CAGW

Using the NASA Planetary Spectrum Generator (PSG) to determine the effect of Infrared absorbing gases on planetary temperature

Abstract

The NASA PSG allows for concentrations of IR absorbing gases in a planetary atmosphere to be specified as input, and generates the resulting transmissivity of the atmosphere as output. From this, values for Radiative Forcing are derived.

By specifying input over a range of concentrations of Carbon Dioxide, Water Vapour and Temperature, ‘maps’ of the atmosphere are constructed, for example this at 288K:

Temperature is calculated from Radiative Forcing data by calibration using the accepted Zero atmosphere temperature of 255K and the ‘pre-industrial’ temperature of 287K. A plot of temperature values across a range of CO₂ concentrations is derived:

For the standard climatology parameter of an increase in temperature for a doubling of CO₂ concentration, a value of 0.92K is obtained for the doubling 275-550 ppmv CO₂. The value for the range 400 – 800 ppmv is different only in the third decimal place.

It is concluded that reported global temperature increases, both present day and predicted for the future, are in excess of those which may be attributed to long wave infrared energy absorption by the range of ‘greenhouse gases’ considered in this analysis.

An interactive tool based on the above, ‘CO₂ – The control knob‘ is provided to allow visualisation of the effect on global temperature of a range of CO₂ concentrations from 200 – 1600 ppmv.

1. Introduction

The NASA PSG takes as input an atmospheric description generated by the MERRA2 project in the form of a configuration file which may be downloaded, edited and resubmitted to the PSG. Of relevance to the determination of atmospheric absorption is the multi-layered description of pressure, temperature and gas concentration which may be viewed graphically in the PSG as shown:

An initial description (Vertical profile with 72 layers (Earth at 0 m, NASA/MERRA2 2020/01/08) was downloaded and modified by changing Temperature (calculated using the lapse rate equation), water vapour concentration (calculated using the Arden Buck equation) and CO₂ concentration over a range of values:

Temperature: 283.15 – 298.15K, Water vapour: 0-100% RH, CO₂ concentration: 0 – 1600ppmv

Each modified description was uploaded to the PSG and the resulting transmittance data downloaded. The atmospheric description for 288.15K, 80%RH 400ppmv CO₂ may be obtained here and the file containing the resulting transmittance data may be obtained here.

2. Data analysis

Data was collated in two stages; First, the raw transmittance data files for the range of Relative Humidity 0-100% at a given temperature and CO₂ concentration were combined into a spreadsheet, an example of which may be obtained here. A total of 63 spreadsheets were created to include data for the 9 temperatures and 7 CO2 concentrations considered. In each of these spreadsheets the transmittance data (in the range 0-1) is multiplied by the value of the Planck equation for the wavelength interval it represents, to produce a value in W/m². The results of the multiplications are then summed to produce a value for the total power transmitted at the given Temperature, RH and CO₂ concentration.

Data from the 63 spreadsheets was then collated into a single spreadsheet that may be downloaded here. In this spreadsheet the transmittance values are converted to absorption values by subtracting from the total radiated power (σT^4), multiplied by the emissivity (0.98) and the ‘Atmospheric Absorption Fraction’ (0.53, ref) to give values for Radiative Forcing.

Data from this spreadsheet was exported to the ‘Spyder’ Python IDE to generate 3D graphs for visualisation:

The graph on the left shows the Radiative Forcing results obtained over the range 0 – 100% RH and 0 – 1600 ppmv CO₂ at 288K. The graph on the right spans the smaller range of 20 – 100% RH and 200 – 1600 ppmv CO₂ in order to show the variation of RF with temperature. The lower (cyan) surface represents 287.15K; each of the following surfaces represents a 1K increase in temperature, to the upper surface representing 293K.

3. Calculations

From the PSG data it is possible to determine a value for climate sensitivity. Assuming a ‘zero atmosphere’ temperature of 255K and the RF value of 143.57 W/m² at temperature 287.15K, 275ppmv CO₂ and 80% RH, the climate sensitivity is:

To determine the increase in temperature caused by an increase in CO₂ concentration from 275 to 550 ppmv, the increase in RF is determined by finding the difference between the 2 concentrations at the same temperature i.e.

The increase in temperature is then:

For a fixed value of Relative Humidity this increase in temperature will result in an increase in water vapour, causing an increase in Radiative Forcing. To determine this increase, an equation is derived from the PSG data. The relationship between RF and temperature at 550ppmv CO₂, 80%RH is shown in this graph:

Using the derived cubic polynomial, the increase in RF for an increase of 0.38K is 0.99 W/m², which in turn leads to an increase in temperature of 0.22K. This calculation is iterated until the temperature difference between successive steps is less that 1×10^-9, at which point the temperature is 288.07K. The temperature increase due to a doubling of CO₂ concentration, including water vapour feedback is therefore:

Repeating the procedure for the CO₂ concentrations in the range 400 – 1600 ppmv, the following values are obtained:

Or displayed graphically:

4. CH₄ and N₂O

PSG data was obtained for a range of concentrations of CH₄ and N₂O, with CO₂ held fixed at 400ppmv and water vapor fixed as 80%RH. Taking the difference in the value of Radiative Forcing obtained for a doubling of the concentration of each gas, it was found that for CH₄:

Applying the same iterative process to account for the resulting increase in temperature and consequent increase in absorption by water vapour, it was found that:

Which translates to an increase in temperature of 0.12K for a doubling of CH₄. Applying the same method, the temperature increase for a doubling of N₂O from 0.32 ppmv to 0.64ppmv was found to be 0.16K.

5. Summary and Conclusions

The atmospheric description provided by the NASA MERRA2 project and the data generated by the Planetary Spectrum Generator are taken to provide a comprehensive description of the planetary atmosphere. The results thus obtained are believed to reflect the relationship between CO₂ concentration and planetary/atmospheric temperature to a good degree of accuracy.

The method employed is not that of mainstream climatology, i.e. summation of individual forcings, rather it considers the behaviour of Planck, water vapour and lapse rate forcings combined. Cloud and aerosol forcings are not included. NASA considers both of these forcings to be negative.

It is concluded that the worst-case contribution to the increase in global temperature by CO₂, through the mechanism of long wave Infrared absorption, is less than half of that claimed in the IPCC 5^th & 6^th Assessment Reports.