International Journal of Greenhouse Gas Control 109 (2021) 103388 2attribute CO2leaks and, at the same time, foster and build public trust in the safety and integrity of a CCS project; requiring the weighing together of many factors in developing a responsible and transparent environ- mental monitoring program in a dialogue with relevant stakeholder groups. In this paper we consider the process of monitoring in the marine environment and discuss an approach to acquiring a sufficient baseline understanding to efficiently underpin such monitoring. Here we use “marine environment”to refer to the upper few meters or so of seabed sediments and the overlying water column, i.e. the zone housing com- plex ecosystems. 1.1.Monitoring and regulatory requirements Unlike many industries with long national traditions and diversified national regulation, CCS law and regulations stem from international cooperation,generatingnationalregulationwithclearsimilarities across jurisdictions. In general, law requires demonstration of storage integrity, absence of environmental impact, and accounting of emissions in the unlikely event of leakage. For emissions accounting, and because of the geological variability amongst sites, an emissions factor approach is not suitable for geological CO2storage, rather a measurement-based approach is required (Dixon et al., 2015). Specific permitting and related monitoring requirements are closely related to CCS site charac- terisation and selection, risk and project impact assessments, stake- holder, and public participation, including access to information. The IPCC Guidelines chapter on carbon dioxide capture and geological storage (IPCC, 2005) set the foundation for all global monitoring regu- lations and can be reduced to the following components as outlined by Dixon and Romanak (2015): 1) site characterisation and identification of potential leakage pathways, 2) assessment of risk of leakage through site characterisation and modelling of CO2behaviour, 3) monitoring of CO2behaviour during injection and subsequent updating of models, and 4) reporting of CO2injected and emissions from storage. Offshore, leakage is defined as CO2flux from beneath the seabed into the ocean (with connection between ocean and atmosphere implied). With respect to the environmental portion of these regulations, the methodology re- quires that a monitoring plan include measurement of background CO2 fluxes through the seabed as well as any leakage flux that may occur. This activity therefore requires methods that can distinguish between the two types of fluxes, known as“attribution”. The resultant protocol for environmental monitoring could be summarized as 1) measurement of background CO2concentration, 2) detection of an anomaly, 3) source attribution of that anomaly, and 4) quantification of leakage emissions if attributed to leakage. CCS is operating in a rapidly changing socio-economic, technolog- ical, and physical environment. Adaptive management facilitated by the latest scientific knowledge on the condition and functioning of the marine environment and the management of human activities at sea will be desirable, (Platjouw and Soininen, 2019). In line with adaptive management, regulations on CCS monitoring advocate management that can adapt and incorporate new information as it becomes available. Typically, CCS monitoring plans are not fixed for the whole lifespan of the storage site, but revised to account for changes to the assessed risk of leakage, risks to the environment and human health, new scientific knowledge, and improvements in best available technology (see i.e. the European Union CCS Directive 2009/31/EC art 13 (2)). The specific monitoringrequirementswithinaCCSprojectaredesignedina dialogue-based process between the operator, proposing the monitoring plan, third-party stakeholders, i.e. the general public, fisheries sector etc. partaking in the impact assessment process, and the regulators. Globally, soft law instruments, or guidelines, recognize CCS as an emissionsreductiontechnology.TheIPCCGuidelinesforNational Greenhouse Gas Inventories, Volume 2, Energy, Chapter 5 (IPCC, 2006, refined IPCC 2019), has inventory methods consistent with the IPCC Special Report on Carbon Dioxide Capture and Storage (IPCC, 2005). The guidelines are foremost aiming for GHG accounting and provide methodologies for estimating and reporting national anthropogenic greenhouse gas sources and sinks. The guidelines (sec. 5.7) state that “the choice of monitoring technologies will need to be made on a site-by-site basis”, and as monitoring technologies are advancing rapidly “it would be good practice to keep up to date on new technologies”. Dixon and Romanak (2015) state that the methodology of the IPPC Guidelines“hasbecomethebasisforallsubsequentinternational regulation and legal guidance for CO2geological storage”. The United Nations Framework Convention on Climate Change (1997), the 1997 Kyoto Protocol, Article 12, has been in force since 2005. The Kyoto Protocol is legally binding upon developed countries, but still only includes non-prescriptive commitments, for example ref. Art 3 nr 1“do not exceed their assigned amounts”. The aim of the Pro- tocol relates to GHG accounting and protection of the environment, particular for developing countries. The Kyoto Protocol offers Interna- tional Emissions Trading, Joint implementation, and the Clean Devel- opmentMechanism,rewardinglow-carbonprojectsindeveloping countries by the creation of carbon credits. In 2011, ‘Modalities and Procedures’for CCS were agreed (Decision 10/CMP.7), stating that the monitoring plan shall“reflect the principles and criteria of international good practice for the monitoring of geological storage sites and consider the range of technologies described in the IPPC Guidelines and other good practice guidance.”Thus, the IPCC guidelines which do not prescribe specific technologies but focus on site-specific monitoring technologies and best available technology, guide the Kyoto Protocol. Leaving the global arena, regional cooperation has led to legally binding commitments for signatory states, as under the 1992 OSPAR Convention and the 1972 London Protocol. The 1996 monitoring pro- tocol of the London Protocol, amended in 2006 to allow CCS, and the monitoring protocol of OSPAR, (OSPAR Guidelines for Risk Assessment and Management of Storage of CO2Streams in Geological Formations, ReferenceNumber:2007–12,OSPAR2007)setout,fromalegal perspective, mere recommendations, both using the phrase that the monitoring“may include”. Read in context they encompass monitoring of sub-seabed geological formations, surrounding geological layers and geological layers above the formation, monitoring to detect migration, monitoring the seafloor and overlaying water to detect leakage, and monitoring seafloor and marine communities (benthic and water col- umn) to detect and measure the effects of leakages on marine organisms. Description of the“normal”or baseline is part of regional and na- tional legally binding impact assessment and monitoring requirements. In the EU this follows from the EIA Directive 2011/92/EU as amended by 2014/52/EU (European Union, 2014). According to§4 nr 1 and Annex I nr. 22, CCS storage sites pursuant to the CCS Directive shall be made subject to an impact assessment. Art 5, 1) requires the developer to provide, prior to any development consent, information on the (a) site and (b) the likely significant effects of the project on the environment,... and (f) any additional information specified in Annex IV relevant to the particular project. This is specified in Annex IV as a“description of the relevant aspects of the current state of the environment (baseline) and an outline of the likely evolution thereof without implementation of the project as far as natural changes from the baseline scenario can be assessed with reasonable effort on the basis of the availability of envi- ronmental information and scientific knowledge”. Further, the CCS Directive Article 7(6) and Article 9(5) requires a monitoring plan sub- mitted to and approved by the competent authority, updated every five years. According to Annex II, (1.1), the monitoring plan shall provide details of the monitoring to be deployed at the main stages of the project, “including baseline, operational and post-closure monitoring”. National legislation in EU and EEA member states shall, according to EU law, fulfil these minimum requirements stemming from the EIA and CCS Directives. In accordance with international soft law instruments, the EU Directives also build on site-specific monitoring programs and the principle of best available technology. Prior to site licensing, the assessment of potential environmental J. Blackford et al.