Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 °N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO₂ concentration stimulate vegetation productivity, taking up CO₂. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 ± 0.3 Pg C yr^?1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 ± 20 Pg C over 1901 levels, of which 17 ± 8 Pg C comes from vegetation (43%) and 21 ± 16 Pg C from the soil (8%). Both CO₂ fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO₂ fertilization effect, permafrost, and the role of soil moisture. Unlike the CO₂ fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid‐latitudes. These effects are further enhanced as a result of positive carbon cycle–climate feedbacks due to additional CO₂ and warming.     

Is carbon within the global terrestrial biosphere becoming more oxidized? Implications for trends in atmospheric O2

Measurements of atmospheric O₂ and CO₂ concentrations serve as a widely used means to partition global land and ocean carbon sinks. Interpretation of these measurements has assumed that the terrestrial biosphere contributes to changing O₂ levels by either expanding or contracting in size, and thus serving as either a carbon sink or source (and conversely as either an oxygen source or sink). Here, we show how changes in atmospheric O₂ can also occur if carbon within the terrestrial biosphere becomes more reduced or more oxidized, even with a constant carbon pool. At a global scale, we hypothesize that increasing levels of disturbance within many biomes has favored plant functional types with lower oxidative ratios and that this has caused carbon within the terrestrial biosphere to become increasingly more oxidized over a period of decades. Accounting for this mechanism in the global atmospheric O₂ budget may require a small increase in the size of the land carbon sink. In a scenario based on the Carnegie–Ames–Stanford Approach model, a cumulative decrease in the oxidative ratio of net primary production (NPP) (moles of O₂ produced per mole of CO₂ fixed in NPP) by 0.01 over a period of 100 years would create an O₂ disequilibrium of 0.0017 and require an increased land carbon sink of 0.1 Pg C yr⁻¹ to balance global atmospheric O₂ and CO₂ budgets. At present, however, it is challenging to directly measure the oxidative ratio of terrestrial ecosystem exchange and even more difficult to detect a disequilibrium caused by a changing oxidative ratio of NPP. Information on plant and soil chemical composition complement gas exchange approaches for measuring the oxidative ratio, particularly for understanding how this quantity may respond to various global change processes over annual to decadal timescales.     

Higher than expected CO2 fertilization inferred from leaf to global observations

Several lines of evidence point to an increase in the activity of the terrestrial biosphere over recent decades, impacting the global net land carbon sink (NLS) and its control on the growth of atmospheric carbon dioxide (c_a). Global terrestrial gross primary production (GPP)—the rate of carbon fixation by photosynthesis—is estimated to have risen by (31 ± 5)% since 1900, but the relative contributions of different putative drivers to this increase are not well known. Here we identify the rising atmospheric CO₂ concentration as the dominant driver. We reconcile leaf‐level and global atmospheric constraints on trends in modeled biospheric activity to reveal a global CO₂ fertilization effect on photosynthesis of 30% since 1900, or 47% for a doubling of c_a above the pre‐industrial level. Our historic value is nearly twice as high as current estimates (17 ± 4)% that do not use the full range of available constraints. Consequently, under a future low‐emission scenario, we project a land carbon sink (174 PgC, 2006–2099) that is 57 PgC larger than if a lower CO₂ fertilization effect comparable with current estimates is assumed. These findings suggest a larger beneficial role of the land carbon sink in modulating future excess anthropogenic CO₂ consistent with the target of the Paris Agreement to stay below 2°C warming, and underscore the importance of preserving terrestrial carbon sinks.     

Global terrestrial carbon storage and uncertainties in its temperature sensitivity examined with a simple model

The future of the land carbon sink is a significant uncertainty in global change projections. Here, key controls on global terrestrial carbon storage are examined using a simple model of vegetation and soil. Equilibrium solutions are derived as a function of atmospheric CO₂ and global temperature, these environmental variables are then linked in an idealized global change trajectory, and the lag between the dynamic and equilibrium solutions is derived for different linear rates of increase in atmospheric CO₂. Terrestrial carbon storage is departing significantly from equilibrium because CO₂ and temperature are increasing on a similar timescale to ecosystem change, and the lag is found to be proportional to the rate of forcing. Thus peak sizes of the land carbon sink, and any future land carbon source, are proportional to the rate of increase of CO₂. A switch from a land carbon sink to a source occurs at a higher CO₂ and temperature under more rapid forcing. The effects of parameter uncertainty in temperature sensitivities of photosynthesis, plant respiration and soil respiration, and structural uncertainty through the effect of fixing the ratio of plant respiration to photosynthesis are explored. In each case, the CO₂ fertilization effect on photosynthesis is constrained to reproduce the 1990 atmospheric CO₂ concentration within a closed global model. New literature compilations are presented for the temperature sensitivities of plant and soil respiration. A lower limit, Q₁₀=1.29, for soil respiration significantly increases future land carbon storage. An upper limit, Q₁₀=3.63, for soil respiration underpredicts the increase in carbon storage since the Last Glacial Maximum. Fixing the ratio of plant respiration to photosynthesis (R/P) at 0.5 generates the largest and most persistent land carbon sink, followed by the weakest land carbon source.     

Evidence for large carbon sink and long residence time in semiarid forests based on 15 year flux and inventory records

The rate of change in atmospheric CO₂ is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001–2016) in a 55 year old 30 km² pine forest growing at the semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145–160 g C m^?2 year^?1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m^?2 year^?1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m² and 372 g N/m². Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.     

Terrestrial carbon sinks in China and around the world and their contribution to carbon neutrality

Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO₂) concentration and to achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (?0.2±0.9) Pg C yr^?1 (1 Pg=10¹⁵ g) in the 1960s to a sink of (1.9±1.1) Pg C yr^?1 in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20–0.25 Pg C yr^?1 in China during the past decades, and predict it to be 0.15–0.52 Pg C yr^?1 by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO₂ concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO₂ concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.

     

The Terrestrial Silica Pump

Silicon (Si) cycling controls atmospheric CO₂ concentrations and thus, the global climate, through three well-recognized means: chemical weathering of mineral silicates, occlusion of carbon (C) to soil phytoliths, and the oceanic biological Si pump. In the latter, oceanic diatoms directly sequester 25.8 Gton C yr⁻¹, accounting for 43% of the total oceanic net primary production (NPP). However, another important link between C and Si cycling remains largely ignored, specifically the role of Si in terrestrial NPP. Here we show that 55% of terrestrial NPP (33 Gton C yr⁻¹) is due to active Si-accumulating vegetation, on par with the amount of C sequestered annually via marine diatoms. Our results suggest that similar to oceanic diatoms, the biological Si cycle of land plants also controls atmospheric CO₂ levels. In addition, we provide the first estimates of Si fixed in terrestrial vegetation by major global biome type, highlighting the ecosystems of most dynamic Si fixation. Projected global land use change will convert forests to agricultural lands, increasing the fixation of Si by land plants, and the magnitude of the terrestrial Si pump.     

Terrestrial carbon stocks across a gradient of urbanization: a study of the Seattle,WA region

Most of our global population and its CO₂ emissions can be attributed to urban areas. The process of urbanization changes terrestrial carbon stocks and fluxes, which, in turn, impact ecosystem functions and atmospheric CO₂ concentrations. Using the Seattle, WA, region as a case study, this paper explores the relationships between aboveground carbon stocks and land cover within an urbanizing area. The major objectives were to estimate aboveground live and dead terrestrial carbon stocks across multiple land cover classes and quantify the relationships between urban cover and vegetation across a gradient of urbanization. We established 154 sample plots in the Seattle region to assess carbon stocks as a function of distance from the urban core and land cover [urban (heavy, medium, and low), mixed forest, and conifer forest land covers]. The mean (and 95% CI) aboveground live biomass for the region was 89±22 Mg C ha^?1 with an additional 11.8±4 Mg C ha^?1 of coarse woody debris biomass. The average live biomass stored within forested and urban land covers was 140±40 and 18±14 Mg C ha^?1, respectively, with a 57% mean vegetated canopy cover regionally. Both the total carbon stocks and mean vegetated canopy cover were surprisingly high, even within the heavily urbanized areas, well exceeding observations within other urbanizing areas and the average US forested carbon stocks. As urban land covers and populations continue to rapidly increase across the globe, these results highlight the importance of considering vegetation in urbanizing areas within the terrestrial carbon cycle.     

Implications of a large global root biomass for carbon sink estimates and for soil carbon dynamics

Recent evidence suggests that significantly more plant carbon (C) is stored below ground than existing estimates indicate. This study explores the implications for biome C pool sizes and global C fluxes. It predicts a root C pool of at least 268 Pg, 68% larger than previously thought. Although still a low-precision estimate (owing to the uncertainties of biome-scale measurements), a global root C pool this large implies stronger land C sinks, particularly in tropical and temperate forests, shrubland and savanna. The land sink predicted from revised C inventories is 2.7 Pg yr(-1). This is 0.1 Pg yr(-1) larger than current estimates, within the uncertainties associated with global C fluxes, but conflicting with a smaller sink (2.4 Pg yr(-1)) estimated from C balance. Sink estimates derived from C inventories and C balance match, however, if global soil C is assumed to be declining by 0.4-0.7% yr(-1), rates that agree with long-term regional rates of soil C loss. Either possibility, a stronger land C sink or widespread soil C loss, argues that these features of the global C cycle should be reassessed to improve the accuracy and precision of C flux and pool estimates at both global and biome scales.     

The production of phytolith‐occluded carbon in China's forests: implications to biogeochemical carbon sequestration

The persistent terrestrial carbon sink regulates long‐term climate change, but its size, location, and mechanisms remain uncertain. One of the most promising terrestrial biogeochemical carbon sequestration mechanisms is the occlusion of carbon within phytoliths, the silicified features that deposit within plant tissues. Using phytolith content–biogenic silica content transfer function obtained from our investigation, in combination with published silica content and aboveground net primary productivity (ANPP) data of leaf litter and herb layer in China's forests, we estimated the production of phytolith‐occluded carbon (PhytOC) in China's forests. The present annual phytolith carbon sink in China's forests is 1.7 ± 0.4 Tg CO₂ yr ^{? 1}, 30% of which is contributed by bamboo because the production flux of PhytOC through tree leaf litter for bamboo is 3–80 times higher than that of other forest types. As a result of national and international bamboo afforestation and reforestation, the potential of phytolith carbon sink for China's forests and world's bamboo can reach 6.8 ± 1.5 and 27.0 ± 6.1 Tg CO₂ yr^?1, respectively. Forest management practices such as bamboo afforestation and reforestation may significantly enhance the long‐term terrestrial carbon sink and contribute to mitigation of global climate warming.     

Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model

The Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ) combines process‐based, large‐scale representations of terrestrial vegetation dynamics and land‐atmosphere carbon and water exchanges in a modular framework. Features include feedback through canopy conductance between photosynthesis and transpiration and interactive coupling between these ‘fast’ processes and other ecosystem processes including resource competition, tissue turnover, population dynamics, soil organic matter and litter dynamics and fire disturbance. Ten plants functional types (PFTs) are differentiated by physiological, morphological, phenological, bioclimatic and fire‐response attributes. Resource competition and differential responses to fire between PFTs influence their relative fractional cover from year to year. Photosynthesis, evapotranspiration and soil water dynamics are modelled on a daily time step, while vegetation structure and PFT population densities are updated annually. Simulations have been made over the industrial period both for specific sites where field measurements were available for model evaluation, and globally on a 0.5°° × 0.5°° grid. Modelled vegetation patterns are consistent with observations, including remotely sensed vegetation structure and phenology. Seasonal cycles of net ecosystem exchange and soil moisture compare well with local measurements. Global carbon exchange fields used as input to an atmospheric tracer transport model (TM2) provided a good fit to observed seasonal cycles of CO₂ concentration at all latitudes. Simulated inter‐annual variability of the global terrestrial carbon balance is in phase with and comparable in amplitude to observed variability in the growth rate of atmospheric CO₂. Global terrestrial carbon and water cycle parameters (pool sizes and fluxes) lie within their accepted ranges. The model is being used to study past, present and future terrestrial ecosystem dynamics, biochemical and biophysical interactions between ecosystems and the atmosphere, and as a component of coupled Earth system models.     

Spatiotemporal heterogeneity of soil and sediment respiration in a river‐floodplain mosaic (Tagliamento,NE Italy)

1. In their natural state, river floodplains are composed of a complex mosaic of contrasting aquatic and terrestrial habitats. These habitats are expected to differ widely in their properties and corresponding ecological processes, although empirical data on their capacity to produce, store and transform organic matter and nutrients are limited. 2. The objectives of this study were (i) to quantify the spatiotemporal variation of respiration, a dominant carbon flux in ecosystems, in a complex river floodplain, (ii) to identify the environmental drivers of respiration within and among floodplain habitat types and (iii) to calculate whole‐floodplain respiration and to put these values into a global ecosystem context. 3. We measured soil and sediment respiration (sum of root and heterotrophic respiration; SR) throughout an annual cycle in two aquatic (pond and channel) and four terrestrial (gravel, large wood, vegetated island and riparian forest) floodplain habitat types in the island‐braided section of the near‐natural Tagliamento River (NE Italy). 4. Floodplain habitat types differed greatly in substratum composition (soil to coarse gravel), organic matter content (0.63 to 4.1% ash‐free dry mass) and temperature (seasonal range per habitat type: 8.6 to 33.1 °C). Average annual SR ranged from 0.54 ± 1.56 (exposed gravel) to 3.94 ± 3.72 μmol CO₂ m^?2 s^?1 (vegetated islands) indicating distinct variation in respiration within and among habitat types. Temperature was the most important predictor of SR. However, the Q₁₀ value ranged from 1.62 (channel habitat) to 4.57 (riparian forest), demonstrating major differences in habitat‐specific temperature sensitivity in SR. 5. Total annual SR in individual floodplain habitats ranged from 160 (ponds) to 1205 g C m^?2 (vegetated islands) and spanned almost the entire range of global ecosystem respiration, from deserts to tropical forests.     

Enhanced regional terrestrial carbon uptake over Korea revealed by atmospheric CO2 measurements from 1999 to 2017

Understanding changes in terrestrial carbon balance is important to improve our knowledge of the regional carbon cycle and climate change. However, evaluating regional changes in the terrestrial carbon balance is challenging due to the lack of surface flux measurements. This study reveals that the terrestrial carbon uptake over the Republic of Korea has been enhanced from 1999 to 2017 by analyzing long‐term atmospheric CO₂ concentration measurements at the Anmyeondo Station (36.53°N, 126.32°E) located in the western coast. The influence of terrestrial carbon flux on atmospheric CO₂ concentrations (ΔCO₂) is estimated from the difference of CO₂ concentrations that were influenced by the land sector (through easterly winds) and the Yellow Sea sector (through westerly winds). We find a significant trend in ΔCO₂ of ?4.75 ppm per decade (p < .05) during the vegetation growing season (May through October), suggesting that the regional terrestrial carbon uptake has increased relative to the surrounding ocean areas. Combined analysis with satellite measured normalized difference vegetation index and gross primary production shows that the enhanced carbon uptake is associated with significant nationwide increases in vegetation and its production. Process‐based terrestrial model and inverse model simulations estimate that regional terrestrial carbon uptake increases by up to 18.9 and 8.0 Tg C for the study period, accounting for 13.4% and 5.7% of the average annual domestic carbon emissions, respectively. Atmospheric chemical transport model simulations indicate that the enhanced terrestrial carbon sink is the primary reason for the observed ΔCO₂ trend rather than anthropogenic emissions and atmospheric circulation changes. Our results highlight the fact that atmospheric CO₂ measurements could open up the possibility of detecting regional changes in the terrestrial carbon cycle even where anthropogenic emissions are not negligible.     

Estimation of the ecosystem carbon budget in South Korea between 1999 and 2008

A carbon flux model, the vegetation integrated simulator for trace gases, was employed to estimate the carbon budgets of vegetation ecosystems in South Korea. The geographic information system was used to prepare the input variables for the model, such as climate, soil, and land-cover data, from reliable national inventories. Model simulation results indicated that the annual average gross primary production, net primary production, and soil respiration (SR) for 10 years were 91.89, 40.16, and 62.91 Tg C year^?1, respectively. The model also estimated a net ecosystem production with a value of 3.51 Tg C year^?1 between 1999 and 2008. Such results indicate that the vegetation ecosystems of South Korea offset 3.3 % of anthropogenic emissions as a net carbon sink. Latitudinal and topographical gradients over the total simulation area were found for all estimates. In addition, the estimates varied between seasons and years, especially in estimates for biomass growth and carbon uptake, because of variations in the weather conditions. Finally, model validation was conducted using measured soil efflux and flux measurement data from the Gwangneung experimental forest (GEF). The estimated SR accounted for 81.6 % of the observed SR at the GEF site (P < 0.005). Further, the model accounted well for the observed phase and amplitude of changes in the summer and autumn seasons.     

The high-latitude terrestrial carbon sink: a model analysis

A dynamic, global vegetation model, hybrid v4.1 ( Friend et al. 1997 ), was driven by transient climate output from the UK Hadley Centre GCM (HadCM2) with the IS92a scenario of increasing atmospheric CO₂ equivalent, sulphate aerosols and predicted patterns of atmospheric N deposition. Changes in areas of vegetation types and carbon storage in biomass and soils were predicted for areas north of 50°N from 1860 to 2100. Hybrid is a combined biogeochemical, biophysical and biogeographical model of natural, potential ecosystems. The effect of periodic boreal forest fires was assessed by adding a simple stochastic fire model. Hybrid represents plant physiological and soil processes regulating the carbon, water and N cycles and competition between individuals of parameterized generalized plant types. The latter were combined to represent tundra, temperate grassland, temperate/mixed forest and coniferous forest. The model simulated the current areas and estimated carbon stocks in the four vegetation types. It was predicted that land areas above 50°N (about 23% of the vegetated global land area) are currently accumulating about 0.4 PgC y^?1 (about 30% of the estimated global terrestrial sink) and that this sink could grow to 0.8–1.0 PgC y^?1 by the second half of the next century and persist undiminished until 2100. This sink was due mainly to an increase in forest productivity and biomass in response to increasing atmospheric CO₂, temperature and N deposition, and includes an estimate of the effect of boreal forest fire, which was estimated to diminish the sink approximately by the amount of carbon emitted to the atmosphere during fires. Averaged over the region, N deposition contributed about 18% to the sink by the 2080 s. As expected, climate change (temperature, precipitation, solar radiation and saturation pressure deficit) and N deposition without increasing atmospheric CO₂ produced a carbon source. Forest areas expanded both south and north, halving the current tundra area by 2100. This expansion contributed about 30% to the sink by the 2090 s. Tundra areas which were not invaded by forest fluctuated from sink to source. It was concluded that a high latitude carbon sink exists at present and, even assuming little effect of N deposition, no forest expansion and continued boreal forest fires, the sink is likely to persist at its current level for a century.     

Reviewing the carbon cycle dynamics and carbon sequestration potential of Cyperus papyrus L. wetlands in tropical Africa

Tropical papyrus wetlands have the ability to assimilate and sequester significant amounts of carbon. However, the spatial extent, productivity and carbon sink strength associated with papyrus wetlands remains poorly characterised. The objective of this study was to collate information from peer-reviewed publications and relevant government and NGO reports to better understand carbon dynamics within papyrus dominated wetlands, and to assess the processes that regulate the magnitude of the carbon sink. Papyrus wetlands were shown to exhibit high rates of photosynthetic carbon dioxide (CO₂) assimilation of up to 40 μmol CO₂ m^?2 s^?1 where the incident photosynthetic photon flux density was ≥1,000 μmol m^?2 s^?1, high rates of net primary production ranging between 14 and 52 g DM m^?2 d^?1 and represent a significant carbon sink where up to 88 t C ha^?1 is stored in the aboveground and belowground components of the papyrus vegetation. Under flooded conditions significant detrital and peat deposits accumulate in excess of 1 m in depth, representing an additional carbon store in the order of 640 t C ha^?1. This study also highlighted the lack of empirical data on emissions of other radiatively important trace gases such as methane and nitrous oxide and also the vulnerability of these carbon sinks to both future changes in climate, in particular periods of hydrological drawdown, and anthropogenic land use change where the papyrus vegetation is removed in favour of subsistence agricultural cropping systems.     

Changes in terrestrial carbon storage in the United States. 1: The roles of agriculture and forestry

• 1 Changes in the areas of croplands and pastures, and rates of wood harvest in seven regions of the United States, including Alaska, were derived from historical statistics for the period 1700–1990. These rates of land‐use change were used in a cohort model, together with equations defining the changes in live vegetation, slash, wood products and soil that follow a change in land use, to calculate the annual flux of carbon to the atmosphere from changes in land use.
• 2 The calculated flux increased from less than 10 TgC/yr in 1700 to a maximum of about 400 TgC/yr around 1880 and then decreased to approximately zero by 1950. The total flux for the 290‐year period was a release of 32.6 PgC. The area of forests and woodlands declined by 42% (160 × 10⁶ ha), releasing 29 PgC, or 90% of the total flux. Cultivation of soils accounted for about 25% of the carbon loss. Between 1950 and 1990 the annual flux of carbon was approximately zero, although eastern forests were accumulating carbon.
• 3 When the effects of fire and fire exclusion (reported in a companion paper) were added to this analysis of land‐use change, the uptake of carbon calculated for forests was similar in magnitude to the uptake measured in forest inventories, suggesting that past harvests account for a significant fraction of the observed carbon sink in forests.
• 4 Changes in the management of croplands between 1965 and 1990 may have led to an additional accumulation of carbon, not included in the 32.6 PgC release, but even with this additional non‐forest sink, the calculated accumulation of carbon in the United States was an order of magnitude smaller than the North American carbon sink inferred recently from atmospheric data and models.

     

Carbon sequestration in wetland soils of the northern Gulf of Mexico coastal region

Coastal wetlands play an important but complex role in the global carbon cycle, contributing to the ecosystem service of greenhouse gas regulation through carbon sequestration. Although coastal wetlands occupy a small percent of the total US land area, their potential for carbon storage, especially in soils, often exceeds that of other terrestrial ecosystems. More than half of the coastal wetlands in the US are located in the northern Gulf of Mexico, yet these wetlands continue to be degraded at an alarming rate, resulting in a significant loss of stored carbon and reduction in capacity for carbon sequestration. We provide estimates of surface soil carbon densities for wetlands in the northern Gulf of Mexico coastal region, calculated from field measurements of bulk density and soil carbon content in the upper 10–15 cm of soil. We combined these estimates with soil accretion rates derived from the literature and wetland area estimates to calculate surface soil carbon pools and accumulation rates. Wetlands in the northern Gulf of Mexico coastal region potentially store 34–47 Mg C ha^?1 and could potentially accumulate 11,517 Gg C year^?1. These estimates provide important information that can be used to incorporate the value of wetlands in the northern Gulf of Mexico coastal region in future wetland management decisions related to global climate change. Estimates of carbon sequestration potential should be considered along with estimates of other ecosystem services provided by wetlands in the northern Gulf of Mexico coastal region to strengthen and enhance the conservation, sustainable management, and restoration of these important natural resources.     

Effects of 21st‐century climate,land use,and disturbances on ecosystem carbon balance in California

Terrestrial ecosystems are an important sink for atmospheric carbon dioxide (CO₂), sequestering ~30% of annual anthropogenic emissions and slowing the rise of atmospheric CO₂. However, the future direction and magnitude of the land sink is highly uncertain. We examined how historical and projected changes in climate, land use, and ecosystem disturbances affect the carbon balance of terrestrial ecosystems in California over the period 2001–2100. We modeled 32 unique scenarios, spanning 4 land use and 2 radiative forcing scenarios as simulated by four global climate models. Between 2001 and 2015, carbon storage in California's terrestrial ecosystems declined by ?188.4 Tg C, with a mean annual flux ranging from a source of ?89.8 Tg C/year to a sink of 60.1 Tg C/year. The large variability in the magnitude of the state's carbon source/sink was primarily attributable to interannual variability in weather and climate, which affected the rate of carbon uptake in vegetation and the rate of ecosystem respiration. Under nearly all future scenarios, carbon storage in terrestrial ecosystems was projected to decline, with an average loss of ?9.4% (?432.3 Tg C) by the year 2100 from current stocks. However, uncertainty in the magnitude of carbon loss was high, with individual scenario projections ranging from ?916.2 to 121.2 Tg C and was largely driven by differences in future climate conditions projected by climate models. Moving from a high to a low radiative forcing scenario reduced net ecosystem carbon loss by 21% and when combined with reductions in land‐use change (i.e., moving from a high to a low land‐use scenario), net carbon losses were reduced by 55% on average. However, reconciling large uncertainties associated with the effect of increasing atmospheric CO₂ is needed to better constrain models used to establish baseline conditions from which ecosystem‐based climate mitigation strategies can be evaluated.