An official website of the United States government
Concern over the impacts of anthropogenic greenhouse gas emissions has led to proposals to offset CO2 emissions by mechanisms that could increase carbon storage in soils. Estimates of the potential efficacy of various strategies vary, but an issue common to all of them is how to verify the magnitude and stability of the results of interventions. Soil sinks are “out of sight” which contrasts with strategies like reforestation. Below-ground carbon sinks, whether they be organic or inorganic, pose substantially more challenging issues for quantification and monitoring. ERW seeks to apply Ca, Mg-rich silicate rocks to enhance the consumption of CO2 by weathering reactions. Measurement of base cation losses from soils is used for quantifying CO2 uptake. Assessing CO2 uptake this way requires assessment of small differences between heterogeneous end members. Geochemical tracers can be used to estimate basalt input assuming that the endmembers are distinct. To compensate for open system behavior normalization to an “immobile” element is necessary. The limitation is typically the highly heterogenous nature of soils. Data from these settings often have high covariances. We reanalyzed published data on amended soils using Monte Carlo uncertainty analysis. We find that in a number of cases the Ca and Mg differences in pre- and post-amendment soils are not significantly different from zero (1 s.e.). High soil chemistry variances makes quantification of small differences difficult. Techniques for estimating relevant sample size and power for noisy data sets and modest effect sizes are well developed in other fields and can be appropriately applied to ERW problems. A simplified example using Lehr’s approximate rule for a two-sided test with s.d. for the pre- and post- data sets = 0.1 and the effect size (net change) = 0.03 yields a sample size n = 178 to obtain a sample power of 0.8 at the 95% CI, an optimistic estimate. Appropriate experimental design for ERW will require substantial sampling effort and rigorous a priori statistical assessment. Current studies so far cannot achieve this. The ocean ∆CO2/∆ALK ratio used to estimate CO2 uptake is important and unlikely to be as high as most studies have assumed.
more »
« less
Assessment of CO consumption from basalt amendments to soils.
Concern over the impacts of anthropogenic greenhouse gas emissions has led to proposals to offset CO2 emissions by mechanisms that could increase carbon storage in soils. Estimates of the potential efficacy of various strategies vary, but an issue common to all of them is how to verify the magnitude and stability of the results of interventions. Soil sinks are “out of sight” which contrasts with strategies like reforestation. Below-ground carbon sinks, whether they be organic or inorganic, pose substantially more challenging issues for quantification and monitoring. ERW seeks to apply Ca, Mg-rich silicate rocks to enhance the consumption of CO2 by weathering reactions. Measurement of base cation losses from soils is used for quantifying CO2 uptake. Assessing CO2 uptake this way requires assessment of small differences between heterogeneous end members. Geochemical tracers can be used to estimate basalt input assuming that the endmembers are distinct. To compensate for open system behavior normalization to an “immobile” element is necessary. The limitation is typically the highly heterogenous nature of soils. Data from these settings often have high covariances. We reanalyzed published data on amended soils using Monte Carlo uncertainty analysis. We find that in a number of cases the Ca and Mg differences in pre- and post-amendment soils are not significantly different from zero (1 s.e.). High soil chemistry variances makes quantification of small differences difficult. Techniques for estimating relevant sample size and power for noisy data sets and modest effect sizes are well developed in other fields and can be appropriately applied to ERW problems. A simplified example using Lehr’s approximate rule for a two-sided test with s.d. for the pre- and post- data sets = 0.1 and the effect size (net change) = 0.03 yields a sample size n = 178 to obtain a sample power of 0.8 at the 95% CI, an optimistic estimate. Appropriate experimental design for ERW will require substantial sampling effort and rigorous a priori statistical assessment. Current studies so far cannot achieve this. The ocean ∆CO2/∆ALK ratio used to estimate CO2 uptake is important and unlikely to be as high as most studies have assumed.
more »
« less
- Award ID(s):
- 2141989
- PAR ID:
- 10563425
- Publisher / Repository:
- Geochemical Society
- Date Published:
- Format(s):
- Medium: X
- Location:
- Chicago, IL
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Agriculture is being called upon to increase carbon (C) storage in soils to reduce greenhouse gas (GHG) accumulation in the atmosphere. Cropping systems research can be used to support GHG mitigation efforts, but we must quantify land management impacts using appropriate assumptions and unambiguous methods. Soil C sequestration is considered temporary because it can be re-emitted as carbon dioxide (CO2) if the effecting practice is not maintained and/or the soil–plant system is disturbed, for example, as the result of changing climate. Because of this, the climate benefit of soil C sequestration depends on the time that C is held out of the atmosphere. When assessing the net GHG impact of management practices, soil C storage is often aggregated with non-CO2 (N2O and CH4) emissions after converting all components to CO2 equivalents (CO2e) and assuming a given time horizon (TH), in what is known as stock change accounting. However, such analyses do not consider potential re-emission of soil C or apply consistent assumptions about time horizons. Here, we demonstrate that tonne-year accounting provides a more conservative estimate of the emissions offsetting potential of soil C storage compared to stock change accounting. Tonne-year accounting can be used to reconcile differences in the context and timeframes of soil C sequestration and non-CO2 GHG emissions. The approach can be applied post hoc to commonly observed cropping systems data to estimate GHG emissions offsets associated with agricultural land management over given THs and with more clearly defined assumptions.more » « less
-
Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize datasets and methodologies to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC) are based on land-use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The global net uptake of CO2 by the ocean (SOCEAN, called the ocean sink) is estimated with global ocean biogeochemistry models and observation-based fCO2 products (fCO2 is the fugacity of CO2). The global net uptake of CO2 by the land (SLAND, called the land sink) is estimated with dynamic global vegetation models. Additional lines of evidence on land and ocean sinks are provided by atmospheric inversions, atmospheric oxygen measurements, and Earth system models. The sum of all sources and sinks results in the carbon budget imbalance (BIM), a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the year 2023, EFOS increased by 1.3 % relative to 2022, with fossil emissions at 10.1 ± 0.5 GtC yr−1 (10.3 ± 0.5 GtC yr−1 when the cement carbonation sink is not included), and ELUC was 1.0 ± 0.7 GtC yr−1, for a total anthropogenic CO2 emission (including the cement carbonation sink) of 11.1 ± 0.9 GtC yr−1 (40.6 ± 3.2 GtCO2 yr−1). Also, for 2023, GATM was 5.9 ± 0.2 GtC yr−1 (2.79 ± 0.1 ppm yr−1; ppm denotes parts per million), SOCEAN was 2.9 ± 0.4 GtC yr−1, and SLAND was 2.3 ± 1.0 GtC yr−1, with a near-zero BIM (−0.02 GtC yr−1). The global atmospheric CO2 concentration averaged over 2023 reached 419.31 ± 0.1 ppm. Preliminary data for 2024 suggest an increase in EFOS relative to 2023 of +0.8 % (−0.2 % to 1.7 %) globally and an atmospheric CO2 concentration increase by 2.87 ppm, reaching 422.45 ppm, 52 % above the pre-industrial level (around 278 ppm in 1750). Overall, the mean of and trend in the components of the global carbon budget are consistently estimated over the period 1959–2023, with a near-zero overall budget imbalance, although discrepancies of up to around 1 GtC yr−1 persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows the following: (1) a persistent large uncertainty in the estimate of land-use change emissions, (2) low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) a discrepancy between the different methods on the mean ocean sink. This living-data update documents changes in methods and datasets applied to this most recent global carbon budget as well as evolving community understanding of the global carbon cycle. The data presented in this work are available at https://doi.org/10.18160/GCP-2024 (Friedlingstein et al., 2024).more » « less
-
Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land-use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based fCO2 products. The terrestrial CO2 sink (SLAND) is estimated with dynamic global vegetation models. Additional lines of evidence on land and ocean sinks are provided by atmospheric inversions, atmospheric oxygen measurements, and Earth system models. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the year 2022, EFOS increased by 0.9 % relative to 2021, with fossil emissions at 9.9±0.5 Gt C yr−1 (10.2±0.5 Gt C yr−1 when the cement carbonation sink is not included), and ELUC was 1.2±0.7 Gt C yr−1, for a total anthropogenic CO2 emission (including the cement carbonation sink) of 11.1±0.8 Gt C yr−1 (40.7±3.2 Gt CO2 yr−1). Also, for 2022, GATM was 4.6±0.2 Gt C yr−1 (2.18±0.1 ppm yr−1; ppm denotes parts per million), SOCEAN was 2.8±0.4 Gt C yr−1, and SLAND was 3.8±0.8 Gt C yr−1, with a BIM of −0.1 Gt C yr−1 (i.e. total estimated sources marginally too low or sinks marginally too high). The global atmospheric CO2 concentration averaged over 2022 reached 417.1±0.1 ppm. Preliminary data for 2023 suggest an increase in EFOS relative to 2022 of +1.1 % (0.0 % to 2.1 %) globally and atmospheric CO2 concentration reaching 419.3 ppm, 51 % above the pre-industrial level (around 278 ppm in 1750). Overall, the mean of and trend in the components of the global carbon budget are consistently estimated over the period 1959–2022, with a near-zero overall budget imbalance, although discrepancies of up to around 1 Gt C yr−1 persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows the following: (1) a persistent large uncertainty in the estimate of land-use changes emissions, (2) a low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) a discrepancy between the different methods on the strength of the ocean sink over the last decade. This living-data update documents changes in methods and data sets applied to this most recent global carbon budget as well as evolving community understanding of the global carbon cycle. The data presented in this work are available at https://doi.org/10.18160/GCP-2023 (Friedlingstein et al., 2023).more » « less
-
Topographic depressions in upland soils experience anaerobic conditions conducive for iron (Fe) reduction following heavy rainfall. These depressional areas can also accumulate reactive Fe compounds, carbon (C), and nitrate, creating potential hot spots of Fe-mediated carbon dioxide (CO2) and nitrous oxide (N2O) production. While there are multiple mechanisms by which Fe redox reactions can facilitate CO2 and N2O production, it is unclear what their cumulative effect is on CO2 and N2O emissions in depressional soils under dynamic redox. We hypothesized that Fe reduction and oxidation facilitate greater CO2 and N2O emissions in depressional compared to upslope soils in response to flooding. To test this, we amended upslope and depressional soils with Fe(II), Fe(III), or labile C and measured CO2 and N2O emissions in response to flooding. We found that depressional soils have greater Fe reduction potential, which can contribute to soil CO2 emissions during flooded conditions when C is not limiting. Additionally, Fe(II) addition stimulated N2O production, suggesting that chemodenitrification may be an important pathway of N2O production in depressions that accumulate Fe(II). As rainfall intensification results in more frequent flooding of depressional upland soils, Fe-mediated CO2 and N2O production may become increasingly important pathways of soil greenhouse gas emissions.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
