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Robust estimates of CO 2 budget, CO 2 exchanged between the atmosphere and terrestrial biosphere, are necessary to better understand the role of the terrestrial biosphere in mitigating anthropogenic CO 2 emissions. Over the past decade, this field of research has advanced through understanding of the differences and similarities of two fundamentally different approaches: "top-down" atmospheric inversions and "bottom-up" biosphere models. Since the first studies were undertaken, these approaches have shown an increasing level of agreement, but disagreements in some regions still persist, in part because they do not estimate the same quantity of atmosphere-biosphere CO 2 exchange. Here, we conducted a thorough comparison of CO 2 budgets at multiple scales and from multiple methods to assess the current state of the science in estimating CO 2 budgets. Our set of atmospheric inversions and biosphere models, which were adjusted for a consistent flux definition, showed a high level of agreement for global and hemispheric CO 2 budgets in the 2000s. Regionally, improved agreement in CO 2 budgets was notable for North America and Southeast Asia. However, large gaps between the two methods remained in East Asia and South America. In other regions, Europe, boreal Asia, Africa, South Asia, and Oceania, it was difficult to determine whether those regions act as a net sink or source because of the large spread in estimates from atmospheric inversions. These results highlight two research directions to improve the robustness of CO 2 budgets: (a) to increase representation of processes in biosphere models that could contribute to fill the budget gaps, such as forest regrowth and forest degradation; and (b) to reduce sink-source compensation between regions (dipoles) in atmospheric inversion so that their estimates become more comparable. Advancements on both research areas will increase the level of agreement between the top-down and bottom-up approaches and yield more robust knowledge of regional CO 2 budgets., (© 2019 John Wiley & Sons Ltd.)

This study examined the predictors of carbon emissions in member countries of the North American Free Trade Agreement (NAFTA). Panel models robust to cross-sectional dependence and slope heterogeneity were used for the study. From the heterogeneity and cross-sectional dependence tests, the studied panel was heterogeneous and cross-sectionally dependent. Also, the unit root and cointegration tests established the series to be first differenced stationary and cointegrated in the long run. Additionally, results of the CCEMG regression estimator in the whole panel affirmed economic growth (GDP) to be a significantly positive predictor of CO 2 emissions, while foreign direct investments (FDI) and population growth (POP) were trivial determinants of CO 2 emissions. The discoveries were however diverse in the individual countries. Finally, there was no causality between GDP and CO 2 emissions and between POP and CO 2 emissions. However, there was a one-way causality from CO 2 emissions to FDI. Policy recommendations are further discussed.

We evaluate spatial structure in North American CO2 flux observations using a simple diagnostic land surface model. The Vegetation Photosynthesis Respiration Model (VPRM) calculates net ecosystem exchange (NEE) using locally observed temperature and photosynthetically active radiation (PAR) along with satellite-derived phenology and moisture. We use observed NEE from a group of 65 North American eddy co-variance tower sites spanning North America to estimate VPRM parameters for these sites. We investigate spatial coherence in regional CO2 fluxes at several different time scales by using geostatistical methods to examine the spatial structure of model data-model residuals. We find that persistent spatial structure does exist in the data-model residuals at a length scale of approximately 1000 km. This spatial structure defines a flux-tower-based VPRM residual covariance matrix. The residual covariance matrix is useful in constructing prior fluxes for atmospheric CO2 concentration inversion calculations, as well as for constructing a VPRM North American CO2 flux map optimized to eddy covariance observations. Finally, the estimated VPRM parameter values do not separate clearly by plant functional type (PFT). This calls into question whether PFTs partition ecosystems by carbon cycle participation when the viewing lens is a simple model. [ABSTRACT FROM AUTHOR]

Atmospheric inversions provide estimates of carbon dioxide (CO2) fluxes between the surface and atmosphere based on atmospheric CO2 concentration observations. The number of CO2 observations is projected to increase severalfold in the next decades from expanding in situ networks and next‐generation CO2‐observing satellites, providing both an opportunity and a challenge for inversions. This study introduces the TRACE Regional Atmosphere–Carbon Ensemble (TRACE) system, which employ an ensemble‐based simultaneous state and parameter estimation (ESSPE) approach to enable the assimilation of large volumes of observations for constraining CO2 flux parameters. TRACE uses an online full‐physics mesoscale atmospheric model and assimilates observations serially in a coupled atmosphere–carbon ensemble Kalman filter. The data assimilation system was tested in a series of observing system simulation experiments using in situ observations for a regional domain over North America in summer. Under ideal conditions with known prior flux parameter error covariances, TRACE reduced the error in domain‐integrated monthly CO2 fluxes by about 97% relative to the prior flux errors. In a more realistic scenario with unknown prior flux error statistics, the corresponding relative error reductions ranged from 80.6% to 88.5% depending on the specification of prior flux parameter error correlations. For regionally integrated fluxes on a spatial scale of 106 km2, the sum of absolute errors was reduced by 34.5%–50.9% relative to the prior flux errors. Moreover, TRACE produced posterior uncertainty estimates that were consistent with the true errors. These initial experiments show that the ESSPE approach in TRACE provides a promising method for advancing CO2 inversion techniques. Plain Language Summary: To gain a better understanding of the main drivers behind atmospheric carbon dioxide (CO2) variations and trends, it is essential to accurately quantify CO2 exchanges—also known as fluxes—between the atmosphere and other components of the Earth system. It is generally not possible to directly measure surface CO2 fluxes at regional scales; however, fluxes can be inferred from a network of atmospheric CO2 concentration observations combined with atmospheric modeling and inversion methods, which seek to find the most likely fluxes given prior knowledge and observational evidence. This paper presents a new regional inversion framework called TRACE Regional Atmosphere–Carbon Ensemble (TRACE) for deriving CO2 fluxes at high resolution in space and time. One of the major innovations in TRACE is the ensemble‐based dual state and parameter estimation approach, which makes it computationally feasible to ingest large volumes of observations into the system. A series of experiments were carried out in a domain over North America to test the new framework in a controlled setting. In the experiments, we used synthetic tower observations of atmospheric CO2 concentrations to constrain parameters controlling terrestrial biogenic and oceanic CO2 fluxes. The results show that TRACE is capable of accurately estimating both the magnitude and spatial pattern of regional CO2 fluxes. Key Points: An ensemble‐based simultaneous state and parameter estimation approach is introduced for regional CO2 flux inversionsThe new dual‐state TRACE Regional Atmosphere–Carbon Ensemble (TRACE) framework provides a flexible platform for conducting coupled atmosphere–carbon data assimilationControlled experiments show that TRACE is effective at constraining regional CO2 fluxes using in situ CO2 concentration observations [ABSTRACT FROM AUTHOR]

Computational requirements often impose limitations on the spatial and temporal resolutions of atmospheric CO2 inversions, increasing aggregation and representation errors. This study enables higher spatial and temporal resolution inversions with spatial and temporal error structures similar to those used in other published inversions by representing the prior flux error covariances as a Kronecker product of spatial and temporal covariances and by using spectral methods for the spatial correlations. Compared to existing inversion systems that are forced to degrade the resolution of the problem in order to bring the dimensionality down to computationally tractable levels, this inversion framework is able to take advantage of mesoscale transport simulations and more of the complexity of spatial and temporal covariances in the surface CO2 fluxes. This approach was successfully implemented over one month with an identical‐twin observing system simulation experiment (OSSE) using a set of assumptions about the prior flux uncertainties compatible both with continental‐scale uncertainty estimates and with comparisons of vegetation models to flux towers. The demonstration illustrates the potential of the newly developed inversion system to use high‐temporal‐resolution information from the North American tower network, to extract high‐resolution information about CO2 fluxes that is inaccessible to coarser resolution inversion systems, and to simultaneously optimize an ensemble of prior estimates. This demonstration sets the stage for regional flux inversions that can take full advantage of the high‐resolution data available in tower CO2 records and mesoscale atmospheric transport reanalyses, include more realistic prior error structures, and explore specifying prior fluxes with ensembles. Plain Language Summary: This paper describes a new inverse framework for deriving surface carbon dioxide (CO2) fluxes at high spatial and temporal resolutions from atmospheric mole fraction measurements. We demonstrate the potential of the system over North America by inverting for subdaily biogenic CO2 fluxes while accounting for the complexity of spatio‐temporal structures in flux error covariances in a data‐based manner. Key Points: A new inversion system uses spectral methods for spatial correlations to do a 27 km subdaily inversion over North AmericaThe software package allows simultaneous optimization of multiple prior mean flux estimatesThe system is demonstrated over continental North America for a network of seven towers [ABSTRACT FROM AUTHOR]

This paper updates the results obtained earlier with the climate model developed at the A.M. Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences (IAP RAS CM) and describing the impact of atmospheric sulfur dioxide on the terrestrial carbon cycle. Since no global data are available for the near-surface SO2 concentration, a statistical model was used to reconstruct its concentration from appropriate data on surface sulfate concentrations; the coefficients of this model were tuned by using the output of the RAMS-CMAQ atmospheric chemistry and transport model. The results obtained in this study are generally consistent with data reported earlier. Specifically, the most significant SO2 impact on the terrestrial carbon cycle was found to be for southeastern North America and for Europe. However, this impact for southeastern Asia obtained in this study is considerably weaker than the value obtained earlier, which can be explained by the excessive moisture content in the atmosphere used in the IAP RAS CM for this region. [ABSTRACT FROM AUTHOR]

Comparisons of carbon uptake estimates from bottom-up terrestrial biosphere models (TBMs) to top-down atmospheric inversions help assess how well we understand carbon dioxide (CO 2) exchange between the atmosphere and terrestrial biosphere. Previous comparisons have shown varying levels of agreement between bottom-up and top-down approaches, but they have almost exclusively focused on large, aggregated scales (e.g., global or continental), providing limited insights into reasons for the mismatches. Here we explore how consistency, defined as the spread in net ecosystem exchange (NEE) estimates within an ensemble of TBMs or inversions, varies with at finer spatial scales ranging from 1∘×1∘ to the continent of North America. We also evaluate how well consistency informs accuracy in overall NEE estimates by filtering models based on their agreement with the variability, magnitude, and seasonality in observed atmospheric CO 2 drawdowns or enhancements. We find that TBMs produce more consistent estimates of NEE for most regions and at most scales relative to inversions. Filtering models using atmospheric CO 2 metrics causes ensemble spread to decrease substantially for TBMs, but not for inversions. This suggests that ensemble spread is likely not a reliable measure of the uncertainty associated with the North American carbon balance at any spatial scale. Promisingly, applying atmospheric CO 2 metrics leads to a set of models with converging flux estimates across TBMs and inversions. Overall, we show that multiscale assessment of the agreement between bottom-up and top-down NEE estimates, aided by regional-scale observational constraints is a promising path towards identifying fine-scale sources of uncertainty and improving both ensemble consistency and accuracy. These findings help refine our understanding of biospheric carbon balance, particularly at scales relevant for informing regional carbon-climate feedbacks. [ABSTRACT FROM AUTHOR]

Recent research has shown that the effect of human activities on climate can be characterised by a single statistic, called cumulative carbon. This statistic is the aggregate amount of carbon emitted in the form of carbon dioxide by activities such as fossilfuel burning and deforestation over the entire time such activities persist. In this paper, the concept is used to address the question of fair allocation of carbon emissions amongst nations or other emitting units. It is concluded that even if emissions prior to the year 2000 are lefi out of the accounting North America would have a just obligation to cease emissions completely in thirteen years, even if the emissions rate were frozen at its current level. China, India, and other developing nations could continue emissions for much longer before exhausting their fair share of the Carbon Commons. If historical emissions are fully taken into account, North America exceeded its fair share of usage in 1970, and has been in carbon overdraft ever since, whereas none of the major nations of the developing world have yet exceeded their fair shares. It is concluded that, based on principles of human equality, North America, and in particular the US, has a strong moral obligation to take the lead in actions that will ultimately reduce global carbon emissions. Western Europe has similar obligations, but has begun measures to take on a fair share of responsibility for emissions reduction, whereas such action has been largely absent in North America. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines two questions: (1) in a high oil price (and operating cost) environment what are typical breakeven prices for CO2? and, (2) are these prices sufficient to incentivize development of large-scale CCS projects? To address these questions we have developed an engineering-economic model for geological storage of CO2 through EOR. In this paper we briefly describe the performance and cost models for CO2-flood EOR, and use them to estimate the breakeven price for CO2 as a function of significant variables. In particular, the relationship between breakeven CO2 price and oil price is developed for four illustrative cases, all of which are, or were, operating EOR projects in North America. The sensitivity of the breakeven CO2 price to variability and uncertainty in reservoir characteristics and other model input parameters is also examined in detail for one of the cases. [Copyright &y& Elsevier]

In the beginnings of mechanical refrigeration, at the end of the nineteenth century, carbon dioxide was one of the first refrigerants to be used in compression-type refrigerating machines, later gaining widespread application mainly onboard refrigerated ships, but common in other sectors of refrigeration as well. It was only immediately after World War II that CO2 was rapidly eclipsed as a refrigerant, due to the advent of the synthesised halogenated working fluids, addressed as safe and ideal refrigerants at that time. Because of the stratospheric Ozone depletion environmental issue, CFC and HCFC working fluids are now in the process of being phased out of use under the Montreal Protocol. The Global Warming environmental issue casts concern over the use of the new HFC fluids as substitute refrigerants, because of their high GWP values, which make them subject to regulations under the Kyoto Protocol. In this mixed situation, CO2 is being revisited as a fully environmentally friendly and safe refrigerant. An intense research activity on its prospective applications is underway in many research establishments in Europe, Japan and North America, and important results have already been reached in exploiting the peculiar characteristics of this high-pressure fluid operated with a transcritical cycle. In some applications CO2 systems have already been commercialised; this applies to heat pump water heaters, as a brine in indirect systems and in the low temperature stage of cascade systems. The paper critically analyses the prospects for the future return of CO2 as a working fluid, or sometimes as a brine with change of phase, in important application areas. These include air conditioning and heat pump systems in the residential and commercial sectors, commercial and transport refrigeration and mobile air conditioning. [ABSTRACT FROM AUTHOR]

This paper examines long-term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under non-water-stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light-class carboxylase-based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process-based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange. [ABSTRACT FROM AUTHOR]

The eddy covariance method is widely used to investigate fluxes of energy, water, and carbon dioxide at landscape scales, providing important information on how ecological systems function. Flux measurements quantify ecosystem responses to environmental perturbations and management strategies, including nature‐based climate‐change mitigation measures. However, due to the high cost of conventional instrumentation, most eddy covariance studies employ a single system, limiting spatial representation to the flux footprint. Insufficient replication may be limiting our understanding of ecosystem behavior. To address this limitation, we deployed eight lower‐cost eddy covariance systems in two clusters around two conventional eddy covariance systems in the Chihuahuan Desert of North America for a period of 2 years. These dryland settings characterized by large temperature variations and relatively low carbon dioxide fluxes represented a challenging setting for eddy covariance. We found very good closure of energy and water balance across all systems (within ±9% of unity). We found very good correspondence between the lower‐cost and conventional systems' fluxes of sensible heat (with concordance correlation coefficient (CCC) of ≥0.87), latent energy (evapotranspiration; CCC ≥ 0.89), and useful correspondence in the net ecosystem exchange ((NEE); with CCC ≥ 0.4) at the daily temporal resolution. Relative to the conventional systems, the low‐frequency systems were characterized by a higher level of random error, particularly in the NEE fluxes. Lower‐cost systems can enable wider deployment affording better replication and sampling of spatiotemporal variability at the expense of greater measurement noise that might be limiting for certain applications. Replicated eddy covariance observations may be useful when addressing gaps in the existing monitoring of critical and underrepresented ecosystems and for measuring areas larger than a single flux footprint. Plain Language Summary: Vegetation around the world is critical to how our planet functions. Plants regulate how sunlight, rain, and even carbon dioxide are used and recycled. In order to learn about plant responses to rain, sunlight, temperature, and climate change, scientists measure the exchanges of water, energy, and carbon between the atmosphere and the land surface. However, the instruments used for these measurements are expensive, so they are only installed in a small number of locations around the entire planet. This is a problem for two reasons: First, these instruments are more common in more wealthy countries giving a distorted view of the world. Second, often only one instrument is installed, meaning we must assume that the area measured is exactly the same as the entire landscape, when this is often not true. To solve this problem, we built a much cheaper version of the instruments, which costs just one‐quarter of the usual price. To test our new instruments, we installed them next to the conventional instruments in two locations. We found strong agreement between our cheaper instruments and the high‐cost ones. This is good because cheaper instruments could be used more widely to improve the measurement of important ecosystems. Key Points: Lower‐cost, low‐frequency eddy covariance (EC) systems yield evapotranspiration and CO2 fluxes that correspond strongly with high‐frequency EC systemsLower‐cost EC systems enable increased spatial sampling and replication, improving confidence in retrieved evapotranspiration and CO2 fluxesLower‐cost EC systems may therefore address the undersampling and spatial biases in existing EC studies and networks [ABSTRACT FROM AUTHOR]

Urban regions emit a large fraction of anthropogenic emissions of greenhouse gases (GHG) such as carbon dioxide (CO2) and methane (CH4) that contribute to modern-day climate change. As such, a growing number of urban policymakers and stakeholders are adopting emission reduction targets and implementing policies to reach those targets. Over the past two decades research teams have established urban GHG monitoring networks to determine how much, where, and why a particular city emits GHGs, and to track changes in emissions over time. Coordination among these efforts has been limited, restricting the scope of analyses and insights. Here we present a harmonized data set synthesizing urban GHG observations from cities with monitoring networks across North America that will facilitate cross-city analyses and address scientific questions that are difficult to address in isolation. Measurement(s) carbon dioxide • methane • carbon monoxide Technology Type(s) spectroscopy Sample Characteristic - Environment city Sample Characteristic - Location North America [ABSTRACT FROM AUTHOR]

Throughout 2011–2012, Norit (now part of Cabot Corporation) performed five full-scale field trials at cement plants within North America. Despite differences in kiln configuration, raw mix composition, activated carbon injection (ACI) location, flue gas temperature, and particulate control systems, results show that test activated carbons significantly reduced mercury emissions. The test programs discussed in this report were developed to assist cement plants in meeting pending regulations for mercury emissions in a cost-effective and reliable manner. Several new carbon sorbents were evaluated with either improved mercury removal performance, concrete compatible characteristics, or both. Depending on the plant locations, EPA Method 30B and/or mercury CEMS were used to determine the mercury removal across the particulate control system. All carbon sorbents tested in the United States reduced mercury emissions to less than 55-lb mercury per million tons of clinker for all cases, thus meeting the cement emissions regulations for existing kilns. Carbon sorbents tested outside of the United States targeted mercury reduction requirements as mandated by the applicable regulating body. [ABSTRACT FROM AUTHOR]

This paper reports the first ebullitive fluxes of hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2) and total gaseous mercury (TGM) from the Cornwall Area of Concern (CAC). Although sediments were contaminated with mercury, bubbling was a negligible source of mercury for the global atmosphere. Indeed, the average emission of TGM through ebullition was 0.04 pg m−2 h−1. Measurements of H2, CO, CH4 and CO2 trapped gas concentrations and fluxes were used as indicators of diagenesis processes. The CAC represented a significant regional source of CH4 since the estimated ebullitive fluxes (3.5 mg m−2 h−1) were similar to the CH4 emissions measured above typical flooded freshwater wetlands. As molecular diffusion is known as the main pathway of CO2 transport from water to the atmosphere, CO2 ebullitive fluxes were weak (0.39 mg m−2 h−1). Bubbling of CO (1.6 μg m−2 h−1) was 10 folds less important than CO fluxes measured over flooded freshwater wetlands. Finally, H2 emissions (0.001 μg m−2 h−1) were negligible since the level of this trace gas is tightly regulated by microorganisms in anaerobic environments. [Copyright &y& Elsevier]

Feedbacks between the climate system and the carbon cycle represent a key source of uncertainty in model projections of Earth's climate, in part due to our inability to directly measure large-scale biosphere–atmosphere carbon fluxes. In situ measurements of the CO2 mole fraction from surface flasks, towers, and aircraft are used in inverse models to infer fluxes, but measurement networks remain sparse, with limited or no coverage over large parts of the planet. Satellite retrievals of total column CO2 (XCO2), such as those from NASA's Orbiting Carbon Observatory-2 (OCO-2), can potentially provide unprecedented global information about CO2 spatiotemporal variability. However, for use in inverse modeling, data need to be extremely stable, highly precise, and unbiased to distinguish abundance changes emanating from surface fluxes from those associated with variability in weather. Systematic errors in XCO2 have been identified and, while bias correction algorithms are applied globally, inconsistencies persist at regional and smaller scales that may complicate or confound flux estimation. To evaluate XCO2 retrievals and assess potential biases, we compare OCO-2 v10 retrievals with in situ data-constrained XCO2 simulations over North America estimated using surface fluxes and boundary conditions optimized with observations that are rigorously calibrated relative to the World Meteorological Organization X2007 CO2 scale. Systematic errors in simulated atmospheric transport are independently evaluated using unassimilated aircraft and AirCore profiles. We find that the global OCO-2 v10 bias correction shifts the distribution of retrievals closer to the simulated XCO2 , as intended. Comparisons between bias-corrected and simulated XCO2 reveal differences that vary seasonally. Importantly, the difference between simulations and retrievals is of the same magnitude as the imprint of recent surface flux in the total column. This work demonstrates that systematic errors in OCO-2 v10 retrievals of XCO2 over land can be large enough to confound reliable surface flux estimation and that further improvements in retrieval and bias correction techniques are essential. Finally, we show that independent observations, especially vertical profile data, such as those from the National Oceanic and Atmospheric Administration aircraft and AirCore programs are critical for evaluating errors in both satellite retrievals and carbon cycle models. [ABSTRACT FROM AUTHOR]

The spatial-seasonal distributions of carbon dioxide (CO2) data derived from satellite observation data are introduced into the coupled BNU-ESM (Beijing Normal University Earth System Model) global model to study the differences in global atmospheric temperature changes caused by the uniform and nonuniform distributions of CO2. A series of ensemble simulations are conducted under different CO2 distributions from 1850 to 2005. From the seasonal to interannual scale, the spatial distributions of CO2 concentration and surface air temperature between the nonuniform and uniform simulations are different. The CO2 concentrations in the nonuniform simulations are higher than those in the uniform simulations in most areas of the Northern Hemisphere and lower in almost all of the Southern Hemisphere; however, the air temperatures are higher in the nonuniform simulations than in the uniform simulations over most land areas, especially in North America. The amplitudes of the temperature increase in both ensemble runs (nonuniform-uniform) from 1986 to 2005 caused by increases in the CO2 concentration are smaller in the Southern Hemisphere than in the Northern Hemisphere. The amplitudes of the temperature increase over the two decades in the ensemble uniform simulation is approximately 0.2~0.3 °C higher than that in the ensemble nonuniform simulations at the global scale. [ABSTRACT FROM AUTHOR]

Carbon balance of terrestrial ecosystems in the northern high latitudes (NHL) is sensitive to climate change. It remains uncertain whether current regional carbon uptake capacity can be sustained under future warming. Here the atmospheric CO2 drawdown rate (CDR) between 1974 and 2014, defined as the CO2 decrease in ppm over the number of days in spring or summer, is estimated using atmospheric CO2 observations at Barrow (now known as Utqiaġvik), Alaska. We found that the sensitivity of CDR to interannual seasonal air temperature anomalies has trended toward less carbon uptake for a given amount of warming over this period. Changes in interannual temperature sensitivity of CDR suggest that relatively warm springs now result in less of a carbon uptake enhancement. Similarly, relatively warm summers now result in greater carbon release. These results generally agree with the sensitivity of net carbon exchange (NCE) estimated by atmospheric CO2 inversion. When NCE was aggregated over North America (NA) and Eurasia (EA), separately, the temperature sensitivity of NCE in NA has changed more than in EA. To explore potential mechanisms of this signal, we also examine trends in interannual variability of other climate variables (soil temperature and precipitation), satellite-derived gross primary production (GPP), and Trends in Net Land–Atmosphere Carbon Exchanges (TRENDY) model ensemble results. Our analysis suggests that the weakened spring sensitivity of CDR may be related to the slowdown in seasonal soil thawing rate, while the summer sensitivity change may be caused by the temporally coincident decrease in temperature sensitivity of photosynthesis. This study suggests that the current NHL carbon sink may become unsustainable as temperatures warm further. We also found that current carbon cycle models do not represent the decrease in temperature sensitivity of net carbon flux. We argue that current carbon–climate models misrepresent important aspect of the carbon–climate feedback and bias the estimation of warming influence on NHL carbon balance. [ABSTRACT FROM AUTHOR]

In this study, we investigate links between Arctic sea ice loss and the variability of 2-m temperatures over a 6-month period (November–April) over two domains centered over northern Eurasia and northern North America. Based on data from the Climate Forecast System Reanalysis (CFSR), there has been an increase (a decrease) in recent seasonal temperature variability over Eurasia (North America), which can be attributed to cooling (warming) during the winter months. Decreases in the intraseasonal variability of temperature anomalies, however, are noted in both regions for the November–April period. This study investigates the role of different forcings on the changes seen in the reanalysis product using Atmospheric Model Intercomparison Project simulations forced with repeating sea surface temperature, sea ice, and carbon dioxide concentration relative to climatologies from two different base periods, 1981–90 and 2005–14. The seasonal temperature and intraseasonal anomaly variabilities are examined, and we find that only the simulations with reduction in sea ice (2005–14 base-period sea ice concentration) produce significant decreases in intraseasonal temperature anomaly variability over these regions, agreeing with the CFSR analysis. Runs that reduce sea ice also result in a significant decrease in the frequency and magnitude of extreme warm and cold temperature anomalies. It is proposed that the weakened latitudinal temperature gradient, resulting from decreased sea ice, leads to reduced meridional temperature advection variability, which in turn contributes to the reduction in the variability of temperature anomalies. [ABSTRACT FROM AUTHOR]

This study systematically examines the regional uncertainties and biases in carbon dioxide (CO2) mole fractions from two of the state‐of‐the‐art global CO2 analysis products, namely, the Copernicus Atmosphere Monitoring Service (CAMS) real‐time atmospheric analysis from the European Centre for Medium‐Range Weather Forecasts (ECMWF) and the CarbonTracker Near‐Real‐Time (CT‐NRT) reanalysis from the National Oceanic and Atmospheric Administration (NOAA), by evaluation against hundreds of hours of airborne in situ measurements from the summer 2016 and winter 2017 Atmospheric Carbon and Transport (ACT)‐America field campaigns. Both the CAMS and CT‐NRT analyses agree reasonably well with the independent ACT‐America airborne CO2 measurements in the free troposphere, with root‐mean‐square deviations (RMSDs) between analyses and observations generally between 1 and 2 ppm but show considerably larger uncertainties in the atmospheric boundary layer where the RMSDs exceed 8 ppm in the lowermost 1 km of the troposphere in summer. There are strong variations in accuracy and bias between seasons, and across three different subregions in the United States (Mid‐Atlantic, Midwest, and South), with the largest uncertainties in the Mid‐Atlantic region in summer. Overall, the RMSDs of the CAMS and CT‐NRT analyses against airborne data are comparable to each other and largely consistent with the differences between the two analyses. The current study provides uncertainty estimates for both analysis products over North America and suggests that these two independent estimates can be used to approximate regional CO2 analysis uncertainties. Both statistics are important in future studies in quantifying the uncertainties in regional CO2 mole fraction and flux estimates. Key Points: Two global CO2 analysis products are compared with airborne in situ data collected during the first two ACT‐America field campaignsBoth analyses agree reasonably well with observations but show considerable biases in CO2 in the Mid‐Atlantic region during summer 2016The two independent analysis products can be used to quantify the overall analysis uncertainties in estimated CO2 mole fractions [ABSTRACT FROM AUTHOR]

We present observing system simulation experiments (OSSEs) to evaluate the impact of a proposed network of ground-based miniaturized laser heterodyne radiometer (mini-LHR) instruments that measure atmospheric column-averaged carbon dioxide (XCO2) with a 1 ppm precision. A particular strength of this passive measurement approach is its insensitivity to clouds and aerosols due to its direct sun pointing and narrow field of view (0.2 ∘). Developed at the NASA Goddard Space Flight Center (GSFC), these portable, low-cost mini-LHR instruments were designed to operate in tandem with the sun photometers used by the AErosol RObotic NETwork (AERONET). This partnership allows us to leverage the existing framework of AERONET's global ground network of more than 500 sites as well as providing simultaneous measurements of aerosols that are known to be a major source of error in retrievals of XCO2 from passive nadir-viewing satellite observations. We show, using the global 3-D GEOS-Chem chemistry transport model, that a deployment of 50 mini-LHRs at strategic (but not optimized) AERONET sites significantly improves our knowledge of global and regional land-based CO2 fluxes. This improvement varies seasonally and ranges 58 %–81 % over southern lands, 47 %–76 % over tropical lands, 71 %–92 % over northern lands, and 64 %–91 % globally. We also show significant added value from combining mini-LHR instruments with the existing ground-based NOAA flask network. Collectively, these data result in improved a posteriori CO2 flux estimates on spatial scales of ∼10 km 2 , especially over North America and Europe, where the ground-based networks are densest. Our studies suggest that the mini-LHR network could also play a substantive role in reducing carbon flux uncertainty in Arctic and tropical systems by filling in geographical gaps in measurements left by ground-based networks and space-based observations. A realized network would also provide necessary data for the quinquennial global stocktakes that form part of the Paris Agreement. [ABSTRACT FROM AUTHOR]

Abstract: Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here we construct such a budget for eastern North America using historical data, empirical models, remote sensing algorithms, and process‐based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they, respectively, make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters, and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling. [ABSTRACT FROM AUTHOR]

We explored the underlying patterns of temporal stream CO partial pressure ( pCO) variability using high-frequency sensors in seven disparate headwater streams distributed across the northern hemisphere. We also compared this dataset of >40,000 pCO records with other published records from lotic systems. Individual stream sites exhibited relatively distinct pCO patterns over time with few consistent traits across sites. Some sites showed strong diel variability, some exhibited increasing pCO with increasing discharge, whereas other streams had reduced pCO with increasing discharge or no clear response to changes in flow. The only 'universal' signature observed in headwater streams was a late summer pCO maxima that was likely driven by greatest rates of organic matter respiration due to highest annual temperatures. However, we did not observe this seasonal pattern in a southern hardwood forest site, likely because the region was transitioning from a severe drought. This work clearly illustrates the heterogeneous nature of headwater streams, and highlights the idiosyncratic nature of a non-conservative solute that is jointly influenced by physics, hydrology, and biology. We suggest that future researchers carefully select sensor locations (within and among streams) and provide additional contextual information when attempting to explain pCO patterns. [ABSTRACT FROM AUTHOR]

This empirical study investigates the determinants of CO2 emission in 18 countries of the Middle East and North African region covering the period from 1971 to 2009. The analysis is based on a dynamic panel data model employing the Generalized Method of Moments (GMM) technique. The potential determinants of carbon emissions identified are per capita gross domestic product, energy usage, energy consumption from fossil fuel, foreign direct investment, urbanization, industrial production, agricultural production and education level. The results show per capita gross domestic product, energy consumption based on fossil fuel, foreign direct investment and agriculture production have significant impact on the growth of carbon emissions in the region. [ABSTRACT FROM AUTHOR]

We evaluate North American carbon fluxes using a monthly global Bayesian synthesis inversion that includes well-calibrated carbon dioxide concentrations measured at continental flux towers. We employ the NASA Parametrized Chemistry Tracer Model (PCTM) for atmospheric transport and a TransCom-style inversion with subcontinental resolution. We subsample carbon dioxide time series at four North American flux tower sites for mid-day hours to ensure sampling of a deep, well-mixed atmospheric boundary layer. The addition of these flux tower sites to a global network reduces North America mean annual flux uncertainty for 2001-2003 by 20% to 0.4 Pg C yr compared to a network without the tower sites. North American flux is estimated to be a net sink of 1.2 ± 0.4 Pg C yr which is within the uncertainty bounds of the result without the towers. Uncertainty reduction is found to be local to the regions within North America where the flux towers are located, and including the towers reduces covariances between regions within North America. Mid-day carbon dioxide observations from flux towers provide a viable means of increasing continental observation density and reducing the uncertainty of regional carbon flux estimates in atmospheric inversions. [ABSTRACT FROM AUTHOR]

1 Natural forest systems constitute a major portion of the world's land area, and are subject to the potentially negative effects of both global climate change and invasion by exotic insects. A suite of invasive weevils has become established in the northern hardwood forests of North America. How these insects will respond to increasing CO2 or O3 is unknown. 2 The present study examined the effects of elevated atmospheric CO2 and O3 on the invasive weevil Polydrusus sericeus Schaller at the Aspen Free Air CO2 Enrichment (FACE) site near Rhinelander, Wisconsin. A performance assay was conducted in the laboratory during the summer of 2007 using mated pairs of P. sericeus fed a combination of aspen, birch and maple foliage. We recorded leaf area consumption, oviposition and adult longevity. We also conducted visual abundance surveys in the field from 2004 to 2007 on aspen and birch at Aspen FACE. 3 Elevated CO2, but not O3, significantly affected P. sericeus performance. Female, but not male, longevity was reduced under elevated CO2. Polydrusus sericeus also produced fewer eggs under elevated CO2 conditions compared with ambient conditions. Adult P. sericeus strongly preferred birch over both aspen and maple, regardless of fumigation treatment. 4 The effects of elevated CO2 on P. sericeus populations at Aspen FACE were minimal, and varied among years and host tree species. Polydrusus sericeus abundance was significantly greater on birch than aspen. Over the long term, elevated CO2 may reduce adult female longevity and fecundity of P. sericeus. Further studies are needed to evaluate how this information may scale to ecosystem impacts. [ABSTRACT FROM AUTHOR]

The Paleocene–Eocene Thermal Maximum (PETM; 55 Ma) is of particular interest since it is regarded as a suitable analog to future climate change. In this study, the PETM climate is investigated using the Community Climate System Model (CCSM3) with atmospheric CO2 concentrations of 4×, 8×, and 16× the preindustrial value. Simulated climate change from 4× to 8× atmospheric CO2 concentration, possibly corresponding to an environmental precursor of the PETM event, leads to a warming of the North Atlantic Ocean Intermediate-Water masses, thus lowering the critical depth for methane hydrate destabilization by ∼500 m. A further increase from 8× to 16×CO2, analogous to a possible massive methane hydrate release, results in global oceanic warming and stratification. The increase in the radiative surface warming, especially at high latitudes, is partially offset by a decrease in the ocean heat transport due to a reduced overturning circulation. Surface temperature values simulated in the 16×CO2 PETM run represent the closest match to surface temperature reconstructions from proxies for this period. Simulated PETM precipitation, characterized by a slight northward shift of the intertropical convergence zone, increases at higher CO2 concentrations, especially for the northern midlatitudes as well as the high latitudes in both hemispheres. Data-inferred precipitation values and gradients for North America and Spain, for instance, are in good agreement with the 16×CO2 simulation. Increasing atmospheric CO2 concentrations might also have favored the release of terrestrial methane through warmer and wetter conditions over land, thus reinforcing the greenhouse gas concentration increase. [ABSTRACT FROM AUTHOR]

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Refinements in the spatial and temporal resolution of North American fossil-fuel carbon dioxide (CO2) emissions provide additional information about anthropogenic aspects of the carbon cycle. In North America, the seasonal and spatial patterns are a distinctive component to characterizing anthropogenic carbon emissions. The pattern of fossil-fuel-based CO2 emissions on a monthly scale has greater temporal and spatial variability than the flux aggregated to the national annual level. For some areas, monthly emissions can vary by as much as 85% for some fuels when compared with monthly estimates based on a uniform temporal and spatial distribution. The United States accounts for the majority of North American fossil carbon emissions, and the amplitude of the seasonal flux in emissions in the United States is greater than the total mean monthly emissions in both Canada and Mexico. Nevertheless, Canada and Mexico have distinctive seasonal patterns as well. For the continent, emissions were aggregated on a 5° × 10° latitude–longitude grid. The monthly pattern of emissions varies on both a north–south and east–west gradient and evolves through the time period analyzed (1990–2007). For many areas in North America, the magnitude of the month-to-month variation is larger than the total annual emissions from land use change, making the characterization of emissions patterns essential to understanding humanity’s influence on the carbon cycle. [ABSTRACT FROM AUTHOR]

Synoptic variations of atmospheric CO2 produced by interactions between weather and surface fluxes are investigated mechanistically and quantitatively in midlatitude and tropical regions using continuous in-situ CO2 observations in North America, South America and Europe and forward chemical transport model simulations with the Parameterized Chemistry Transport Model. Frontal CO2 climatologies show consistently strong, characteristic frontal CO2 signals throughout the midlatitudes of North America and Europe. Transitions between synoptically identifiable CO2 air masses or transient spikes along the frontal boundary typically characterize these signals. One case study of a summer cold front shows CO2 gradients organizing with deformational flow along weather fronts, producing strong and spatially coherent variations. In order to differentiate physical and biological controls on synoptic variations in midlatitudes and a site in Amazonia, a boundary layer budget equation is constructed to break down boundary layer CO2 tendencies into components driven by advection, moist convection, and surface fluxes. This analysis suggests that, in midlatitudes, advection is dominant throughout the year and responsible for 60-70% of day-to-day variations on average, with moist convection contributing less than 5%. At a site in Amazonia, vertical mixing, in particular coupling between convective transport and surface CO2 flux, is most important, with advection responsible for 26% of variations, moist convection 32%, and surface flux 42%. Transport model sensitivity experiments agree with budget analysis. These results imply the existence of a recharge-discharge mechanism in Amazonia important for controlling synoptic variations of boundary layer CO2, and that forward and inverse simulations should take care to represent moist convective transport. Due to the scarcity of tropical observations at the time of this study, results in Amazonia are not generalized for the tropics, and future work should extend analysis to additional tropical locations. [ABSTRACT FROM AUTHOR]

We use the GEOS-Chem chemistry transport model to interpret the sources and sinks of CO2 that determine variability of column-averaged volume mixing ratios (CVMRs), as observed by the SCIAMACHY satellite instrument, during the 2003 North American growing season. GEOS-Chem generally reproduces the magnitude and seasonal cycle of observed CO2 surface VMRs across North America and is quantitatively consistent with column VMRs in later years. However, it cannot reproduce the magnitude or variability of FSI-WFM-DOAS SCIAMACHY CVMRs. We use model tagged tracers to show that local fluxes largely determine CVMR variability over North America, with the largest individual CVMR contributions (1.1%) from the land biosphere. Fuel sources are relatively constant while biomass burning makes a significant contribution only during midsummer. We also show that non-local sources contribute significantly to total CVMRs over North America, with the boreal Asian land biosphere contributing close to 1% in midsummer at high latitudes. We used the monthly-mean Jacobian matrix for North America to illustrate that: 1) North American CVMRs represent a superposition of many weak flux signatures, but differences in flux distributions should permit independent flux estimation; and 2) the atmospheric e-folding lifetimes for many of these flux signatures are 3-4 months, beyond which time they are too well-mixed to interpret. These long lifetimes will improve the efficacy of observed CVMRs as surface CO2 flux constraints. [ABSTRACT FROM AUTHOR]

Linear regression was used to relate modes of tropospheric circulation variability to estimates of gross ecosystem production (GEP) and ecosystem respiration (ER) measured at 14 midlatitude North American eddy covariance (EC) towers. The North Atlantic Oscillation (NAO) exhibited a north–south gradient in its effect on fluxes, with negative influence on fluxes at central and northeastern stations and positive influence on fluxes at southeastern stations. During spring, average values of GEP and ER within the northern “cold” sector decreased by 22 and 12 g C m-2 (18% and 11%), respectively, in response to a unit increase (+1 standard deviation) in the expansion coefficient of the NAO mode. Despite a northward advancement of the “warm” sector during summer, GEP and ER remained negatively correlated with the NAO at northern stations, decreasing on average by 48 and 30 g C m-2 (8% and 6%), respectively. During spring, the North Pacific Oscillation (NPO) reduced GEP and ER at central and northeastern stations on average by 20 and 7 g C m-2 (16% and 6%) and increased GEP and ER at southern and west coast stations on average by 53 and 49 g C m-2 (12% and 17%) in response to a unit increase in the NPO. This pattern persisted into summer, only shifted northward, with flux decreases of 19 and 24 g C m-2 (3% and 5%) at northern stations and increases of 72 and 82 g C m-2 (9% and 16%) at central stations. The direction of the flux response in each case was supported by synoptic conditions inferred from composite maps of North American circulation and gridded surface air temperature anomalies. The magnitude and timing of the relationships differed between stations and was attributed to differences in geographic location and plant functional type. Difficulty in the interpretation of significant correlations was attributed to the short sample length of typical EC records and unmodeled variability, including, for example, modulation by the NAO during high NPO. Despite these limitations, long-term monitoring EC stations show promise in characterizing the regional and ecosystem-specific carbon cycle response to low-frequency modes of tropospheric circulation variability and may play a critical role in validating ecosystem model responses to such phenomena. [ABSTRACT FROM AUTHOR]

The trillion barrel oil shale and oil sand deposits in North America offer the potential to make the less energy dependent on unreliable foreign sources. However, if these unconventional deposits were produced using existing combustion processes, substantial CO2 emissions would be injected into air. To avoid this green house gas problem and yet produce liquid fuels, a wind powered electrothermal energy storage system is described. It stores the unpredictable intermittent wind electrical energy as thermal energy over long periods in thick fossil hydrocarbon deposits. Because thermal diffusion time is very slow in such deposits, thermal energy is effectively trapped in a defined section of the hydrocarbon deposit. This allows time for the thermal energy to convert hydro-carbons into gaseous and liquid fuels. The process is highly energy. efficient and makes available considerably more energy than was expended during the heating. In addition, the method can increase the reliability of the grid and provide a load-leveling function. The wind-powered electrothermal conversion method produces substantially less CO2 than traditional shale oil extraction processes or renewable energy processes that employ a combustion step to produce the fuel. [ABSTRACT FROM AUTHOR]

The Eocene–Oligocene transition towards a cool climate (∼33.5 million years ago) was one of the most pronounced climate events during the Cenozoic era. The marine record of this transition has been extensively studied. However, significantly less research has focused on continental climate change at the time, yielding partly inconsistent results on the magnitude and timing of the changes. Here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high-resolution (about 40,000 years) continental temperature record for the Eocene–Oligocene transition. We find a large drop in mean annual temperature of 8.2 ± 3.1 °C over about 400,000 years, the possibility of a small increase in temperature seasonality, and no resolvable change in aridity across the transition. The large change in mean annual temperature, exceeding changes in sea surface temperatures at comparable latitudes and possibly delayed in time with respect to marine changes by up to 400,000 years, explains the faunal turnover for gastropods, amphibians and reptiles, whereas most mammals in the region were unaffected. Our results are in agreement with modelling studies that attribute the climate cooling at the Eocene–Oligocene transition to a significant drop in atmospheric carbon dioxide concentrations. [ABSTRACT FROM AUTHOR]

Many models in the framework of the Paleoclimate Modelling Intercomparison Project have undertaken simulations of the mid-Holocene (6 kyr ago) climate change. Analysis of the results have mainly focused on the North African summer monsoon that was enhanced 6 kyr ago, in all models, in response to the prescribed enhanced summer insolation. The magnitude of the simulated increase in total rainfall is very different, however, among the models, and so is the prescribed mean hot desert albedo, which varies from 19% to 38%. The appropriate prescription of hot desert's brightness, in the simulation of present-day climate, is known to be a key parameter since the work of Charney, which has been confirmed by many subsequent studies. There is yet no consensus, however, on the albedo climatological values to be used by climate modelers. Here, it is questioned whether changes in the prescription of hot desert albedo may also affect the simulated intensity of climate change. Using the Laboratoire de Météorologie Dynamique atmospheric general circulation model, two sets of simulations, with a mean hot desert albedo of respectively 35% and 28%, have been carried out. The simulated mid-Holocene summer monsoon change in northern Africa is significantly larger when the background hot desert albedo is the lowest (i.e., 28%). The associated increased northward penetration of monsoon rains allows a greater reduction of hot desert area that is in better agreement with paleodata. At least three good reasons have been found to explain these changes, one of them being that when hot desert albedo is relatively low, the atmosphere above is more unstable and the same increase in solar forcing leads to larger changes in precipitable water. The implication of such a study is that differences in models' responses to any external forcing (insolation, increased atmospheric CO[sub 2] , etc.) may be partly explained by differences in the prescription of land surface properties. Th... [ABSTRACT FROM AUTHOR]

Summary • Direct inhibition of dark respiration by elevated atmospheric concentrations of CO2 could alter the carbon balance of plants and ecosystems. The short-term response of leaf dark respiration to elevated CO2 concentrations are reported here in 12 grass and forb species of a North American grassland community. • Specific respiration rates at 25°C and a range of measurement CO2 concentrations were determined for detached leaves of each species field-grown in monoculture. • On average, respiration rates were 1.8% lower at 700 than at 360 µmol mol-1 CO2 . Among species, responses ranged from a 6.4% inhibition to a 2.4% stimulation and were generally not statistically significant. Across a range of CO2 concentrations from 360 to 1300 µmol mol-1 , respiration rates declined linearly and were 11% lower at 1300 than 360 µmol mol-1 CO2 . • Direct inhibition of leaf respiration is small compared with other longer-term, indirect effects of CO2 on carbon exchange. The direct effects of rising atmospheric CO2 concentrations on respiration rates should result in minimal effects on plant carbon exchange in grasslands. [ABSTRACT FROM AUTHOR]

Amaranthus retroflexus L. (Amaranthaceae), Redroot pigweed, is native to North America, but has become a weed of agriculture worldwide. Previous research into competition with food crops found it significantly reduces yields. Additionally, taxonomy, biomass allocation, physiological responses to light intensity, water stress, elevated CO2, and herbicide resistance have been investigated. To extend other research findings, we investigated growth and biomass yield in response to (i) soil moisture stress, and (ii) drought and elevated CO2. Additionally, we investigated seed germination rates following exposure to three elevated temperatures for two different time periods. Overall, moisture stress reduced plant height, stem diameter, and number of leaves. Elevated CO2 (700 ppm) appeared to reduce negative impacts of drought on biomass productivity. Heating seeds at 120 °C and above for either 180 or 300 s significantly reduced germination rate. These results inform an understanding of potential responses of A. retroflexus to future climate change and will be used to predict future occurrence of this weed. The finding that exposing seeds to high temperatures retards germination suggests fire could be used to prevent seed germination from soil seed banks, particularly in no-till situations, and therefore may be used to address infestations or prevent further spread of this weed. [ABSTRACT FROM AUTHOR]

Hydrologic margins of wetlands are narrow, transient zones between inundated and dry areas. As water levels fluctuate, the dynamic hydrology at margins may impact wetland greenhouse gas (GHG) fluxes that are sensitive to soil saturation. The Prairie Pothole Region of North America consists of millions of seasonally-ponded wetlands that are ideal for studying hydrologic transition states. Using a long-term GHG database with biweekly flux measurements from 88 seasonal wetlands, we categorized each sample event into wet to wet (W→W), dry to wet (D→W), dry to dry (D→D), or wet to dry (W→D) hydrologic states based on the presence or absence of ponded water from the previous and current event. Fluxes of methane were 5-times lower in the D→W compared to W→W states, indicating a lag 'ramp-up' period following ponding. Nitrous oxide fluxes were highest in the W→D state and accounted for 20% of total emissions despite accounting for only 5.2% of wetland surface area during the growing season. Fluxes of carbon dioxide were unaffected by transitions, indicating a rapid acclimation to current conditions by respiring organisms. Results of this study highlight how seasonal drying and re-wetting impact GHGs and demonstrate the importance of hydrologic transitions on total wetland GHG balance. [ABSTRACT FROM AUTHOR]