Salinity and relative humidity: climatological relevance and metrological needs

Abstract

Water plays the leading thermodynamic role in Earth's 'steam engine' climate. Followed by clouds and CO₂, water vapour in the atmosphere is dominating the greenhouse effect. Evaporation from the ocean surface is the main route of energy export from the ocean, the rate of which is known with poor 20 % uncertainty only. Regional climatic trends in evaporation and precipitation are reflected in small changes of ocean surface salinity.
Observational data of salinity and relative humidity need to be globally comparable within requisite uncertainties over decades and centuries, but both quantities rely on century-old provisional standards of unclear stability, and on ambiguous definitions. This increasingly urgent and long-pending problem can only be solved 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 such as those available from the recent oceanographic standard TEOS-10 need to 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 IAPSO/SCOR/IAPWS Joint Committee on Seawater JCS is targeting at these aims in cooperation with BIPM, WMO and other international bodies.