About the role of water vapor in greenhouse effect
Aleksandr Zhitomirskiy
To assess the role of water vapor in the greenhouse effect, the values of average temperatures and amounts of precipitation in places located at the same latitude were compared. The lack of a reliable correlation between these values casts doubt on the existence of the greenhouse effect for water vapor.
Outlining the hypothesis of the greenhouse effect, S. Arrhenius (1896) named two greenhouse gases in the Earth's atmosphere - water vapor and carbon dioxide [1]. Later, G.S. Callendar (1938) expressed the opinion that the increase in the "average world" temperature of the Earth (according to his estimate 0.005 oC/ year) is due to an increase in the concentration of carbon dioxide in the atmosphere [2]. According to NASA researchers (2008), the contribution of water vapor to the greenhouse effect of the atmosphere is twice the contribution of carbon dioxide [3]. However, the IPCC (1990) report argues that “Carbon dioxide has been responsible for over half the combined greenhouse effect in the past” [4, p.19]. At the same time, in the page 22 of the same IPCC report, water vapor is called “the largest contributor”, but nevertheless it is not included in the general list of greenhouse gases.
To understand the problem, let's see how substantiated the IPCC's arguments are against taking into account the role of water vapor in the greenhouse effect. On page 23 of the above-mentioned report, it says: “Water vapor has the largest greenhouse effect, but its concentration in the troposphere is determined internally within the climate system, and, on a global scale, is not affected by human sources and sinks. Water vapor will increase in response to global warming and further enhance it, this process is included in climate models”.
This explanation can hardly be considered logical. If water vapor is considered a greenhouse gas (because its molecules absorb infrared radiation), then it should exert its effect regardless of whether its sources are "human" or natural. Moreover, the statement itself that human activity does not affect the content of water vapor in the atmosphere is also false. Water vapor, along with carbon dioxide, is formed during the combustion of all types of fuel, with the exception of certain types of coal. For example, when natural gas methane is burned, 2 H2 O molecules are formed for each CO 2 molecule A huge
amount of water evaporates into the atmosphere as a result of agricultural work. One of the most vivid examples of this is the drying up of the Aral Sea in the second half of the last century, caused by excessive and irrational use of water from the rivers feeding this sea for irrigation of fields.
Unlike carbon dioxide, water vapor is extremely unevenly distributed in the atmosphere. More precisely, we know much more about the distribution of water vapor, since at each meteorological station, along with the temperature, the relative humidity of the air is measured and the amount of precipitation is recorded. Therefore, if in the case of CO 2 it is possible to talk about the greenhouse effect on a global scale, then in the case of water vapor it is possible to trace the relationship between temperature and the concentration of the greenhouse gas (if any) in natural conditions.
The greenhouse effect of water vapor has already been discussed for explanation of the difference between day and night temperatures in the desert in one of the chemistry textbooks for universities [5, p.781]:
“In very dry desert climates, where the water vapor concentration is unusually low, it may be extremely hot during the day but very cold at night. In the absence of extensive layer of water vapor to absorb and then radiate part of the infrared radiation back to Earth, the surface loses this radiation into space and cools off very rapidly”.
In fact, it is not the water vapor concentration that is "unusually low", but the relative humidity, which is defined as the ratio of the partial pressure of water vapor present in air to the value needed for saturation. For example, in the Sahara Desert on a summer day, the air temperature can reach 40 °C with a relative humidity of 25%. From the experimental data on the temperature dependence of the water vapor pressure, it follows that at 40 °C the water vapor pressure at saturation is 55.3 Torr and, therefore, at 25% relative humidity it will be 13.8 Torr. Since the concentration for a gas can be defined as the ratio of the partial pressure to the total pressure, the concentration of water vapor in this case will be 13.8/760 = 0.0182, or 1.82 %. This concentration of water vapor corresponds to 100% relative humidity at 16 °C. For comparison: now in Paris 1 o C, relative humidity 95%, atmospheric pressure 744 Torr. These conditions correspond to H 2 O concentration of 0.63%, that is 3 times less than in Sahara. Consequently, the explanation of the effect of water vapor proposed in this textbook is incorrect, not to mention the fact that the effect of other greenhouse gases, primarily CO2, is not manifested in any way.
The increase in body temperature upon heating, according to the heat capacity
equation, is inversely proportional to the body mass and its heat capacity. Sand has a low thermal conductivity, therefore, it heats up to a shallow depth during the day, and the mass of sand that has absorbed heat is relatively small. The heat capacity of sand is also low, so it heats up to a significantly higher temperature than soil covered with vegetation, not to mention water. After the sun sets, the sand cools, according to Newton's law of cooling, much faster. In addition, there is no vegetation in the desert, and there are no clouds above the desert that can reflect the thermal radiation emanating from the earth.
Since the values of temperature and humidity in different places are known, it is possible to analyze the correlation between them to estimate the greenhouse effect of water vapor. For such a comparison, it is necessary to choose places which located at the same geographical latitude and do not differ significantly in height above sea level. Of course, many factors affecting temperature and humidity cannot be taken into account, but the reliability of assessing the presence of the greenhouse effect in this case is certainly higher than when operating with the global average temperature.
Temperatures (monthly) and average annual precipitation for different locations are available on the website timeanddate.com (climate and weather averages) for the period 1985-2015. It is assumed that the average annual precipitation is correlated with the average concentration of water vapor. Data for locations selected in the Northern and Southern Hemispheres are shown in Tables 1 and 2.
Table 1. Average temperature and precipitation for 7 cities in Northern Hemisphere
The correlation between the average temperature (t) and average precipitation (p) is described by the equation: t = 78.14 – 0.8956 p (r = -0.1677).
Table 2. Average temperature and precipitation for 8 cities in Southen Hemisphere City (Country)
= 64.3 – 0.03387
Based on these results, one could speak of a negative correlation between temperature and humidity. However, given the low absolute value of correlation coefficient r, it would be more correct to assert that there is no reliable correlation between these values. In other words, it is impossible to detect the change in temperature depending on the content of water vapor in the atmosphere in places that absorb approximately equal amounts of solar heat. The amount of water vapor differs several times. Hence the conclusion is: there is no evidence of the existence of the greenhouse effect of water vapor.
The question naturally arises: if in natural conditions it is impossible to find evidence of the greenhouse effect for H2O, the concentration of which may differ several times, then how can one judge the greenhouse effect of CO2, the content of which has increased by about 30% over more than 100 years? This issue deserves a separate consideration.
References
1. S.Arrhenius. On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground. Philosophical Magazine and Journal of Science, Series 5, Vol.41, 237-276, 1896. https://www.rsc.org/images/Arrhenius1896_tcm18
2. G.S.Callendar. The artificial production of carbon dioxide and its influence on temperature. - Quarterly Journal of the Royal Meteorological Society, 64, 223240, 1938.
http://climatepositions.com/wp-content/uploads/2014/03/qjcallender38.pdf
3. Water Vapor Confirmed As Major Player In Climate Change. - NASA/Goddard Space Flight Center. Nov.18, 2008. https://www.sciencedaily.com/releases/2008/11/081117193013.htm
4. Climate Change. The IPCC Scientific Assessment. 1990. 414 pp. https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_full_report.pdf
5. Th.L.Brown, H.E. LeMay, Jr. a.o. Chemistry. The Central Science. Pearson Education. 2009. ISBN 978-0-13-235-848-4. 1117 pp.