David Coe, Walter Fabinski, Gerhard Wiegleb, The Impact of CO2, H2O and Other “Greenhouse Gases” on Equilibrium Earth Temperatures, International Journal of Atmospheric and Oceanic Sciences. Vol. 5, No. 2, 2021, pp. 29-40. doi: 10.11648/j.ijaos.20210502.12
Abstract: It has long been accepted that the “greenhouse effect”, where the atmosphere readily transmits short wavelength incoming solar radiation but selectively absorbs long wavelength outgoing radiation emitted by the earth, is responsible for warming the earth from the 255K effective earth temperature, without atmospheric warming, to the current average temperature of 288K. It is also widely accepted that the two main atmospheric greenhouse gases are H2O and CO2. What is surprising is the wide variation in the estimated warming potential of CO2, the gas held responsible for the modern concept of climate change. Estimates published by the IPCC for climate sensitivity to a doubling of CO2 concentration vary from 1.5 to 4.5°C based upon a plethora of scientific papers attempting to analyse the complexities of atmospheric thermodynamics to determine their results. The aim of this paper is to simplify the method of achieving a figure for climate sensitivity not only for CO2, but also CH4 and N2O, which are also considered to be strong greenhouse gases, by determining just how atmospheric absorption has resulted in the current 33K warming and then extrapolating that result to calculate the expected warming due to future increases of greenhouse gas concentrations. The HITRAN database of gaseous absorption spectra enables the absorption of earth radiation at its current temperature of 288K to be accurately determined for each individual atmospheric constituent and also for the combined absorption of the atmosphere as a whole. From this data it is concluded that H2O is responsible for 29.4K of the 33K warming, with CO2 contributing 3.3K and CH4 and N2O combined just 0.3K. Climate sensitivity to future increases in CO2 concentration is calculated to be 0.50K, including the positive feedback effects of H2O, while climate sensitivities to CH4 and N2O are almost undetectable at 0.06K and 0.08K respectively. This result strongly suggests that increasing levels of CO2 will not lead to significant changes in earth temperature and that increases in CH4 and N2O will have very little discernable impact.
Keywords: Carbon Dioxide, Climate Sensitivity, Greenhouse Effect, Climate Change
5. Other Considerations
5.1. The Impact of Clouds
The obvious impact of clouds is to increase the reflectivity of the earth thus reducing the level of incoming solar radiance I0 which will have a cooling influence on the earth. However, this paper is concerned with the effects of retained IR emissions from the earth. Cloud cover will not affect the absorbance of atmospheric greenhouse gases, but it will impact upon the total energy absorbed and possibly upon the energy retention factor “n”, in ways that would be difficult to quantify. However, by using the current average earth condition, which includes cloud, as a calibration point to determine an effective value for ”n” consistent with the mean earth temperature of 288K, the current average impact of cloud has, in effect, already been taken into account. This however does not, in itself, identify what this impact is. The structure of clouds is diverse and complex. It is close to impossible to derive a set of equations to describe the formation, structure and impact of clouds on the retention of absorbed energy and hence the radiative balance of the earth. This paper has so far relied upon the extensive HITRAN spectral database, basic physics and simple mathematics to determine values for climate sensitivity. Any attempts to estimate the further impact of cloud would be based upon speculation only and would not be appropriate for this paper.
5.2. Effect of Recently Increased Atmospheric CO2
It is of some interest to calculate the increase in temperature that has occurred due to the increase in atmospheric CO2 levels from the 280ppm prior at the start of the industrial revolution to the current 420ppm registered at the Mona [Mauna!] Loa Observatory. (K. W. Thoning et. al. 2019) [17]. The HITRAN calculations show that atmospheric absorptivity has increased from 0.727 to 0.730 due to the increase of 140ppm CO2, resulting in a temperature increase of 0.24Kelvin. This is, therefore, the full extent of anthropogenic global warming to date.
6. Conclusions
In order to satisfy radiative equilibrium at the “top of the atmosphere” (TOA) at an average earth temperature of 288Kelvin, only 61.5% of the earth’s radiated energy should be transmitted through to space, leaving 38.5% to be absorbed and retained by the atmosphere/earth. Use of the HITRAN data base of gaseous absorption spectra shows the current atmospheric absorption to be 73.0% of total radiative emissions of which 52.74% must be retained by the earth/atmosphere to satisfy the current TOA equilibrium. This is a simple expression of the current earth temperature equilibrium. The 38.5% retained radiation absorption comprises 35.3% attributed to H2O, 3.0% to CO2 and a mere 0.2% to CH4 and N2O combined. From this it follows that the 33Kelvin warming of the earth from 255Kelvin, widely accepted as the zero-atmosphere earth temperature, to the current average temperature of 288Kelvin, is a 29.4K increase attributed to H2O, 3.3K to CO2 and 0.3K to CH4 and N2O combined. H2O is by far the dominant greenhouse gas, and its atmospheric concentration is determined solely by atmospheric temperature. Furthermore, the strength of the H2O infra-red absorption bands is such that the radiation within those bands is quickly absorbed in the lower atmosphere resulting in further increases in H2O concentrations having little further effect upon atmospheric absorption and hence earth temperatures. An increase in average Relative Humidity of 1% will result in a temperature increase of 0.03Kelvin. By comparison CO2 is a bit player. It however does possess strong spectral absorption bands which, like H2O, absorb most of the radiated energy, within those bands, in the lower atmosphere. It also suffers the big disadvantage that most of its absorption bands are overlapped by those of H2O thus reducing greatly its effectiveness. In fact, the climate sensitivity to a doubling of CO2 from 400ppm to 800ppm is calculated to be 0.45 Kelvin. This increases to 0.50 Kelvin when feedback effects are taken into account. This figure is significantly lower than the IPCC claims of 1.5 to 4.5 Kelvin. The contribution of CH4 and N2O is miniscule. Not only have they contributed a mere 0.3Kelvin to current earth temperatures, their climate sensitivities to a doubling of their present atmospheric concentrations are 0.06 and 0.08 Kelvin respectively. As with CO2 their absorption spectra are largely overlapped by the H2O spectra again substantially reducing their impact. It is often claimed that a major contributor to global warming is the positive feedback effect of H2O. As the atmosphere warms, the atmospheric concentration of H2O also increases, resulting in a further increase in temperature suggesting that a tipping point might eventually be reached where runaway temperatures are experienced. The calculations in this paper show that this is simply not the case. There is indeed a positive feedback effect due to the presence of H2O, but this is limited to a multiplying effect of 1.183 to any temperature increase. For example, it increases the CO2 climate sensitivity from 0.45K to 0.53K. A further feedback, however, is caused by a reduction in atmospheric absorptivity as the spectral radiance of the earth’s emitted energy increases with temperature, with peak emissions moving slightly towards lower radiation wavelengths. This causes a negative feedback with a temperature multiplier of 0.9894. This results in a total feedback multiplier of 1.124, reducing the effective CO2 climate sensitivity from 0.53 to 0.50 Kelvin. Feedback effects play a minor role in the warming of the earth. There is, and never can be, a tipping point. As the concentrations of greenhouse gases increase, the temperature sensitivity to those increases becomes smaller and smaller. The earth’s atmosphere is a near perfect example of a stable system. It is also possible to attribute the impact of the increase in CO2 concentrations from the pre-industrial levels of 280ppm to the current 420ppm to an increase in earth mean temperature of just 0.24Kelvin, a figure entirely consistent with the calculated climate sensitivity of 0.50 Kelvin. The atmosphere, mainly due to the beneficial characteristics and impact of H2O absorption spectra, proves to be a highly stable moderator of global temperatures. There is no impending climate emergency and CO2 is not the control parameter of global temperatures, that accolade falls to H2O. CO2 is simply the supporter of life on this planet as a result of the miracle of photosynthesis.
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