The strengths of GH gases
The common features of longwave absorption in the atmosphere
The curve of each GH gas in Fig.1 is calculated when it is the only gas in the AGA conditions. The real combined absorption of GH gases is not a simple summary of the band areas of single GH gases. The real total absorption can be calculated only when all the GH gases are present at the same time. The total absorption is depicted by the purple line. Therefore for example, the total absorption curve does not follow the green line of CO2 absorption curve, because it is essentially caused by the total absorption of H2O and CO2 present at the same time in the atmosphere.
Some important conclusions can be drawn from the absorption graphs. The GH gases have different effects on the total absorption when compared to the absorption caused by water. Ozone pushes the total absorption curve effectively upward, but CH4 and N2O only minimally increase the total absorption in comparison to water absorption. The radiation flux transmitted into space in the clear sky conditions is 83.2 Wm-2, and it is the only potential factor for increased absorption caused by higher GH gas concentrations. In all-sky conditions, clouds absorb about 66% of the transmitted flux, and thus about 28.3 Wm-2 - that is, only about 7 % of the emitted LW radiation - escapes directly into space.
Because the effects of GH gases are very nonlinear, the above contributions are not the actual strengths of GH gases for the changes around the present concentrations. The author has calculated the relative strengths of GH gases (Ollila, 2013a) based on the increased IR absorptions from 1990 to 2005. The most important GH gas is water and its strength in respect to CO2 impact (value = 1) is 15.2. The same values of other GH gases are: CH4 0.144, N2O 0.168, and O3 0.629. Fig. 1 shows that any impact of GH gases that could actually increase warming must do it in the wavelength zone from 7.5 µm to 14 µm in the so-called atmospheric window.
Dr. Ollila graph
It is not generally known that the absorption LW (longwave) radiation emitted from the Earth’s surface proceeds very quickly (Ollila 2012, Ohmura).
This feature is illustrated in Fig. 2, showing that at the height of 1 km, already 90% of the total absorption has been achieved.
Dr. Ollila graph
Contributions of greenhouse gases in greenhouse phenomenon
IPCC (2013) claims in its newest report AR5 that “The contribution of water vapor to the natural greenhouse effect relative to that of carbon dioxide (CO2) depends on the accounting method, but can be considered to be approximately two to three times greater.” There are no references to any scientific papers supporting this claim. IPCC has referred in its 2007 report to the article of Kiehl & Trenberth. The author (Ollila, 2013a) has shown that using the same US Standard Atmosphere 76 with 12% less water he can get the same results: H2O 60% and CO2 26% (60/26 = 2.3). This atmosphere contains only 50 % of the real average global atmosphere (AGA). The author has concluded that the number “three times greater” could refer to the article of Pierrehumbert (2011), which says that CO2 absorption is not close to saturation and its contribution in the tropical climate is about 33%. Pierrehumbert shows no detailed calculations – only the claim above.
As referred to earlier Miskolczi and Mlynczak were the first in their finding that the CO2 accounts only 9% greenhouse (GH) phenomenon, which is much less than 26% reported by Kiehl and Trenberth. Ollila (2012) has reported that in using USST 76 atmosphere the CO2 impact would be 26% and in average global atmosphere (AGA) it would be 11% (9.5% if nonlinearity is observed).
Kiehl and Trenberth have referred to the atmosphere specified by Liou. Even though Liou calls it “Standard Atmosphere”, it turns out that it is exactly the same USST 76 atmosphere as specified in Spectral Calculator. Because Kiehl and Trenberth used actually modified USST 76 atmosphere containing 12% less water than the original USST 76, The author carried out further analyses, using exactly the same atmosphere. In these calculations Spectral Calculator program was applied, which utilizes line-by-line (LBL) calculation method and HITRAN database. The details are the same as described by Ollila (2012).
The Table 1 summarizes the calculation results. Utilising the modified USST 76 atmosphere up to 120 km altitude, the results are very close to Kiehl and Trenberth. The biggest difference is in the warming effects of methane and nitrogen oxidule.
When the calculation was carried out up till 11 km, the CO2 impact was 29%, and it decreased to the value of 27%, when it was calculated up till 120 km. This may look like an illogical result knowing that CO2 concentration is practically the same from the ground up to 80 km. The explanation is the increased absorption of ozone. The maximum concentration of ozone in the stratosphere is even 100 times greater than that in the troposphere. The ozone absorption band is between 9 - 10 µm, where water absorption is very low. These are the reasons why ozone absorption increases from 2% to 7% as the altitude increases.
Table 1. Contributions of GH gases
Myhre and Stordal have achieved the same kind of results concerning the warming effects of CO2 in the upper atmosphere. They even make a conclusion that CO2 has a cooling effect in the stratosphere.
The author’s conclusion is that CO2 is susch powerful GH gas in its absorption wavelength zone that the absorption does not increase above the troposphere at all. Actually, the CO2 effect is fully developed already at the altitude of 2 km. Because ozone increases the total absorption, the relative portion of CO2 decreases but this is not a cooling effect. The total absorption increase from 11 km (302.71 Wm-2) to 120 km (308.94 Wm-2) was found to be divided in the following way: O3 81%, H2O 14%, and CH4& N2O 5%. Because the absorption areas of CH4 and N2O overlap almost totally, it makes sense to calculate the combined effect.
The behaviour of CO2 absorption in the upper atmosphere is one of the good reasons to restrict the GH phenomenon into the troposphere. The exact altitude is not easy to specify. Miskolczi has calculated that the GH effect stops at the altitude of 12.3 km. The author has used the average troposphere altitude of 11 km. The choice has a very small effect on the results. The calculations show that the total absorption increases only 0.3% from 11 km to 13 km.
GWP values and the real effects of concentration increases
Global warming potential (GWP) definition means how much a GH gas can absorb infrared energy, if 1 kg has been released into the atmosphere over a specified time period (20, 100, 500 years) when compared to the same amount of CO2 gas. The 100 year GWP of methane is 25, which means that if the same mass of methane and carbon dioxide were introduced into the atmosphere, methane would trap 25 times more heat than the carbon dioxide over the next 100 years.
The definition of GWP is highly theoretical, because the warming effects of 1 kg of a GH gas pulse are impossible to calculate in the real conditions. In order to illustrate this problem the GWP values were calculated in AGA 2005 (Average Global Atmosphere) conditions for pulse sizes from 1 ppm to 50 ppm of CH4 and N2O. Because methane is lighter gas than CO2, the pulse size must be 2.744 times bigger than CO2. IPCC has used the lifetime of 12 years for CH4, and 114 years for N2O, and these values were used also in this study. The lifetime of CO2 is complicated to calculate, and the used formula produces the lifetime about 45 years.The discounting rate of a pulse over time was the same exponential form as defined by IPCC, and the time horizon was 100 years. The results calculated in AGA 2005 conditions are depicted in Figure 3.
All other GH gases except CO2 and H2O have very low concentrations in the atmosphere. It means that small increases of these gases have relative high warming effects (Beer-Lambert law) in comparison to CO2 which has a small warming effect because of rather saturated absorption capability in the AGA conditions. This is illustrated in Figure 4.
The GWP values do not give the right total picture of CH4 and N2O warming effects, because they are calculated for the unrealistic small amounts of CH4 and N2O. If the CH4 and N2O concentrations would have increased to 379 ppm, the effects were smaller than those of CO2.
The only reliable way to calculate the real warming effects of a GH gas is to use spectral absorption calculations in the real climate conditions for each case. The GWP values are applicable only for comparing relatively small emissions caused by human activities. As illustrated in Figure 4, CH4 and N2O have smaller warming effects than CO2, if increased to the CO2 concentration of about 379 ppm.
Dr. Ollila graph
Dr. Ollila graph
Saturation and the real warming capacities of GH gases
The available LW radiation from the Earth’s surface determines that all GH gases have limited capacity to increase the GH effect and global warming. These limitations can be seen in Figure 7 very clearly. The concentrations of CH4 and N2O are so low that they are not close to saturation, but the growth rates of the concentrations are so small that the real warming effects are very small for the coming centuries.
H2O and CO2 have the main roles in global warming, because the CO2 concentration increases steadily (about 1.9 ppm annually) and water has a dominant role as a GH gas. One can see from Figure 4 that water has much steeper angle coefficient than CO2, indicating the higher warming capacity of water. The behavior of water and the temperature can be seen in Figure 5.
When the water concentration increases from 2.6 prcm to 4.1 prcm, the absorption increase is prominent. A simple explanation for this is that water absorbs LW radiation in the whole radiation range from 5 to 100 µm, including the maximum LW radiation band around 10 µm. In Figure 4, one can see that CO2 concentration increase to 580 ppm creates a rather small increase in the temperature.
Pierrehumbert has come to a conclusion that CO2 is not near to saturation and that its contribution in the tropical climate is about 33%. Figures 4 and 5 show this conclusion to be incorrect. The warming values calculated for a 100 ppm increase of CO2 concentration for the indicated bands are as follows: 280 – 379 ppm 0.2 °C, 379 – 560 ppm 0.14 °C, 560 – 800 ppm 0.11 °C, 800-1200 ppm 0.088 °C, and 1200-2400 ppm 0.055 °C. Nonlinear warming capacity is imminent here. The total saturation has not yet been reached, but the effects are smaller than generally believed.
The total contributions of GH gases (see Table 1) are subject to speculation: some researchers think that smaller concentrations of CO2 would cause also water content to decrease because lower temperatures would increase precipitation. Other researchers think that water is such a strong GH gas that the atmosphere could compensate the lower CO2 concentration by evaporating more water from the oceans.
Actually, this question has value only in creating images for GH gases: is the total contribution of CO2 9% or 33%? In reality the key figures of the kind presented in Table 1 are not needed in warming calculations. The present GH gas concentrations determine the operating point of the climate today. It is more important to know what the strengths of today are in the prevailing atmosphere. The precipitation of water is not a question at all because of the increasing concentration of CO2. Table 2 lists the results of the calculations for the GH gas concentration changes from 1990 to 2005 except that of water and ozone, which are based on 1% change.
Miskolczi has calculated that the doubling of CO2 concentration of 280 ppm can be compensated by the water increase of 2.77%. This means relative strength of 36.1.
Dr. Ollila graph
IPCC has reported that the climate has a positive feedback of water and that water can cause 50% of warming. This principle can be applied also to the historical warming till 2005, because the year 2005 is the turning point between the models. The mechanism is that CO2 increase of 99 ppm (35.4% addition) warms the climate first by 0.38 °C, and then because of constant RH (Relative Humidity), the temperature will increase the same amount caused by increased water amount. I calculated the absolute humidity values based on a constant 0.38 °C increase of the temperature profile in the troposphere. This change increased the total water amount from 2.6 prcm to 2.66 prcm (2.3%). These values mean that the strength of water is 15.4 in comparison to CO2, which is very close to the value as calculated in AGA conditions.
The OLR (Outgoing Longwave Radiation) changes and the warming values of different CO2 concentrations are summarized in Table 4. The global warming caused by the CO2 concentration increase from 280 ppm to 393 ppm calculated through OLR change is 0.24 °C without water feedback. The logarithmic fitting gives the following equation between RF (Radiative Forcing) values and CO2 concentrations in Table 3
RF = 3.12 * ln(C/280), (8)
Where RF is the radiative forcing in Wm-2, and C is the CO2 concentration in ppm. This equation has the same form as the equation introduced by Myhre et al. but the coefficient is different (5.35 versus 3.12). The author analyzes the reasons in the section "IPCC results".
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