Terrestrial Carbon Fluxes Research Articles

The winter Arctic Oscillation, the timing of spring, and carbon fluxes in the Northern Hemisphere

Various observations show trends toward warmer and earlier springs in the Northern Hemisphere. We hypothesize that the positive trend in the winter Arctic Oscillation (AO) has led to higher winter temperatures, advanced spring, and increased seasonal amplitudes in atmospheric CO2. To test this hypothesis, we modeled leaf‐out and terrestrial carbon fluxes using the Simple Biosphere model, Version 2 (SiB2) and the National Centers for Environmental Prediction (NCEP) reanalysis for 1958–2002. We found that our modeled leaf‐out trends were consistent with observed phenology and that the winter AO trend can statistically explain 20–70% of the modeled leaf‐out trends in the eastern United States and northern Europe. We also found that warmer winter temperatures associated with the positive trend in the winter AO increased winter respiration. At the same time, these warmer winter temperatures advanced the date of leaf‐out, increasing the total spring uptake of atmospheric CO2 by plants. These seasonally asymmetric trends toward increased respiration in winter and increased photosynthesis in spring can help explain the trend toward increased seasonal amplitudes in observed atmospheric CO2 concentration.

Two decades of terrestrial carbon fluxes from a carbon cycle data assimilation system (CCDAS)

This paper presents the space‐time distribution of terrestrial carbon fluxes for the period 1979–1999 generated by a terrestrial carbon cycle data assimilation system (CCDAS). CCDAS is based around the Biosphere Energy Transfer Hydrology model. We assimilate satellite observations of photosynthetically active radiation and atmospheric CO2 concentration observations in a two‐step process. The control variables for the assimilation are the parameters of the carbon cycle model. The optimized model produces a moderate fit to the seasonal cycle of atmospheric CO2 concentration and a good fit to its interannual variability. Long‐term mean fluxes show large uptakes over the northern midlatitudes and uptakes over tropical continents partly offsetting the prescribed efflux due to land use change. Interannual variability is dominated by the tropics. On interannual timescales the controlling process is net primary productivity (NPP) while for decadal changes the main driver is changes in soil respiration. An adjoint sensitivity analysis reveals that uncertainty in long‐term storage efficiency of soil carbon is the largest contributor to uncertainty in net flux.

How uncertainties in future climate change predictions translate into future terrestrial carbon fluxes

AbstractWe forced a global terrestrial carbon cycle model by climate fields of 14 ocean and atmosphere general circulation models (OAGCMs) to simulate the response of terrestrial carbon pools and fluxes to climate change over the next century. These models participated in the second phase of the Coupled Model Intercomparison Project (CMIP2), where a 1% per year increase of atmospheric CO2was prescribed. We obtain a reduction in net land uptake because of climate change ranging between 1.4 and 5.7 Gt C yr−1at the time of atmospheric CO2doubling. Such a reduction in terrestrial carbon sinks is largely dominated by the response of tropical ecosystems, where soil water stress occurs. The uncertainty in the simulated land carbon cycle response is the consequence of discrepancies in land temperature and precipitation changes simulated by the OAGCMs. We use a statistical approach to assess the coherence of the land carbon fluxes response to climate change. The biospheric carbon fluxes and pools changes have a coherent response in the tropics, in the Mediterranean region and in high latitudes of the Northern Hemisphere. This is because of a good coherence of soil water content change in the first two regions and of temperature change in the high latitudes of the Northern Hemisphere.Then we evaluate the carbon uptake uncertainties to the assumptions on plant productivity sensitivity to atmospheric CO2and on decomposition rate sensitivity to temperature. We show that these uncertainties are on the same order of magnitude than the uncertainty because of climate change. Finally, we find that the OAGCMs having the largest climate sensitivities to CO2are the ones with the largest soil drying in the tropics, and therefore with the largest reduction of carbon uptake.

Triple oxygen isotope composition of tropospheric carbon dioxide as a tracer of terrestrial gross carbon fluxes

Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO2 (for which Δ17O = δ17O − 0.516 × δ18O ≠ 0). This isotope anomaly is not lost until air returns to the troposphere and CO2 undergoes isotope exchange with water primarily in the terrestrial biosphere and oceans. A two‐box model is used to investigate the contribution of stratospheric production and contemporary surface carbon fluxes to tropospheric Δ17OCO2. The predicted magnitude of ∼0.15‰ is large enough that measurement of a globally averaged tropospheric Δ17OCO2 should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, Δ17OCO2 should be complementary to the primary isotopic tracer of gross carbon exchanges, δ18OCO2, but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, Δ17OCO2 could serve as a direct tracer of gross carbon exchanges and their variations.

Accounting for the effect of transport errors on tracer inversions

Errors in atmospheric transport of tracers lead to errors in estimates of tracer fluxes based upon concentration observations. Typically, such “inverse” methods either neglect transport errors or only assess their effects roughly. We describe a method to quantitatively account for transport errors by incorporating uncertainties in winds into stochastic motions of air parcels. The magnitude of errors in wind fields, as well as their spatiotemporal covariances, are determined by direct comparison of assimilated winds to radiosonde observations. These statistics of transport errors are propagated through stochastic motions of air parcels in a Lagrangian model (STILT). We illustrate this method by conducting an inverse analysis using simulated CO2 observations over the continent and examine the effect of transport errors on estimates of regional terrestrial carbon fluxes. The inverse analysis demonstrates that transport errors can cause significantly biased estimates. We show that the proposed method properly accounts for these errors.

El Niño–Southern Oscillation–induced variability in terrestrial carbon cycling

We examined the response of terrestrial carbon fluxes to climate variability induced by the El Niño–Southern Oscillation (ENSO). We estimated global net primary production (NPP) from 1982 to 1999 using a light use efficiency model driven by satellite‐derived canopy parameters from the Advanced Very High Resolution Radiometer and climate data from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. We estimated a summed heterotrophic respiration and fire carbon flux as the residual between NPP and the terrestrial net carbon flux inferred from an atmospheric inversion model, excluding the impacts of land use change. We propose that for global applications this approach may be more robust than traditional, biophysically based approaches of simulating heterotrophic respiration. NPP interannual variability was significantly related to ENSO, particularly at lower latitudes (22.5°N–22.5°S) but was weakly related to global temperature. Global heterotrophic respiration and fire carbon fluxes were strongly correlated with global temperature (7.9 pgC/°C). Our results confirm the dependence of global heterotrophic respiration and fire carbon fluxes on interannual temperature variability and strongly suggest that ENSO‐mediated NPP variability influences the atmospheric CO2 growth rate.

Potential of MODIS ocean bands for estimating CO2 flux from terrestrial vegetation: A novel approach

A physiologically‐driven spectral index using two ocean‐color bands of MODIS satellite sensor showed great potential to track seasonally changing photosynthetic light use efficiency (LUE) and stress‐induced reduction in net primary productivity (NPP) of terrestrial vegetation. Based on these findings, we developed a simple “continuous field” model solely based on remotely sensed spectral data that could explain 88% of variability in flux‐tower based daily NPP. For the first time, such a procedure is successfully tested at landscape level using satellite imagery. These findings highlight the unexplored potential of narrow‐band satellite sensors to improve estimates of spatial and temporal distribution in terrestrial carbon flux.

Toward constraining regional‐scale fluxes of CO2 with atmospheric observations over a continent: 2. Analysis of COBRA data using a receptor‐oriented framework

We present an analysis framework and illustrate its potential to constrain terrestrial carbon fluxes at the regional scale using observations of CO2 and CO over North America acquired during the CO2 Budget and Rectification Airborne (COBRA) study in 2000. The COBRA data set, presented in detail in a companion paper [Gerbig et al., 2003] provides dense spatial coverage and extensive profiling in the lower atmosphere, revealing strong CO2 signatures of land surface fluxes in the active and relic mixed layers of the atmosphere. We introduce a “receptor‐oriented” analysis framework designed to quantitatively interpret the atmospheric signatures of surface processes by linking concentrations at measurement locations (receptors) to surface fluxes in upwind regions. The framework incorporates three main components: (1) the Stochastic Time‐Inverted Lagrangian Transport (STILT) model, driven with assimilated winds and running backward in time to map out the source‐receptor relationship (footprint) at high temporal and spatial resolution; (2) an observation‐based lateral boundary condition for CO2, resolving vertical and meridional gradients; and (3) a simple parameterization for biosphere‐atmosphere fluxes that uses eddy covariance observations from the AmeriFlux network as prior estimates for fluxes. This framework allows quantitative comparison between the top‐down constraint on fluxes from airborne observations of CO2 with the bottom‐up constraint of eddy flux measurements in a Bayesian synthesis inversion. The model is used to investigate the observed representation error (mismatch between point measurements and grid‐cell‐averaged values in models), evaluated in the companion paper, showing that unresolved spatial variability of surface fluxes gives rise to most of the representation error over the continent. Thus the representation error reflects the effect of aggregation errors. Discrepancies between simulated and observed CO2 distributions are assessed to indicate where improvements are needed, including improved empirical representation of biosphere‐atmosphere exchange process and better simulation of convective processes in atmospheric transport models.

Postglacial marine environmental changes in Maxwell Bay, King George Island, West Antarctica

Sediment textural properties and total organic carbon (TOC) contents of three sediment cores from Maxwell Bay, King George Island, West Antarctica, record changes in Holocene glaciomarine sedimentary environments. The lower sedimentary unit is mostly composed of TOC-poor diamictons, indicating advanced coastal glacier margins and rapid iceberg discharge in proximal glaciomarine settings with limited productivity and meltwater supply. Fine-grained, TOC-rich sediments in the upper lithologic unit suggest more open water and warm conditions, leading to enhanced biological productivity due to increased nutrient-rich meltwater supply into the bay. The relationship between TOC and total sulfur (TS) indicates that the additional sulfur within the sediment has not originated from in situ pyrite formation under the reducing condition, but rather may be attributed to the detrital supply of sand-sized pyrite from the hydrothermal-origin, quartz-pyrite rocks widely distributed in King George Island. The evolution of bottom-water hydrography after deglaciation was recorded in the benthic foraminiferal stable-isotopic composition, corroborated by the TOC and lithologic changes. The O18O values indicate that bottom-water in Maxwell Bay was probably mixed gradually with intruding 18O-rich seawater from Bransfield Strait. In addition, the O13C values reflect a spatial variability in the carbon isotope distribution in Maxwell Bay, depending on marine productivity as well as terrestrial carbon fluxes by meltwater discharge. The distinct lithologic transition, dated to approximately 8000 yr BP (uncorrected) and characterized by textural and geochemical contrasts, highlights the postglacial environmental change by a major coastal glacier retreat in Maxwell Bay.

Global teleconnections of climate to terrestrial carbon flux

We have applied association analysis to 17 years of climate index observations and predicted net ecosystem production on land to infer short‐term (monthly to yearly) teleconnections between atmosphere‐ocean climate forcing and terrestrial carbon cycles. The analysis suggests that on a global level, climate indices can be significantly correlated to net ecosystem carbon fluxes over more than 58% of the nondesert/ice‐covered land surface, commonly with a lead period of 2–6 months. The Southern Oscillation (SO) and Arctic Oscillation (AO) indices explain nearly equal portions of these significantly correlated area carbon fluxes. These significant teleconnections detected between surface climate and seasonal carbon gain or loss in terrestrial vegetation offer important capabilities for making inferences about the variability in the terrestrial carbon cycle of natural and agricultural ecosystems worldwide.

Estimating regional terrestrial carbon fluxes for the Australian continent using a multiple-constraint approach. I. Using remotely sensed data and ecological observations of net primary production

We have developed a modelling framework that synthesizes various types of field measurements at different spatial and temporal scales. We used this modelling framework to estimate monthly means and their standard deviations of gross photosynthesis, total ecosystem production, net primary production (NPP) and net ecosystem production (NEP) for eight regions of the Australian continent between 1990 and 1998. Annual mean NPP of the Australian continent varied between 800 and 1100 Mt C yr−1 between 1990 and 1998, with a coefficient of variation that is defined as the ratio of standard deviation and mean between 0.24 and 0.34. The seasonal variation of NPP for the whole continent varied between 50 and 110 Mt C month−1 with two maxima, one in the autumn and another in the spring. NEP was most negative in the winter (a carbon sink) and was most positive (a carbon source) in the summer. However, the coefficient of variation of monthly mean NEP was very large (> 4), and consequently confidence in the predicted net carbon fluxes for any month in the period 1990–1998 for the whole continent was very low. A companion paper will apply atmospheric inverse technique to measurements of CO2 concentration to further constrain the continental carbon cycle and reduce uncertainty in estimated mean monthly carbon fluxes.

Postglacial marine environmental changes in Maxwell Bay, King George Island, West Antarctica

Sediment textural properties and total organic carbon (TOC) contents of three sediment cores from Maxwell Bay, King George Island, West Antarctica, record changes in Holocene glaciomarine sedimentary environments. The lower sedimentary unit is mostly composed of TOC-poor diamictons, indicating advanced coastal glacier margins and rapid iceberg discharge in proximal glaciomarine settings with limited productivity and meltwater supply. Fine-grained, TOC-rich sediments in the upper lithologic unit suggest more open water and warm conditions, leading to enhanced biological productivity due to increased nutrient-rich meltwater supply into the bay. The relationship between TOC and total sulfur (TS) indicates that the additional sulfur within the sediment has not originated from in situ pyrite formation under the reducing condition, but rather may be attributed to the detrital supply of sand-sized pyrite from the hydrothermal-origin, quartz-pyrite rocks widely distributed in King George Island. The evolution of bottom-water hydrography after deglaciation was recorded in the benthic foraminiferal stable-isotopic composition, corroborated by the TOC and lithologic changes. The O18O values indicate that bottom-water in Maxwell Bay was probably mixed gradually with intruding 18O-rich seawater from Bransfield Strait. In addition, the O13C values reflect a spatial variability in the carbon isotope distribution in Maxwell Bay, depending on marine productivity as well as terrestrial carbon fluxes by meltwater discharge. The distinct lithologic transition, dated to approximately 8000 yr BP (uncorrected) and characterized by textural and geochemical contrasts, highlights the postglacial environmental change by a major coastal glacier retreat in Maxwell Bay.

Hydrochemical modeling of coupled C and N cycling in high-elevation catchments: Importance of snow cover

Several ecosystems in the western US are already undergoing nitrogen (N)saturation, a condition where previously N limited ecosystems are no longer Nlimited. This state of N saturation leads to adverse impacts on terrestrialecology and water quality. Due to the complexities of terrestrialcarbon-nitrogen cycling, integrated hydrologic-biogeochemical modeling providesa tool to improve our understanding and discern between the impacts of changesin N deposition from changes in other ecosystem processes. A model ofbiogeochemical processing in alpine watersheds was developed and applied to theEmerald Lake watershed. Simulations of major terrestrial carbon and nitrogenpools and fluxes were adequate. The use of snow cover information to estimatesoil temperatures improved model simulations indicating that snow coverprocesses need to be incorporated into biogeochemical models of seasonally snowcovered areas. The model simulated mineral nitrogen processes well butsignificant changes in denitrification and dissolved organic nitrogen exportprocesses appear to be necessary. Ourresults also showed that variations in snow cover duration have more of animpact on mineral N export, plant uptake and mineralization than appearspossible due to changes in atmospheric deposition.

Estimating regional terrestrial carbon fluxes for the Australian continent using a multiple-constraint approach I. Using remotely sensed data and ecological observations of net primary production

We have developed a modelling framework that synthesizes various types of field measurements at different spatial and temporal scales. We used this modelling framework to estimate monthly means and their standard deviations of gross photosynthesis, total ecosystem production, net primary production (NPP) and net ecosystem production (NEP) for eight regions of the Australian continent between 1990 and 1998. Annual mean NPP of the Australian continent varied between 800 and 1100 Mt C yr−1 between 1990 and 1998, with a coefficient of variation that is defined as the ratio of standard deviation and mean between 0.24 and 0.34. The seasonal variation of NPP for the whole continent varied between 50 and 110 Mt C month−1 with two maxima, one in the autumn and another in the spring. NEP was most negative in the winter (a carbon sink) and was most positive (a carbon source) in the summer. However, the coefficient of variation of monthly mean NEP was very large (> 4), and consequently confidence in the predicted net carbon fluxes for any month in the period 1990–1998 for the whole continent was very low. A companion paper will apply atmospheric inverse technique to measurements of CO2 concentration to further constrain the continental carbon cycle and reduce uncertainty in estimated mean monthly carbon fluxes.

Increasing terrestrial carbon uptake from the 1980s to the 1990s with changes in climate and atmospheric CO2

Atmospheric measurements suggest that the terrestrial carbon sink increased from the 1980s to the 1990s, but the causes of the increase are not well understood yet. In this study we investigated the responses of global net primary production in (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) to atmospheric CO2increases and climate variation in the period 1981–1998. Our results show that the unusual climate variability in this period associated with strong warming and El Niño caused high interannual variations in terrestrial ecosystem carbon fluxes; nevertheless NPP and NEP increased consistently from the 1980s to 1990s. Annual global NPP and HR varied with a similar magnitude and contributed about equally to the interannual variations in NEP. Global NEP fluctuated between −0.64 and 1.68 Gt C yr−1with a mean value of 0.62 Gt C yr−1, its decadal means increased from 0.23 Gt C yr−1in the 1980s to 1.10 Gt C yr−1in the 1990s. Total and vegetation carbon storage increased with increases of NPP, but soil carbon storage declined because of higher HR than litter inputs. The tropics (20°N–20°S) had higher mean NEP than the north (>20°N), however, they contributed similarly to the global NEP increase from the 1980s and 1990s. Our estimated terrestrial ecosystem carbon uptake, in response to climate variation and atmospheric CO2increase, accounted for only about 15 to 30% of the total terrestrial carbon sink but contributed 73% of its increase from the 1980s to the 1990s.

Biomass carbon of truncated diameter classes in Indian forests

One major uncertainty in estimating terrestrial carbon flux is the biomass density of the forests being cleared or re-growing. Several studies estimated the forest biomass density in different ways. But most of the studies have focused on forest volume inventories in estimating biomass of different forest types. The major drawback of these inventories is that they often report stand/stock tables only above a minimum diameter, reflecting the dominant interest in inventories of commercial volumes. Especially, in countries like India, where most of the forests are disturbed, they might contain important proportions of the stand biomass. The paper estimates the missing number of trees in the inventories using the frequency of stems in the higher diameter classes. Once the stem frequencies have been estimated completed stand tables may be used for biomass estimation. The study concludes that the error of omission of these small diameter classes is substantial as the inventories fail to include 29.7% of the total stand biomass.

Long‐term variations of climate and carbon fluxes over the Amazon basin

The Amazon basin contains some of the most productive ecosystems on the planet, yet we have little understanding of the long‐term behavior. By examining historical climate records over the Amazon, we identify several modes of climatic variability—including previously undocumented long‐term modes. Furthermore, using a process‐based ecosystem model, we show that these variations in climate generate variations in terrestrial carbon fluxes on short (3–4 year), intermediate (8–9 year), and long (24–28 year) time scales. The long‐term cycles in terrestrial carbon balance have not been previously suggested. Finally, we find that time‐lags between productivity and decomposition enhance the short‐term variations in net carbon balance, while slightly dampening the long‐term variations. Given the worldwide attention on terrestrial carbon cycling, and the potential for “carbon sinks”, we suggest that an improved understanding of long‐term climatic and ecosystem processes is crucial. Other regions should be examined for potential long‐term carbon cycle variations.

The importance of rapid, disturbance‐induced losses in carbon management and sequestration

AbstractManagement of terrestrial carbon fluxes is being proposed as a means of increasing the amount of carbon sequestered in the terrestrial biosphere. This approach is generally viewed only as an interim strategy for the coming decades while other longer‐term strategies are developed and implemented — the most important being the direct reduction of carbon emissions. We are concerned that the potential for rapid, disturbance‐induced losses may be much greater than is currently appreciated, especially by the decision‐making community. Here we wish to: (1) highlight the complex and threshold‐like nature of disturbances — such as fire and drought, as well as the erosion associated with each — that could lead to carbon losses; (2) note the global extent of ecosystems that are at risk of such disturbance‐induced carbon losses; and (3) call for increased consideration of and research on the mechanisms by which large, rapid disturbance‐induced losses of terrestrial carbon could occur. Our lack of ability as a scientific community to predict such ecosystem dynamics is precluding the effective consideration of these processes into strategies and policies related to carbon management and sequestration. Consequently, scientists need to do more to improve quantification of these potential losses and to integrate them into sound, sustainable policy options.

Initiative to quantify terrestrial carbon sources and sinks

Questions related to the distribution and spatio‐temporal dynamics of the terrestrial carbon fluxes are at the core of current scientific and policy debates. In recent years, the primary concern has been the increasing CO2 content in the atmosphere, its effect on climate, and the associated role of terrestrial ecosystems in mitigating the increase and impact of climate change. However, terrestrial carbon dynamics is also closely related to biodiversity land degradation, and other pressing policy and assessment questions. Yet at the global level, no system in place now can provide quantitative information about carbon sources and sinks systematically, reliably, and accurately.

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

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.