Observational Evidence for the Greenhouse Effect

In a previous post I looked at evidence of the greenhouse effect from empirical data. In short, an analysis of empirical data for CO2 and GMST shows a one-way causation with CO2 causing warming since the mid-twentieth century. This paper conclusively establishes causation from empirical time series of CO2 and temperature. That is, not only is CO2 a good predictor of temperature, but  uncertainty is reduced in future values of temperature given past values of CO2. This evidence is conclusive, but  even without this, we have observational evidence for the greenhouse effect. In this post I want to consider how this is true generally in satellite observations of earth's emission spectrum at the top of the atmosphere and in individual studies that have made observational determinations of the greenhouse effect.

General Observations

The graph above quite literally shows the greenhouse effect. This can be readily determined, but we need to begin by calculating the effective temperature of earth with empirical data. The equation is:

Tp = Ts√(Rs√[(1-α)/ε]/2D) where:
Ts = 5778 K (temperature of sun)
Rs = 6.96x10^8 m (radius of sun)
D = 1.496 x 10^11 m (distance to sun)
α = 0.306 (albedo of earth)
ε = 1 (emissivity of earth)
Tp = 255 K or -18 C

This is fairly standard and not controversial. If the earth was had no atmosphere, the average surface temperature of the earth would be about -18 C. However, we know that currently, the earth is about 33 C warmer than this, even though I'm using current values for Ts and α. This means necessarily that neither solar variability nor changes in albedo can explain why the earth is 33 C warmer than its effective temperature. It's neither energy entering the earth's climate system nor the amount of solar energy reflected back into space that can explain the discrepancy. The 33 C difference must be caused by something else, and the above diagram of the Earth's emission spectrum shows us what is causing the warming.

The above graph measures the longwave radiation escaping the earth's climate system from the top of the atmosphere into space. There are dips in this emission spectrum that correlate with the absorption wavelengths of the earth's major greenhouse gases, H2O, CO2, CH4 and O3. In other words, this is visible confirmation that longwave radiation is being trapped in the earth's climate system, or more accurately, its escape is slowed down by greenhouse gases. The greenhouse effect is an observable component of the climate system and the rational explanation for the fact that earth is 33 C warmer than what its temperature would be if it existed without an atmosphere. This is one reason (among many) that climate scientists simply do not disagree anymore about whether there is a greenhouse effect and whether increasing GHGs will also push climate towards warming. Scientific discussion currently revolves around how much global temperatures warm (that is, constraining values for ECS), not whether increasing CO2 pushes global temperatures toward warming.

Studies from Top of the Atmosphere

To my knowledge the first study to provide an observational determination of the greenhouse effect was Harries et al 2001[1]. Harries compared outgoing longwave radiation from two orbiting spacecraft, allowing the comparison of observations from 1970 and 1997. Differences in the relevant spectral bands could then be attributed to rising GHG concentrations. The differences in the spectra point to changes in atmospheric CH4, CO2, O3 and CFCs, and provided "direct experimental evidence for a significant increase in the Earth's greenhouse effect." These measurements contained significant uncertainties, since satellites have difficulty distinguishing between instantaneous radiative forcing and the climate radiative response.[2] Nevertheless, this was probably the first observational confirmation of the theoretical expectations of the greenhouse effect.

Later, Kramer et al 2021[3] was able to disentangle the climate's radiative response from instantaneous radiative forcing to calculate that "all-sky instantaneous radiative forcing has increased 0.53 ± 0.11 W/m2 from 2003 to 2018." The increase can be attributed to "a combination of rising concentrations of well-mixed greenhouse gases and recent reductions in aerosol emissions." In other words, the study shows that increasing GHGs push climate towards warming by slowing down the escape of IR light to space, and decreasing aerosol emissions allow more solar energy to be absorbed by the earth to be radiated back to space as IR light and then be trapped by greenhouse gases.

Studies from the Surface

Feldman et al 2015 found observational evidence of the greenhouse effect by studying the increase in radiative forcing in two locations, one in Oklahoma and the other on the north slope of Alaska from 2000 to 2010. The clear-sky radiative forcing they measured showed "statistically significant trends of 0.2 W m−2 per decade (with respective uncertainties of ±0.06 W m−2 per decade and ±0.07 W m−2 per decade) and have seasonal ranges of 0.1–0.2 W m−2."

The study utilized Atmospheric Emitted Radiance Interferometers (AERI) in each location to measure the downwelling infrared radiation that can detect the spectral signature of IR light from CO2. With careful analysis, Feldman and his colleagues were able to isolate the radiative forcing from CO2. The resulting trend was statistically identical at both locations at 0.2 W/m^2/decade with a 22 ppm increase in CO2.

Conclusion

We've understood that the earth is warmer than it should be, given its distance from the Sun, for almost 200 years. We've understood that CO2 and other greenhouse gases play a role in this since the 1850s. Scientists began to arrive at the amount of warming that will occur from doubling CO2 (about 3 C) in the 1950s.[5] Empirical data from CO2 and GMST datasets, observations from the top of the atmosphere, and observations from the surface all provide independent, empirical determinations of the greenhouse effect. There is simply no credible, scientific debate over this point any longer.

References

[1] Harries, J. E., H. E. Brindley, P. J. Sagoo, and R. J. Bantges, 2001: Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997. Nature, 410, 355-357.
https://www.researchgate.net/publication/12065270_Increases_in_greenhouse_forcing_inferred_from_the_outgoing_longwave_radiation_spectra_of_the_Earth_in_1970_and_1997

[2] Jiang, Y., Aumann, H. H., Wingyee-Lau, M., & Yung, Y. L. (2011). Climate change sensitivity evaluation from AIRS and IRIS measurements. In Earth Observing Systems XVI (Vol. 8153, p. 81531Z). Proceedings of SPIE. International Society for Optics and Photonics. http://dx.doi.org/10.1117/12.892817

[3] Kramer, R. J., He, H., Soden, B. J., Oreopoulos, L., Myhre, G., Forster, P. M., & Smith, C. J. (2021). Observational evidence of increasing global radiative forcing. Geophysical Research Letters, 48, e2020GL091585.
https://doi.org/10.1029/2020GL091585

[4] Feldman DR, Collins WD, Gero PJ, Torn MS, Mlawer EJ, Shippert TR. Observational determination of surface radiative forcing by CO2 from 2000 to 2010. Nature. 2015;519(7543):339‐343. doi:10.1038/nature14240
https://escholarship.org/content/qt3428v1r6/qt3428v1r6_noSplash_b5903aebfe105b4071103e11197138f8.pdf

[5] Plass, G.N. (1956), The Carbon Dioxide Theory of Climatic Change. Tellus, 8: 140-154. https://doi.org/10.1111/j.2153-3490.1956.tb01206.x