Atmospheric inversions for estimating CO2 fluxes: methods and perspectives

Abstract. The Indian Ocean (44° S–30° N) plays an important role in the global carbon cycle, yet it remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea–air CO2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea–air CO2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea–air CO2 uptake of −0.37 ± 0.06 PgC yr−1 is consistent with the −0.24 ± 0.12 PgC yr−1 calculated from observations. The fluxes from the southern Indian Ocean (18–44° S; −0.43 ± 0.07 PgC yr−1 are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), overestimation in the northeastern region (Bay of Bengal) and underestimation of the CO2 sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but overestimate the magnitude. The predicted sea–air CO2 fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO2 flux, whereas the atmospheric inversions predicted an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than that found by atmospheric inversions. Prediction of such weak interannual variability in CO2 fluxes by atmospheric inversions was mainly caused by a lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990–2009 of −0.01 PgC decade−1. This is inconsistent with the observations in the southwestern Indian Ocean that shows the growth rate of oceanic pCO2 was faster than the observed atmospheric CO2 growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s.

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European CO2 fluxes from atmospheric inversions using regional and global transport models

Climatic Change ◽

10.1007/s10584-010-9908-4 ◽

2010 ◽

Vol 103 (1-2) ◽

pp. 93-115 ◽

Cited By ~ 17

Author(s):

L. Rivier ◽

◽

Ph. Peylin ◽

Ph. Ciais ◽

M. Gloor ◽

...

Keyword(s):

Co2 Fluxes ◽

Transport Models ◽

Global Transport ◽

Atmospheric Inversions

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Sea–air CO2 fluxes in the Southern Ocean for the period 1990–2009

Biogeosciences ◽

10.5194/bg-10-4037-2013 ◽

2013 ◽

Vol 10 (6) ◽

pp. 4037-4054 ◽

Cited By ~ 123

Author(s):

A. Lenton ◽

B. Tilbrook ◽

R. M. Law ◽

D. Bakker ◽

S. C. Doney ◽

...

Keyword(s):

Carbon Cycle ◽

Southern Ocean ◽

Interannual Variability ◽

Seasonal Cycle ◽

Co2 Flux ◽

Co2 Fluxes ◽

Biogeochemical Models ◽

And Inversion ◽

Annual Flux ◽

Atmospheric Inversions

Abstract. The Southern Ocean (44–75° S) plays a critical role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Different approaches have been used to estimate sea–air CO2 fluxes in this region: synthesis of surface ocean observations, ocean biogeochemical models, and atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Southern Ocean sea–air CO2 fluxes between 1990–2009. Using all models and inversions (26), the integrated median annual sea–air CO2 flux of −0.42 ± 0.07 Pg C yr−1 for the 44–75° S region, is consistent with the −0.27 ± 0.13 Pg C yr−1 calculated using surface observations. The circumpolar region south of 58° S has a small net annual flux (model and inversion median: −0.04 ± 0.07 Pg C yr−1 and observations: +0.04 ± 0.02 Pg C yr−1), with most of the net annual flux located in the 44 to 58° S circumpolar band (model and inversion median: −0.36 ± 0.09 Pg C yr−1 and observations: −0.35 ± 0.09 Pg C yr−1). Seasonally, in the 44–58° S region, the median of 5 ocean biogeochemical models captures the observed sea–air CO2 flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in the net flux. South of 58° S, neither atmospheric inversions nor ocean biogeochemical models reproduce the phase and amplitude of the observed seasonal sea–air CO2 flux, particularly in the Austral Winter. Importantly, no individual atmospheric inversion or ocean biogeochemical model is capable of reproducing both the observed annual mean uptake and the observed seasonal cycle. This raises concerns about projecting future changes in Southern Ocean CO2 fluxes. The median interannual variability from atmospheric inversions and ocean biogeochemical models is substantial in the Southern Ocean; up to 25% of the annual mean flux, with 25% of this interannual variability attributed to the region south of 58° S. Resolving long-term trends is difficult due to the large interannual variability and short time frame (1990–2009) of this study; this is particularly evident from the large spread in trends from inversions and ocean biogeochemical models. Nevertheless, in the period 1990–2009 ocean biogeochemical models do show increasing oceanic uptake consistent with the expected increase of −0.05 Pg C yr−1 decade−1. In contrast, atmospheric inversions suggest little change in the strength of the CO2 sink broadly consistent with the results of Le Quéré et al. (2007).

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Sea–air CO2 fluxes in the Indian Ocean between 1990 and 2009

Biogeosciences Discussions ◽

10.5194/bgd-10-10759-2013 ◽

2013 ◽

Vol 10 (7) ◽

pp. 10759-10810

Author(s):

V. V. S. S. Sarma ◽

A. Lenton ◽

R. Law ◽

N. Metzl ◽

P. K. Patra ◽

...

Keyword(s):

Indian Ocean ◽

Carbon Cycle ◽

Interannual Variability ◽

Atmospheric Co2 ◽

Co2 Uptake ◽

Co2 Fluxes ◽

Southwest Indian Ocean ◽

Biogeochemical Models ◽

The Indian Ocean ◽

Atmospheric Inversions

Abstract. The Indian Ocean (44° S–30° N) plays an important role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea–air CO2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea–air CO2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea–air CO2 uptake of −0.37 ± 0.06 Pg C yr–1, is consistent with the −0.24 ± 0.12 Pg C yr–1 calculated from observations. The fluxes from the Southern Indian Ocean (18° S–44° S; −0.43 ± 0.07 Pg C yr–1) are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), over estimation in the northeastern region (Bay of Bengal) and underestimation of the CO2 sink in the subtropical convergence zone. These differences were mainly driven by a lack of atmospheric CO2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but over estimate the magnitude. The predicted sea–air CO2 fluxes by Ocean BioGeochemical Models (OBGM) respond to seasonal variability with strong phase lags with reference to climatological CO2 flux, whereas the atmospheric inversions predict an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than atmospheric inversions. Prediction of such weak interannual variability in CO2 fluxes by atmospheric inversions was mainly caused by lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990–2009 of −0.01 Pg C decade–1. This is inconsistent with the observations in the southwest Indian Ocean that shows the growth rate of oceanic pCO2 was faster than the observed atmospheric CO2 growth, a finding attributed to the trend of the Southern Annual Mode (SAM) during the 1990s.

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Interannual-to-decadal variability of North Atlantic air-sea CO2 fluxes

Ocean Science Discussions ◽

10.5194/osd-2-437-2005 ◽

2005 ◽

Vol 2 (4) ◽

pp. 437-472 ◽

Cited By ~ 2

Author(s):

S. Raynaud ◽

O. Aumont ◽

K. B. Rodgers ◽

P. Yiou ◽

J. C. Orr

Keyword(s):

North Atlantic ◽

Co2 Flux ◽

Atmospheric Co2 ◽

Global Ocean ◽

Co2 Fluxes ◽

The North ◽

Basin Scale ◽

The North Atlantic ◽

Ocean Models ◽

Atmospheric Inversions

Abstract. The magnitude of the interannual variability of North Atlantic air-sea CO2 fluxes remains uncertain. Fluxes inferred from atmospheric inversions have large variability, whereas those simulated by ocean models have small variability. Part of the difference is that unlike typical atmospheric inversions, ocean models come with spatial resolution at the sub-basin scale. Here we explore sub-basin-scale spatiotemporal variability in the North Atlantic in one ocean model in order to better understand why the the North Atlantic basin may well contribute very little to the global variability of air-sea CO2 flux. We made two simulations with a biogeochemical model coupled to a global ocean general circulation model (OGCM), which itself was forced by 55-year NCEP reanalysis fields. In the first simulation, atmospheric CO2 was maintained at the preindustrial level (278 ppmv); in the second simulation, atmospheric CO2 followed the observed increase. Simulated air-sea CO2 fluxes and associated variables were analysed with a statistical tool known as multichannel singular spectrum analysis (MSSA). We found that the subtropical gyre is not the largest contributor to the overall, basin-wide variability, in contrast to previous suggestions. The subpolar gyre and the inter-gyre region (the transition area between subpolar and subtropical gyres) also contribute with multipolar anomalies at multiple frequencies: these tend to cancel one another in terms of the basin-wide air-sea CO2 flux. We found a strong correlation between the air-sea CO2 fluxes and the North Atlantic Oscillation (NAO), but only if one takes into account time lags as does MSSA (maximum r=0.64 for lags between 1 and 3 years). The contribution of anthropogenic CO2 to total variability was negligible at interannual time scales, whereas at the decadal (13-year) time scale, it increased variability by 30%.

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Sea-air CO2 fluxes in the Southern Ocean for the period 1990–2009

Biogeosciences Discussions ◽

10.5194/bgd-10-285-2013 ◽

2013 ◽

Vol 10 (1) ◽

pp. 285-333 ◽

Cited By ~ 13

Author(s):

A. Lenton ◽

B. Tilbrook ◽

R. Law ◽

D. Bakker ◽

S. C. Doney ◽

...

Keyword(s):

Carbon Cycle ◽

Southern Ocean ◽

Interannual Variability ◽

Seasonal Cycle ◽

Co2 Flux ◽

Co2 Fluxes ◽

Biogeochemical Models ◽

And Inversion ◽

Annual Flux ◽

Atmospheric Inversions

Abstract. The Southern Ocean (44° S–75° S) plays a critical role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Different approaches have been used to estimate sea-air CO2 fluxes in this region: synthesis of surface ocean observations, ocean biogeochemical models, and atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Southern Ocean sea-air CO2 fluxes between 1990–2009. Using all models and inversions (26), the integrated median annual sea-air CO2 flux of −0.42 ± 0.07 Pg C yr−1 for the 44° S–75° S region is consistent with the −0.27 ± 0.13 Pg C yr−1 calculated using surface observations. The circumpolar region south of 58° S has a small net annual flux (model and inversion median: −0.04 ± 0.07 Pg C yr−1 and observations: +0.04 ± 0.02 Pg C yr−1), with most of the net annual flux located in the 44° S to 58° S circumpolar band (model and inversion median: −0.36 ± 0.09 Pg C yr−1 and observations: −0.35 ± 0.09 Pg C yr−1). Seasonally, in the 44° S–58° S region, the median of 5 ocean biogeochemical models captures the observed sea-air CO2 flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in the net flux. South of 58° S, neither atmospheric inversions nor ocean biogeochemical models reproduce the phase and amplitude of the observed seasonal sea-air CO2 flux, particularly in the Austral Winter. Importantly, no individual atmospheric inversion or ocean biogeochemical model is capable of reproducing both the observed annual mean uptake and the observed seasonal cycle. This raises concerns about projecting future changes in Southern Ocean CO2 fluxes. The median interannual variability from atmospheric inversions and ocean biogeochemical models is substantial in the Southern Ocean; up to 25% of the annual mean flux with 25% of this inter-annual variability attributed to the region south of 58° S. Trends in the net CO2 flux from the inversions and models are not statistically different from the expected increase of –0.05 Pg C yr−1 decade−1 due to increasing atmospheric CO2 concentrations. However, resolving long term trends is difficult due to the large interannual variability and short time frame (1990–2009) of this study.

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CO2 Fluxes on the South Taiga Bog in the European Part of Russia in Summer

Сибирский экологический журнал ◽

10.15372/sej20170201 ◽

2017 ◽

Keyword(s):

European Part ◽

Co2 Fluxes ◽

The South ◽

European Part Of Russia

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CO2 flux determination by closed-chamber methods can be seriously biased by inappropriate application of linear regression

Biogeosciences ◽

10.5194/bg-4-1005-2007 ◽

2007 ◽

Vol 4 (6) ◽

pp. 1005-1025 ◽

Cited By ~ 190

Author(s):

L. Kutzbach ◽

J. Schneider ◽

T. Sachs ◽

M. Giebels ◽

H. Nykänen ◽

...

Keyword(s):

Linear Regression ◽

Nonlinear Regression ◽

Co2 Flux ◽

Exponential Model ◽

Co2 Uptake ◽

Co2 Fluxes ◽

Closed Chamber ◽

Exponential Regression ◽

Closure Time ◽

Over Time

Abstract. Closed (non-steady state) chambers are widely used for quantifying carbon dioxide (CO2) fluxes between soils or low-stature canopies and the atmosphere. It is well recognised that covering a soil or vegetation by a closed chamber inherently disturbs the natural CO2 fluxes by altering the concentration gradients between the soil, the vegetation and the overlying air. Thus, the driving factors of CO2 fluxes are not constant during the closed chamber experiment, and no linear increase or decrease of CO2 concentration over time within the chamber headspace can be expected. Nevertheless, linear regression has been applied for calculating CO2 fluxes in many recent, partly influential, studies. This approach has been justified by keeping the closure time short and assuming the concentration change over time to be in the linear range. Here, we test if the application of linear regression is really appropriate for estimating CO2 fluxes using closed chambers over short closure times and if the application of nonlinear regression is necessary. We developed a nonlinear exponential regression model from diffusion and photosynthesis theory. This exponential model was tested with four different datasets of CO2 flux measurements (total number: 1764) conducted at three peatlands sites in Finland and a tundra site in Siberia. Thorough analyses of residuals demonstrated that linear regression was frequently not appropriate for the determination of CO2 fluxes by closed-chamber methods, even if closure times were kept short. The developed exponential model was well suited for nonlinear regression of the concentration over time c(t) evolution in the chamber headspace and estimation of the initial CO2 fluxes at closure time for the majority of experiments. However, a rather large percentage of the exponential regression functions showed curvatures not consistent with the theoretical model which is considered to be caused by violations of the underlying model assumptions. Especially the effects of turbulence and pressure disturbances by the chamber deployment are suspected to have caused unexplainable curvatures. CO2 flux estimates by linear regression can be as low as 40% of the flux estimates of exponential regression for closure times of only two minutes. The degree of underestimation increased with increasing CO2 flux strength and was dependent on soil and vegetation conditions which can disturb not only the quantitative but also the qualitative evaluation of CO2 flux dynamics. The underestimation effect by linear regression was observed to be different for CO2 uptake and release situations which can lead to stronger bias in the daily, seasonal and annual CO2 balances than in the individual fluxes. To avoid serious bias of CO2 flux estimates based on closed chamber experiments, we suggest further tests using published datasets and recommend the use of nonlinear regression models for future closed chamber studies.

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Atmospheric inversions for estimating CO2 fluxes: methods and perspectives