title: Metrological challenges for measurements of key climatological observables: Oceanic salinity and pH, and atmospheric humidity. Part 1: Overview.
abstract: Water in its three ambient phases plays the central thermodynamic role in the terrestrial climate system. Clouds control Earth's radiation balance, atmospheric water vapour is the strongest "greenhouse" gas, and non-equilibrium relative humidity at the air-sea interface drives evaporation and latent heat export from the ocean. On climatic time scales, melting ice caps and regional deviations of the hydrological cycle result in changes of seawater salinity, which in turn may modify the global circulation of the oceans and their ability to store heat and to buffer anthropogenically produced carbon dioxide. In this paper, together with three companion articles, we examine the climatologically relevant quantities ocean salinity, seawater pH and atmospheric relative humidity, noting fundamental deficiencies in the definitions of those key observables, and their lack of secure foundation on the International System of Units, the SI. The metrological histories of those three quantities are reviewed, problems with their current definitions and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10. It is concluded that the International Bureau of Weights and Measures, BIPM, in cooperation with the International Association for the Properties of Water and Steam, IAPWS, along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for these long standing metrological problems in climatology.
authors.affiliation: Leibniz Institute for Baltic Sea Research (IOW), D-18119 Warnemünde, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Feistel
authors.affiliation: Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, F-92312 Sèvres Cedex France.
authors.forename: R
authors.initials: R
authors.lastname: Wielgosz
authors.affiliation: National Physical Laboratory (NPL), Hampton Road, Teddington, Middlesex, TW11 0LW, UK.
authors.forename: S A
authors.initials: SA
authors.lastname: Bell
authors.affiliation: Centro de Química Estrutural, Faculdade de Ciências, University of Lisbon (FCUL), 1749-016 Lisbon, Portugal.
authors.forename: M F
authors.initials: MF
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authors.affiliation: Queen Mary, University of London (QMUL), Mile End Road, London, E1 4NS, UK.
authors.forename: J R
authors.initials: JR
authors.lastname: Cooper
authors.affiliation: Bureau of Meteorology (ABN), GPO Box 1289, Melbourne, VIC 3001, Australia.
authors.forename: P
authors.initials: P
authors.lastname: Dexter
authors.affiliation: University of California, San Diego (UCSD), La Jolla, CA 92093-0244, USA.
authors.forename: A G
authors.initials: AG
authors.lastname: Dickson
authors.affiliation: Laboratoire National de Métrologie et d'Essais (LNE), F-75724 Paris Cedex 15, France.
authors.forename: P
authors.initials: P
authors.lastname: Fisicaro
authors.affiliation: National Institute of Standards and Technology (NIST), Boulder, CO 80305-3337, USA.
authors.forename: A H
authors.initials: AH
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authors.affiliation: MIKES Metrology, VTT Technical Research Centre of Finland Ltd, Tekniikantie 1, FI-02151 Espoo, Finland.
authors.forename: M
authors.initials: M
authors.lastname: Heinonen
authors.affiliation: Leibniz Institute for Tropospheric Research (TROPOS), D-04318 Leipzig, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Hellmuth
authors.affiliation: Zittau/Goerlitz University of Applied Sciences (HSZG), D-02763 Zittau, Germany.
authors.forename: H-J
authors.initials: HJ
authors.lastname: Kretzschmar
authors.affiliation: Measurement Standards Laboratory (MSL), PO Box 31-310, Lower Hutt, New Zealand.
authors.forename: J W
authors.initials: JW
authors.lastname: Lovell-Smith
authors.affiliation: University of New South Wales (UNSW), Sydney, NSW 2052, Australia.
authors.forename: T J
authors.initials: TJ
authors.lastname: McDougall
authors.affiliation: University of British Columbia (UBC), Vancouver, B.C., V6T 1Z4, Canada.
authors.forename: R
authors.initials: R
authors.lastname: Pawlowicz
authors.affiliation: Ocean Scientific International Ltd. (OSIL), Culkin House, Penner Road, Havant, PO9 1QN, UK.
authors.forename: P
authors.initials: P
authors.lastname: Ridout
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), D-38116 Braunschweig, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Seitz
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), D-38116 Braunschweig, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Spitzer
authors.affiliation: Laboratoire National de Métrologie et d'Essais (LNE), F-75724 Paris Cedex 15, France.
authors.forename: D
authors.initials: D
authors.lastname: Stoica
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), D-38116 Braunschweig, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Wolf