This is the stage where we explain what a greenhouse gas does. The two spectra are crucial to the understanding of the role of greenhouse gases in the atmosphere.
These are two MODTRAN generated emission spectra, the higher one is for the full atmosphere with 380 ppmv of CO2 and the lower one is exactly the same except for the removal of all the CO2. The two spectra make obvious the overlapping of the water and CO2 spectra in the 600-750 cm-1 range. The spectra represent the emission towards space at an altitude of 15 km; the mean altitude of the tropopause where the temperature is ~218 K.
This section concentrates on CO2 since it is the most important greenhouse gas whose concentration in the atmosphere is changing because of fossil fuel burning [an important subject dealt with subsequently]. The important region in the Earth's emission spectrum shown in the diagram on page 20 is outlined and lies between 580-750 cm-1. Whereas the majority of the emission spectrum lies roughly along a curve which approximates to a blackbody curve, the emission in the specified region is of a much lower intensity. The 'well' in the curve occurs because CO2 absorbs strongly in that region and only emits radiation to space from a much lower temperature than the emissions from the rest of the spectrum. Much of the CO2 emission originates from the atmosphere at a temperature of about 218 K [-55 oC]. This part of the atmosphere is around 15 km altitude and is known as the tropopause, the top of the troposphere in which around 90% of the atmosphere occurs. The tropopause is the dividing layer between the tropopause and the stratosphere. You have probably noticed when flying long-haul that the outside temperature is around -55 oC and that is because the plane is operating around the tropopause. Emission from the CO2 occurs at that level because the air is 'thin' and does not absorb radiation to anything like the same extent as it does at sea level.
The lower diagram shows that the 'well' is filled to a large extent with some very weak water vapour lines when CO2 is absent. The atmosphere is less opaque to radiation in the 580-750 cm-1 range and more radiation escapes than when CO2 is present.
One important conclusion is that CO2 restricts the passage of radiation from the surface to space; an important characteristic of a greenhouse gas.
To further explain the 'filling' of the well by weak water lines in the absence of CO2 the spectra below were simulated by using the hitranpc programme with the default concentrations of CO2 (330 ppmv) and water vapour (7750 ppmv) with path lengths of 100 metres. The spectra are presented as transmissions, T = 0 is zero transmission, complete absorption; T = 1 is complete transmission, zero absorption.
The CO2 spectrum shows the strongly absorbing fundamental of the bending mode centred at 667.4 cm-1 with the bands with higher J (rotational quantum number) values of the rotational quantum numbers somewhat less strong than those of the central parts and the Q branch absorptions. At either side of the main bands are the two transitions ν21(1) → ν20(2) with its Q branch at 618 cm-1 and ν21(1) → ν1(1) with its Q branch at 720 cm-1. The P branch of the 618 cm-1 transition is obvious, but its R branch overlaps with the P branch of the fundamental bending mode. The R branch of the 720 cm-1 transition is obvious, but its P branch overlaps the R branch of the fundamental bending mode.
The water vapour spectrum shows the considerable number of rotational transitions of the molecule in the 570-770 cm-1 region. Very few of the bands show zero transmission and the spectrum is generally much weaker in absorption than the CO2 spectrum shown above. These weak water lines do contribute to the emission to space, but by no means as much as the CO2 transitions do. The spectrum below emphasizes the small (but not insignificant) role played by the water transitions in the 570-770 cm-1 region. The individual transmissions are multiplied together to give the result shown in the spectrum which is the green spectrum including the main green (CO2) transitions and the water vapour transitions. The almost blotted out spectrum in red is the pure CO2 transmission. The presence of water vapour reduces the transmission over the wavenumber range. If CO2 is absent the emissions from the weak water vapour bands arise at a temperature of ~280 K, i.e., at a considerably lower altitude than the CO2 emissions which are from near the tropopause.
To reach the second important conclusion requires a calculation.
The energy flux values are derived by the MODTRAN programme. The flux leaving the troposphere when we have the full atmosphere is 246.5 W m-2 [watts per square metre]. When the CO2 'block' is removed the flux rises to 285.6 W m-2, an increase of 39.1 W m-2. If this did come about the radiation balance of the atmosphere would be upset. To redress this imbalance and bring the outgoing flux of energy back to 246.5 W m-2 the temperature of the surface would have to be reduced and the extent of the reduction may be estimated by using the Stefan-Boltzmann equation.
New temperature = Fourth root [(288.24 x 246.5/285.6] = 277.8 K
This is 288.2 - 277.8 = 10.4 K lower than the surface would be in the presence of the CO2!
The second important conclusion is that the presence of CO2 causes the surface of the Earth to be 10.4oC warmer than it would be in its absence; it is a greenhouse gas.
The next simulated spectra are those for 380, 760 and 1000 ppmv of CO2 respectively looking down from an altitude of 70 km and hopefully show the slight broadening of the 'well' that is crucial to the understanding of why more CO2 leads to a little more warming.
380 ppmv 760 ppmv 1000 ppmv
The 'wells' in the spectra become broader with increased concentration of CO2 which causes the emission heights of the weaker absorption bands to increase. This means that the emission arises from colder regions. It is of some interest that the emission in the centre of the absorption band is at slightly higher temperatures and this is consistent with the emissions [corresonding to very strong absorptions] occurring in the stratosphere where the temperature increases with increasing altitude.
The broadening of the wells arises from the greater absorption by the bands which emit from a cooler part of the troposphere. The parts of the Planck curves are for temperatures of 280 K (dark blue), 260 K (red), 240 K (light blue) and 220 (yellow), the latter corresponding approximately to the temperature of the tropopause (218K). The parts of the spectra that are 'saturated' in the troposphere appear at higher temperatures as the concentration of CO2 increases. Those parts have their emission levels in the stratosphere and are by no means saturated.
The most obvious contribution to the warming of the troposphere is the movement of the P branch of the 618 cm-1 absorption, seen at both sides of the 240 K Planck curve (light blue) in the left-hand diagram for 380 ppmv CO2. As the CO2 concentration increases that particular P branch moves to lower emission temperatures. In the right-hand 1000 ppmv CO2 case its emission temperature range is between the 240 K and 220 K curves.
Another way of understanding the effect of increasing CO2 concentration is to regard the broadening of the well on the high wavenumber side as reducing the wavenumber range of the IR window. So, for an increase from 300 ppmv to 600 ppmv the reduction in the flux going through the window is (most recent value) 4.1 W/m2, making the flux through the window 40 - 4.1 = 35.9 W/m2 making it essential for the remainder of the fluxes to combine to restore the balance to space.