# The Integrated Carbon Observation System in Europe

Jouni Heiskanen, Christian Brümmer, Nina Buchmann, Carlo Calfapietra, Huilin Chen, Bert Gielen, Thanos Gkritzalis, Samuel Hammer, Susan Hartman, Mathias Herbst, et al.

► To cite this version:

Jouni Heiskanen, Christian Brümmer, Nina Buchmann, Carlo Calfapietra, Huilin Chen, et al. The Integrated Carbon Observation System in Europe. Bulletin of the American Meteorological Society, 2022, 103 (3), pp.E855-E872. 10.1175/bams-d-19-0364.1. hal-03411211

HAL Id: hal-03411211

https://hal.science/hal-03411211

Submitted on 2 Nov 2021

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

1 The Integrated Carbon Observation System in Europe

2 Authors

3 Jouni Heiskanen, Head Office, Integrated Carbon Observation System European Research

4 Infrastructure Consortium, Helsinki, Uusimaa, Finland, and Faculty of Biological and

5 Environmental Sciences, University of Helsinki, Helsinki, Uusimaa, Finland. Corresponding

6 author: email jouni.heiskanen@helsinki.fi

7 Christian Brümmer, Thünen Institute of Climate-Smart Agriculture, Braunschweig, Germany

8 Nina Buchmann, Department of Environmental Systems Science, ETH Zurich, Zurich,

9 Switzerland

10 Carlo Calfapietra, Institute of Research on Terrestrial Ecosystems, National Research

11 Council, Porano, Italy

12 Huilin Chen, Centre for Isotope Research, University of Groningen, Groningen, The

13 Netherlands

14 Bert Gielen, Department of Biology, University of Antwerp, Wilrijk, Belgium

15 Thanos Gkritzalis, Flanders Marine Institute, Ostend, Belgium

16 Samuel Hammer, Institut für Umweltphysik, Heidelberg University, Heidelberg, Germany

17 Susan Hartman, National Oceanography Centre, Southampton, United Kingdom

18 Mathias Herbst, Centre for Agrometeorological Research, German Meteorological Service,

19 Braunschweig, Germany

20 Ivan A. Janssens, Department of Biology, University of Antwerp, Wilrijk, Belgium

21 Armin Jordan, Max-Planck-Institute for Biogeochemistry, Jena, Germany

22 Eija Juurola, Institute for Atmospheric and Earth System Research, University of Helsinki,

23 Helsinki, Uusimaa, Finland

24 Ute Karstens, ICOS ERIC, Carbon Portal, Lund, Sweden

25 Ville Kasurinen, Head Office, Integrated Carbon Observation System European Research

26 Infrastructure Consortium, Helsinki, Uusimaa, Finland

27 Bart Kruijt, Department of Environmental Sciences, Wageningen University & Research,

28 Wageningen, the Netherlands

29 Harry Lankreijer, ICOS ERIC, Carbon Portal, Lund, Sweden

30 Ingeborg Levin, Institut für Umweltphysik, Heidelberg University, Heidelberg, Germany
31 Maj-Lena Linderson, Department of Physical Geography and Ecosystem Science, Lund

32 University, Lund, Sweden

33 Denis Loustau, INRAE, ISPA, Villenave d'Ornon, France.

34 Lutz Merbold, Agroscope, Research Division Agroecology and Environment,

35 Reckenholzstrasse 191, 8046 Zurich, Switzerland – previously at Department of

36 Environmental Systems Science, ETH Zurich, Zurich, Switzerland

37 Cathrine Lund Myhre, Atmosphere and Climate Department, Norwegian Institute for Air

38 Research, Kjeller, Norway

39 Dario Papale, Department for Innovation in Biological Agro-food and Forest systems,

40 University of Tuscia, Viterbo, Italy and euroMediterranean Center on Climate Change

41 (CMCC), Lecce, Italy

42 Marian Pavelka, Department of Matter and Energy Fluxes, Global Change Research Institute,

43 CAS, Brno, Czech Republic

44 Kim Pilegaard, Department of Environmental Engineering, Technical University of Denmark,

45 2800 Kgs. Lyngby, Denmark

46 Michel Ramonet, Université Paris-Saclay, CEA, CNRS, UVSQ, Laboratoire des Sciences du

47 Climat et de l'Environnement (LSCE/IPSL), Gif-sur-Yvette, France

48 Corinna Rebmann, Helmholtz Centre for Environmental Research, GmbH – UFZ, Leipzig,

49 Germany

50 Janne Rinne, Department of Physical Geography and Ecosystem Science, Lund University,

51 Lund, Sweden

52 Léonard Rivier, ATC ICOS, Université Paris-Saclay, CEA, CNRS, UVSQ, Laboratoire des

53 Sciences du Climat et de l'Environnement (LSCE/IPSL), Gif-sur-Yvette, France

54 Elena Saltikoff, Head Office, Integrated Carbon Observation System European Research

55 Infrastructure Consortium, Helsinki, Uusimaa, Finland

56 Richard Sanders, Climate Department, Norwegian Research Centre, Bergen, Norway

57 Martin Steinbacher, Laboratory for Air Pollution / Environmental Technology, Empa,

58 Duebendorf, Switzerland

59 Tobias Steinhoff, Chemical Oceanography, GEOMAR Helmholtz Centre for Ocean Research

60 Kiel, Kiel, Germany and NORCE Norwegian research Centre AS, Bergen, Norway.

61 Andrew Watson, College of Life and Environmental Sciences, University of Exeter, Exeter,

62 United Kingdom

63 Alex T. Vermeulen, ICOS ERIC, Carbon Portal, Lund, Sweden

64 Timo Vesala, Institute For Atmosphere and Earth System Research/Physics, Faculty of

65 Science, University of Helsinki, Helsinki, Uusimaa, Finland, and Institute for Atmosphere and

66 Earth System Research/Forest Sciences, PO Box 27, Faculty of Agriculture and Forestry,
67 University of Helsinki, Helsinki, Uusimaa, Finland, and Yugra State University, 628012,

68 Khanty-Mansiysk, Russia

69 Gabriela Vítková, Global Change Research Institute of the Czech Academy of Sciences,

70 Bělidla 986/4°, Brno, Czech Republic

71 Werner Kutsch, Head Office, Integrated Carbon Observation System European Research

72 Infrastructure Consortium, Helsinki, Uusimaa, Finland

73
74 **Abstract**

Since 1750, land use change and fossil fuel combustion has led to a 46 % increase in the atmospheric carbon dioxide (CO₂) concentrations, causing global warming with substantial societal consequences. The Paris Agreement aims to limiting global temperature increases to well below 2°C above pre-industrial levels. Increasing levels of CO₂ and other greenhouse gases (GHGs), such as methane (CH₄) and nitrous oxide (N₂O), in the atmosphere are the primary cause of climate change. Approximately half of the carbon emissions to the atmosphere is sequestered by ocean and land sinks, leading to ocean acidification but also slowing the rate of global warming. However, there are significant uncertainties in the future global warming scenarios due to uncertainties in the size, nature and stability of these sinks. Quantifying and monitoring the size and timing of natural sinks and the impact of climate change on ecosystems are important information to guide policy-makers’ decisions and strategies on reductions in emissions. Continuous, long-term observations are required to quantify GHG emissions, sinks, and their impacts on Earth systems. The Integrated Carbon Observation System (ICOS) was designed as the European *in situ* observation and information system to support science and society in their efforts to mitigate climate change. It provides standardized and open data currently from over 140 measurement stations across 12 European countries. The stations observe GHG concentrations in the atmosphere and carbon and GHG fluxes between the atmosphere, land surface and the oceans. This article describes how ICOS fulfills its mission to harmonize these observations, ensure the related long-term financial commitments, provide easy access to well-documented and reproducible high-quality data and related protocols and
96 tools for scientific studies, and deliver information and GHG-related products to

97 stakeholders in society and policy.

98
99

Capsule

100

ICOS is a distributed Research Infrastructure conducting standardized, high-precision and

101

long-term observations and facilitating research to understand the carbon cycle and to

102

provide necessary information on greenhouse gases.

103
104 **Introduction**

Since the industrial revolution the combination of land use change and fossil fuel combustion has led to a 46 % increase in the atmospheric CO2 concentrations totaling to a buildup of 2200 +/- 320 GtCO2 in the atmosphere (Friedlingstein et al. 2020; Rogelj et al. 2018). Consensus is that this has led to a significant warming of the atmosphere and increased heat storage of the upper ocean with subsequent effects of considerable societal importance. Human-induced increase in atmospheric concentrations of carbon dioxide (CO2) and other greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O), are the primary cause of the ongoing climate warming (IPCC 2018). The atmospheric buildup of CO2 would have been about twice as large had approximately half of the carbon emitted to the atmosphere not been sequestered by ocean and land sinks, leading to the rate of warming being reduced (Friedlingstein et al. 2020). However, the size, nature and stability of these sinks are uncertain, which together with the uncertainty of the speed of release of the heat stored in the ocean surfaces leads to large uncertainties in the projected global climate warming with different GHG mitigation scenarios (Ma et al. 2020; Rhein et al. 2013). Improving the quantification and reducing the uncertainty of these projections is important to support policy-making and the size and timing of reductions in global emissions. There are uncertainties in the emission sources but the largest cause for uncertainty in the global carbon budget estimates are likely due to the lack of understanding of land and ocean sinks (Friedlingstein et al. 2020). Understanding of these sinks, sources, and the related processes can only be achieved with research based on spatially and temporally comprehensive and precise data. This is ever more important now that a specific goal has been set at limiting average global surface temperature
126 increases to well below 2°C above pre-industrial levels (The Paris Agreement (United Nations
127 2015a) ratified by 189 countries to guide the actions to combat climate change).

128 Anthropogenic emissions of GHGs to the atmosphere are superimposed on with the much
129 larger natural GHG exchange fluxes between the atmosphere and the terrestrial ecosystems
130 and ocean, which are further affected by ongoing climate warming. Quantifying the
131 anthropogenic perturbation therefore depends on quantifying both natural and
132 anthropogenic emissions and sinks and understanding the drivers of feedback mechanisms
133 over both.

134 The Integrated Carbon Observation System (ICOS), which currently includes over 140 stations,
135 was designed as the European *in situ* observation and information system to support science
136 and society in their efforts to mitigate climate change. ICOS is motivated by understanding
137 the sources, sinks and cycling of greenhouse gases in the atmosphere-biosphere-hydrosphere
138 continuum. The European Commission, Belgium, Finland, France, Germany, Italy, the
139 Netherlands, Norway, Sweden, and Switzerland committed to this mission when the ICOS
140 ERIC (European Research Infrastructure Consortium) was established in 2015.

141 Key aspects of climate science addressed by ICOS have been elaborated in earlier articles,
142 with Schulze et al. (2009) emphasizing the importance of N₂O and CH₄ in the European
143 greenhouse gas budget, Peters et al. (2010) quantifying European net terrestrial CO₂
144 exchange, Gielen et al. (2017) briefly summarizing the different components of the network,
145 Franz et al. (2018) giving an overview on ICOS ecosystem observations, Steinhoff et al. (2019)
146 describing the ocean network, and Levin et al. (2020) addressing the atmospheric network.
147 This article provides a comprehensive overview of the ICOS Research Infrastructure (RI),
148 including a historical overview, describing the structure, operations and financial
sustainability of the ICOS RI, elaborating present and future scientific questions, and
149

150 discussing lessons learned and challenges addressed by ICOS.

151 **The rationale and path into Integrated Carbon Observation System**

Even though the connection between human actions and climate change had been made by
152
the end of the 20th century (IPCC 1992), many important questions were still open, such as
153
how much CO₂ from fossil fuel burning remains in the atmosphere and how much was taken
154
up by oceans and terrestrial ecosystems (Keeling 1978). A major obstacle in answering these
155
questions was limited data availability and the use of different observational methods, units,
156
and scales by different countries and sites. This required global harmonization of
157
observations, first started in the atmosphere by the World Meteorological Organization
158
Global Atmosphere Watch (WMO GAW) programme in 1989 (WMO 2014) and with the
159
FLUXNET ecosystem global network in 1996 (Baldocchi et al. 2001).
160

Another obstacle was how to draw conclusions from various pieces of data and information.
161
This called for a framework how to systematically provide scientific knowledge in global scale,
162
giving birth to the Intergovernmental Panel on Climate Change (IPCC), established in the end
163
of 1980s. Eyes turned next to land, where various methods had been developed to
164
understand highly diverse and complex terrestrial ecosystems. This posed challenges to
165
compare the results, and the Global Climate Observing System (GCOS) was established to
166
harmonize terrestrial observations and to define a set of Essential Climate Variables (GCOS
167
1994, 2016; WMO 2009). Quantifying relatively small long-term trends in CO₂ and other GHG
168
concentration and fluxes against a background of much larger short-term variations caused
169
by the 'natural' carbon cycle requires highly precise and accurate observations. To decrease
170
uncertainties by improving the quality of observations, and to draw general conclusions,
research- and investigator-based European ecosystem networks, with focus on CO₂, energy and
water exchange, emerged in the 1990s with the support of the European Commission funding
programs (EuroFlux, CarboEurope IP and GHG Europe). During 1998-2002, the Euroflux
network covered 30 stations mainly in forest ecosystems across Europe (Janssens et al. 2003),
which later developed into the network of ecosystem stations within ICOS.

At the beginning of the 1990s, the Global Ocean Observing System (GOOS) was established to
coordinate and harmonize ocean observations together with GCOS. The scientific community
undertook the task to provide open access to global ocean surface CO₂ data via the Surface
Ocean CO₂ Atlas, SOCAT (Pfeil et al. 2013). These data are essential to estimate ocean carbon
budget and acidification. As a community effort, SOCAT depends heavily on voluntary data
submission and secondary quality control, and the Ocean Carbon Data System of National
Oceanic and Atmospheric Administration and ICOS support SOCAT and contribute
significantly to its data operations and development.

The development of observation networks had been fragmented into various projects in
Europe (see Fig. 1 in Franz et al. (2018)). By the beginning of the 2000s, it was possible to
estimate the European terrestrial carbon budget by either using the few ecosystem network
data available (e.g. Papale and Valentini 2003) or by methods using atmospheric network
data, but these provided dissimilar and highly uncertain results (Janssens et al. 2003). The
results suggested that increase in ecosystem representation and data would reduce the
uncertainty in the bottom-up approach and that including more atmospheric stations would
improve the accuracy of top-down estimates. The EU-funded CarboEurope Integrative Project
(2004-2008), was a major step towards integrated studies, harmonized observations and data
flows, covering atmosphere and ecosystem sciences (Schulze et al. 2009). In parallel, the CarboOcean IP conducted over 2005-2009 developed systematic ocean carbon observations and analysis across Europe. The observations collected in the context of these projects were an important example to demonstrate how a large and coordinated network could provide a unique dataset valuable for the modeling activities to estimate continental scale GHG fluxes (e.g. Luyssaert et al. 2010; Schulze et al. 2009; Vetter et al. 2008).

European countries have been at the forefront of setting-up the Paris Agreement to reduce emissions. Implementation of climate change mitigation is done by individual nations, but to effectively curb the increase of GHG concentrations in the future, a comprehensive strategy of emission reductions and natural sink conservation must be designed collectively. The success of the scientific projects showing capability of the scientific community to provide quantitative information at a European scale paved way for the political will to develop ICOS – an observation system that will narrow down future uncertainties and provide observational evidence of the current state of the carbon cycle perturbation. Throughout the development of ICOS, the policy-makers, funders and scientists have been in constant dialogue to improve the scientific foundation of decision-making and obtain the political and financial commitments across European countries.

ICOS foundation required negotiating the concept for such as system, with clear purpose and governance as well as financial structure and responsibilities of each participants, in which the countries could then commit. This was the purpose of ICOS preparatory phase project in 2008-2013 (reference, see also Appendix 1).

User-centric approach drove the development of a centralized data provision hub for all ICOS data, the Carbon Portal. The problem of different type of observations in atmosphere,
ecosystem, and ocean stations was addressed by centralizing the quality control and data processing in three respective Thematic Centres with specific experience and knowledge. To allow measurements of required precision, the Central Analytical Laboratories was designed to provide calibration gases to atmospheric and ocean monitoring stations. The process for scientific development was planned on the interactions between these components and the Monitoring Station Assemblies (MSAs) which include all station Principal Investigators (PIs).

The financial challenges were tackled by acquiring commitments from various countries interested to build a national network of ICOS stations or propose a Central Facility. The host countries provide the majority of the financial support by direct governmental grants (ICOS Ecosystem Thematic Center is hosted by Italy, France, and Belgium, Atmosphere Thematic Center by France and Finland, Ocean Thematic Center by the Norway and the UK, the Central Analytical Laboratories by Germany, and the Carbon Portal by Sweden and the Netherlands).

The stations are maintained by individual countries, and each country also contributes to the general costs for the upkeep of the RI. The principles for sharing the financial responsibilities were written in the ICOS financial rules.

With scientific, technical, and financial concepts in place, the last challenge was how to coordinate such an infrastructure across many countries. The solution was to establish a legal entity designed to manage Research Infrastructures and recognized in all European countries, called ICOS ERIC (European Research Infrastructure Consortium (https://ec.europa.eu/info/research-and-innovation/strategy/european-research-infrastructures/eric_en)), hosted by Finland and France with participation from all member countries), to coordinate the whole research infrastructure and to report to and consult with the ministerial stakeholders.
The mission of ICOS is to harmonize European carbon and GHG observations, ensure the related long-term financial commitments, provide easy access to well-documented and reproducible high-quality data and related protocols and tools for scientific studies, and to deliver GHG-related products to stakeholders in society and policy. The first five years of ICOS from 2015-2019 focused on establishing an operational infrastructure, and as an acknowledgement of successful implementation, ICOS ERIC was included in the European Strategy Forum on Research Infrastructures' strategy as a Landmark infrastructure (ESFRI 2016). Since becoming operational in 2015, the Czech Republic, Denmark, the United Kingdom, and Spain have joined ICOS, and Poland has announced to join ICOS, considerably expanding the network, and negotiations are currently under way with Estonia, Greece, Hungary, Ireland, Portugal and Romania. The second phase of ICOS, described in the ICOS strategy published in 2019 (ICOS 2019), and its associated implementation plan, will place emphasis on the use of data and on enhancing the network's capability to analyze anthropogenic impacts on the carbon cycle. We foresee the new European Green Deal launched by EU Commission at the end of 2019 (https://ec.europa.eu/info/strategy/priorities-2019-2024/european-green-deal_en), designed to make Europe the first net-zero continent, will further strengthen ICOS' role in the forthcoming years.

## Description of ICOS observations and data

ICOS provides the core network of highly accurate, long-term European in situ observations of carbon and GHGs (see Appendix 2 for full list of observed variables). The terrestrial network of over 100 stations ranges from Sweden (Latitude 68°N in WGS84 coordinates) to the
262 Mediterranean Sea (Lat 36°N), from the UK (Longitude 3°W) to Finland (Lon 30°E), from the lowlands near sea level to alpine regions (2168 m a.s.l., Italy), with stations also outside of Europe (e.g. in French Guyana, Greenland and Democratic Republic of the Congo). The marine network of over 20 stations and vessels extends from polar areas to the equator and from coasts to open ocean.

267 The atmospheric network of tall towers, mountain, and coastal stations covers large parts of Europe with continuous measurements of CO₂ and CH₄ mole fractions. When coupled with an atmospheric model, these data provide an integrated view of all natural and anthropogenic fluxes. In fully equipped ICOS stations, meteorological variables, N₂O and ²²²Radon are observed to link concentration variations to atmospheric mixing. In addition, N₂O, SF₆, H₂, and for CO₂ source apportionment CO, ¹³C-CO₂, ¹⁴C-CO₂, ¹⁸O-CO₂, and O₂/N₂, are analyzed in air sampled by automated flask samplers at the most extensively equipped stations, called Class 1 stations (ICOS 2017a; Levin et al. 2020; Appendix 2).

275 GHG fluxes in different terrestrial ecosystems (forests, croplands, grasslands, mires, wetlands, shrublands, lakes, Mediterranean savannas, urban sites) are observed at comprehensively equipped stations to quantify the exchange of carbon, GHGs and energy between the atmosphere and the ecosystems (Franz et al. 2018), by using the eddy covariance technique (Rebmann et al. 2018). Biosphere-atmosphere exchange measurements at flux towers represent the only direct method to provide detailed data at ecosystem scale, and they are valuable also for different user communities: e.g. sensible and latent heat fluxes are important to understand the water cycle and to improve weather forecasts, turbulence data are used in studying boundary layer physics. In a subset of stations, fluxes of CH₄ and N₂O are observed (Nemitz et al. 2018). Complementary data comprise e.g. soil organic carbon content,
285 Green Area Index, litterfall, aboveground biomass, records of disturbances, and vegetation
286 properties such as leaf nutrients and phenological status, as well as management activities
287 (Arrouays et al. 2018; Gielen et al. 2018; Hufkens et al. 2018; Loustau et al. 2018; Op de Beeck
288 et al. 2018; Pavelka et al. 2018; Saunders et al. 2018). These various observations are used to
289 estimate the contributions of different components of the ecosystem, such as soil or
290 vegetation, to the seasonal and interannual variability of the carbon and GHG budget of the
291 whole ecosystem, as well as when upscaling carbon fluxes to regional and global scales (Jung
292 et al. 2020).

293 The ocean observations are conducted either on fixed platforms (e.g. moorings and surface
294 buoys) or on ships operating predominantly in the North Atlantic, Nordic, Baltic and
295 Mediterranean Seas, but occasionally also in Polar regions and equatorial Atlantic. Partial
296 pressure of sea surface CO2 is used in conjunction with other parameters (temperature,
297 salinity, mixed layer depth) and satellite remote sensing products including wind fields and
298 chlorophyll to calculate oceanic uptake of CO2 (ICOS 2017b, 2020b; Steinhoff et al. 2019).
299 Other carbon cycle parameters (pH, alkalinity, dissolved inorganic carbon) and related
300 properties such as nutrients and oxygen are used to investigate ocean transports and controls
301 over carbon uptake. This latter work involves collaboration across various elements of the
302 European RI landscape and components of the Global Ocean Observing System (GOOS)
303 including Euro-Argo (European consortium for operating Argo floats), EMSO (the European
304 Multidisciplinary Seafloor and water column Observatory), and GO-SHIP (the Global Ocean
305 Ship-based Hydrographic Investigations Program). While the main policy framework that ICOS
306 contributes to is the Paris Agreement, also Agenda 2030 (United Nations 2015b) and its
307 Sustainable Development Goal 14.3 is supported with monitoring of ocean acidification.
The variables and related costs for all ICOS observations are detailed in the ICOS Handbook, released every 2 years (ICOS 2020a). For example, to build one fully equipped atmosphere or ecosystem station (Class 1) costs between 0.5 and 1 M€ (not including personnel costs), whereas costs for stations having only a subset of observations (Class 2) can be between 0.1 and 0.3 M€. Maintenance requirements and collection of ancillary data are also provided, with the most significant component being person-power, ranging from 0.3 to 4 Full Time Equivalents per annum depending on the type of the station.

## Quality Assurance and Control (QA/QC)

Within the CarboEurope IP project (Schulze et al. 2010) the challenges to achieve highly compatible atmospheric data became obvious. Large efforts were undertaken to assess the compatibility of atmospheric measurements done by different laboratories at different observational sites. Yet, results from these exercises repeatedly yielded evidence that the WMO compatibility goals were not met by all participants and for all tracers. Biases between laboratories could sometimes be of the same order of magnitude as the atmospheric signals that should be captured and it was not possible to define a network data compatibility. This was motivating the ICOS concept with highly standardized measurement approaches at the observatories (including aspects such as instrumentation, procedures to account for atmospheric humidity, and calibration procedures) and the establishment of central facilities that assess the adequate performance of all installed analyzers and assure transparent data processing (ATC), as well as the consistency of sample measurement results and reference gas assignments that are used within the monitoring network (CAL). To have the ability to make a defendable uncertainty assessment that is required for observational data (WMO 2020b) the following QA/QC approaches are applied that cover all levels of the observational
system (stations as well as central facilities). To minimize uncertainties in both observations provided by single stations and studies using data from multiple stations, several steps are taken. The instruments themselves have strict requirements, they are systematically calibrated, their setup is based on stringent protocols, and the data are processed by the Thematic Centers with proven and standardized methodologies (Hazan et al. 2016; El Yazidi et al. 2018; Vitale et al. 2020).

Scientists in the ICOS atmosphere community, coordinated via the Atmospheric MSA, have defined and approved protocols for instrumentation setup and sampling strategies (ICOS 2017a, Levin et al. 2020) to ensure that atmospheric measurements comply with the compatibility goals set by the WMO for measurements of major GHGs and associated tracers (WMO 2020b). Stringent network compatibility within ICOS and with other networks is key when using the observations in concert with atmospheric transport models to quantify GHG sources and sinks. Calibration gases are prepared and calibrated centrally for the network by the Flask and Calibration Laboratory (FCL) of the Central Analytical Laboratories (CAL) that maintains tight links to the WMO Central Calibration Laboratory to ensure the traceability of ICOS data to internationally accepted WMO calibration scales by one unique path. To assess the accuracy of the implementation of these scales at the FCL maintains several ongoing round robin exercises with the NOAA laboratories as quality control. The FCL is also responsible for flask analyses except for ¹⁴C-CO₂, which is analyzed by the Central Radiocarbon Laboratory of the CAL. The precision and stability of all GHG analyzers are tested at the Atmosphere Thematic Centre prior to deployment (Yver Kwok et al. 2015). A comprehensive overview of the optimization of the quality management as part of the labelling process of atmosphere stations is given in Yver-Kwok et al. (2021). For quality control of the continuous in situ measurements, automated QC figures are generated on a daily level by the ATC that
355 summarize the statistics of the measurement precision (repeatability and target gas bias)
356 which provide a basis to quantify the measurement uncertainty. Additional auditing is made
357 with travelling instrumentation (ICOS Mobile Lab) operated at selected stations for a couple
358 of weeks, and by ongoing comparison of flask results with *in situ* observations at Class 1
359 stations (Levin et al. 2020).

360 To ensure high quality of observations in diverse ecosystems, with various drivers influencing
361 the carbon and GHG fluxes, the observation methods need careful attention. Since diverse
362 observation methods were established for different climate regions and ecosystem types in
363 the past decades, a community-driven effort was necessary to define key and ancillary
364 components to be observed in each ecosystem type to analyze carbon and GHG fluxes. Also,
365 much effort has been put into defining the specifications and methodology of observations
366 by the community, together with the Ecosystem MSA and the Ecosystem Thematic Centre.
367 Both optimal sets of variables and practical feasibility were considered when harmonizing the
368 observations, which resulted in a compromise suitable for high quality and long-term
369 continental scale observation system (Franz et al. 2018). Over 100 scientists' efforts were
370 acknowledged in a set of publications describing the ecosystem measurement protocols in
371 2018 (International Agrophysics, vol. 32(4), 2018). Starting from the protocols, more practical
372 and detailed Instruction documents were prepared and published by the ETC
373 (http://www.icos-etc.eu/documents/instructions) that are revised and updated regularly,
374 following the newest developments and know-how.

375 For the ocean observations, the major challenges are the complexity of the carbonate system,
376 the often remote location of stations, and suitability of different observing methods for
377 different types of stations. Tailored solutions are needed in order for each station to deliver
378 the best possible data, and the ICOS ocean community, supported by the Ocean Thematic
379 Centre, has adopted and adapted existing and proven best practice guidelines and protocols
380 (Dickson et al. 2007) for observations made by different types of stations (Steinhoff et al.
381 2019). ICOS is the first multi-national entity within the marine community that has
382 standardized CO2 observations (Steinhoff et al. 2019). The Fixed Ocean Stations' maintenance
383 and calibration are done during the visit by research vessels, ideally several times per year,
384 whereas observations on Ships of Opportunity (SOOP) are calibrated even more frequently.
385 Inclusion of marine towers with direct flux observations is currently under development
386 (Steinhoff et al. 2019). CO2 observations are calibrated with standards traceable to the WMO
387 calibration scales (ICOS 2020b). Data quality, control and uniformity is also supported by a
388 customized QuinCe tool developed by the OTC.

389 To guarantee the quality of observations, in all three network components (atmosphere,
390 ecosystem, and ocean), it is necessary for a station to pass an ICOS station certification
391 process. Here, the station characteristics are evaluated, its compliance with measurement
392 protocols and standards is analyzed, and data transfer and quality are evaluated by the
393 respective Thematic Centres during a test measurement period of a few months. After
394 successfully completing this process which typically takes two to three years, the station
395 receives the ICOS certificate. This means the station meets the high standards of the ICOS
396 network and continuously provides ICOS data. Of the over 140 stations in the ICOS network,
397 over 60 stations have been certified by the end of 2020.

398 Open data access

399 ICOS has addressed the major challenge of data access and simplification of data use (Fig. 2)
400 thanks to the PI and Central Facilities work that agreed on a continuous data submission and
401 adoption of an open data license (Creative Commons Attribution 4.0 International, which also allows commercial use). Additionally, the services provided to make data distribution easier and assignment of Digital Object Identifier (doi) to datasets are major advances to improve and promote open data.
402
403
404

405 To serve various user needs, different levels of the data are openly accessible with different level of processing and quality check. Much attention has been paid to the metadata that follow the specifications defined by the Carbon Portal in collaboration with the Thematic Centres, also considering existing international standards. All steps of data flows were designed based on the FAIR (Findable, Accessible, Interoperable, Reusable) principles (Wilkinson et al., 2016), giving the user sufficient tools to interpret the data (ICOS 2015).
406
407
408
409
410

411 Different levels of data are stored throughout the process, from raw sensor data (Level 0), to the automatically calibrated near-real-time data (Level 1; available within 24 hours of the measurement) to the final, quality-checked data (Level 2). All the data are passed on to the Carbon Portal, which provides free and open access to ICOS data. The data are minted with Persistent Identifiers to provide unique identification and citation of the datasets and their contributors (ICOS 2019). The Carbon Portal offers tailor-made tools and services (Fig. 3) and distributes products (Level 3) that are created by the scientific community based on ICOS data and possibly from other data sources (Fig. 4). The Carbon Portal has started to develop and provide tools for online analysis of data and model results (see e.g. ICOS 2020c). These enable transparent analyses of data by station PIs, interactive collaboration with the data users and utilization of cloud services as virtual working environments.
412
413
414
415
416
417
418
419
420
421
422

Major scientific questions and glimpse to the future

423

Many major scientific questions have guided the development of systematic, continental
observations of carbon and GHG budgets. Scientists have been able to answer how much of
emitted CO2 from fossil fuels have accumulated in the atmosphere, oceans and terrestrial
ecosystems (Friedlingstein et al. 2020). Many advancements have been made in defining how
terrestrial ecosystems are affected by and how they feed back to climate change, e.g. by
changes in evapotranspiration or albedo.

424

425

426

427

428

429

With the ICOS network reaching maturity via station certification, the compilation of the
European carbon and GHG budget, which was previously possible as one-time effort (Schulze
et al. 2009), can soon be produced annually at high spatial resolution and with reduced
uncertainty. This is a significant step forward in assessing changes and trends on the
continental scale. Advancements have been made in providing detailed information on the
dominantly studied ecosystems, e.g. forests, grasslands and croplands, while we still have
only rudimentary understanding of some other ecosystem types, e.g. lakes, rivers, peatlands,
Mediterranean savannas and Arctic tundra (Baldocchi 2014; Schulze et al. 2010), or on urban
systems. Mitigation capacity of urban areas as well as their adaptation capacity will need
much deeper attention as the urban population is continuously growing and urban areas
represent the major sources of GHGs in Europe and in most of the continents (Calfapietra et
al. 2015).

430

431

432

433

434

435

436

437

438

439

440

441

ICOS data is widely used in publications from various scientific fields. The amount of ICOS-
related publications per year have increased from 30 in 2012 to roughly 200 in 2020, and the
citations from 600 to 11 000, respectively (https://www.icos-cp.eu/science-and-
impact/society-impact/references). The publications are associated to almost 60 categories

442

443

444
with the two largest being meteorology and atmospheric sciences (37% of all publications) and environmental sciences (34%) (ICOS 2021). The cross-domain integration in ICOS allows us to comprehensively address the biogeochemical fluxes of carbon and GHGs and to identify and study existing gaps in knowledge. A recent example are the 17 publications, based on data from more than 100 stations, following the drought in Europe in 2018. The drought was analyzed from how it was detectable in the atmospheric station network and how it affected ecosystem processes and GHG budgets, to regional assessments of its influences on ecosystem carbon exchange, and relations to major crops (Peters et al. 2020). ICOS made this rapid scientific response possible by building the foundation for fast action, by harmonizing observations and centralized data processing, by analyzing the data in near-real time to detect anomalies in drivers and ecosystem responses, by facilitating networking of scientists, and by providing virtual solutions for joint work. The results show that the drought affected more the productivity of crops and grasslands than forests, which protected themselves by reducing their evaporation and growth, leading to decreased uptake of carbon dioxide (Peters et al. 2020). In general, carbon sinks decreased by 18% in a study covering 56 ecosystem sites (Graf et al. 2020). The dry conditions even turned some mires from sinks into sources (Rinne et al. 2020). In some parts of Europe, winter 2018 was wet, leaving a lot of soil moisture in the ground, while spring was sunny and came early - this caused the vegetation to grow in spring more than average. In some places, this early spring growth was enough to offset the reduction of carbon uptake later in summer (e.g. Smith et al. 2020). Currently, there is a joint effort of similar magnitude under preparation analyzing the warm winter 2019-2020. The above mentioned are also reflected in the biennial ICOS Science Conference that brings scientists from different disciplines together to discuss besides science, also e.g.
468 methodological improvements and societal relevance of long-term observations of climate-
469 related variables.

470 Now, with the Paris Agreement having clear processes to guide the nations with climate
471 change mitigation, the pressure is increasing to provide robust information to support the
472 review of the impact of these actions (Art 14.1 in the Paris Agreement). ICOS is actively
473 engaged with GCOS to provide observations of Essential Climate Variables and to draft a
474 suitable indicator representing terrestrial ecosystems. ICOS provides data and participates to
475 the development of Global Carbon Budget to reduce the uncertainty of the global estimates
476 and to build a solid foundation for some of the global data sources, such as SOCAT and
477 FLUXNET, the global network of gas flux observations between ecosystems and the
478 atmosphere (Papale 2020). ICOS is in active dialogue with UNFCCC Subsidiary Body for
479 Scientific and Technological Advice to facilitate discussion between science and policy. ICOS
480 is currently focusing on providing the needed information at national and regional levels with
481 the separation of natural and anthropogenic fluxes. For example, VERIFY (H2020 project
482 776810) aims to improve national GHG inventories with top-down (atmospheric inversions)
483 and bottom-up (inventories made with complementary methods and data than used by
484 governmental authorities) scientific approaches (Petrescu et al. 2020).

485 The capability to disentangle the natural cycle and the anthropogenic disturbance has made
486 progress, and consensus exists that the required next step is to link tightly *in situ* and remote
487 sensing observations and modelling to more accurately quantify anthropogenic CO₂ emissions
488 (Copernicus 2015, 2019). The calibration and verification of satellite products and models
489 within this system aim to rely on the *in situ* ICOS network, including potential atmospheric
490 vertical profiling of GHGs using AirCore (Karion et al. 2010) and collaboration with the Total
Carbon Column Observing Network (TCCON). This system is currently developed by scientists involved in ICOS and peers in the CoCO2 project (H2020 project 958927). Further developments could include the provision of more accurate observations of hot spots of human activities, mainly in urban areas (WMO 2019). The ¹⁴C methodology is used for quantifying the CO₂ emissions from fossil fuel burning, as the fossil energy sources are void of ¹⁴C. Their contribution can be derived from measurements of the ¹⁴C/¹²C ratio in atmospheric CO₂ (Basu et al. 2020; Levin et al. 2003, 2011). These observations are systematically made in ICOS but mostly in sparsely populated locations. The concept of an urban observatory has been tested in some European and US cities (Breon et al. 2015; Lauvaux et al. 2020) but more development, probably combining atmospheric observations and modelling with flux observations, is needed before the methodology is mature enough to be incorporated into a research infrastructure such as ICOS. This system for greenhouse gas measurements in urban areas is developed by ICOS in the H2020 project PAUL (Pilot Application in Urban Landscapes – towards integrated city observatories for greenhouse gases). Additionally, the flux towers are invaluable to provide data on carbon sinks over vegetated urban surfaces at neighborhood-scale (e.g. Nordbo et al. 2012).

Lack of sufficient geographical coverage of observations is a source of major uncertainty in most regions of the world (WMO 2020a). Even in Europe where the *in situ* observations of CO₂ and other GHGs were brought together by ICOS, large parts of Eastern Europe are still to join this network. The benefits are understood at the scientific level but as membership in a research infrastructure obliges sustained funding and commitments, the discussions must enter the political level. Currently, the discussions are ongoing with seven countries to join ICOS.
ICOS is collaborating with complementary networks in Europe and in other continents towards more harmonized standardization of observations and data processing, common data policies, and common data citation system. Examples are the Long-Term Ecosystem Research (eLTER), the European Research Infrastructure for the observation of Aerosol, Clouds and Trace Gases (ACTRIS), the AmeriFlux Management Project and the National Ecological Observatory Network (NEON) in the US and the US Carbon Cycle Science Program, the Chinese Ecosystem Research Network, the Terrestrial Ecosystem Research Network (TERN) in Australia, and the National Institute for Environmental Studies in Japan. ICOS also supports and develops the global data networks, such as WMO Global Atmosphere Watch and the World Data Centre of Greenhouse Gases for atmospheric observations, FLUXNET for ecosystem GHG fluxes, Surface Ocean CO₂ Atlas (SOCAT), and Global Ocean Data Analysis Project (GLODAP). ICOS contributes to the development of harmonized observations in Africa via design study and capacity building (Lopéz-Ballesteros et al. 2018).

Many global coordination frameworks are in place to address the global environmental challenges. However, there is a huge distance between global frameworks and local actions. Developments are needed on different scales to improve the observational capacity and to transform the data into information useful for local and national decision makers, non-governmental organizations and the private sector. Examples of the urgently needed developments are defining how changing climate affects the ability of natural terrestrial and ocean sink to sequester carbon, supporting the verification of national GHG inventories, and understanding and validating the efficacy of mitigation actions. ICOS provides the European in situ observations of CO₂ and other GHGs, and is well positioned to coordinate the in situ component of a comprehensive system that caters to different information needs at different
537 spatial and temporal scales (Copernicus 2019). Combating climate change needs reliable

538 information, and ICOS is here to deliver.

539 Acknowledgements

540 Academy of Finland (Grants nr 281255, 319871, 320124, 329221), the Danish Agency for

541 Science and Higher Education, Danish Ministry of Energy, Utilities and Climate, EU (Grants nr

542 211574, 730944), Finnish Ministry of Transport and Communication, Flemish Fund for

543 Scientific Research (FWO Grant nr G0H3317), French Ministry of Research (MESRI), German

544 Ministry of Education and Research (BMBF), German Ministry of Transport and Digital

545 Infrastructure (BMVI), the Italian Ministry of Universities and Research, The Netherlands

546 Ministry of Education, Culture and Science, the Netherlands Organisation for Scientific

547 Research., the Ministry of Education, Youth and Sports of the Czech Republic, The Natural

548 Environment Research Council of the UK, Norwegian Environmental Agency, Norwegian

549 Ministry of Climate and Environment, Research Council of Norway, Swedish Research

550 Council (Grant nr 2019-00205), Swiss National Science Foundation (ICOS-CH Phase 1 and

551 Phase 2, 20FI21_148992 and 20FI20_173691) and the ETH domain.

552
553 **Appendix**

554 **Appendix 1. ICOS Governance and Funding**

555 **Governance**

556 The European Strategy Forum for Research Infrastructures (ESFRI) produced a roadmap of RIs
557 in 2006, including the preparation of ICOS to cover European carbon dioxide and other GHG
558 observations (ESFRI 2006). The ICOS Preparatory Phase Project (2008-2013) focused on
559 integrating the already existing stations into a single network and establishing a model for
560 sustained funding since the ICOS observations at the stations was usually limited to the
561 lifetime of regular research projects. Besides harmonized observations, data management
562 and archiving, this unprecedented effort covered also administrative, financial and legal
563 aspects. ICOS ERIC (was established in 2015 with the member (Belgium, Finland, France,
564 Germany, Italy, the Netherlands, Norway, Sweden) and observer (Switzerland) countries
565 committing to the long-term funding of the RI.

566 The governance and operational structure of Integrated Carbon Observation System
567 European Research Infrastructure Consortium (ICOS ERIC) and ICOS Research Infrastructure
568 (RI) is shown in Appendix Fig. 1. The data are provided by the ICOS National Networks which
569 are networks of stations operated at national level and form the backbone of ICOS RI. The
570 Central Facilities receive, quality control and process the data measured at the ICOS stations.
571 The Central Facilities are operated by one or several Host Institutions either at national or at
572 multi-national level, and they include the Atmosphere Thematic Centre (ATC), Ecosystem
573 Thematic Centre (ETC), Ocean Thematic Centre (OTC) and Central Analytical Laboratories
574 (CAL). Monitoring Station Assemblies (MSAs) in atmosphere, ecosystem and ocean domains,
575 gather together the ICOS Station Principal Investigators (PIs) to discuss technical and scientific
576 topics. All data from raw, near-real time to final quality controlled data are stored and
577 published through the ICOS Carbon Portal, part of ICOS ERIC.

578 The task sharing between ICOS ERIC and Central Facilities is clearly defined and agreed upon
579 in ICOS ERIC - Central Facilities agreements. Furthermore, the basic management and internal
580 distribution of the work is organized by the Central Facilities host institutions, and
581 employment practices are carried out according to the respective institutional practices.
582 Financial governance follows the similar approach: the host institutions have their own
583 responsibility but have to comply with common rules and are monitored by ICOS ERIC.

584 The decision-making body in ICOS ERIC is the General Assembly consisting of delegates from
585 all Member and Observer countries. The Research Infrastructure Committee, with
586 representatives from the ICOS ERIC and Central Facilities and MSAs, advises the Director
587 General and the General Assembly on scientific and organizational topics. Scientific Advisory
588 Board and Ethical Advisory Boards are external bodies for giving strategic guidance for ICOS
589 RI.

590 **Funding**

591 The national networks are funded mainly by the national funding agencies and respective
592 ministries, with additional support by the host institutions of the measuring stations. A
593 substantial part of the total costs of the Central Facilities and ICOS ERIC is covered by
594 contributions of the hosting country/ies (host premium contribution) of the Central Facilities,
595 Head Office and Carbon Portal.

596 Member and Observer countries of ICOS ERIC pays annual membership contributions. Total
597 membership contributions are formed by the following elements: common basic
598 contribution, common GNI based contribution, and number and type of stations. The latter
599 part of the membership contribution is redistributed to the activities in the ICOS Central

600 Facilities. The Central Facilities are also supported by the host organizations. ICOS ERIC seeks

601 funding opportunities from European Commission and other sources.
602 **Appendix 2. Observed Variables at ICOS Stations**

603 All the observed variables at ICOS stations are presented in Appendix 2 tables 1-4.

604
References

606 Arrouays, D., and Coauthors, 2018: Soil sampling and preparation for monitoring soil carbon.

607 *International Agrophysics*, **32**(4), 633–643, doi:10.1515/intag-2017-0047.

608 Baldocchi, D., 2014: Measuring fluxes of trace gases and energy between ecosystems and

609 the atmosphere - the state and future of the eddy covariance method. *Global Change*

610 *Biology*, **20**(12): 3600–3609, https://doi.org/10.1111/gcb.12649.

611 Baldocchi, D., and Coauthors, 2001: FLUXNET: A New Tool to Study the Temporal and Spatial

612 Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities.

613 *Bulletin of the American Meteorological Society*, **82**(11), 2415-2434,

614 https://doi.org/10.1175/1520-0477(2001)082<2415:FANTTS>2.3.CO;2.

615 Basu, S., and Coauthors, 2020: Estimating US fossil fuel CO₂ emissions from measurements

616 of ¹⁴C in atmospheric CO₂. *Proceedings of the National Academy of Sciences*, **117** (24),

617 13300-13307, DOI: 10.1073/pnas.1919032117

618 Breon, F. M., and Coauthors, 2015: An attempt at estimating Paris area CO₂ emissions from

619 atmospheric concentration measurements. *Atmos. Chem. Phys.*, **15**, 1707–1724,

620 www.atmos-chem-phys.net/15/1707/2015/.

621 Calfapietra C., Ü., Niinemets, and J. Peñuelas, 2015: Urban Plant Physiology: adaptation-

622 mitigation strategies under permanent stress. *Trends in Plant Science*, **20**, 72-75.

623 Copernicus, 2015: Towards a European Operational Observing System to Monitor Fossil CO₂

624 emissions. https://edgar.jrc.ec.europa.eu/news_docs/CO2_report_22-10-2015.pdf.
625 Copernicus, 2019: An Operational Anthropogenic CO₂ Emissions Monitoring & Verification

626 Support Capacity – Needs and high level requirements for in situ measurements.

627 https://doi.org/10.2760/182790.

628 Dickson, A. G., C. L. Sabine, and J. R. Christian (Eds.) 2007: Guide to Best Practices for Ocean

629 CO₂ Measurements. PICES Special Publication 3, pp 191, ISSN: 1813-8519.

630 El Yazidi, A., and Coauthors, 2018: Identification of spikes associated with local sources in

631 continuous time series of atmospheric CO, CO₂ and CH₄. *Atmospheric Measurement*

632 *Techniques*, **11**(3), 1599-1614.

633 ESFRI, 2006: European Roadmap for Research Infrastructures. Report 2006. 84pp,

634 https://www.esfri.eu/sites/default/files/esfri_roadmap_2006_en.pdf.

635 ESFRI, 2016: Strategy report on Research Infrastructures. 208pp,

636 https://www.esfri.eu/sites/default/files/20160309_ROADMAP_browsable.pdf.

637 Franz, D. and Coauthors, 2018: Towards long-term standardised carbon and greenhouse gas

638 observations for monitoring Europe's terrestrial ecosystems: A review. *International*

639 *Agrophysics*, **32** (4), 439–55, https://doi.org/10.1515/intag-2017-0039.

640 Friedlingstein, P., and Coauthors, 2020: Global carbon budget 2020. *Earth System Science*

641 *Data*, **12**, 3269–3340, https://doi.org/10.5194/essd-12-3269-2020.

642 GCOS, 1994: Report of the GCOS/GTOS Terrestrial Observation Panel. WMO/TD- No.

643 642; GCOS- No. 08. https://library.wmo.int/doc_num.php?explnum_id=3804.

644 GCOS, 2016: The Global Observing System for Climate: Implementation Needs. GCOS- No.

645 200. https://library.wmo.int/doc_num.php?explnum_id=3417.
646 Gielen, B., M. Op de Beeck, D. Loustau, R. Ceulemans, A. Jordan, and D. Papale, 2017:

647 Integrated Carbon Observation System (ICOS): An Infrastructure to Monitor the European

648 Greenhouse Gas Balance. In *Terrestrial Ecosystem Research Infrastructures: Challenges and

649 *Opportunities*, 1st edition, CRC Press, ISBN 9781498751315.

650 Gielen, B., and Coauthors, 2018: Ancillary data measurements at ICOS sites. *International*

651 *Agrophysics*, **32**(4), 645-664, doi: https://doi.org/10.1515/intag-2017-0048.

652 Graf, A., and Coauthors, 2020: Altered energy partitioning across terrestrial ecosystems in

653 the European drought year 2018. *Philosophical Transactions of the Royal Society B*, **375**, doi:

654 10.1098/rstb.2019.0524.

655 Hazan, L., J. Tarniewicz, M. Ramonet, O. Laurent, and A. Abbaris, 2016: Automatic

656 processing of atmospheric CO₂ and CH₄ mole fractions at the ICOS Atmosphere Thematic

657 Centre. *Atmospheric Measurement Techniques*, **9**(9), 4719-4736.

658 Hufkens, K., and Coauthors, 2018: Assimilating phenology datasets automatically across

659 ICOS ecosystem stations. *Int. Agrophys.*, **32**(4), 677-687, doi: https://doi.org/10.1515/intag-

660 2017-0050.

661 ICOS, 2015: ICOS RI Data Policy. *ICOS ERIC*, 11pp, https://www.icos-

662 cp.eu/sites/default/files/cmis/ICOS%20RI%20Data%20Policy.pdf.

663 ICOS, 2017a: ICOS Atmospheric Station specifications, Version 1.3. *ICOS ATC*, 55pp,

664 https://doi.org/10.18160/SDW6-BX90.
665 ICOS, 2017b: ICOS Marine Station Labelling Step 2. ICOS OTC, 23pp, https://otc.icos-

666 cp.eu/sites/default/files/2018-

667 05/ICOS%20Marine%20Station%20Labelling%20Step%20%20v4_crb.docx_.pdf.

668 ICOS, 2019: ICOS Strategy. ICOS ERIC, 44pp, https://doi.org/10.18160/TV1J-4XQY.

669 ICOS, 2020a: ICOS Handbook 2020. ICOS ERIC, 152pp, ISBN 978-952-69501-1-2.

670 ICOS, 2020b. ICOS Ocean Station Labelling Step 2. ICOS OTC, 20pp, doi: 10.18160/8SDC-

671 K4FR.

672 ICOS, 2020c: Jupyter Notebook for ICOS Flask Sampling Strategy (Version 1.0). ICOS ERIC.

673 https://doi.org/10.18160/FSS2-SH26.

674 ICOS, 2021: ICOS five year evaluation 2020. ICOS ERIC, 127pp, ISBN 978-952-69501-3-6.

675 International Agrophysics, vol. 32(4), 2018. Special Issue on ICOS Ecosystem Protocols.

676 http://www.international-agrophysics.org/Issue-4-2018,7048.

677 IPCC, 1992: Climate change 1992. The supplementary report to the IPCC scientific

678 assessment. ISBN 0 521 43829 2.

679 IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global

680 warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission

681 pathways, in the context of strengthening the global response to the threat of climate

682 change, sustainable development, and efforts to eradicate poverty. Masson-Delmotte, V., P.

683 Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R.

684 Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock,

685 M. Tignor, and T. Waterfield, Eds., 630 pp, in press.
686 Janssens, I. and Coauthors, 2003: Europe's Terrestrial Biosphere Anthropogenic CO₂

687 Emissions. *Science*, **300**, 1538–42, https://doi.org/10.1126/science.1083592.

688 Jung, M. and Coauthors, 2020: Scaling carbon fluxes from eddy covariance sites to globe:

689 synthesis and evaluation of the FLUXCOM approach. *Biogeosciences*, **17**, 1343-1365,

690 https://doi.org/10.5194/bg-17-1343-2020.

691 Karion, A., C. Sweeney, P. Tans, and T. Newberger, 2010: AirCore: An innovative atmospheric

692 sampling system. *Journal of Atmospheric and Oceanic Technology*, **27** (11), 1839–1853, doi:

693 10.1175/2010JTECHA1448.1.

694 Keeling, C. D., 1978: The influence of Mauna Loa Observatory on the development of

695 atmospheric CO₂ research. In *Mauna Loa Observatory. A 20th Anniversary Report*, J. Miller,

696 Ed., 36–54. U.S. Department of Commerce.

697 Lauvaux, T. and Coauthors, 2020: Policy-Relevant Assessment of Urban CO₂ Emissions,

698 *Environmental Science & Technology*, **54** (16), 10237-10245, doi: 10.1021/acs.est.0c00343.

699 Levin, I., S. Hammer, E. Eichelmann, and F. R. Vogel, 2011: Verification of greenhouse gas

700 emission reductions: The prospect of atmospheric monitoring in polluted areas.

701 *Philosophical Transactions of the Royal Society A*, **369** (1943), 1906–1924,

702 doi:10.1098/rsta.2010.0249.

703 Levin, I., B. Kromer, M. Schmidt, and H. Sartorius, 2003: A novel approach for independent

704 budgeting of fossil fuel CO₂ over Europe by ¹⁴CO₂ observations. *Geophysical Research*

705 *Letters*, **30** (23), 1–5, doi:10.1029/2003GL018477.
706

Levin, I., and Coauthors, 2020: A dedicated flask sampling strategy developed for ICOS

707

stations based on CO₂ and CO measurements and STILT footprint modelling. *Atmospheric*

708

*Chemistry and Physics Discussions*, 1–29, doi:10.5194/acp-2020-185.

709

López-Ballesteros, A., and Coauthors, 2018: Towards a feasible and representative pan-

710

African research infrastructure network for GHG observations. *Environmental Research*

711

*Letters*, **13** (8), doi:10.1088/1748-9326/aad66c.Loustau, D., and Coauthors, 2018: Sampling

712

and collecting foliage elements for the determination of the foliar nutrients in ICOS

713

ecosystem stations. *Int. Agrophys.*, **32**(4), 665-676, doi: https://doi.org/10.1515/intag-2017-

714

0038.

715

Luyssaert, S., and Coauthors, 2010: The European carbon balance. Part 3: Forests. *Global*

716

*Change Biology*, **16**, 1429-1450, doi: 10.1111/j.1365-2486.2009.02056.x.

717

Ma, X., W. Liu, R. J. Allen, G. Huang, and X. Li, 2020: Dependence of regional ocean heat

718

uptake on anthropogenic warming scenarios. *Science Advances*, **6**(45), eabc0303,

719

doi:10.1126/sciadv.abc0303.

720

Nemitz, E., and Coauthors, 2018: Standardisation of eddy-covariance flux measurements of

721

methane and nitrous oxide. *International Agrophysics*, **32** (4), 517–549, doi:10.1515/intag-

722

2017-0042.

723

Nordbo, A., L. Järvi, S. Haapanala, C. R. Wood, and T. Vesala, 2012: Fraction of natural area

724

as main predictor of net CO₂ emissions from cities. *Geophysical Research Letters*, **39** (20), 1–

725

5, doi:10.1029/2012GL053087.
726 Op de Beeck, M., and Coauthors, 2018. Soil-meteorological measurements at ICOS

727 monitoring stations in terrestrial ecosystems. *International Agrophysics*, **32**(4), 619-631,

728 https://doi.org/10.1515/intag-2017-0041.

729 Papale, D., 2020: Ideas and perspectives: enhancing the impact of the FLUXNET network of

730 eddy covariance sites. *Biogeosciences*, **17**, 5587-5598, doi: 10.5194/bg-17-5587-2020.

731 Papale, D., and R. Valentini, 2003: A new assessment of European forests carbon exchanges

732 by eddy fluxes and artificial neural network spatialization. *Global Change Biology*, **9**, 525-

733 535, https://doi.org/10.1046/j.1365-2486.2003.00609.x.

734 Pavelka, M., and Coauthors, 2018: Standardisation of chamber technique for CO₂, N₂O and

735 CH₄ fluxes measurements from terrestrial ecosystems. *International Agrophysics*, **32**(4),

736 569-587, doi:10.1515/intag-2017-0045.

737 Peters, W., and Coauthors, 2010: Seven years of recent European net terrestrial carbon

738 dioxide exchange constrained by atmospheric observations. *Global Change Biology*, **16**(4),

739 1317-1337, doi:https://doi.org/10.1111/j.1365-2486.2009.02078.x.

740 Peters, W., Bastos, A., Ciais, P., Vermeulen, A., and Kutsch, W. (eds.), 2020: Impacts of the

741 2018 severe drought and heatwave in Europe: from site to continental scale. Special issue in

742 *Philosophical Transactions of the Royal Society B*,

743 https://royalsocietypublishing.org/toc/rstb/375/1810.

744 Petrescu, A. M. R., and Coauthors, 2020: The consolidated European synthesis of CH₄ and

745 N₂O emissions for EU27 and UK: 1990-2018, *Earth System Science Data Discussion*. preprint

746 in review, https://doi.org/10.5194/essd-2020-367.
747 Pfeil, B., and Coauthors, 2013: A uniform, quality controlled Surface Ocean CO₂ Atlas

748 (SOCAT). *Earth System Science Data*, **5**, 125-143, doi:10.5194/essd-5-125-2013.

749 Rebmann, C., and Coauthors, 2018: ICOS eddy covariance flux-station site setup: a review.

750 *International Agrophysics*, **32**, 471-494, doi: 10.1515/intag-2017-0044.

751 Rhein, M., and Coauthors, 2013: Observations: ocean. *Climate Change 2013: The Physical*

752 *Science Basis*. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A.

753 Nauels, Y. Xia, V. Bex and P. M. Midgley, Eds., 60 pp, Cambridge University Press,
754 Cambridge.

755 Rinne, J., and Coauthors, 2020: Effect of the 2018 European drought on methane and
756 carbon dioxide exchange of northern mire ecosystems. *Philosophical Transactions of the*
757 *Royal Society B*, **375**, doi:10.1098/rstb.2019.0517.

758 Rogelj, J., and Coauthors, 2018: Mitigation Pathways Compatible with 1.5°C in the Context
759 of Sustainable Development. *Global Warming of 1.5°C. An IPCC Special Report on the*
760 *impacts of global warming of 1.5°C above pre-industrial levels and related global*
761 *greenhouse gas emission pathways, in the context of strengthening the global response to*
762 *the threat of climate change, sustainable development, and efforts to eradicate poverty*, V.

763 Masson-Delmotte, P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W.

764 Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I.

765 Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield, Eds., 82 pp. In press.

766 Saunders, M., and Coauthors, 2018: Importance of reporting ancillary site characteristics,
767 and management and disturbance information at ICOS stations. *International Agrophysics*,
768 **32** (4), 457-469, doi:10.1515/intag-2017-0040.
769 Schulze, E. D., and Coauthors, 2009: Importance of methane and nitrous oxide for Europe's
770 terrestrial greenhouse-gas balance. *Nature Geoscience*, **2**, 842–850.

771 Schulze, E. D., and Coauthors, 2010: The European carbon balance. Part 4: Integration of
772 carbon and other trace-gas fluxes. *Global Change Biology*, **16**, 1451–69,
773 https://doi.org/10.1111/j.1365-2486.2010.02215.x.

774 Smith, N. E., and Coauthors, 2020: Spring enhancement and summer reduction in carbon
775 uptake during the 2018 drought in northwestern Europe. *Philosophical Transactions of the
776 Royal Society B*, **375**, doi:10.1098/rstb.2019.0509.

777 Steinhoff, T., and Coauthors, 2019: Constraining the Oceanic Uptake and Fluxes of
778 Greenhouse Gases by Building an Ocean Network of Certified Stations: The Ocean

779 Component of the Integrated Carbon Observation System, ICOS-Oceans. *Frontiers in Marine
780 Science*, **6**, 1–15, https://doi.org/10.3389/fmars.2019.00544.

781 Thompson, R., Broquet, G., Gerbig, C., Monteil, G., Lang, M. and Koch, F.-T., 2019: Drought-
782 2018 ensemble of inversion results for 2009–2018, doi:10.18160/YQ8P-P7CF.

783 Thompson R. L., Broquet G., Gerbig C., Koch T., Lang M., Monteil G., Munassar S., Nickless
784 A., Scholze M., Ramonet M., Karstens U., van Schaik E., Wu Z. and Rödenbeck C., 2020:

785 Changes in net ecosystem exchange over Europe during the 2018 drought based on
786 atmospheric observations. *Philosophical Transactions of the Royal Society B*, **375**:20190512,
787 http://doi.org/10.1098/rstb.2019.0512.

788 United Nations, 2015a: Paris Agreement.

789 https://unfccc.int/files/meetings/paris_nov_2015/application/pdf/paris_agreement_english
790 _ .pdf.
791 United Nations, 2015b: Transforming our world: the 2030 Agenda for Sustainable

792 Development.

793 https://www.un.org/en/development/desa/population/migration/generalassembly/docs/gl

794 obalcompact/A_RES_70_1_E.pdf

795 Vetter, M., and Coauthors, 2008: Analyzing the causes and spatial pattern of the European

796 2003 carbon flux anomaly using seven models. Biogeosciences, 5, 561-583,

797 https://doi.org/10.5194/bg-5-561-2008.

798 Wilkinson, M. D., and Coauthors, 2016: The FAIR Guiding Principles for scientific data

799 management and stewardship. Scientific Data, 3:160018, doi: 10.1038/sdata.2016.18.

800 Vitale, D., Fratini, G., Bilancia, M., Nicolini, G., Sabbatini, S., and D. Papale, 2020: A robust

801 data cleaning procedure for eddy covariance flux measurements. Biogeosciences, 17, 1367-

802 1391, doi: 10.5194/bg-17-1367-2020.

803 WMO, 2009: A history of climate activities. WMO Bulletin, 58 (3), 141,

804 https://library.wmo.int/doc_num.php?explnum_id=6945.

805 WMO, 2014: The Global Atmosphere Watch Programme: 25 Years of Global Coordinated

806 Atmospheric Composition Observations and Analyses. WMO-No. 1143,

807 https://library.wmo.int/doc_num.php?explnum_id=7886

808 WMO, 2019: An Integrated Global Greenhouse Gas Information System (IG3IS) Science

809 Implementation Plan. GAW Report, 245,

810 https://library.wmo.int/doc_num.php?explnum_id=10034.

811 WMO, 2020a: WMO WDCGG Data summary. WDCGG No. 44,

812 https://gaw.kishou.go.jp/static/publications/summary/sum44/sum44.pdf
813 WMO, 2020b: 20th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases and

814 Related Measurement Techniques (GGMT-2019). GAW Report 255, 19-23.

815 https://library.wmo.int/doc_num.php?explnum_id=10353

816 Yver Kwok, C., and Coauthors, 2015: Comprehensive laboratory and field testing of cavity

817 ring-down spectroscopy analyzers measuring H₂O, CO₂, CH₄ and CO. *Atmospheric*

818 *Measurement Techniques*, **8**(9), 3867-3892.

819 Yver-Kwok, C., and Coauthors, 2021: Evaluation and optimization of ICOS atmosphere

820 station data as part of the labeling process. *Atmospheric Measurement Techniques*, **14**, 89–

821 116, https://doi.org/10.5194/amt-14-89-2021.
<table><thead><tr><th>Category</th><th>Gases, continuous sampling</th><th>Gases, periodical sampling</th><th>Meteorology, continuous</th><th>Eddy Fluxes</th></tr></thead><tbody><tr><td>Class 1<br>Mandatory parameters</td><td><ul><li>CO<sub>2</sub>, CH<sub>4</sub>, CO: at each sampling height</li></ul></td><td><ul><li>CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, SF<sub>6</sub>, CO, H<sub>2</sub>, <sup>13</sup>C and <sup>18</sup>O in CO<sub>2</sub>: weekly sampled at highest sampling height</li><li><sup>14</sup>C (radiocarbon integrated samples): at highest sampling height</li></ul></td><td><ul><li>Air temperature, relative humidity, wind direction, wind speed: at highest and lowest sampling height*</li><li>Atmospheric Pressure</li><li>Planetary Boundary Layer Height**</li></ul></td><td></td></tr><tr><td>Class 2<br>Mandatory parameters</td><td><ul><li>CO<sub>2</sub>, CH<sub>4</sub>: at each sampling height</li></ul></td><td></td><td><ul><li>Air temperature, relative humidity, wind direction, wind speed: at highest and lowest sampling height*</li><li>Atmospheric Pressure</li></ul></td><td></td></tr><tr><td>Recommended parameters**<br>*</td><td><ul><li><sup>222</sup>Rn, N<sub>2</sub>O, O<sub>2</sub>/N<sub>2</sub> ratio</li><li>CO for Class 2 stations</li></ul></td><td><ul><li>CH<sub>4</sub> stable isotopes, O<sub>2</sub>/N<sub>2</sub> ratio for class 1 stations: weekly sampled at highest</li></ul></td><td></td><td><ul><li>CO<sub>2</sub>: at one sampling height</li></ul></td></tr></tbody></table>
<table><tr><td></td><td>sampling height</td><td></td><td></td></tr></table>

824

825

* Atmospheric temperature and relative humidity recommended at all sampling heights

826

** Only required for continental stations

827

*** Recommended for its scientific value but support from ATC in terms of protocols, database, spare analyser

828

will not be ensured as long as the parameters are not mandatory

829
<table><thead><tr><th>Variables</th><th>Forest</th><th>Grassland</th><th>Cropland</th><th>Wetland*</th><th>Marine*</th><th>Lakes*</th></tr></thead><tbody><tr><td>CO<sub>2</sub>, H<sub>2</sub>O and H fluxes (eddy covariance, including profile for storage)</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td></tr><tr><td>CH<sub>4</sub> and N<sub>2</sub>O fluxes (eddy covariance, including profile for storage)</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>Air H<sub>2</sub>O concentration</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>Incoming, Outgoing and Net SW and LW radiations</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1</td><td>1</td></tr><tr><td>Incoming SW radiation (high quality)</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td></tr><tr><td>Incoming PPFD</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td></tr><tr><td>PPFD below canopy + ground reflected</td><td>Fac</td><td>Fac</td><td>Fac</td><td>N.R.</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Outgoing PPFD</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>Fac</td><td>Fac</td></tr><tr><td>Diffuse PPFD and/or SW radiation</td><td>1</td><td>1</td><td>1</td><td>1</td><td>Fac</td><td>Fac</td></tr><tr><td>Spectral reflectance</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td></tr><tr><td>Soil Heat flux</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Air Temperature and Humidity profile</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>Fac</td><td>Fac</td></tr><tr><td>Backup meteo station (TA, RH, SW_IN, Precipitation)</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td></tr><tr><td>Total high accuracy precipitation</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td></tr><tr><td>Snow height</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>Fac</td><td>Fac</td></tr><tr><td>Soil Water Content profile</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Soil Temperature profile</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr></tbody></table>
<table><tr><td>Air Pressure</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td></tr><tr><td>Trunk and branches temperature</td><td>Fac</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Water Table Depth</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Tree diameter (continuous)</td><td>1</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Phenology/camera</td><td>1</td><td>1</td><td>1</td><td>1</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Soil CO2 automatic chambers</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>CH4 and N2O fluxes by automatic chambers</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>Wind speed and wind direction (additional to 3D sonic)</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>GAI</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Above Ground Biomass</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Soil carbon content</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Litterfall</td><td>1</td><td>1</td><td>1</td><td>1</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Leaf nutrients content</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Soil water N content</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td><td>N.R.</td><td>N.R.</td></tr><tr><td>DOC concentration</td><td>Fac</td><td>Fac</td><td>Fac</td><td>Fac</td><td>N.R.</td><td>N.R.</td></tr><tr><td>C and N import/export by management</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>N.R.</td><td>N.R.</td></tr><tr><td>Oxygen and pCO2 surface concentration</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>Fac</td><td>2</td><td>2</td></tr><tr><td>Oxygen, pCO2 and pN2O concentration profile</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>Fac</td><td>1</td><td>1</td></tr><tr><td>Salinity</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>1 & 2</td><td>N.R.</td></tr><tr><td>Wave properties</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>Fac</td><td>Fac</td></tr><tr><td>Water temperature profile</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>N.R.</td><td>1</td><td>1</td></tr><tr><td>Management and disturbances information</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td><td>1 & 2</td></tr></table>
833 Fac = Facultative variable; N.R. = Not Relevant for the ecosystem

834 * Wetland includes all different water inundated or saturated ecosystems according to Joosten and Clark 2002

835 ** List of variables for Lake, Marine and Urban sites under discussion

836
<table><thead><tr><td>VARIABLE</td><td>FREQUENCY</td><td>ACCURACY</td><td>REQUIRED FOR CLASS</td></tr></thead><tbody><tr><td>Sea surface fCO<sub>2</sub></td><td>Quasi-continuous</td><td>± 2 µatm</td><td>1 &amp; 2</td></tr><tr><td>Intake temperature (SST)</td><td>Continuous</td><td>± 0.05 °C</td><td>1 &amp; 2</td></tr><tr><td>Equilibrator temperature</td><td>Continuous</td><td>± 0.05 °C</td><td>1 &amp; 2</td></tr><tr><td>ΔT (Intake/Equilibrator temperature difference)</td><td>Continuous</td><td>&lt; 1.5 °C (normal) &lt; 3 °C (ice-edge)</td><td>1 &amp; 2</td></tr><tr><td>Water vapour pressure*</td><td>Continuous</td><td>± 0.5 mbar</td><td>1 &amp; 2</td></tr><tr><td>Equilibrator pressure</td><td>Continuous</td><td>± 2.0 mbar</td><td>1 &amp; 2</td></tr><tr><td>Atmospheric pressure/sea level pressure</td><td>Continuous</td><td>± 1.0 mbar</td><td>1 &amp; 2</td></tr><tr><td>Sea surface salinity (SSS)</td><td>Continuous</td><td>± 0.1 PSU</td><td>1 &amp; 2</td></tr><tr><td>Dissolved oxygen</td><td>Continuous</td><td>± 2%</td><td>1</td></tr><tr><td>Total alkalinity (TA)**</td><td>***</td><td>± 10 µmol kg<sup>-1</sup></td><td>1</td></tr><tr><td>Dissolved inorganic carbon (DIC)**</td><td>***</td><td>± 5 µmol kg<sup>-1</sup></td><td>1</td></tr></tbody></table>

838 *If the analysed headspace gas is not dried completely prior to measurement.

839 ** At least one of these variables must be provided.

840 *** The frequency of these additional variables will be decided on during the labelling process based

841 on the area where the station is operating.

842
<table><thead><tr><td>VARIABLE</td><td>FREQUENCY</td><td>ACCURACY</td><td>REQUIRED FOR CLASS</td></tr></thead><tbody><tr><td>Sea surface pCO<sub>2</sub></td><td>&gt; 1/day (open ocean)<br/>&gt; 3/day (coastal)</td><td>± 10 µatm</td><td>1 &amp; 2</td></tr><tr><td>Sea surface temperature</td><td>&gt; 1/day (open ocean)<br/>&gt; 3/day (coastal)</td><td>± 0.02 °C</td><td>1 &amp; 2</td></tr><tr><td>Sea surface salinity</td><td>&gt; 1/day (open ocean)<br/>&gt; 3/day (coastal)</td><td>± 0.1 PSU</td><td>1 &amp; 2</td></tr><tr><td>Pressure (depth)</td><td>&gt; 1/day (open ocean)<br/>&gt; 3/day (coastal)</td><td>± 3 dbar</td><td>1 &amp; 2</td></tr><tr><td>Dissolved oxygen</td><td>&gt; 1/day (open ocean)<br/>&gt; 3/day (coastal)</td><td>± 2%</td><td>1 &amp; 2</td></tr><tr><td>Total alkalinity (TA)*</td><td>**</td><td>± 4 µmol kg<sup>-1</sup></td><td>1 &amp; 2</td></tr><tr><td>Dissolved inorganic carbon (DIC)*</td><td>**</td><td>± 2 µmol kg<sup>-1</sup></td><td>1 &amp; 2</td></tr><tr><td>pH***</td><td>**</td><td>± 0.003</td><td>1 &amp; 2</td></tr><tr><td>Dissolved nutrients ****</td><td>**</td><td>± 1-3%****</td><td>1</td></tr></tbody></table>

844 * At least one of these variables must be provided.

845 ** The frequency of these additional variables will be decided on during the labelling process based

846 on the area where the station is operating.

847 *** pH (together with TA or DIC) is ONLY required for validation of the pCO₂ data. pH should NOT be used

848 together with pCO₂ to calculate the full carbonate system due to high resulting uncertainty.

849 **** At least two out of the three dissolved nutrients nitrate (NO₃), phosphate (PO₄), and silicate (Si(OH)₄) must

850 be provided.

851 ***** The accuracy refers to samples without conservation. If conservation is used (freezing is the most used

852 method) the accuracy might increase.

853
<figure><img src="image_3.png" /></figure>

855

856 **Figure 1.** Map of ICOS stations. The dots represent fixed stations in different domains

857 (ocean, ecosystem, atmosphere) and lines the Ships of Opportunities. Up-to-date details

858 (e.g. station class, contact info, data) from each station can be found from

859 https://www.icos-cp.eu/observations/station-network.
<figure><img src="image_4.png" /></figure>

860

861 Figure 2. Schematic figure of the carbon cycle and related data collection process and user

862 access to all the data via the Carbon Portal. The color-coding links the areas of the

863 biogeochemical carbon cycle to the respective stations and Thematic Centres. The green
864 color indicates the exchange of carbon, GHGs and energy between the atmosphere and
865 ecosystems (vertical arrows), the red color the atmospheric gas concentrations, chemistry
866 and transport processes (horizontal arrows), and the blue color the ocean-atmosphere gas
867 exchange (vertical arrows), observed within the ICOS stations of respective domains (dots in
868 the lower part and also in Fig. 1). The observations are centrally processed within the
869 Thematic Centres and the data stored to ICOS repository, with Carbon Portal serving as one-
870 stop-shop for all ICOS data products.

871
<figure><img src="image_5.png" /></figure>

872

873 Figure 3. An example of ICOS tool to analyze the potential impact of natural and
874 anthropogenic emissions to the CO₂ concentrations in the atmosphere, based on model
875 simulation of Lagrangian transport model STILT (Stochastic Time Inverted Lagrangian
876 Transport; Lin et al 2013) together with emission-sector and fuel-type specific emissions
877 from a pre-release of the EDGARv4.3 inventory (EC-JRC/PBL, 2015). Figure shows results for
878 the ICOS Class 1 Jungfraujoch station in Switzerland, including biospheric and anthropogenic
879 carbon emissions. Panel A) shows modelled footprint and wind directions influencing the
880 measurement tower signal; panel B) shows selected towers, location of atmospheric tower
881 in Europe and variables that are available for interactive visualization; while panel C) shows
882 the time series of a selected variable, including measured and modelled concentrations.

883
<figure><img src="image_6.png" /></figure>

884

885

886

887

888

889

890

<caption>Figure 4. Example of a regional-scale atmospheric inversion result, estimating the net ecosystem exchange (NEE) based on atmospheric observations from ICOS and other stations. The presented example is part of a multi-model ensemble of atmospheric inversions, available at Carbon Portal (Thompson et al, 2019), that was used to estimate the effect of the 2018 drought on net ecosystem exchange over Europe (Thompson et al., 2020).</caption>
<figure><img src="image_7.png" /><figcaption>891</figcaption></figure>

892

Appendix 1 Figure 1. The governance structure of ICOS.