Carbon Cycle Model Research Articles

Large-scale CO2 disposal/storage in bedded rock salt caverns of China: An evaluation of safety and suitability

As a major greenhouse gas, carbon dioxide (CO2) causes climate warming and weather changes. On the basis of CO2 disposal/storage in salt caverns in this study, a new carbon cycle model is proposed, which provides a new way for carbon capture and storage. The safety and suitability evaluation of CO2 disposal/storage in bedded rock salt caverns in China was carried out. Long-term disposal (Time ≥1000 years), medium-term disposal (several hundred years), and short-term storage (0–30 years) were studied to meet permanent geological isolation of carbon and temporary carbon cycle. The results show that: 1) For long-term and medium-term disposal/storage, it is feasible to carry out permanent geological isolation at proper depth and operating pressure. 2) For short-term storage, the stability of CO2 and CH4 storage in bedded rock salt has a little difference by controlling the operating pressure constant withdrawal-injection cycle. However, the stored CO2 has a much larger storage density and working density than the stored CH4. 3) If dozens of such caverns can be used in a salt mine, the potential for disposal or storage is much considerable. Therefore, the utilization of CO2 storage in salt caverns also acts as an attractive way of carbon neutralization and carbon cycle.

Changes in carbon inputs affect soil respiration and its temperature sensitivity in a broadleaved forest in central China

Soil respiration, as an important process in global carbon cycle, is drastically stimulated by warming. Changing plant carbon inputs caused by intensified human disturbances and climate change can complexly influence soil respiration and its temperature sensitivity (Q10). Although a number of experiments have been established in the world to explore the effects of changing carbon inputs, it remains a challenge to predict the direction and magnitude of changes in soil respiration and Q10 due to high spatiotemporal variability of climatic zone and ecosystem. Therefore, we conducted a field manipulative experiment to examine the impact of litter removal and root exclusion on soil respiration and Q10 in a deciduous broad-leaved forest in transition zone between subtropical and warm temperate zone. The results showed that litter removal significantly reduced soil respiration, microbial biomass carbon, soil organic carbon, total nitrogen and soil moisture. In addition, litter removal altered soil microbial community composition, expressed in increase of Gram-positive / Gram-negative bacteria ratio and percentage of actinomycetes, and enhanced microbial physiological stress index. Root exclusion significantly increased Q10 by elevating soil C/N ratio, which supporting “C quality-temperature” hypothesis. The study indicates that root exclusion may accelerate soil respiration under warming. Our study provides insights to improve carbon cycling models and accurately predict the consequences of climate change.

Weakly reversible CF-decompositions of chemical kinetic systems

This paper studies chemical kinetic systems which decompose into weakly reversible complex factorizable (CF) systems. Among power law kinetic systems, CF systems (denoted as PL-RDK systems) are those where branching reactions of a reactant complex have identical rows in the kinetic order matrix. Mass action and generalized mass action systems (GMAS) are well-known examples. Schmitz's global carbon cycle model is a previously studied non-complex factorizable (NF) power law system (denoted as PL-NDK). We derive novel conditions for the existence of weakly reversible CF-decompositions and present an algorithm for verifying these conditions. We discuss methods for identifying independent decompositions, i.e., those where the stoichiometric subspaces of the subnetworks form a direct sum, as such decompositions relate positive equilibria sets of the subnetworks to that of the whole network. We then use the results to determine the positive equilibria sets of PL-NDK systems which admit an independent weakly reversible decomposition into PL-RDK systems of PLP type, i.e., the positive equilibria are log-parametrized, which is a broad generalization of a Deficiency Zero Theorem of Fortun et al. (2019).

Leaf and root phenology and biomass ofEriophorum vaginatumin response to warming in the Arctic

AbstractThe response of plant leaf and root phenology and biomass in the Arctic to global change remains unclear due to the lack of synchronous measurements of above- and belowground parts. Our objective was to determine the phenological dynamics of the above- and belowground parts of Eriophorum vaginatum in the Arctic and its response to warming. We established a common garden located at Toolik Lake Field Station; tussocks of E. vaginatum from three locations, Coldfoot, Toolik Lake and Sagwon, were transplanted into the common garden. Control and warming treatments for E. vaginatum were set up at the Toolik Lake during the growing seasons of 2016 and 2017. Digital cameras, a handheld sensor and minirhizotrons were used to simultaneously observe leaf greenness, normalized difference vegetation index and root length dynamics, respectively. Leaf and root growth rates of E. vaginatum were asynchronous such that the timing of maximal leaf growth (mid-July) was about 28 days earlier than that of root growth. Warming of air temperature by 1 °C delayed the timing of leaf senescence and thus prolonged the growing season, but the temperature increase had no significant effect on root phenology. The seasonal dynamics of leaf biomass were affected by air temperature, whereas root biomass was correlated with soil thaw depth. Therefore, we suggest that leaf and root components should be considered comprehensively when using carbon and nutrient cycle models, as above- and belowground productivity and functional traits may have a different response to climate warming.

Assimilation of NEON Observations Into a Process‐Based Carbon Cycle Model Reveals Divergent Mechanisms of Carbon Dynamics in Temperate Deciduous Forests

AbstractTerrestrial ecosystems can potentially alleviate or exacerbate climate change by regulating atmospheric CO2 concentration. Divergent predictions of the terrestrial C sink by the Earth System Models (ESMs) indicate no unified mechanism regarding abiotic and biotic response to climate change. The amount and diversity of observations of the terrestrial C cycle create an opportunity to improve the predictive capacity of C‐cycle models. Modeling study with observations could provide valuable insights into the controls on interannual variability of the terrestrial C sink. In this study, we used data from three deciduous forest sites in the National Ecological Observatory Network (NEON) to do site‐specific parameterizations in the Terrestrial ECOsystem model (TECO), and explore controls of the net C uptake. Calibrated TECO explained 58%–83% of variation in leaf area index and 35%–40% in net ecosystem C exchange. Root mean square (percentage) errors were 131–436 gC/m2 (0.9%–3.3%) for wood C pool, 3–73 gC/m2 (0.5%–11.5%) for fine root C, and 170–763 gC/m2 (1.9%–8.0%) for soil C pool. Calibrated parameters revealed site‐specific processes in phenology and turnover of leaves, fine roots, slow‐decomposing soil C, and temperature sensitivity of organic matter decomposition among the three deciduous forest sites. These findings imply the presence of bias in models using parameters at plant functional type (PFT) level, which is the case for many ESMs relying on land surface components (i.e., PFTs), rather than site‐specific parameters. Lastly, the magnitude of the terrestrial C sink in deciduous forests increased with temperature and this increase was caused by the temperature‐driven stimulation of the gross primary production.

Definitions and methods to estimate regional land carbon fluxes for the second phase of the REgional Carbon Cycle Assessment and Processes Project (RECCAP-2)

Abstract. Regional land carbon budgets provide insights into the spatial distribution of the land uptake of atmospheric carbon dioxide and can be used to evaluate carbon cycle models and to define baselines for land-based additional mitigation efforts. The scientific community has been involved in providing observation-based estimates of regional carbon budgets either by downscaling atmospheric CO2 observations into surface fluxes with atmospheric inversions, by using inventories of carbon stock changes in terrestrial ecosystems, by upscaling local field observations such as flux towers with gridded climate and remote sensing fields, or by integrating data-driven or process-oriented terrestrial carbon cycle models. The first coordinated attempt to collect regional carbon budgets for nine regions covering the entire globe in the RECCAP-1 project has delivered estimates for the decade 2000–2009, but these budgets were not comparable between regions due to different definitions and component fluxes being reported or omitted. The recent recognition of lateral fluxes of carbon by human activities and rivers that connect CO2 uptake in one area with its release in another also requires better definitions and protocols to reach harmonized regional budgets that can be summed up to a globe scale and compared with the atmospheric CO2 growth rate and inversion results. In this study, using the international initiative RECCAP-2 coordinated by the Global Carbon Project, which aims to be an update to regional carbon budgets over the last 2 decades based on observations for 10 regions covering the globe with a better harmonization than the precursor project, we provide recommendations for using atmospheric inversion results to match bottom-up carbon accounting and models, and we define the different component fluxes of the net land atmosphere carbon exchange that should be reported by each research group in charge of each region. Special attention is given to lateral fluxes, inland water fluxes, and land use fluxes.

A comprehensive framework for assessing the accuracy and uncertainty of global above-ground biomass maps

Over the past decade, several global maps of above-ground biomass (AGB) have been produced, but they exhibit significant differences that reduce their value for climate and carbon cycle modelling, and also for national estimates of forest carbon stocks and their changes. The number of such maps is anticipated to increase because of new satellite missions dedicated to measuring AGB. Objective and consistent methods to estimate the accuracy and uncertainty of AGB maps are therefore urgently needed. This paper develops and demonstrates a framework aimed at achieving this. The framework provides a means to compare AGB maps with AGB estimates from a global collection of National Forest Inventories and research plots that accounts for the uncertainty of plot AGB errors. This uncertainty depends strongly on plot size, and is dominated by the combined errors from tree measurements and allometric models (inter-quartile range of their standard deviation (SD) = 30–151 Mg ha−1). Estimates of sampling errors are also important, especially in the most common case where plots are smaller than map pixels (SD = 16–44 Mg ha−1). Plot uncertainty estimates are used to calculate the minimum-variance linear unbiased estimates of the mean forest AGB when averaged to 0.1∘. These are used to assess four AGB maps: Baccini (2000), GEOCARBON (2008), GlobBiomass (2010) and CCI Biomass (2017). Map bias, estimated using the differences between the plot and 0.1∘ map averages, is modelled using random forest regression driven by variables shown to affect the map estimates. The bias model is particularly sensitive to the map estimate of AGB and tree cover, and exhibits strong regional biases. Variograms indicate that AGB map errors have map-specific spatial correlation up to a range of 50–104 km, which increases the variance of spatially aggregated AGB map estimates compared to when pixel errors are independent. After bias adjustment, total pantropical AGB and its associated SD are derived for the four map epochs. This total becomes closer to the value estimated by the Forest Resources Assessment after every epoch and shows a similar decrease. The framework is applicable to both local and global-scale analysis, and is available at https://github.com/arnanaraza/PlotToMap. Our study therefore constitutes a major step towards improved AGB map validation and improvement.

Combining tree-ring width and carbon isotope data to investigate stem carbon allocation in an evergreen coniferous species

Carbon allocation in tree stems critically affects how trees respond to climate variability and extreme weather. However, it is difficult to directly measure nonstructural carbohydrates in tree stems, and monitoring them over time is expensive and usually lasts only a few years at most. Hence, we utilized a method substituting nonstructural carbohydrates with the basal area increment (BAI) and intrinsic water-use efficiency (iWUE) to study tree stem carbon allocation. The results indicated that the values of the RAW (BAI-based tree-ring width index) and iWUEres (iWUE residual serial) in the current year could reveal information about early growing season climatic conditions. Thus, coniferous species might utilize carbon produced by photosynthesis during the early growing season to support tree growth. Wet years caused the RAW in the next year to significantly increase, and the RAW in drought years showed a significant correlation with the iWUEres in the previous year, while the RAW in the next year showed little correlation with the iWUEres in the current year. These results suggested that favourable climatic conditions could stimulate the accumulation of carbon, which promoted radial growth in the following year. In addition, drought in semiarid areas has a 1–3-year legacy effect on tree growth, suppressing the growth rate below the normal rate. The decline in tree growth was most noticeable in the year after drought, suggesting that inter-annual allocation of carbon reserves is a long-term process, yet trees prioritize using the newest carbon source for growth during adjacent years. This study broadens the understanding of nonstructural carbohydrates and their supply in evergreen coniferous species and has important implications for forest climate–carbon cycle models under changing climates.

A review of forest carbon cycle models on spatiotemporal scales

The carbon cycle of forest ecosystems plays an important role in cutting down global environmental pressure, reducing greenhouse effects, and assimilating carbon dioxide. Model simulation has developed rapidly and become an irreplaceable method in researching carbon cycle mechanisms and estimating carbon fluxes of forest ecosystems. So far, little research has systematically reviewed the progresses in forest carbon cycle models. A review of forest carbon cycle models on spatiotemporal scales will be timely and valuable. In this paper, the forest carbon cycle models on spatiotemporal scales were reviewed, and the advantages and disadvantages of forest carbon cycle models at different spatiotemporal scales were compared. The existing problems of forest carbon cycle models were analysed. Proposals on the prospects of forest carbon cycle models in the future were presented. This review can provide new ideas and a comprehensive understanding on the assessment method development of current forest carbon cycle.

Progress in Dust Modelling, Global Dust Budgets, and Soil Organic Carbon Dynamics

Dust emission is an important corollary of the soil degradation process in arid and semi-arid areas worldwide. Soil organic carbon (SOC) is the main terrestrial pool in the carbon cycle, and dust emission redistributes SOC within terrestrial ecosystems and to the atmosphere and oceans. This redistribution plays an important role in the global carbon cycle. Herein, we present a systematic review of dust modelling, global dust budgets, and the effects of dust emission on SOC dynamics. Focusing on selected dust models developed in the past five decades at different spatio-temporal scales, we discuss the global dust sources, sinks, and budgets identified by these models and the effect of dust emissions on SOC dynamics. We obtain the following conclusions: (1) dust models have made considerable progress, but there are still some uncertainties; (2) a set of parameters should be developed for the use of dust models in different regions, and direct anthropogenic dust should be considered in dust emission estimations; and (3) the involvement of dust emission in the carbon cycle models is crucial for improving the accuracy of carbon assessment.

Short-Term Recovery of the Aboveground Carbon Stock in Iberian Shrublands at the Extremes of an Environmental Gradient and as a Function of Burn Severity

The degree to which burn severity influences the recovery of aboveground carbon density (ACD) of live pools in shrublands remains unclear. Multitemporal LiDAR data was used to evaluate ACD recovery three years after fire in shrubland ecosystems as a function of burn severity immediately after fire across an environmental and productivity gradient in the western Mediterranean Basin. Two large mixed-severity wildfires were assessed: an Atlantic site, dominated by resprouter shrubs and located at the most productive extreme of the gradient, and a Mediterranean site, dominated by obligate seeders and located at the less productive extreme. Initial assessment of burn severity was performed using the differenced Normalized Burn Ratio index computed from Landsat imagery. Thresholds for low and high burn severity categories were established using the Composite Burn Index (CBI). LiDAR canopy metrics were calibrated with field measurements of mean shrub height and cover at plot level in a post-fire situation. Pre-fire and post-fire ACD estimates, and their ratio (ACDr) to calculate carbon stock recovery, were computed from the predictions of LiDAR grid metrics at landscape level using shrubland allometric relationships. Overall, ACDr decreased both with high burn severity and low productivity, although the burn severity impact was not homogeneous within the gradient. In the Atlantic site, ACDr was similar under low and high burn severity, whereas it decreased with burn severity in the Mediterranean site. These results suggest that carbon cycling models could be biased by not accounting for both fire severity and species composition of shrublands under different environmental conditions.

New constraints on the postglacial shallow-water carbonate accumulation in the Great Barrier Reef

More accurate global volumetric estimations of shallow-water reef deposits are needed to better inform climate and carbon cycle models. Using recently acquired datasets and International Ocean Discovery Program (IODP) Expedition 325 cores, we calculated shallow-water CaCO3 volumetrics and mass for the Great Barrier Reef region and extrapolated these results globally. In our estimates, we include deposits that have been neglected in global carbonate budgets: Holocene Halimeda bioherms located on the shelf, and postglacial pre-Holocene (now) drowned coral reefs located on the shelf edge. Our results show that in the Great Barrier Reef alone, these drowned reef deposits represent ca. 135 Gt CaCO3, comparatively representing 16–20% of the younger Holocene reef deposits. Globally, under plausible assumptions, we estimate the presence of ca. 8100 Gt CaCO3 of Holocene reef deposits, ca. 1500 Gt CaCO3 of drowned reef deposits and ca. 590 Gt CaCO3 of Halimeda shelf bioherms. Significantly, we found that in our scenarios the periods of pronounced reefal mass accumulation broadly encompass the occurrence of the Younger Dryas and periods of CO2 surge (14.9–14.4 ka, 13.0–11.5 ka) observed in Antarctic ice cores. Our estimations are consistent with reef accretion episodes inferred from previous global carbon cycle models and with the chronology from reef cores from the shelf edge of the Great Barrier Reef.

Carbon Emission and Redistribution among Forest Carbon Pools, and Change in Soil Nutrient Content after Different Severities of Forest Fires in Northeast China

Forest fires are a significant factor that affects the boreal forest carbon distribution which emits carbon into the atmosphere and leads to carbon redistribution among carbon pools. However, knowledge about how much carbon was transferred among pools and the immediate changes in soil nutrient contents in areas that were burned by fires of various severities are still limited. In this study, we surveyed eight wildfire sites that are located in northeast China within three months after the fires occurred. Our results indicate that the total soil nitrogen, phosphorus, and organic carbon contents significantly increased after moderate- and high-severity fires. The carbon emissions were 3.84, 5.14, and 12.86 Mg C/ha for low-, moderate-, and high-severity fires, respectively. The amount of carbon transferred among pools increased with fire severity except for the charcoal pool, storing the highest amounts of carbon in moderate-severity fires. Although the charcoal and ash pools accounted for a small proportion of the total ecosystem, they are important for biogeochemical cycles and are worthy of attention. The carbon redistribution information in our study is important for accurately estimating the forest carbon budget and providing crucial parameters for forest carbon cycling models to incorporate the carbon transfer process.

Intensified continental chemical weathering and carbon-cycle perturbations linked to volcanism during the Triassic\u2013Jurassic transition

Direct evidence of intense chemical weathering induced by volcanism is rare in sedimentary successions. Here, we undertake a multiproxy analysis (including organic carbon isotopes, mercury (Hg) concentrations and isotopes, chemical index of alteration (CIA), and clay minerals) of two well-dated Triassic–Jurassic (T–J) boundary sections representing high- and low/middle-paleolatitude sites. Both sections show increasing CIA in association with Hg peaks near the T–J boundary. We interpret these results as reflecting volcanism-induced intensification of continental chemical weathering, which is also supported by negative mass-independent fractionation (MIF) of odd Hg isotopes. The interval of enhanced chemical weathering persisted for ~2 million years, which is consistent with carbon-cycle model results of the time needed to drawdown excess atmospheric CO2 following a carbon release event. Lastly, these data also demonstrate that high-latitude continental settings are more sensitive than low/middle-latitude sites to shifts in weathering intensity during climatic warming events.

Stem Carbon Dioxide Efflux of Lignophytes Exceeds That of Cycads and Arborescent Monocots

Tree stem CO2 efflux (Es) can be substantial and the factors controlling ecosystem-level Es are required to fully understand the carbon cycle and construct models that predict atmospheric CO2 dynamics. The majority of Es studies used woody lignophyte trees as the model species. Applying these lignophyte data to represent all tree forms can be inaccurate. The Es of 318 arborescent species was quantified in a common garden setting and the results were sorted into four stem growth forms: cycads, palms, monocot trees that were not palms, and woody lignophyte trees. The woody trees were comprised of gymnosperm and eudicot species. The Es did not differ among the cycads, palms, and non-palm monocots. Lignophyte trees exhibited Es that was 40% greater than that of the other stem growth forms. The Es of lignophyte gymnosperm trees was similar to that of lignophyte eudicot trees. This extensive species survey indicates that the Es from lignophyte tree species do not align with the Es from other tree growth forms. Use of Es estimates from the literature can be inaccurate for understanding the carbon cycle in tropical forests, which contain numerous non-lignophyte tree species.

Modeling the Carbon Cycle Dynamics and the Greenhouse Effect

A carbon cycle model is proposed that predicts changes in the atmospheric CO2 concentration, and provides a global temperature estimate from an empirical correlation of the two variables. The model is validated by simulating the anthropogenic carbon emissions and deforestation since the industrial revolution, and comparing the predicted and measured atmospheric CO2 concentration and global temperature data. The temperature data are also compared with those predicted by a greenhouse effect model based on the effective emission temperature hypothesis. The result suggests that radiative forcing by CO2 alone can account for only about half of the measured global warming. The model requires further elaboration for the other half, in order to be applicable to simulation of potential climate control.

New Insights into the Carbon Cycle and Depositional Models of the Eocene Saline Lake, Jianghan Basin, China

New Insights into the Carbon Cycle and Depositional Models of the Eocene Saline Lake, Jianghan Basin, China

Estimating photosynthetic capacity from optimized Rubisco–chlorophyll relationships among vegetation types and under global change

The maximum rate of carboxylation (Vcmax), a key parameter indicating photosynthetic capacity, is commonly fixed as a constant by vegetation types and/or varies according to empirical scaling functions in Earth system models (ESMs). As such, the setting of Vcmax results in uncertainties of estimated carbon assimilation. It is known that the coupling between leaf chlorophyll and Rubisco (ribulose-1,5-biphosphate carboxylase-oxygenase) contents can be applied to estimate Vcmax. However, how this coupling is affected by environmental changes and varies among plant functional types (PFTs) has not been well investigated yet. The effect of varying coupling between chlorophyll and Rubisco contents on the estimation of Vcmax is still not clear. In this study, we compiled data from 76 previous studies to investigate the coupling between Chlorophyll (Chl) and Rubisco (Rub), in different PFTs and under different environmental conditions. We also assessed the ability of a Rub-based semi-mechanistic model to estimate Vcmax normalized to 25 °C (Vcmax25) based on the Rub–Chl relationship. Our results revealed strong, linear Rub-Chl relationships for different PFTs (R 2 = 0.73, 0.67, 0.54 and 0.72 for forest, crop, grass and shrub, and C4 plants, respectively). The Rub–Chl slope of natural C3 PFTs was consistent and significantly different from those of crops and C4 plants. A meta-analysis indicated that reduced light intensity, elevated CO2, and nitrogen addition strongly altered Rub/Chl. A semi-mechanistic model based on PFT-specific Rub–Chl relationships was able to estimate Vcmax25 with high confidence. Our findings have important implications for improving global carbon cycle modeling by ESMs through the improved parameterization of Vcmax25 using remotely sensed Chl content.

Pemodelan Sistem Karbonat di Laut Jawa

<p class="Papertext"><strong>Modeling Carbonate System in the Java Sea</strong>. Besides the global fossil fuel burning activities, forest fires in Kalimantan could potentially increase atmospheric CO<sub>2</sub> concentrations, impacting air-sea CO<sub>2</sub> gas exchange in the Java Sea and changing the balance of the marine carbonate system. This study uses a marine carbonate model to examine the processes that control CO<sub>2</sub> flux in the Java Sea and their relationship to CO<sub>2</sub> increase in the atmosphere. OCMIP-2 (<em>Ocean Carbon-Cycle Model Intercomparison Model Project, Phase-2</em>) is performed in this marine carbonate model coupled with the marine ecosystem model. The model results show that the quantity of carbon air flux differs during February and October 2000. More considerable flux is produced during February 2000, where the wind speeds are higher than in October 2000. However, the wind speeds have less impact when the CO<sub>2</sub> level in the atmosphere rises significantly. Due to the influence of a relatively high surface temperature in the tropical Java sea, the Java Sea functions as a carbon source to the atmosphere in general. In this case, the role of the <em>solubility pump</em> is more significant than that of biological processes in carbon absorption. Moreover, increased CO<sub>2</sub> in the atmosphere could alter the partial pressure equilibrium. In the case of 2002 forest fires (atmospheric CO<sub>2</sub> = 460 ppm), the carbon source of the Java Sea was less than before forest fires and even became carbon sink when atmospheric CO<sub>2</sub> rose to 1135.2 ppm based on the highest SSP scenario in 2100. This modeling also reveals marine acidification issues and could rapidly assess the future changes in marine ecosystems due to CO<sub>2</sub> levels rising in the atmosphere.</p>

Global to local impacts on atmospheric CO2 from the COVID-19 lockdown, biosphere and weather variabilities

The worldwide lockdown in response to the COVID-19 pandemic in year 2020 led to an economic slowdown and a large reduction in fossil fuel CO2 emissions (Le Quéré 2020 Nat. Clim. Change 10 647–53, Liu 2020 Nat. Commun. 11); however, it is unclear how much it would slow the increasing trend of atmospheric CO2 concentration, the main driver of climate change, and whether this impact can be observed considering the large biosphere and weather variabilities. We used a state-of-the-art atmospheric transport model to simulate CO2, and the model was driven by a new daily fossil fuel emissions dataset and hourly biospheric fluxes from a carbon cycle model forced with observed climate variability. Our results show a 0.21 ppm decrease in the atmospheric column CO2 anomaly in the Northern Hemisphere latitude band 0–45° N in March 2020, and an average of 0.14 ppm for the period of February–April 2020, which is the largest decrease in the last 10 years. A similar decrease was observed by the carbon observing satellite GOSAT (Yokota et al 2009 Sola 5 160–3). Using model sensitivity experiments, we further found that the COVID and weather variability are the major contributors to this CO2 drawdown, and the biosphere showed a small positive anomaly. Measurements at marine boundary layer stations, such as Hawaii, exhibit 1–2 ppm anomalies, mostly due to weather and the biosphere. At the city scale, the on-road CO2 enhancement measured in Beijing shows a reduction by 20–30 ppm, which is consistent with the drastically reduced traffic during the COVID lockdown. A stepwise drop of 20 ppm during the city-wide lockdown was observed in the city of Chengdu. The ability of our current carbon monitoring systems in detecting the small and short-lasting COVID signals at different policy relevant scales (country and city) against the background of fossil fuel CO2 accumulated over the last two centuries is encouraging. The COVID-19 pandemic is an unintended experiment. Its impact suggests that to keep atmospheric CO2 at a climate-safe level will require sustained effort of similar magnitude and improved accuracy, as well as expanded spatiotemporal coverage of our monitoring systems.