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Metrological challenges for measurements of key climatological observables: Oceanic salinity and pH, and atmospheric humidity. Part 1: overview



Rainer Feistel, Robert Wielgosz, Stephanie A. Bell, M F. Camoes, J R. Cooper, P Dexter, A G. Dickson, P Fisicaro, Allan H. Harvey, M Heinonen, Olaf Hellmuth, H J. Kretzschmar, J W. Lovell-Smith, Trevor J. McDougall, R Pawlowicz, S Seitz, Petra Spitzer, D Stoica, H Wolf, P Ridout


Water in its three ambient phases plays the dominant thermodynamic role for the functioning principles of the terrestrial climate system. Together with clouds, water vapour in the atmosphere is the strongest greenhouse gas (followed by clouds and carbon dioxide, CO2). The observed seasonal amplitudes of ocean temperatures can only be explained by strong evaporation from the sea surface, the rate of which is poorly known (20 % uncertainty). Evaporation and precipitation rates are controlled by solar irradiation, cloud formation and relative humidity, which apparently show no significant global climatic trend over the sea. Regional trends in evaporation and precipitation are reflected in small changes of oceanic salinity on climatic time scales. Increasing atmospheric CO2 levels may cause severe damage to marine ecosystems by ocean acidification. Observational data of salinity, relative humidity and seawater pH need to be globally comparable within requisite uncertainties over time spans of centuries. This increasingly urgent and long-pending goal can only be achieved by proper metrological traceability to the International System of Units (SI). Consistent with such SI-based definitions, state-of-the-art correlation equations for thermophysical properties of water, seawater, ice and humid air should be developed and adopted as joint international standards for all branches of climate research, in oceanography, meteorology and glaciology for data analysis and numerical models. The IAPWS/SCOR/IAPSO Joint Committee on Seawater JCS is targeting at these aims in cooperation with BIPM, WMO and other international bodies. This paper reviews the historical development and the current definition and measurement practice of seawater salinity, pH and relative humidity. Related deficiencies and problems are analysed and suggestions are made for the development of future uniform SI-based definitions and SI-traceable, long-term comparable measurements of those key climatological pa


climate, humidity, metrology, pH, salinity, seawater, water


Feistel, R. , Wielgosz, R. , Bell, S. , Camoes, M. , Cooper, J. , Dexter, P. , Dickson, A. , Fisicaro, P. , Harvey, A. , Heinonen, M. , Hellmuth, O. , Kretzschmar, H. , Lovell-Smith, J. , McDougall, T. , Pawlowicz, R. , Seitz, S. , Spitzer, P. , Stoica, D. , Wolf, H. and Ridout, P. (2015), Metrological challenges for measurements of key climatological observables: Oceanic salinity and pH, and atmospheric humidity. Part 1: overview, Metrologia, [online], (Accessed June 21, 2024)


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Created December 14, 2015, Updated October 12, 2021