Atmospheric and Oceanic Sciences

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesise datasets and methodologies to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy and cement production data. Emissions from land-use change (ELUC) are estimated by bookkeeping models based on land-use data. The global atmospheric CO2 growth rate (GATM) is computed from changes in concentration measured at surface stations. The global net uptake of CO2 by the ocean (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based fCO2-products. The global net uptake of CO2 by the land (SLAND) is estimated with dynamic global vegetation models. Additional lines of evidence are provided by atmospheric inversions, atmospheric oxygen measurements, ocean interior observation-based estimates, and Earth System Models. This year, we introduced corrections on the ELUC, SOCEAN and SLAND estimates. The sum of all sources and sinks results in the carbon budget imbalance (BIM), a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as ± 1σ. For the year 2024, EFOS increased by 1.1 % relative to 2023, with fossil emissions at 10.3 ± 0.5 GtC yr−1 (including the cement carbonation sink, 0.2 GtC yr−1), ELUC was 1.3 ± 0.7 GtC yr−1, for total anthropogenic CO2 emissions of 11.6 ± 0.9 GtC yr−1 (42.4 ± 3.2 GtCO2 yr−1). Also, for 2024, GATM was 7.9 ± 0.2 GtC yr−1 (3.73 ± 0.1 ppm yr−1), 2.2 GtC above the 2023 growth rate. SOCEAN was 3.4 ± 0.4 GtC yr−1 and SLAND was 1.9 ± 1.1 GtC yr−1, leaving a large negative BIM (−1.7 GtC yr−1), suggesting that the total sink or GATM is strongly overestimated in 2024. The global atmospheric CO2 concentration averaged over 2024 reached 422.8 ± 0.1 ppm. Preliminary data for 2025 suggest an increase in EFOS relative to 2024 of +1.0 % (0.2 % to 1.7 %) globally, and atmospheric CO2 concentration increasing by 2.1 ppm reaching 425.6 ppm, 53 % above the pre-industrial level (around 278 ppm in 1750). Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2024, with a near-zero overall budget imbalance, although discrepancies of up to around 1 GtC yr−1 persist for the representation of annual to decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows: (1) a persistent large uncertainty in the estimate of land-use change emissions, (2) a low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) a discrepancy between the different methods on the mean ocean sink. This living data update documents changes in methods and datasets applied to this most-recent global carbon budget as well as evolving community understanding of the global carbon cycle. The data presented in this work are available at https://doi.org/10.18160/GCP-2025 (Friedlingstein et al., 2025c).

Abstract Despite its strong influence on relative sea-level (RSL) rise, there is still low confidence in estimates of vertical land motion (VLM) and its contribution to RSL change. To address this problem, we synergize diverse VLM data, which now cover almost 65% of the coastal population, and are key to resolve small scale subsidence, including East, South, and Southeast Asian cities and populated deltaic regions, largely not covered by earlier geodetic measurements. We find that the average modern (1995-2020) global RSL rise experienced by coastal populations (6 mm/year) is about twice the climate-driven absolute sea-level rise. This reflects a strong tendency for higher rates of subsidence in densely populated areas, with 71% of the global coastal population living in subsiding regions. Paired with community efforts to extend consistent observations, these data are essential to ensure reliable estimates of present and future RSL rise to support risk and adaptation assessment.

The collapse of Northern Hemisphere ice sheets has been deemed as a trigger of positive climate feedback during Quaternary glacial terminations. Increasing boreal summer insolation is widely considered the primary driver; however, the precise initiating mechanisms remain elusive. Here we report an unambiguous warming trend in the southern Nordic Seas since the late Last Glacial Maximum, coinciding with the marked increase in 65°N summer insolation, and subsequently followed by a distinct surface cooling linked to intense freshwater discharge. Our reconstructions indicate that the initial collapse during the Last Termination began within the Eurasian Ice Sheet. Increasing boreal insolation weakened the latitudinal insolation gradient and displaced the westerlies northward, promoting poleward oceanic heat transport and ensuing warming in the Nordic Seas. This warming accelerated the disintegration of marine-terminating glaciers of the Eurasian Ice Sheet, leading to a catastrophic meltwater release. The resulting freshwater perturbation contributed to the early weakening of the Atlantic Meridional Overturning Circulation and ultimately triggered the destabilization of the Laurentide Ice Sheet, further accelerating the deglaciation process. Rising boreal insolation enhanced poleward heat transport and Nordic Seas warming since the late LGM, triggering the initial Eurasian Ice Sheet collapse. The resulting meltwater led to early AMOC weakening and acceleration of the deglaciation process.

Abstract. The Arctic is warming nearly four times faster than the global average, leading to widespread permafrost thaw degradation with profound implications for ecosystems and infrastructure. While gradual permafrost thaw occurs over decades, abrupt thaw events – such as thermokarst formation or retrogressive thaw slumps – can rapidly alter ecosystems and severely damage infrastructure. Although abrupt thaw is increasingly widespread, comprehensive datasets that map its spatial distribution at regional scales for land managers and local governments are still lacking. To address this gap, we created the Alaska Permafrost Thaw Database, an open-access, collaborative database which compiles 19 540 permafrost thaw and thermokarst locations across Alaska from 44 sources, integrating field observations, remote sensing products, and the published literature. This database spans observations from 1950 through the present and incorporates datasets of varying spatial resolution, ranging from field-based point measurements to remotely sensed products (1–125 m), providing statewide coverage across Alaska. The dataset includes abrupt thaw features and sites experiencing gradual top-down thaw that can help to support comparative analysis and predictive modeling. We used this database to explore relationships between thaw type (abrupt vs. non-abrupt) and topographic metrics (i.e., slope, relative elevation, and potential incoming solar radiation), analyze the distribution of various thaw features across Alaska's major ecoregions, and compare the database to current spatial datasets of ground ice and Yedoma. Our analysis shows abrupt thaw features are more prevalent in lowlands and depressions while gradual top-down and lateral thaw features are more commonly associated with areas receiving higher potential incoming solar radiation such as south facing slopes and open clearings. We also found substantial mismatches between ice-driven thaw processes and existing ground ice and Yedoma maps, likely reflecting the coarse resolution of current mapping products relative to the fine-scale nature of field measurements and highlighting the limitations of current datasets for local-scale prediction. The database provides direct, empirical evidence of actively thawing and stable permafrost locations and can be used to inform and validate ground ice mapping. By comparing the database with physiographic characteristics and remotely sensed measurements, the database can guide future field campaigns in areas with little to no observations. As permafrost thaw transforms Arctic landscapes, high-resolution, accessible spatial data – such as our thaw database – will be critical for informing mitigation and adaptation strategies. The Alaska Permafrost Thaw Database is openly available at Zenodo (https://doi.org/10.5281/zenodo.16996415, Webb et al., 2025b), which provides a link to the GitHub repository and access to all versions; this paper describes version 2.0.0.

ABSTRACT To address the lack of research on the multidimensional characteristics and combined risks of humid heatwave‐heavy precipitation compound events, this study proposes a copula‐based compound humid heatwave and heavy precipitation joint risk analysis method (CHHRA) following the identification of such events. This method systematically assesses the multidimensional combined risks of humid heatwave‐heavy precipitation compound events in East China under both ‘simultaneous occurrence (AND)’ and ‘at least one occurrence (OR)’ scenarios. The main conclusions are as follows: (1) Under return periods of 20, 50 and 100 years, both the average intensity and duration of compound events increase significantly with global warming. Under high‐emission scenarios (SSP370 and SSP585), the increase is approximately double the historical level. (2) Spatially, compound events with higher average intensity and longer duration will concentrate in eastern coastal and southern regions. Meanwhile, under both ‘AND’ and ‘OR’ scenarios, extreme compound events with shorter return periods will predominantly occur in western inland and northern areas. (3) The combined risk under the SSP585 scenario is substantially higher than under SSP126; the recurrence intervals for 50‐year ‘AND’ and ‘OR’ events are reduced to within 30 and 5 years, respectively. These findings provide data support for the optimisation of extreme weather monitoring and early warning systems, and offer a scientific basis for policy formulation by emergency management authorities.

Central Asia (CA) experiences large interannual hydroclimate variability that profoundly affects ecosystems, agriculture, and socioeconomic stability. While the El Niño–Southern Oscillation (ENSO) has long been recognized as the leading driver, a significant proportion of observed CA precipitation variability remains unexplained. Here, we reveal that Antarctic ozone variability in October–November serves as a robust and independent precursor to winter CA precipitation (WCAP). Antarctic ozone modulates WCAP through two primary pathways. First, enhanced Antarctic ozone drives a persistent negative phase of the Southern Annular Mode (SAM) that persists into the following winter. The ensuing ozone–SAM coupling shifts two Ferrel cells equatorward and enhances low-level convergence and ascent over CA, thereby increasing WCAP. Second, the negative SAM induces central–eastern South Pacific warming, triggering Rossby wave trains that establish a meridional circulation dipole around CA and strengthen subtropical westerlies, moisture transport, and WCAP. Exposure analyses further indicate that combined ozone–ENSO effects substantially exacerbate precipitation-related socioeconomic risks across CA, adding ~34 billion person-days of population exposure and >34 billion dollar-days of economic exposure by mid-century relative to ENSO alone. Our findings identify Antarctic ozone variability as a critical yet previously underappreciated predictor of CA hydroclimate, revealing subtle interhemispheric connections beyond the traditional ENSO-centric paradigm.

Dissolved oxygen (DO), as a vital material sustaining aquatic ecosystems, has declined markedly in oceans, lakes, and coastal waters, yet unbiased understandings of changing DO concentrations in each individual river segment globally remain a challenge. Here, we estimate DO concentrations in 21,439 rivers globally between 1985 and 2023, based on Landsat observations and climatic data, and examine their patterns and trends. We find sustained deoxygenation in global rivers, at a rate of −0.045 mg liter −1 decade −1 , with 78.8% experiencing fluvial deoxygenation, driven mainly by oxygen solubility and temperature. Moreover, short-term heatwaves and dam impoundment exert non-neglecting influence on these changes. Future projections demonstrate that global fluvial DO concentrations decline by 1.1% ± 1.6% under SSP1–2.6 and 4.7% ± 2.7% under SSP5–8.5 throughout the 21st century. Our study provides an unbiased baseline for escalating deoxygenation in global fluvial ecosystems that underscores targeted measures to mitigate deoxygenation threats and protect ecosystem health.

Abstract Decreases in anthropogenic aerosols will reduce fine particulate matter (PM 2.5 ); however, meteorological feedbacks alter dust emissions, modifying air quality gains. We use eight Earth System Models from the Regional Aerosol Model Intercomparison Project (RAMIP) simulations to assess African climate and air quality responses to anthropogenic aerosol emission perturbations, including meteorological feedbacks on dust emissions. By 2050, African and global emissions reductions drive the largest continent‐average PM 2.5 decrease (0.92 ± 0.17 μg m −3 ; 5% and 1.35 ± 0.50 μg m −3 ; 7%, respectively) relative to SSP3‐7.0, though regional dust increases partially offset these reductions. Anthropogenic emissions reductions in the U.S. and Europe also lower African PM 2.5 by 0.29 ± 0.32 μg m −3 (2%) due to teleconnections of Northern Hemisphere warming influencing the Intertropical Convergence Zone. Inter‐model variability in dust and total PM 2.5 reflects differences in meteorological responses and dust emission parameterizations. Meteorological responses explain 90% of dust emissions variability across regions. Aerosol‐driven climate feedbacks on dust account for up to 70% of total PM 2.5 changes in the Sahara and Namib, offsetting up to 20% of anthropogenic PM 2.5 reductions across Africa. Under 2050 global and Africa‐wide anthropogenic aerosol reductions, 96,000 (95% CI: 54,000–137,000) and 84,000 (95% CI: 43,000–125,000) PM 2.5 ‐related deaths are avoided in Africa, respectively. Dust PM 2.5 contributes an uncertain 3.4% of the avoided deaths under global reductions and has no net effect under Africa‐wide reductions. Aerosol‐driven climate feedbacks may partially offset direct air quality gains, though their continental‐scale contribution remains small and uncertain.

Earthquakes of magnitude ( M ) >5.5 on oceanic transform faults (OTFs) repeatedly rupture the same locked patches, sometimes quasiperiodically. These patches are separated by “barriers” that halt earthquake propagation and slip mostly aseismically. However, the physical processes governing this systematic behavior remain unclear. We analyzed two barriers along the Gofar transform fault that have arrested ~15 M 6 earthquakes over the past three decades. Ocean bottom seismometer data indicate that the barriers hosted intense microseismicity before the mainshocks and comprise multistrand faults and transtensional stepovers with 100- to 400-m lateral offset. These characteristics contradict earthquake rupture termination models invoking velocity-strengthening friction or large geometric steps and instead point to damage-enhanced porosity and dilatancy-strengthening mechanisms. By isolating rupture segments, the barriers regulate the quasiperiodic recurrence of OTF earthquakes.

Declining nitrogen oxide (NO x = NO + NO 2 ) emissions have transformed oxidation pathways in urban atmospheres, with implications for air quality. Organic peroxy radicals (RO 2 ), key intermediates in volatile organic compound oxidation, typically react with NO to form ozone (O 3 ). Under lower-NO conditions, alternative RO 2 fates, including isomerization forming highly oxidized organic molecules (HOMs), can enhance secondary organic aerosol (SOA) production. We combine aircraft observations over four major North American cities with geostationary satellite data to characterize isoprene-derived RO 2 fate across urban environments. We infer RO 2 bimolecular lifetimes (τ bi ) as a proxy for isomerization potential, finding longer τ bi (17 ± 11 seconds) in New York, Chicago, and Toronto compared to Los Angeles (7 ± 6 seconds). Satellite measurements reveal that long τ bi is widespread across urban North America, suggesting that declining NO x is likely to lead to greater HOM formation in urban regions. These findings indicate that atmospheric models omitting RO 2 isomerization chemistry may incorrectly simulate organic oxidation and the subsequent oxidation state of volatile organic compounds and SOA.