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 (E-FOS) are based on energy and cement production data. Emissions from land-use change (E-LUC) are estimated by bookkeeping models based on land-use data. The global atmospheric CO2 growth rate (G(ATM)) is computed from changes in concentration measured at surface stations. The global net uptake of CO2 by the ocean (S-OCEAN) is estimated with global ocean biogeochemistry models and observation-based fCO(2)-products. The global net uptake of CO2 by the land (S-LAND) 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 E-LUC, S-OCEAN and S-LAND estimates. The sum of all sources and sinks results in the carbon budget imbalance (B-IM), a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as +/- 1 sigma. For the year 2024, E-FOS 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)), E-LUC 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 GtCO(2) yr(-1)). Also, for 2024, G(ATM) was 7.9 +/- 0.2 GtC yr(-1) (3.73 +/- 0.1 ppm yr(-1)), 2.2 GtC above the 2023 growth rate. S-OCEAN was 3.4 +/- 0.4 GtC yr(-1) and S-LAND was 1.9 +/- 1.1 GtC yr(-1), leaving a large negative B-IM (-1.7 GtC yr(-1)), suggesting that the total sink or G(ATM) 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 E-FOS 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.

Organic carbon remineralization rate (OCRR) and the calcium carbonate production rate (CCPR) are influential variables on the efficiency of the biological carbon pump (BCP) but are not well understood in Red Sea. We used historical cruise data of carbonate chemistry, oxygen, and transient tracers from five locations along the north–south central axis of the Red Sea to estimate OCRR and CCPR from tracer‐based water mean ages (Γ), apparent oxygen utilization (AOU), and alkalinity utilization (AU). This resulted in the first basin‐wide and depth‐resolving (100–1,000 m) OCRR and CCPR estimates. Spatial distributions for Γ, AOU, and AU were strongly influenced by the large‐scale circulation and showed maxima intermediate depths (400–500 m). Conversely, OCRR and CCPR showed no statistically significant latitudinal differences and peaked (6.5 ± 4.3 and 11.9 ± 4.6 mmol C m −3 yr −1 , respectively) at 100‐m depth, which decreased to nearly constant values (3.8 ± 0.7 and 1.4 ± 0.3 mmol C m −3 yr −1 , respectively) at 300 m and deeper. By depth‐integrating CCPR, we estimated annual calcium carbonate production (CCP) of (0.8 ± 0.3) × 10 12 mol, or 0.6% of global ocean production, in the Red Sea, which has only 0.12% of the world ocean area. High correlation between AU and Γ indicated in situ alkalinity removal taking place also in subsurface and deep waters, probably due to chemical precipitation, which has been previously reported for the area. CCP‐induced AU affects the carbonate chemistry in the Red Sea water column, and we hypothesize that it also impacts that of the Gulf of Aden through the outflowing Red Sea Outflow Water.

We evaluate changes in dissolved inorganic carbon (DIC) in the Greenland Sea between 2002 and 2016, a period characterized by increasing convection depths. We find a mid-depth maximum in anthropogenic carbon (Cant) accumulation that occurred as waters at these depths were rejuvenated by deeper reaching convection; broadly, these waters have caught up with the atmospheric CO2 rise that had happened between the last time they were ventilated and 2002 and also tracked the atmospheric CO2 rise 2002-2016. The overlying waters only tracked the atmospheric CO2 rise 2002-2016. The mid-depth maximum in Cant accumulation was not evident in estimates generated with commonly used multiple linear regression (MLR) methods. We analyze the reasons why and show that the eMLR(C*) method may not fully capture nonsteady state changes in Cant when applied along a single hydrographic section as done here. This nonsteady component equates to redistribution of C*, whose spatial gradients in the Greenland Sea are dominated by Cant. We also show that the regular extended multiple linear regression method is sensitive to loss of spatial DIC gradients, which now happens as more and more Cant enters the ocean. Our findings demonstrate that MLR-based estimates of the Cant accumulation rate should not be taken at face value in highly dynamical ocean regions, such as the Greenland Sea, and the need for also considering the total change in DIC and how this is affected by natural processes. Further investigations into the ability of MLR methods to reproduce nonsteady state changes in Cant are encouraged. The ocean holds vast quantities of carbon. Each year this inventory increases as the ocean absorbs a quarter of our CO2 emissions. Keeping track of ocean carbon is a key climate change research priority. Observations from the Greenland Sea indicate at first glance a steady rise in DIC concentrations in the upper approximately 1,500-2,000 m of the water column, roughly equal to what one would expect from the atmospheric CO2 rise. This is unusually deep compared to the rest of the global ocean but reflects the deep-water formation that occurs in this region. A closer inspection of the data, however, reveals that the seemingly uneventful rise in carbon in this region is the net result of several counteracting processes. In response to deeper convection, mid-depth waters have lost inorganic carbon generated by the remineralization of organic matter, natural carbon. This has been counteracted by an unusually large rise in their content of man-made, or anthropogenic carbon. Widely adopted methods for estimating decadal rises in anthropogenic carbon struggle to quantify these changes, such that our ability to detect the nature of effects of climate variability and change on the efficiency of the ocean carbon sink can be questioned. Deeper convection caused a mid-depth maximum in the rate of anthropogenic carbon increase in the Greenland Sea from 2002 to 2016 The mid-depth maximum in anthropogenic carbon accumulation was not evident in estimates generated with multiple linear regression methods Nonsteady state anthropogenic carbon accumulation may bias the eMLR(C*) method when applied along a single hydrographic section

Maps of surface water fugacity of CO2 (fCO(2)) over eastern Fram Strait, south-western Nansen Basin, and the north-western Barents Sea (73-84 degrees N, 5-46 degrees E) from September 1997 to December 2020 were made and used to investigate seasonal and temporal trends. The mapping utilized a neural network technique, the self-organizing map (SOM), that was trained with different combinations of satellite/observational/model data of sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), chlorophyll a (Chl a), sea ice concentration, and atmospheric mole fraction of CO2 (xCO(2)). The trained SOM was labelled with available surface ocean fCO(2) data, and the labelled SOM was subsequently used to map the fCO(2). The produced maps reveal that fCO(2) in northern Barents Sea, at the border of the Nansen Basin, has increased significantly over the last decades by between 4.2 and 5.5 +/- 0.6-1.1 mu atm yr(-1) over the winter to summer seasons. These rates are twice the rate of atmospheric CO2 increase, which was about 2 mu atm yr(-1). The spatial pattern coincides with the strongest decreases in sea ice concentration as well as with a salinification of the surface water. The former allows for a prolongation of the air-sea CO2 flux with resultant oceanic CO2 uptake in previously ice-covered waters, and the latter is caused by a shift from Arctic Water dominance to more saline waters containing more dissolved inorganic carbon, most likely of Atlantic Water origin although brine-release influenced deep water may also contribute.

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 synthesize data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (E-FOS) are based on energy statistics and cement production data, while emissions from land-use change (E-LUC), mainly deforestation, are based on land-use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (G(ATM)) is computed from the annual changes in concentration. The ocean CO2 sink (S-OCEAN) is estimated with global ocean biogeochemistry models and observation-based fCO(2) products. The terrestrial CO2 sink (S-LAND) is estimated with dynamic global vegetation models. Additional lines of evidence on land and ocean sinks are provided by atmospheric inversions, atmospheric oxygen measurements, and Earth system models. The resulting carbon budget imbalance (B-IM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as +/- 1 sigma. For the year 2022, E-FOS increased by 0.9% relative to 2021, with fossil emissions at 9.9 +/- 0.5 GtC yr(-1) (10.2 +/- 0.5 GtC yr(-1) when the cement carbonation sink is not included), and E-LUC was 1.2 +/- 0.7 GtC yr(-1), for a total anthropogenic CO2 emission (including the cement carbonation sink) of 11.1 +/- 0.8 GtC yr(-1) (40.7 +/- 3.2 GtCO(2) yr(-1)). Also, for 2022, G(ATM) was 4.6 +/- 0.2 GtC yr(-1) (2.18 +/- 0.1 ppm yr(-1); ppm denotes parts per million), S-OCEAN was 2.8 +/- 0.4 GtC yr(-1), and S-LAND was 3.8 +/- 0.8 GtC yr(-1), with a B-IM of 0.1 GtC yr(-1) (i.e. total estimated sources marginally too low or sinks marginally too high). The global atmospheric CO2 concentration averaged over 2022 reached 417.1 +/- 0.1 ppm. Preliminary data for 2023 suggest an increase in E-FOS relative to 2022 of +/- 1:1% (0.0% to 2.1 %) globally and atmospheric CO2 concentration reaching 419.3 ppm, 51% above the pre-industrial level (around 278 ppm in 1750). Overall, the mean of and trend in the components of the global carbon budget are consistently estimated over the period 1959-2022, with a near-zero overall budget imbalance, although discrepancies of up to around 1 Gt Cyr(-1) persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows the following: (1) a persistent large uncertainty in the estimate of land-use changes 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 strength of the ocean sink over the last decade. This living-data update documents changes in methods and data sets 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-2023 (Friedlingstein et al., 2023).

<div> <p><span data-contrast="auto">Formulating appropriate monitoring programs for offshore geological CO</span><sub><span data-contrast="auto">2</span></sub><span data-contrast="auto"> storage projects, from either a regulatory or operator viewpoint, is difficult to achieve without a properly quantified cost-benefit analysis of what that monitoring could and should achieve. In addition, communicating risks and uncertainties is a challenge for offshore storage projects, and tools assisting in dialogue with stakeholders, governments and public at large will be of value.  </span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">The monitoring programs will have a role in communicating risks and benefits for storage projects and assure against unjustified accusations for having adverse environmental effects but cannot be seen in isolation from the multi-leveled CCUS (Carbon Capture, Usage and Storage) management systems.</span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">Evaluations of CO</span><sub><span data-contrast="auto">2</span></sub><span data-contrast="auto"> storage monitoring techniques usually aim to determine the suitability to user‐defined project scenario (e.g., IEAGHG monitoring selection tool, <span lang="FR">https://ieaghg.org/ccs-resources/monitoring-selection-tool</span></span><span data-contrast="auto">) or to assess the availability of sensors that can measure variables that are likely to fluctuate under a seepage scenario, or processes that are sensitive to CO</span><sub><span data-contrast="auto">2</span></sub><span data-contrast="auto">-related stress. Less focus has been on how they perform relative to regulatory requirements, cost efficiency, and user friendliness. </span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">We can use observations and models to characterise the natural variability of the marine system, or the noise from which an anomalous signal must be detected. We can use models to simulate hypothetical leak events thereby defining the monitoring target(s). We have algorithms that assess the cost-benefit of a range of anomaly criteria – i.e., a signal that would provoke a more concerted monitoring campaign and finally algorithms that can derive the optimal deployment strategy – i.e., where to monitor and when. The challenge is to collate these abilities into a coherent whole, which then allows the presentation of an evaluated monitoring system that can be judged against regulatory and societal expectations.</span><span data-ccp-props="{"> </span></p> </div> <div> <p>We outline the approach chosen in the ACTOM project (<span lang="FR">https://actom.w.uib.no)</span> to develop procedures for design and execution of appropriate, rigorous, and cost-effective monitoring of offshore carbon storage, aligning industrial, societal, and regulative expectations with technological capabilities and limitations. </p> </div> <div> <p><span data-contrast="auto">The ACTOM toolbox is capable of simulating “what if” seep scenarios, as well as monitoring deployments, that can be used to deliver environmental impact assessments as required under the CCS (Carbon Capture and Storage) and EIA directives. As a result, recommended monitoring strategies could be delivered autonomously and be dependent on established generic operational marine simulation models, both factors reducing costs. </span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">We will demonstrate use of the toolbox on three sites in the Gulf of Mexico, in southern North Sea and off the coast of Norway, each with distinctive features and availability of data. </span><span data-ccp-props="{"> </span></p> </div>

Due to low calcium carbonate saturation states, and winter mixing that brings anthropogenic carbon to the deep ocean, the Nordic Seas and their cold-water corals are vulnerable to ocean acidification. Here, we present a detailed investigation of the changes in pH and aragonite saturation in the Nordic Seas from preindustrial times to 2100, by using in situ observations, gridded climatological data, and projections for three different future scenarios with the Norwegian Earth System Model (NorESM1-ME). During the period of regular ocean biogeochemistry observations from 1981–2019, the pH decreased with rates of 2–3 × 10−3 yr−1 in the upper 200 m of the Nordic Seas. In some regions, the pH decrease can be detected down to 2000 m depth. This resulted in a decrease in the aragonite saturation state, which is now close to undersaturation in the depth layer of 1000–2000 m. The model simulations suggest that the pH of the Nordic Seas will decrease at an overall faster rate than the global ocean from the preindustrial era to 2100, bringing the Nordic Seas' pH closer to the global average. In the esmRCP8.5 scenario, the whole water column is projected to be undersaturated with respect to aragonite at the end of the 21st century, thereby endangering all cold-water corals of the Nordic Seas. In the esmRCP4.5 scenario, the deepest cold-water coral reefs are projected to be exposed to undersaturation. Exposure of all cold-water corals to corrosive waters can only be avoided with marginal under the esmRCP2.6 scenario. Over all timescales, the main driver of the pH drop is the increase in dissolved inorganic carbon (CT) caused by the raising anthropogenic CO2, followed by the temperature increase. Thermodynamic salinity effects are of secondary importance. We find substantial changes in total alkalinity (AT) and CT as a result of the salinification, or decreased freshwater content, of the Atlantic water during all time periods, and as a result of an increased freshwater export in polar waters in past and future scenarios. However, the net impact of this decrease (increase) in freshwater content on pH is negligible, as the effects of a concentration (dilution) of CT and AT are canceling.

The North Sea is one of the busiest and most exploited marine areas on Earth and is home to many highly migratory and economically important species. At the northern boundary, a 300-km corridor between Norway and Shetland forms a major egress point from the North Sea to the Norwegian Sea and broader Atlantic Ocean, which is known to be used by many focal species including Atlantic salmon, European eel, European sturgeon, Atlantic bluefin tuna, basking shark, Atlantic mackerel, spiny dogfish, among others. We argue that this relatively shallow 300-km corridor is a critical area for ecological and oceanographic research in the North Sea to understand species distribution, migratory patterns, responses to climate, fisheries, and more. Instrumentation of the Bergen-Shetland Corridor with a line of oceanographic and biological tracking infrastructure would help capture the spatiotemporal dynamics of the ocean and its major fauna between the North Sea and Norwegian Sea, a boundary between management areas from ICES and OSPAR.

Carbon capture and storage is a key mitigation strategy proposed for keeping the global temperature rise below 1.5 degrees C. Offshore storage can provide up to 13% of the global CO2 reduction required to achieve the Intergovernmental Panel on Climate Change goals. The public must be assured that potential leakages from storage reservoirs can be detected and that therefore the CO2 is safely contained. We conducted a controlled release of 675 kg CO2 within sediments at 120 m water depth, to simulate a leak and test novel detection, quantification and attribution approaches. We show that even at a very low release rate (6 kg day(-1)), CO2 can be detected within sediments and in the water column. Alongside detection we show the fluxes of both dissolved and gaseous CO2 can be quantified. The CO2 source was verified using natural and added tracers. The experiment demonstrates that existing technologies and techniques can detect, attribute and quantify any escape of CO2 from sub-seabed reservoirs as required for public assurance, regulatory oversight and emissions trading schemes.

Optimised marine monitoring strategies for CCS can be developed through several computational approaches. Many of these methods exist already including simulating hypothetical release events through models of hydrodynamics and biogeochemistry, quantifying highly sensitive criteria to distinguish from anomalous background conditions as an anomaly indicator, and the use of complex algorithms or machine learning to increases the certainty of detecting anomalies whilst utilising the most cost effective mobile or stationary monitoring platform distributions. However, one of the biggest challenges is the accessibility of these approaches, including closed source programs and inaccessible data, along with the need for programming skills and high-powered supercomputers. We integrate a number of these approaches and make them accessible through the ACTOM toolbox, the end-product of which is to aid users in defining a monitoring plan that will satisfy local stakeholders The toolbox uses pre-existing hydrodynamic and marine biogeochemical data that can be readily attained from existing high-resolution models and simulations, combined with available in-situ measurements, allowing the methodologies to be applied coherently to multiple offshore storage sites and to answer specific stakeholder led questions around required sensitivity and assurance levels. The ACTOM toolbox is designed to provide value over a range of field cases with diverse subsurface geology and environmental marine characteristics. Here we present the technical details of the ACTOM toolbox, and its components. The preparation steps for various sites, and how the toolbox can address regional concerns and be tuned to local characteristics is presented in another contribution to GHGT-16.

The ACTOM toolbox is designed to provide value over a range of field cases with diverse subsurface geology and environmental marine characteristics. Whereas the end-product of the toolbox is to aid users in defining a monitoring plan that will satisfy local stakeholders, one intermediate outcome from our study is to understand how data quality and availability as well as site variability will affect the development of the monitoring plan. For this assessment we collect and use available data from three representative sites as input to the toolbox: the Gulf of Mexico and northern and southern North Sea to visualize how the output from the semi- automated planning toolbox works in practice. In an ideal setting, hydrodynamic and marine biogeochemical data can be readily attained from calibrated high-resolution models and simulations, combined with available in-situ measurements. The process of identifying data availability, and later gathering and pre-processing the data, will have to be site specific. The technical details of the toolbox, and its components will be presented in another contribution to GHGT-16, here we will demonstrate the preparation steps for the three sites, and how the toolbox can address regional concerns and be tuned to local characteristics.

Towards better understanding of carbon and oxygen biogeochemical rates in the Red Sea Salma Elageed1,3 , A M. Omar2, Emil Jeansson2, Elsheikh B. Ali1 , Ingunn Skjelvan2 , Knut Barthel3 , Truls Johannessen3, P.Zhai4 1Institute of Marine Research, Red Sea University, Port Sudan, Sudan 2 NORCE, Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway 3 Geophysical Institute, University of Bergen, Bergen, Norway 4 Geoscience Dept., Princeton University, USA Abstract The Red Sea is one of the warmest and saltiest seas in the world, with surface water temperatures of 26–30°C and salinities of 36–41. The sea gains heat in the south and loses heat in the north and this gives a large-scale thermohaline circulation pattern with a northward surface flow and a southward flow at sill depth. At smaller spatial scales, along-coastal currents and upwelling occur. Here we summarise the main results from two studies that are parts of a PhD-study. We demonstrate how multi-spatial scale circulation and biological processes influence rates of: air-sea flux of carbon dioxide (CO2), oxygen utilization (OU), and removal of total alkalinity by calcification and sedimentation, i.e., alkalinity utilization (AU). In the first study, based on cruise data collected in the Red Sea in 2011 and 1982 (Aegaeo and MEROU cruises, respectively), we combine depth profiles of tracer-based water mass ages, AU, and OU to derive the first-ever basin-wide, long time integrated utilization rates of alkalinity (AUR) and oxygen (OUR). Results reveal that the large-scale circulation impacts the water masse ages and OU while remineralization of organic matter and calcification also influences in depth variations of OU and AU. The highest rates for OUR and AUR occur in the surface water followed by a swift attenuation of the rates towards zero for AUR and ~5 µmol kg-1 for OUR at 500 m depth. In the second study, new carbon and hydrography data from the Sudanese coastal Red Sea were used to investigate seasonal dynamics of sea surface partial pressure of CO2 (pCO2) and air–sea CO2 exchange. The results show that seasonal pCO2 change was primarily driven by temperature changes while along-coast advection, upwelling of CO2-rich deep water, and uptake of atmospheric CO2 also contributed to changes in dissolved inorganic carbon and total alkalinity. Furthermore, based on a compilation of historical and our new data, the region seems to have transformed from being a source of CO2 to the atmosphere throughout the year to becoming a sink of CO2 during parts of the year.

The dynamics of sea surface pCO2 (pCO2w) and air–sea CO2 exchange of the Sudanese coastal Red Sea has for the first time been studied over a full annual cycle (October 2014–October​ 2015) based on semi-continuous measurements from moored autonomous sensors. pCO2w showed a seasonal amplitude of approximately 70 μatm, overlaid by a high frequency (3-4 days) signal of around 10 μatm. The highest values, of about 440 μatm occurred during summer and fall, while the lowest values of about 370 μatm occurred during winter. The monthly pCO2w change was primarily driven by temperature, i.e., heating and cooling of the water surface. Additionally, Dissolved Inorganic Carbon (DIC) and Total Alkalinity (AT) contributed significantly to the observed change in pCO2w as a consequence of along-coast advection and upwelling of CO2-rich deep water, and likely biological production, and uptake of atmospheric CO2. The area is a net annual source for atmospheric CO2 of 0.180 ± 0.009 mol CO2 m−2 y−1. Based on a compilation of historic and our new data, altogether covering the years 1977 to 2015, long term trends of pCO2w were determined for the seasons winter–spring (1.75 ± 0.72 μatm y−1) and summer -fall (180 ± 0.41 μatm y−1), both weaker than the atmospheric trend (1.96 ± 0.02 μatm y−1). We are suggesting that the study region has transformed from being a source of CO2 to the atmosphere throughout the year to becoming a sink of CO2 during parts of the year. The long term pCO2w trend was to a large degree driven by increasing DIC, but increasing AT and temperature also played a role.

Over the last few years a number of computational approaches have been developed that enable the optimisation of marine monitoring strategies for CCS. These include methods to: simulate and quantify hypothetical release events, identify highly sensitive criteria by which to distinguish anomalous biogeochemistry which may indicate a seep and define minimum deployments of monitoring platforms which guarantee an acceptable chance of detection. In order for these tools to be operationally useful it is necessary to place them in an accessible and integrated digital format that allows the methodologies to be applied coherently to multiple offshore storage sites and to answer specific stakeholder led questions around required sensitivity, cost and assurance levels. In this work we describe progress towards achieving this integration as well as the site specific data necessary to generate bespoke monitoring guidance.

Environmental monitoring of offshore Carbon Capture and Storage (CCS) complexes requires robust methodologies and cost-effective tools to detect, attribute and quantify CO2 leakage in the unlikely event it occurs from a sub-seafloor reservoir. Various approaches can be utilised for environmental CCS monitoring, but their capabilities are often undemonstrated and more detailed monitoring strategies need to be developed. We tested and compared different approaches in an offshore setting using a CO2 release experiment conducted at 120 m water depth in the Central North Sea. Tests were carried out over a range of CO(2 )injection rates (6 - 143 kg d(-1)) comparable to emission rates observed from abandoned wells. Here, we discuss the benefits and challenges of the tested approaches and compare their relative cost, temporal and spatial resolution, technology readiness level and sensitivity to leakage. The individual approaches demonstrate a high level of sensitivity and certainty and cover a wide range of operational requirements. Additionally, we refer to a set of generic requirements for site-specific baseline surveys that will aid in the interpretation of the results. Critically, we show that the capability of most techniques to detect and quantify leakage exceeds the currently existing legal requirements.