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 CO<sub>2</sub>, 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.<br /> 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.<br /> The IAPSO/SCOR/IAPWS Joint Committee on Seawater JCS is targeting at these aims in cooperation with BIPM, WMO and other international bodies.

Thermodynamic properties of seawater, ice and humid air: TEOS-10, before and beyond

“…This estimate is known to be in error by typically 2 % because of the lowered vapour pressure of seawater compared to that of pure water. To get an idea of the relevance of this error, note that a small change in the global latent heat flux by 1 %, or about 1 W m −2 , would exceed by a factor of 200 the flux responsible for the currently observed greenhouse warming of the atmosphere, about 0.005 W m −2 Feistel, 2015Feistel, , 2017. The estimated energy imbalance of 0.4-0.8 W m −2 of the warming ocean (Cheng et al, 2016) is also within this error range.…”

“…Much less known, however, is the lecture by Heinrich Hertz given in 1885 in which he analysed the thermodynamics of the hydrological cycle in the climate system as a "gigantic steam engine" (Mulligan and Hertz, 1997, p. 41). In fact, rather than CO 2 , water in the troposphere in the form of humidity and clouds contributes the major part to the overall greenhouse effect (Abbot and Fowle Jr., 1908;Emden, 1913;Trenberth et al, 2007;Lacis et al, 2010;Schmidt et al, 2010;Feistel and Ebeling, 2011;Feistel, 2015Feistel, , 2017Lovell-Smith et al, 2016). The global water cycle, along with its observation and modelling, poses a fundamental challenge for climate research (Sherwood et al, 2010;Reid and Valdés, 2011;Tollefsen, 2012;Fasullo and Trenberth, 2012;Josey et al, 2013;Stevens and Bony, 2013;IPCC, 2013).…”

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

“…Following this approach, establishing traceability to SI of salinity, pH and relative-humidity measurement results may include (Hellmuth et al, 2014; Feistel, 2013, 2015)…”

Reply to the short comments of Hannes Schmidt of 04 Apr 2018

“…This estimate is known to be in error by typically 2 % because of the lowered vapour pressure of seawater compared to that of pure water. To get an idea of the relevance of this error, note that a small change in the global latent heat flux by 1 %, or about 1 W m −2 , would exceed by a factor of 200 the flux responsible for the currently observed greenhouse warming of the atmosphere, about 0.005 W m −2 (Lovell-Smith et al, 2016;Feistel, 2015Feistel, , 2017. The estimated energy imbalance of 0.4-0.8 W m −2 of the warming ocean (Cheng et al, 2016) is also within this error range.…”

“…Much less known, however, is the lecture by Heinrich Hertz given in 1885 in which he analysed the thermodynamics of the hydrological cycle in the climate system as a "gigantic steam engine" (Mulligan and Hertz, 1997, p. 41). In fact, rather than CO 2 , water in the troposphere in the form of humidity and clouds contributes the major part to the overall greenhouse effect (Abbot and Fowle Jr., 1908;Emden, 1913;Trenberth et al, 2007;Lacis et al, 2010;Schmidt et al, 2010;Feistel and Ebeling, 2011;Feistel, 2015Feistel, , 2017Lovell-Smith et al, 2016). The global water cycle, along with its observation and modelling, poses a fundamental challenge for climate research (Sherwood et al, 2010;Reid and Valdés, 2011;Tollefsen, 2012;Fasullo and Trenberth, 2012;Josey et al, 2013;Stevens and Bony, 2013;IPCC, 2013).…”