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The CO2 seasonal cycle amplitude (SCA) in the Northern Hemisphere has increased since the 1960s--a feature attributed mainly to enhanced vegetation activity along climate warming and CO2 increase. We identified a temporal change in the sign of the correlation between SCA and air temperature (T) from positive to negative around the year 2000 at most Northern Hemisphere ground stations, consistent with signals from satellite column CO2 measurements since the mid-2000s. Further, we explored potential causes of this change using net biome productivity estimates from three atmospheric inversions for the period 1980-2015. The change in the SCA-T relationship is primarily attributable to changes in the net biome productivity-T relationship: positive correlations weakened in the spring in the high latitudes, confirming a limit to the "warmer spring-bigger carbon sink" mechanism; negative correlations diminished in the autumn/winter in the mid-to-high latitudes, challenging the "warmer winter-larger carbon release" assumption and highlighting the complexity of carbon processes outside the peak growing season. [ABSTRACT FROM AUTHOR]

Vegetation Optical Depth (VOD) has emerged as a valuable metric to quantify water stress on vegetation's carbon uptake from a remote sensing perspective. However, existing spaceborne microwave remote sensing platforms face limitations in capturing the diurnal VOD variations and global products lack site‐level validation against plant physiology. To address these challenges, we leveraged the Global Navigation Satellite System (GNSS) L‐band microwave signal, measuring its attenuation by the canopy of a temperate broadleaf forest using a pair of GNSS receivers. This approach allowed us to collect continuous VOD observations at a sub‐hourly scale. We found a significant seasonal‐scale correlation between VOD and leaf water potential. The VOD diurnal amplitude is affected by soil moisture, plant transpiration and leaf surface water. Additionally, VOD can help independently estimate plant transpiration. Our findings pave the way for a deeper understanding of response of the vegetation to water stress at finer temporal scales. Plain Language Summary: Microwave satellite measurements offer valuable insights into Vegetation Optical Depth (VOD), which reflects the amount of water present in plants. However, most VOD products currently available only offer observations at two times of the day, which fail to capture the variations of VOD over the course of the day. In this study, we installed Global Navigation Satellite System (GNSS) signal receivers at the top and bottom of a temperate broadleaf forest canopy located at a flux tower in Missouri, USA. This setup allowed us to obtain continuous observations of VOD at sub‐hourly scale. The strong correlation observed between GNSS VOD and local leaf water potential suggests that this robust and cost‐effective technique can complement other labor‐intensive measurements. The VOD diurnal amplitude is influenced by soil water content, evapotranspiration, and leaf surface water in the form of dew and interception. GNSS VOD also exhibits the potential to aid in estimating plant transpiration, which can be further used for partitioning evapotranspiration. The investigation of plant water content dynamics in this study will enhance our understanding of the carbon‐water coupling at a finer temporal scale. Key Points: Sub‐hourly Vegetation Optical Depth (VOD) retrieved from Global Navigation Satellite Systems attenuation relates to soil and leaf water dynamicsDaily maximum VOD correlates well with field measurements of predawn leaf water potentialPlant transpiration can be reconstructed from VOD principal components [ABSTRACT FROM AUTHOR]

The coupled chemistry of methane, carbon monoxide (CO), and hydroxyl radical (OH) can modulate methane's 9‐year lifetime. This is often ignored in methane flux inversions, and the impacts of neglecting interactive chemistry have not been quantified. Using a coupled‐chemistry box model, we show that neglecting the effect of methane source perturbation on [OH] can lead to a 25% bias in estimating abrupt changes in methane sources after only 10 years. Further, large CO emissions, such as from biomass burning, can increase methane concentrations by extending the methane lifetime through impacts on [OH]. Finally, we quantify the biases of including (or excluding) coupled chemistry in the context of recent methane and CO trends. Decreasing CO concentrations, beginning in the 2000's, have notable impacts on methane flux inversions. Given these nonnegligible errors, decadal methane emissions inversions should incorporate chemical feedbacks for more robust methane trend analyses and source attributions. Plain Language Summary: Methane inversion studies commonly assume that atmospheric methane has a 9‐year lifetime, but the decay rate of methane perturbations can be extended by 40%. This effect is from interactions of other atmospheric compounds with methane's main sink, the hydroxyl radical. This is important for estimating global emissions over recent decades. We show that one of these compounds, carbon monoxide (CO), emitted from wildfires during El Niño, can lead to large increases in methane concentrations by extending the methane lifetime. Moreover, ignoring these effects can lead up to a 25% error in estimating methane emissions changes after a decade. Finally, we show that the effect of decreasing CO on methane has reduced the methane lifetime and has led to potential biases in calculating methane emissions. Thus, attributing causes of recent methane emissions trends are dependent on the consideration of compounds indirectly affecting the methane lifetime, which may have implications for future mitigation plans. Key Points: Neglecting chemical feedbacks can bias estimates of methane emissions perturbations by up to 25% over 10 yearsStrong biomass burning events, such as El Niño, can indirectly increase the methane growth rate through emissions of COAttributions of decadal trends in methane are dependent on the assumptions about both OH and CO [ABSTRACT FROM AUTHOR]

Methods developed to explore the luminescent properties of the moon facilitated the development of techniques to infer terrestrial solar‐induced chlorophyll fluorescence (SIF) from satellite instruments. While single SIF retrievals are inherently noisy, averaging many retrievals allows us to obtain highly accurate estimates. We analyzed several years of aggregated SIF data collected by the TROPOspheric Monitoring Instrument (TROPOMI) and the Orbiting Carbon Observatory‐2 (OCO‐2) over nonvegetated areas to explore the potential of SIF retrievals beyond the realm of photosynthesis. The fundamentally different retrievals at varying wavelengths in the near‐infrared reveal that about 10% of all barren surfaces are weakly luminescent, while a few areas luminesce strongly—amounts comparable to SIF from vegetation. By means of lithological maps, we attributed the strongest luminescence signals to exposed carbonate sedimentary rocks. Besides a detailed evaluation of the signal properties, we discussed implications for SIF data sets and other remote sensing products. Plain Language Summary: During the last decade, advancements in remote sensing techniques allowed us to infer solar‐induced chlorophyll fluorescence, a faint electromagnetic signal emitted by vegetation, from space‐borne measurements. Here, we explored the potential of such measurements to also observe mineral luminescence, a signal that is typically even weaker than fluorescence from vegetation, which explains why it has not been documented to be detectable from space. We identified luminescent regions by means of several years of vegetation fluorescence data derived from two satellites (OCO‐2 and Sentinel‐5P) over nonvegetated areas and analyzed the composition of luminescent surfaces. According to our analysis, about 10% of all barren areas on Earth are weakly luminescent, whereas some areas display surprisingly strong signals. Key Points: First report of mineral luminescence observed from spaceStrongest signals originate from carbonate sedimentary rocksEmission appears to be spectrally flat in the near‐infrared and Lambertian [ABSTRACT FROM AUTHOR]

Methods developed to explore the luminescent properties of the moon facilitated the development of techniques to infer terrestrial solar‐induced chlorophyll fluorescence (SIF) from satellite instruments. While single SIF retrievals are inherently noisy, averaging many retrievals allows us to obtain highly accurate estimates. We analyzed several years of aggregated SIF data collected by the TROPOspheric Monitoring Instrument (TROPOMI) and the Orbiting Carbon Observatory‐2 (OCO‐2) over nonvegetated areas to explore the potential of SIF retrievals beyond the realm of photosynthesis. The fundamentally different retrievals at varying wavelengths in the near‐infrared reveal that about 10% of all barren surfaces are weakly luminescent, while a few areas luminesce strongly—amounts comparable to SIF from vegetation. By means of lithological maps, we attributed the strongest luminescence signals to exposed carbonate sedimentary rocks. Besides a detailed evaluation of the signal properties, we discussed implications for SIF data sets and other remote sensing products. Plain Language Summary: During the last decade, advancements in remote sensing techniques allowed us to infer solar‐induced chlorophyll fluorescence, a faint electromagnetic signal emitted by vegetation, from space‐borne measurements. Here, we explored the potential of such measurements to also observe mineral luminescence, a signal that is typically even weaker than fluorescence from vegetation, which explains why it has not been documented to be detectable from space. We identified luminescent regions by means of several years of vegetation fluorescence data derived from two satellites (OCO‐2 and Sentinel‐5P) over nonvegetated areas and analyzed the composition of luminescent surfaces. According to our analysis, about 10% of all barren areas on Earth are weakly luminescent, whereas some areas display surprisingly strong signals. Key Points: First report of mineral luminescence observed from spaceStrongest signals originate from carbonate sedimentary rocksEmission appears to be spectrally flat in the near‐infrared and Lambertian [ABSTRACT FROM AUTHOR]

Timely and accurate monitoring of crops is essential for food security. Here, we examine how well solar‐induced chlorophyll fluorescence (SIF) can inform crop productivity across the United States. Based on tower‐level observations and process‐based modeling, we find highly linear gross primary production (GPP):SIF relationships for C4 crops, while C3 crops show some saturation of GPP at high light when SIF continues to increase. C4 crops yield higher GPP:SIF ratios (30–50%) primarily because SIF is most sensitive to the light reactions (does not account for photorespiration). Scaling to the satellite, we compare SIF from the TROPOspheric Monitoring Instrument (TROPOMI) against tower‐derived GPP and county‐level crop statistics. Temporally, TROPOMI SIF strongly agrees with GPP observations upscaled across a corn and soybean dominated cropland (R2 = 0.89). Spatially, county‐level TROPOMI SIF correlates with crop productivity (R2 = 0.72; 0.86 when accounting for planted area and C3/C4 contributions), highlighting the potential of SIF for reliable crop monitoring. Plain Language Summary: Crop monitoring is essential for ensuring food security, but reliable, instantaneous production estimates at the global scale are lacking. The monitoring of crop production in a changing climate is of paramount importance to sustainable food security. Accurate estimates of crop production are dependent on adequately quantifying crop photosynthesis. Our paper demonstrates that solar‐induced chlorophyll fluorescence (SIF), an emission of red to far‐red light from chlorophyll is highly correlated with crop photosynthesis. We show that a new high spatial resolution satellite SIF data set is highly correlated with crop productivity in the United States, which is benchmarked by the United States Department of Agriculture county‐level crop statistics. These results will improve the understanding of crop production and carbon flux over agricultural lands, as well as provide an accurate, large‐scale, and timely monitoring method for global crop production estimates. Key Points: The photosynthetic pathway (C3, C4) impacts the relationship between CO2 uptake and SIF, which helps to interpret satellite signalsTROPOMI SIF agrees well with the seasonality of crop gross primary production (GPP) when accounting for C3/C4 fractionationTROPOMI SIF is highly correlated with USDA reported crop productivity at the county scale [ABSTRACT FROM AUTHOR]

We quantify the impact of atmospheric transport and limited marine boundary layer sampling on changes in global and regional methane burdens estimate using tracer transport model simulations with annually repeating methane emissions and sinks but varying atmospheric transport patterns. We find the 1σ error due to this transport and sampling effect on annual global methane increases to be 1.11 ppb/year and on zonal growth rates to be 3.8 ppb/year, indicating that it becomes more critical at smaller spatiotemporal scales. We also find that the trends in inter‐hemispheric and inter‐polar difference of methane are significantly influenced by the effect. Contrary to a negligible trend in the inter‐hemispheric difference of measurements, we find, after adjusting for the transport and sampling, a trend of 0.37 ± 0.06 ppb/year. This is consistent with the emission trend from a 3‐D inversion of the measurements, suggesting a faster increase in emissions in the Northern Hemisphere than in the Southern Hemisphere. Plain Language Summary: Changes in global and regional atmospheric burdens of methane are determined by the net effect of sources and sinks and atmospheric transport. Many studies approximate the burdens based on measurements from a network of globally distributed surface air sampling sites. Here we quantify the impact of atmospheric transport and limited marine boundary layer sampling (transport and sampling) on changes in global and regional methane burden estimates using tracer transport model simulation with annually repeating methane emissions and sinks but varying atmospheric transport patterns. We find that for assessing the annual global methane increases, a measurement‐only approach is fairly accurate. However, extending the measurement‐only analysis to hemispheric or latitudinal variations is more problematic as transport influences quickly become significant or even dominant. We find a large impact of transport and sampling effect on the inter‐hemispheric and inter‐polar difference. Key Points: Transport gives rise to CH4 variations in the spatiotemporal gradients, which are not caused by source or sink changesWe find the transport and sampling error on annual global CH4 increases to be 1.11 ppb/year and on zonal growth rates to be 3.8 ppb/yearThe transport‐adjusted inter‐hemispheric difference has a trend of 0.37 ppb/year, consistent with the emission trend from a 3‐D inversion [ABSTRACT FROM AUTHOR]

Abstract: To assess global carbon cycle variability, we decompose the net land carbon sink into the sum of gross primary productivity (GPP), terrestrial ecosystem respiration (TER), and fire emissions and apply a Bayesian framework to constrain these fluxes between 1980 and 2014. The constrained GPP and TER fluxes show an increasing trend of only half of the prior trend simulated by models. From the optimization, we infer that TER increased in parallel with GPP from 1980 to 1990, but then stalled during the cooler periods, in 1990–1994 coincident with the Pinatubo eruption, and during the recent warming hiatus period. After each of these TER stalling periods, TER is found to increase faster than GPP, explaining a relative reduction of the net land sink. These results shed light on decadal variations of GPP and TER and suggest that they exhibit different responses to temperature anomalies over the last 35 years. [ABSTRACT FROM AUTHOR]

Satellite estimates of burned area, associated carbon monoxide (CO) emission estimates, and CO column retrievals do not agree on the peak fire month in Africa, evident in both Northern and Southern Africa though distinct in the burning seasonality. Here we analyze this long‐standing problem using (1) a top‐down Bayesian inversion of Measurements Of Pollution In The Troposphere CO columns during 2005–2016 into surface CO emissions and (2) the bottom‐up Global Fire Emissions Database 4.1 s. We show that Global Fire Emissions Database 4.1 s underestimates CO emissions by 12–62% in the late fire season and hypothesize that this is partly because it assumes seasonally static emission factors. However, the degree to which emission factors would have to vary through the season to bring top‐down and bottom‐up in agreement cannot be confirmed by past field‐based measurements. Improved observational constraint on the seasonality of burned area, fuel combustion, and emission factors would further reduce the discrepancy between bottom‐up and top‐down emission estimates. Plain Language Summary: Fire is an integral component of African ecosystems, which emits large amounts of trace gases, pollutants, and aerosols into the atmosphere. The accurate assessment of this impact is currently hampered by the poor knowledge of fire emission estimates in Africa. One known issue is that the bottom‐up estimates of carbon monoxide (CO) emissions using satellite‐based burned area are generally underestimated and also describe a CO emission peak 1–2 months earlier than satellite CO column retrievals and their inversion into emissions. Here we used a Bayesian atmospheric inversion approach to analyze the seasonal variation in CO emissions, CO emission factors, and combustion efficiency of African savanna fires. The results lead us to hypothesize that under actual conditions, flaming‐ and smoldering‐dominated combustions tend to appear successively when a moving fire front passes through a savanna. This mechanism is not sufficiently considered in current bottom‐up fire emission models, which partly explains the emission underestimate and a phase shift to the early season in the seasonality of bottom‐up emissions data. Key Points: CO emission factors of African fires likely rise up higher than the seasonally constant value used by GFED in the late fire seasonThe increase in emission factors is partly due to a flaming to smoldering transition in relation with rainfall and fire radiative powerThe flaming to smoldering transition partly explains why the peak month in African fire emissions occurs later than GFED estimates [ABSTRACT FROM AUTHOR]