Astronomy - Ch. 9.1: Earth's Atmosphere (51 of 61) The CO2 and H2O Overlap Spectrum

In this video I will compare the absorption spectrum of CO2 with the absorption spectrum of H2O and its transmission window at various wavelengths for the radiated energy from the Earth's surface.

Next video in this series can be seen at:

[frankhuggins]

CO2 is the main GHG in the atmosphere over deserts. Yet, all deserts cool down very rapidly once the Sun sets. Not so for the humid areas of the world. CO2 has a minimal effect.

[daNorse]

The x-axis is the wavelength in µm, but what is the unit of the y-axis?

[honestbrian]

When you add methane to the mix what is the net impact?

[lengthmuldoon]

Thank you for your video.

Can the proportion of co2 not overlapped by water vapour be expressed as a % of the ~400ppm in the atmosphere and can a temp rise figure be attributed to it ?


Seems the panic over carbon reduction actually boils down to removing this incredibly tiny "active" co2 portion in the tenuous belief this will cool the planet (we quite like "warming" in the UK btw).

[seankellycrypto]

Hi. It's the same guy as for the video number 21.

[hughgeiger]

Descriptions of radiative transfer models in texts such as Carling and Kasting (2017, sec 2.4, sec 2.5, App A) are very complex (yet incomplete). Numerical approachee require massive compute power to handle millions of spectral lines and the interactions between constituent gases. Ignoring convection (other than to establish a temperature/pressure profile at dry or moist adiabat), upward and downward fluxes and heating can be calculated at each level in the atmosphere, brought to equilibrium, and the perturbed by eg doubling CO2 (and feedbacks). Energy is absorbed in proportion to the concentrations of the molecules (eg CO2, H2O), the absorbance of the molecule, and the temperature and pressure of the gas. In the troposphere and stratosphere, the energy is immediately thermalized into molecular motion (heat, ie temperature) and is then available for any molecule to emit, at any spectral line. In thermodynamic equilibrium, Kirchhoff law states that molecules in a gas absorb and emit the same amount of energy at a given frequency. A simplification is that a layer absorbs and emits as a gray body (extinction coefficient is independent of wavelength - extinction is absorption plus scattering). But one might imagine that in reality, thermalization modifies the flux spectrum, ie Kirchhoffs law doesn't hold. My incomplete understanding is that there are huge Jacobians required to calculate the coupled effects of different gases at each spectral line. Now throw in convection, aerosols and water vapour, evaporation and condensation. I realize that Michel knows his approach is overly simplified (which he states, along with many other complications), but is the transmission at 10 km a reasonable approach to producing a ballpark estimate? His explanation of band wings and widening with increased concentration is essential to understand, as is 3D path length - clearly doubling CO2 does not double the temperature increase (or worse, exponential as implied by the hockey stick - everyone needs to understand Michel's explanation - the effect of CO2 is exponential the other way - decreasing effect with increased concentration - in fact CO2 needs to quadruple in abundance to double band averaged absorption, and that doubling of absorption is not linear in temperature). But can we consider the overlap of CO2 and H2O at 10 km transmitance around 15 um, and some proportion of areas, to be representative of the actual coupled effect? If so, this needs to be taught to everyone!