Eric J. Morgan

The global ocean and land biosphere absorb roughly half of fossil fuel carbon dioxide (CO ₂ ) emissions. The temporal evolution of these sinks remains difficult to quantify owing to data sparsity. Leveraging 30+ years of atmospheric oxygen (O ₂ ) and CO ₂ observations, we find that the ocean carbon sink increased by 0.63 ± 0.19 petagrams of carbon per year per decade (Pg C year ⁻¹ decade ⁻¹ ) (a mean of 2.8 ± 0.4 Pg C year ⁻¹ ), accounting for 35% of fossil emissions between 1990 and 2019. The net land sink shows no robust trend (with a mean of 1.0 ± 0.5 Pg C year ⁻¹ , accounting for 13% of fossil emissions), despite large decadal variability. These results challenge many process-based model simulations of the land and ocean sinks, questioning their ability to accurately predict future climate states.

The ocean is the largest natural sink for anthropogenic CO ₂ and excess heat, and this uptake drives its acidification, warming, and deoxygenation. Air-sea heat, oxygen, and carbon exchanges are linked through solubility and biological cycling, yet their budgets are built independently and rarely checked for consistency. Here we use the Ocean Circulation Inverse Model (OCIM) to quantify three coefficients that couple these budgets and constrain the climate-driven CO ₂ flux independently of the anthropogenic sink. The thermal coupling of air-sea CO ₂ flux to ocean heat content change is -0.167 ± 0.020 PgC (10 ²² J) ^-1 , only 54% of the expected solubility-driven outgassing from warming, because waters subduct before equilibrating with the atmosphere. The corresponding thermal O ₂ flux coefficient (-14.20 ± 0.38 Tmol (10 ²² J) ^-1 ) achieves 96% of the solubility-driven response, reflecting the rapid O ₂ equilibration. The non-thermal O ₂ :CO ₂ flux coupling coefficient (-2.24 ± 0.29) is more negative than the assumed Redfield O ₂ :C ratio of -1.4, reflecting the slower equilibration of CO ₂ . These coefficients, combined with observed ocean heat uptake and deoxygenation over 2005-2024, yield a thermal CO ₂ outgassing of 0.20 ± 0.03 PgC yr ^-1 and a non-thermal CO ₂ uptake of 0.20 ± 0.09 PgC yr ^-1 , giving a net climate-driven CO ₂ uptake of 0.01 ± 0.09 PgC yr ^-1 , whereas the ocean biogeochemical models (GOBM) in the Global Carbon Budget 2025 (GCB2025) simulate a source of 0.16 ± 0.05 PgC yr ^-1 . Combined with the OCIM anthropogenic CO ₂ sink estimate, our total ocean CO ₂ uptake is significantly larger than the GOBM ensemble in GCB2025

Tropical lands play an important role in the global carbon cycle yet their contribution remains uncertain owing to sparse observations. Satellite observations of atmospheric carbon dioxide (CO₂) have greatly increased spatial coverage over tropical regions, providing the potential for improved estimates of terrestrial fluxes. Despite this advancement, the spread among satellite‐based and in‐situ atmospheric CO₂ flux inversions over northern tropical Africa (NTA), spanning 0–24°N, remains large. Satellite‐based estimates of an annual source of 0.8–1.45 PgC yr⁻¹ challenge our understanding of tropical and global carbon cycling. Here, we compare posterior mole fractions from the suite of inversions participating in the Orbiting Carbon Observatory 2 (OCO‐2) Version 10 Model Intercomparison Project (v10 MIP) with independent in‐situ airborne observations made over the tropical Atlantic Ocean by the National Aeronautics and Space Administration (NASA) Atmospheric Tomography (ATom) mission during four seasons. We develop emergent constraints on tropical African CO₂ fluxes using flux‐concentration relationships defined by the model suite. We find an annual flux of 0.14 ± 0.39 PgC yr⁻¹ (mean and standard deviation) for NTA, 2016–2018. The satellite‐based flux bias suggests a potential positive concentration bias in OCO‐2 B10 and earlier version retrievals over land in NTA during the dry season. Nevertheless, the OCO‐2 observations provide improved flux estimates relative to the in situ observing network at other times of year, indicating stronger uptake in NTA during the wet season than the in‐situ inversion estimates.

Seasonal change of atmospheric potential oxygen (APO ∼ O₂ + CO₂) is a tracer for air‐sea O₂ flux with little sensitivity to the terrestrial exchange of O₂ and CO₂. In this study, we present the tropospheric distribution and inventory of APO in each hemisphere with seasonal resolution, using O₂ and CO₂ measurements from discrete airborne campaigns between 2009 and 2018. The airborne data are represented on a mass‐weighted isentropic coordinate (M_θe) as an alternative to latitude, which reduces the noise from synoptic variability in the APO cycles. We find a larger seasonal amplitude of APO inventory in the Southern Hemisphere relative to the Northern Hemisphere, and a larger amplitude in high latitudes (low M_θe) relative to low latitudes (high M_θe) within each hemisphere. With a box model, we invert the seasonal changes in APO inventory to yield estimates of air‐sea flux cycles at the hemispheric scale. We found a larger seasonal net outgassing of APO in the Southern Hemisphere (518 ± 52.6 Tmol) than in the Northern Hemisphere (342 ± 52.1 Tmol). Differences in APO phasing and amplitude between the hemispheres suggest distinct physical and biogeochemical mechanisms driving the air‐sea O₂ fluxes, such as fall outgassing of photosynthetic O₂ in the Northern Hemisphere, possibly associated with the formation of the seasonal subsurface shallow oxygen maximum. We compare our estimates with four model‐ and observation‐based products, identifying key limitations in these products or in the tools used to create them.

Tropical lands play an important role in the global carbon cycle yet their contribution remains uncertain owing to sparse observations. Satellite observations of atmospheric carbon dioxide (CO ₂ ) have greatly increased spatial coverage over tropical regions, providing the potential for improved estimates of terrestrial fluxes. Despite this advancement, the spread among satellite-based and in-situ atmospheric CO ₂ flux inversions over northern tropical Africa (NTA), spanning 0-24◦N, remains large. Satellite-based estimates of an annual source of 0.8-1.45 PgC yr ⁻¹ challenge our understanding of tropical and global carbon cycling. Here, we compare posterior mole fractions from the suite of inversions participating in the Orbiting Carbon Observatory 2 (OCO-2) Version 10 Model Intercomparison Project (v10 MIP) with independent in-situ airborne observations made over the tropical Atlantic Ocean by the NASA Atmospheric Tomography (ATom) mission during four seasons. We develop emergent constraints on tropical African CO ₂ fluxes using flux-concentration relationships defined by the model suite. We find an annual flux of 0.14 ± 0.39 PgC yr ⁻¹ (mean and standard deviation) for NTA, 2016-2018. The satellite-based flux bias suggests a potential positive concentration bias in OCO-2 B10 and earlier version retrievals over land in NTA during the dry season. Nevertheless, the OCO-2 observations provide improved flux estimates relative to the in situ observing network at other times of year, indicating stronger uptake in NTA during the wet season than the in-situ inversion estimates.

Understanding terrestrial ecosystems and their response to anthropogenic climate change requires quantification of land‐atmosphere carbon exchange. However, top‐down and bottom‐up estimates of large‐scale land‐atmosphere fluxes, including the northern extratropical growing season net flux (GSNF), show significant discrepancies. We developed a data‐driven metric for the GSNF using atmospheric carbon dioxide concentration observations collected during the High‐Performance Instrumented Airborne Platform for Environmental Research Pole‐to‐Pole Observations and Atmospheric Tomography Mission flight campaigns. This aircraft‐derived metric is bias‐corrected using three independent atmospheric inversion systems. We estimate the northern extratropical GSNF to be 5.7 ± 0.3 Pg C and use it to evaluate net biosphere productivity from the Coupled Model Intercomparison Project phase 5 and 6 (CMIP5 and CMIP6) models. While the model‐to‐model spread in the GSNF has decreased in the CMIP6 models relative to that of the CMIP5 models, there is still disagreement on the magnitude and timing of seasonal carbon uptake with most models underestimating the GSNF and overestimating the length of the growing season relative to the observations. We also use an emergent constraint approach to estimate annual northern extratropical gross primary productivity to be 56 ± 17 Pg C, heterotrophic respiration to be 25 ± 13 Pg C, and net primary productivity to be 28 ± 12 Pg C. The flux inferred from these aircraft observations provides an additional constraint on large‐scale gross fluxes in prognostic Earth system models that may ultimately improve our ability to accurately predict carbon‐climate feedbacks.

Understanding terrestrial ecosystems and their response to anthropogenic climate change requires quantification of land-atmosphere carbon exchange. However, top-down and bottom-up estimates of large-scale land-atmosphere fluxes, including the northern extratropical growing season net flux (GSNF), show significant discrepancies. We develop a data-driven metric for the GSNF using atmospheric carbon dioxide concentration observations collected during the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom) flight campaigns. This aircraft-derived metric is bias corrected using three independent atmospheric inversion systems. We estimate the northern extratropical GSNF to be 5.7 ± 0.2 Pg C and use it to evaluate net biosphere productivity from the Coupled Model Intercomparison Project phase 5 and 6 (CMIP5 and CMIP6) models. While the model-to-model spread in the GSNF has decreased in CMIP6 models relative to that of the CMIP5 models, there is still disagreement on the magnitude and timing of seasonal carbon uptake with most models underestimating the GSNF and overestimating the length of the growing season relative to the observations. We also use an emergent constraint approach to estimate annual northern extratropical gross primary productivity to be 56 ± 15 Pg C, heterotrophic respiration to be 25 ± 11 Pg C, and net primary productivity to be 28 ± 10 Pg C. The flux inferred from these aircraft observations provides an additional constraint on large-scale, gross fluxes in prognostic Earth system models that may ultimately improve our ability to accurately predict carbon-climate feedbacks.

Up in the air: Understanding ocean-atmospheric carbon dioxide (CO ₂ ) fluxes in the Southern Ocean is necessary for quantifying the global CO ₂ budget, but measurements in the harsh conditions there make collecting good data difficult, so a quantitative picture still is out of reach. Long et al . present measurements of atmospheric CO ₂ concentrations made by aircraft and show that the annual net flux of carbon into the ocean south of 45°S is large, with stronger summertime uptake and less wintertime outgassing than other recent observations have indicated. —HJS

The air‐sea exchange of oxygen (O₂) is driven by changes in solubility, biological activity, and circulation. The total air‐sea exchange of O₂ has been shown to be closely related to the air‐sea exchange of heat on seasonal timescales, with the ratio of the seasonal flux of O₂ to heat varying with latitude, being higher in the extratropics and lower in the subtropics. This O₂/heat ratio is both a fundamental biogeochemical property of air‐sea exchange and a convenient metric for testing earth system models. Current estimates of the O₂/heat flux ratio rely on sparse observations of dissolved O₂, leaving it fairly unconstrained. From a model ensemble we show that the ratio of the seasonal amplitude of two atmospheric tracers, atmospheric potential oxygen (APO) and the argon‐to‐nitrogen ratio (Ar/O₂), exhibits a close relationship to the O₂/heat ratio of the extratropics (40–). The amplitude ratio, /, is relatively constant within the extratropics of each hemisphere due to the zonal mixing of the atmosphere. / is not sensitive to atmospheric transport, as most of the observed spatial variability in the seasonal amplitude of APO is compensated by similar variations in (Ar/). From the relationship between /heat and / in the model ensemble, we determine that the atmospheric observations suggest hemispherically distinct /heat flux ratios of 3.3 0.3 and 4.7 0.8 nmol between 40 and in the Northern and Southern Hemispheres respectively, providing a useful constraint for and heat air‐sea fluxes in earth system models and observation‐based data products.

Eastern Boundary Upwelling Systems (EBUSs) are coastal hotspots of the potent greenhouse gas nitrous oxide (N₂O). However, estimates of their emissions suffer from large uncertainties due to their significant spatial and temporal heterogeneity. Here, we derive the first multiyear, monthly resolution N₂O emissions from three of the four major EBUSs using high‐frequency coastal atmospheric measurements and an inverse method. We find average combined N₂O emissions from the northern California, Benguela, and southern Canary upwelling systems to be 57.7 (51.4–63.9) Gg‐N yr⁻¹. We also find an offshore region near the Benguela EBUS that exhibits large pulses of emissions with emissions that reach 677 Gg‐N yr⁻¹ in 1 month. Our findings highlight that atmospheric measurements coupled with inverse modeling can capture the large variability in EBUS emissions by quantifying emissions over large spatial distances and over long time periods compared to previous methods using traditional oceanographic measurements.