Monthly CO2 Fluxes Research Articles

Interannual characteristics and driving mechanism of CO2 fluxes during the growing season in an alpine wetland ecosystem at the southern foot of the Qilian Mountains.

The carbon process of the alpine ecosystem is complex and sensitive in the face of continuous global warming. However, the long-term dynamics of carbon budget and its driving mechanism of alpine ecosystem remain unclear. Using the eddy covariance (EC) technique-a fast and direct method of measuring carbon dioxide (CO2) fluxes, we analyzed the dynamics of CO2 fluxes and their driving mechanism in an alpine wetland in the northeastern Qinghai-Tibet Plateau (QTP) during the growing season (May-September) from 2004-2016. The results show that the monthly gross primary productivity (GPP) and ecosystem respiration (Re) showed a unimodal pattern, and the monthly net ecosystem CO2 exchange (NEE) showed a V-shaped trend. With the alpine wetland ecosystem being a carbon sink during the growing season, that is, a reservoir that absorbs more atmospheric carbon than it releases, the annual NEE, GPP, and Re reached -67.5 ± 10.2, 473.4 ± 19.1, and 405.9 ± 8.9 gCm-2, respectively. At the monthly scale, the classification and regression tree (CART) analysis revealed air temperature (Ta) to be the main determinant of variations in the monthly NEE and GPP. Soil temperature (Ts) largely determined the changes in the monthly Re. The linear regression analysis confirmed that thermal conditions (Ta, Ts) were crucial determinants of the dynamics of monthly CO2 fluxes during the growing season. At the interannual scale, the variations of CO2 fluxes were affected mainly by precipitation and thermal conditions. The annual GPP and Re were positively correlated with Ta and Ts, and were negatively correlated with precipitation. However, hydrothermal conditions (Ta, Ts, and precipitation) had no significant effect on annual NEE. Our results indicated that climate warming would be beneficial to the improvement of GPP and Re in the alpine wetland, while the increase of precipitation can weaken this effect.

Tall tower eddy covariance measurements of CO2 fluxes in Vienna, Austria

This study reports on three years (2018–2020) of turbulent CO2 fluxes measured at 144 m above the city of Vienna, Austria using an eddy covariance system installed on the A1 Arsenal radio tower. Overall CO2 flux data availability after quality control filtering (49%) was comparable to that observed by other studies of urban and suburban eddy covariance measurements. Average monthly CO2 fluxes followed seasonal trends in temperature-based proxies of heating demand, with the highest fluxes observed in the winter months. When analysing separate trends for the two dominating wind directions, this seasonality was most pronounced for the flows from the north-west when flux footprints overlapped with Vienna's most populated and urbanised districts and fluxes. For flows from the south-east, when flux footprints covered areas of built-up and industrial land yet were dominated by green urban spaces and arable land, the seasonality was much less marked and average monthly CO2 fluxes were generally lower, particularly in winter. Weekday-weekend patterns as well as weekday diurnal variations in CO2 generally followed respective trends in local traffic counts; however, flux correlations with hourly traffic counts on weekends and public holidays were much weaker. Weekend/public holiday diurnal trends in CO2 fluxes indicated an influence of vegetation fluxes on net CO2 flux variations between midday and late afternoon in spring and summer. Furthermore, variations in weekend/public holiday CO2 fluxes from night-time to morning periods suggested a partial night-time decoupling between turbulent exchange at 144 m and the net fluxes at the surface. While the influence of CO2 advection could not be quantified, the pronounced morning flux peaks indicated that CO2 released to the atmosphere during these partially decoupled, night-time periods accumulated to some extent and was subsequently mixed vertically with the morning increase in boundary layer depth. Indeed, the calculated annual fluxes suggested that positive and negative 30-min biases were largely offset when averaging over longer time intervals. The 2018 flux of 10.89 kt CO2 km−2 was in close agreement with the annual CO2 emissions estimate of 10.19 kt CO2 km−2 derived from an official emissions inventory (excluding large point source emissions). A similar annual flux was observed in 2019; however, the substantially lower flux in 2020 likely reflected emissions reductions associated with the COVID-19 pandemic. Comparison of normalised daytime CO2 fluxes between years indicated that the initial lockdown led to a ca. 50–60% reduction in net CO2 emissions from Vienna's most populated and urbanised districts. This study thus provides further evidence that defensible estimates of urban CO2 fluxes can be achieved with tall tower eddy covariance.

Interannual and seasonal variations in carbon exchanges over an alpine meadow in the northeastern edge of the Qinghai-Tibet Plateau, China.

The alpine meadow is highly sensitive to global climate change due to its high elevation and cold environment. To understand the dynamics of ecosystem carbon cycling, CO2 fluxes were measured over the Suli alpine meadow, which is located at the upper reach of the Shule River basin at the northeastern edge of the Qinghai-Tibet Plateau (QTP), China. The measurements were taken from October 2008 to September 2012 using the eddy covariance technique. Obvious seasonal and inter-annual variations were observed in the CO2 flux. The annual net carbon exchange ranged from -195.28 g·CO2·m-2 to -118.49 g·CO2·m-2, indicating that the alpine meadow ecosystem in this area played a role as a carbon sink. The inter-annual variability in the net carbon exchange was significantly related to the length of the growing season for the alpine meadow. The results showed that the months of June, July and August were the strongest CO2 absorption periods, while April, May and October were the strongest CO2 release periods. The annual net exchanges of CO2 in the four years were -118.49 g·CO2·m-2, -130.75 g·CO2·m-2, -195.83 g·CO2·m-2 and -160.65 g·CO2·m-2, and the average value was -151.43 g·CO2·m-2. On a seasonal scale, the monthly CO2 fluxes were largely controlled by temperature. At the annual scale, there was no dominant factor that influenced the interannual variations in the CO2 flux.

Increasing summer net CO<sub>2</sub> uptake in high northern ecosystems inferred from atmospheric inversions and comparisons to remote-sensing NDVI

Abstract. Warmer temperatures and elevated atmospheric CO2 concentrations over the last several decades have been credited with increasing vegetation activity and photosynthetic uptake of CO2 from the atmosphere in the high northern latitude ecosystems: the boreal forest and arctic tundra. At the same time, soils in the region have been warming, permafrost is melting, fire frequency and severity are increasing, and some regions of the boreal forest are showing signs of stress due to drought or insect disturbance. The recent trends in net carbon balance of these ecosystems, across heterogeneous disturbance patterns, and the future implications of these changes are unclear. Here, we examine CO2 fluxes from northern boreal and tundra regions from 1985 to 2012, estimated from two atmospheric inversions (RIGC and Jena). Both used measured atmospheric CO2 concentrations and wind fields from interannually variable climate reanalysis. In the arctic zone, the latitude region above 60° N excluding Europe (10° W–63° E), neither inversion finds a significant long-term trend in annual CO2 balance. The boreal zone, the latitude region from approximately 50–60° N, again excluding Europe, showed a trend of 8–11 Tg C yr−2 over the common period of validity from 1986 to 2006, resulting in an annual CO2 sink in 2006 that was 170–230 Tg C yr−1 larger than in 1986. This trend appears to continue through 2012 in the Jena inversion as well. In both latitudinal zones, the seasonal amplitude of monthly CO2 fluxes increased due to increased uptake in summer, and in the arctic zone also due to increased fall CO2 release. These findings suggest that the boreal zone has been maintaining and likely increasing CO2 sink strength over this period, despite browning trends in some regions and changes in fire frequency and land use. Meanwhile, the arctic zone shows that increased summer CO2 uptake, consistent with strong greening trends, is offset by increased fall CO2 release, resulting in a net neutral trend in annual fluxes. The inversion fluxes from the arctic and boreal zones covering the permafrost regions showed no indication of a large-scale positive climate–carbon feedback caused by warming temperatures on high northern latitude terrestrial CO2 fluxes from 1985 to 2012.

China’s sizeable and uncertain carbon sink: a perspective from GOSAT

Despite the agreement that China’s terrestrial ecosystems can provide a carbon sink and offset carbon dioxide (CO2) emissions from fossil fuels, the magnitude and spatial distribution of the sink remain uncertain. Accurate quantification of the carbon sequestration capacity of China’s terrestrial ecosystems has profound scientific and policy implications. Here, we report on the magnitude and patterns of China’s terrestrial carbon sink using the global monthly CO2 flux data product from the Greenhouse gases Observing SATellite (GOSAT), the world’s first satellite dedicated to global greenhouse gas observation. We use the first year’s data from GOSAT (June 2009–May 2010) that are currently available to assess China’s biospheric carbon fluxes. Our results show that China’s terrestrial ecosystems provide a carbon sink of −0.21 Pg C a−1. The consumption of fossil fuels in China leads to carbon dioxide emissions of 1.90 Pg C a−1 into the atmosphere, approximately 11.1 % of which is offset by China’s terrestrial ecosystems. China’s terrestrial ecosystems play a significant role in offsetting fossil fuel emissions and slowing down the buildup of CO2 in the atmosphere. Our analysis based on GOSAT data offers a new perspective on the magnitude and distribution of China’s carbon sink. Our results show that China’s terrestrial ecosystems provide a sizeable and uncertain carbon sink, and further research is needed to reduce the uncertainty in its magnitude and distribution.

Above‐ground woody carbon sequestration measured from tree rings is coherent with net ecosystem productivity at five eddy‐covariance sites

• Attempts to combine biometric and eddy-covariance (EC) quantifications of carbon allocation to different storage pools in forests have been inconsistent and variably successful in the past. • We assessed above-ground biomass changes at five long-term EC forest stations based on tree-ring width and wood density measurements, together with multiple allometric models. Measurements were validated with site-specific biomass estimates and compared with the sum of monthly CO₂ fluxes between 1997 and 2009. • Biometric measurements and seasonal net ecosystem productivity (NEP) proved largely compatible and suggested that carbon sequestered between January and July is mainly used for volume increase, whereas that taken up between August and September supports a combination of cell wall thickening and storage. The inter-annual variability in above-ground woody carbon uptake was significantly linked with wood production at the sites, ranging between 110 and 370 g C m(-2) yr(-1) , thereby accounting for 10-25% of gross primary productivity (GPP), 15-32% of terrestrial ecosystem respiration (TER) and 25-80% of NEP. • The observed seasonal partitioning of carbon used to support different wood formation processes refines our knowledge on the dynamics and magnitude of carbon allocation in forests across the major European climatic zones. It may thus contribute, for example, to improved vegetation model parameterization and provides an enhanced framework to link tree-ring parameters with EC measurements.

Diel and seasonal variation of methane and carbon dioxide fluxes at Site Guojiaba, the Three Gorges Reservoir

In order to investigate the CH4 and CO2 fluxes across the water-gas interface and identify their controlling factors, four diel field campaigns and one monthly sampling campaign during June 2010–May 2011 were carried out at a site near the Three Gorges Dam, China. The averaged CH4 and CO2 fluxes across the air-water interface from the site were much less than those reported from reservoirs in tropic and temperate regions, and from the natural river channels of the Yangtze River. CH4 Fluxes at the site were very low compared to most other reservoirs or natural lakes. One of the most important reasons may be due to the oxidation of CH4 in the water column owing to the great water depth and high DO in water in the Three Gorges Reservoir. The averaged monthly CH4 and CO2 fluxes at the site during the observation year were 0.05 mg/(m2·hr) and 104.43 mg/(m2·hr) respectively with the maximum occurred in July 2010. The monthly CO2 fluxes during the observation year were positively correlated to the surface water temperature, and negatively correlated to the air pressure and the surface water pH. The CO2 flux showed a positive correlation with DOC to some extent, although not significantly, which indicated that allochthonous organic C was a major source of CO2 and biogeochemical processes in this reservoir were C-limited. The significantly positive correlation between the reservoir outflow and the seasonal gas flux indicate the disturbance condition of the water body dominated the seasonal gas emission.

Variability of fCO2 in the Eastern Tropical Atlantic from a moored buoy

A fCO2 sensor, based on a colorimetric method used for the CARIOCA buoys, has been installed on a Pilot Research Moored Array in the Tropical Atlantic (PIRATA) mooring at 6°S, 10°W, in the gulf of Guinea, in June 2006 during the EGEE 3 cruise. Hourly fCO2 data recorded from June to December 2006 are presented. An alkalinity‐salinity relationship has been determined using data from different cruises, which allows the calculation of dissolved inorganic carbon. Although the tropical Atlantic is an important source of CO2, an unexpected area of low CO2 concentrations is observed in the South Equatorial Counter Current with fCO2 values close to equilibrium conditions or even slightly undersaturated with respect to the atmospheric fCO2 value of 367.7 μatm measured during the cruise. At the end of June, an increase of seawater fCO2 to 400 μatm is consistent with the beginning of the upwelling season occurring from July to September. Although the mooring is not located within the upwelling area, the spreading of the cold tongue explains the large CO2 outgassing. The monthly CO2 flux ranges from 1.19 mmol m−2 d−1 in June to a maximum of 8.37 mmol m−2 d−1 in October, when high fCO2 values above 420 μatm are maintained by the warming of surface water. Most of the fCO2 distribution can be explained by physical processes and a strong relationship between fCO2 and SST is determined for the upwelling season. From mid‐September, diurnal cycles can be detected. Using a dissolved inorganic carbon budget, periods where net community production or diurnal warming and cooling dominates are observed.

Global monthly CO2 flux inversion with a focus over North America

A nested inverse modelling system was developed for estimating carbon fluxes of 30 regions in North America and 20 regions for the rest of the globe. Monthly inverse modelling was conducted using CO2 concentration measurements of 3 yr (2001–2003) at 88 sites. Inversion results show that in 2003 the global carbon sink is -2.76 ± 0.55 Pg C. Oceans and lands are responsible for 88.5% and 11.5% of the sink, respectively. Northern lands are the largest sinks with North America contributing a sink of -0.97 ± 0.21 Pg C in 2003, of which Canada’s sink is -0.34 ± 0.14 Pg C.For Canada, the inverse results show a spatial pattern in agreement, for the most part, with a carbon source and sink distribution map previously derived through ecosystem modelling. However, discrepancies in the spatial pattern and in flux magnitude between these two estimates exist in certain regions. Numerical experiments with a full covariance matrix, with the consideration of the error structure of the a priori flux field based on meteorological variables among the 30 North America regions, resulted in a small but meaningful improvement in the inverted fluxes. Uncertainty reduction analysis suggests that new observation sites are still needed to further improve the inversion for these 30 regions in North America.

Temperature regulation of soil carbon dioxide production in the Humid Pampa of Argentina: estimation of carbon fluxes under climate change

Our objectives were to quantify the effect of abiotic factors on CO2 emissions in the Humid Pampa of Argentina and estimate the potential increase in CO2 fluxes from this agricultural soil as a consequence of climate change. The experimental site was located at Pergamino (33°56′S, 60°34′W), on a fine, illitic, thermic Typic Argiudoll soil. In situ CO2 production presented an exponential relationship with air temperature. C liberated annually by mineralization was estimated by integration of monthly respiration measurements and amounted to 8.4 t C ha–1 year–1. Future monthly CO2 fluxes were calculated for the climate change scenario (doubled atmospheric C concentration) using mean monthly temperatures predicted for Pergamino. An increase of around 50% in CO2 emission from agricultural soils in the Humid Pampa could be expected as a consequence of climate change. The effect of the climate change scenario projected by the global climate models for the Humid Pampa indicates a reduction of the biomass production of cereal crops. Consequently, the predicted decrease in C inputs to soil for this region and an important increase in soil C mineralization would result in marked future C losses.