of those results with the measured seasonal cycle of CO2 at various remote monitoring sites. While before including the satellite data into model calculations, some simulations within the error range lead to a CO2 seasonal cycle outside the observations, there is a good agreement with the additional constraint. The conclusion is that the constraint delivered by the satellite data is at least as significant as that delivered by atmospheric CO2 measurements. We also show that the CO2 data mainly reflect the activity of northern vegetation, in particular conifers and C3 grasses. This suggests that satellite measurements provide the most useful global data currently available for checking and improving terrestrial vegetation models and that consistency with CO2 measurements is a necessary but not a sufficient requirement for their realism.

IPCC Working Group 1 Third Assessment Report

Abstract: The concentration of CO2 in the atmosphere has risen from close to 280 parts per million (ppm) in 1800, at first slowly and then progressively faster to a value of 367 ppm in 1999, echoing the increasing pace of global agricultural and industrial development. This is known from numerous, well-replicated measurements of the composition of air bubbles trapped in Antarctic ice. Atmospheric CO2 concentrations have been measured directly with high precision since 1957; these measurements agree with ice-core measurements, and show a continuation of the increasing trend up to the present.

Exchanges of Atmospheric CO2 and 13CO2 with the Terrestrial Biosphere and Oceans from 1978 to 2000. II. A Three-Dimensional Tracer Inversion Model to Deduce Regional Fluxes

Abstract: Author(s): Piper, Stephen C; Keeling, Charles D; Heimann, Martin; Stewart, Elisabeth F | Abstract: A three-dimensional tracer inversion model is described that couples atmospheric CO2 transport with prescribed and adjustable source/sink components of the global car- bon cycle to predict atmospheric CO2 concentration and 13C/12C isotopic ratio taking account of exchange fluxes of atmospheric CO2 with the terrestrial biosphere and the oceans. Industrial CO2 emissions are prescribed from fuel production data. Transport of CO2 is prescribed by a model, TM2, that employs 9 vertical levels from the earth’s surface to 10 mb, a numerical time step of 4 hours, and a grid spacing of approxi- mately 8° of latitude and 10° of longitude. Horizontal advection is specified from analyzed observations of wind. Vertical advection is consistent with mass conservation of wind within each grid box. Convective mixing and vertical diffusion are determined at each time step from meteorological data. The source/sink components represent various CO2 exchanges, some sources to the atmosphere, others sinks. The study focuses on establishing interannual variability in net terrestrial biospheric and net oceanic fluxes with the atmosphere revealed by variability in atmospheric CO2, taking account of possible stimulation of land plant growth ("CO2 fertilization") and oceanic CO2 uptake, as well as industrial CO2 emissions. Net primary production of land plants (NPP) and heterotrophic respiration are specified to vary only seasonally, on the basis of data averaged from 1982-1990, inclusive. NPP is determined from a vegetative index, NDVI, derived from remotely sensed radiometric data from satellites. Heterotrophic respiration is a function of surface air temperature. Oceanic exchange of CO2 varies seasonally as specified by a coefficient of CO2 gas exchange. Spatial varia- bility of all source/sink components is specified for each 8° x 10° grid box of TM2, a priori, for 5 terrestrial biospheric and 5 oceanic source/sink components, and with respect to emissions of industrial CO2. Spatial variations of terrestrial exchange are made proportional to NPP. Heterotrophic respiration similarly varies by setting its annual average for each grid box equal to NPP. Spatial variations in oceanic CO2 exchange take account of gas exchange dependence on wind speed and temperature and, in the tropics, on a time-invariant spatially variable specification of the partial pressure of CO2 of surface sea water, based on direct observations. Carbon-isotopic fractionation is taken into account for all chemical processes modeled. To produce an optimal fit to observations of atmospheric CO2, the inversion model adjusts the magnitude of 7 additional source/sink components divided with respect to tropical, temperate, and polar geographic zones. There are 4 terrestrial zones, excluding a southern polar zone of negligible importance. There are 3 oceanic zones: one tropical, and one combined temperate/polar zone in each hemisphere. Calculations are carried out in a quasi-stationary mode that repeats a single annual cycle 4 times, and saves the results for the final year. Alternatively, the model has been run in an extended response mode that takes account of a 4-year history of atmospheric CO2 response to a pulse introduced during the first year of this history. Interannual variations in exchange are established by adjusting the model to predict atmospheric CO2 concentration and 13C/12C ratio averaged for annual periods at overlapping 6-month intervals. Net CO2 exchange fluxes, seasonally adjusted, were determined from 1981-1999, inclusive, using atmospheric CO2 data reported by Keeling et al. [2001].

The effects of CO2, climate and land-use on terrestrial carbon balance, 1920-1992: An analysis with four process-based ecosystem models

Abstract: The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term (1920-1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO 2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO 2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr '•, which is within the uncertainty of analysis based on CO2 and 02 budgets. Three of the four models indicated (in accordance with 02 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO 2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the E1 Ni•o/Southern Oscillation (ENSO)-scale variability in the atmospheric CO 2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from standlevel process tudies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system. •Authorship after McGuire and Sitch is alphabetical. 2Also at U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska, Fairbanks 3potsdam Institute for Climate Impact Research, Potsdam, Germany. 4Institute of Arctic Biology, University of Alaska, Fairbanks. 5Institute for Plant Ecology, Justus-Liebig-University, Giessen, Germany. 6Climate, People, and Environment Program, Institute for Environmental Studies, University of Wisconsin, Madison. 7Max-Planck-Institut fur Biogeochemie, J na, Germany. 8Physics Institute, University of Bern, Bern, Switzerland. øThe Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts. •øComplex Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durhmn. •Electric Power Research Institute, Palo Alto, California. Copyright 2001 by the American Geophysical Union. Paper number 2000GB001298 0886-6236/0 !/2000GB001298512.00

The cycle of atmospheric molecular oxygen and its isotopes

Abstract: Publisher Summary This chapter reviews recent applications of using oxygen and its isotopes as diagnostic tracers of the global carbon cycle. The focus is on atmospheric O2 and CO2 and the oxygen isotope ratios in each of these molecules. Because of the vigorous mixing in the atmosphere, spatiotemporal variations of these species in atmospheric air reflect large-scale surface processes. In particular, the oxygen isotope ratio measurements in atmospheric O2 are only now becoming precise enough to reveal spatial and temporal patterns in the present-day atmosphere. If they were so precise, however, the potential of an integrative approach would be substantial: The tracers discussed in the chapter provide a total of six independent constraints that may be used in a combined way to quantitatively deduce the six major carbon fluxes of interest including photosynthesis and respiration on land and in the sea, together with the gross air-sea gas exchange fluxes. The application of this approach requires the knowledge of: (1) the processes occurring at linkage points between the oxygen cycle and the carbon cycle; (2) the fractionation processes involved at the phase transitions; and (3) the isotopic composition of the water that is imparted to O2 and CO2 formed during photosynthesis or respiration. This information must be available on the temporal and spatial scales of interest. It remains a research challenge for the next few years to develop a modeling framework into which the tracer information can be integrated, possibly by means of advanced data assimilation methods.