Misuse of Feldman's Paper on the Greenhouse Effect

Back in 2015, Feldman et al published a paper showing observational and empirical validation of the greenhouse effect. They did this in a pretty ingenious way. They set up two RS-80 sensors at two locations and pointed these at the sky to record downwelling longwave radiation (dLWR) at frequencies absorbed and emitted by CO2. They monitored these sites for 10 years and recorded observations under clear-sky conditions, which would isolate dLWR from CO2 in the atmosphere apart from clouds. Their results showed a 0.2 W/m^2 increase in dLWR, and these results confirmed theoretical predictions regarding the greenhouse effect. In their words:

The time series both show 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. This is approximately ten per cent of the trend in downwelling longwave radiation. These results confirm theoretical predictions of the atmospheric greenhouse effect due to anthropogenic emissions, and provide empirical evidence of how rising CO2 levels, mediated by temporal variations due to photosynthesis and respiration, are affecting the surface energy balance.

During the study period, CO2 increased by 22 ppm from 370 to 392 ppm, which would indicate an increase in ERF of 0.38 W/m^2. So these results do confirm the theoretical (modeled) predictions of climate science.

Since it's publication, there have been several attempts to misuse these results to give the impression that Feldman's paper shows CO2 has a very small effect on global temperatures. In order to say this, though, they have to misunderstand what Feldman's results were, and they have to ignore what Feldman says to address their misuse of this study. Here are two examples:

1. Some argue that Feldman measured only a 0.2 W/m^2 per decade increase in downwelling LWR, and this means that CO2 can't have a large warming effect, especially since doubling CO2 causes an ERF of 3.7 W/m^2 and a change from 370 to 392 ppm would produce an ERF that is twice what is observed in Feldman's paper. However, Feldman's study did not measure ERF, which is a stratosphere-adjusted change in radiative forcing assessed at the tropopause. Instead, as is clearly indicated in the title of the paper, the study observed change in surface radiative forcing--that is, a change in dLWR at wavelengths absorbed and emitted by CO2. This is not the same as ERF; the change in downward LWR is proportional to but smaller than change in ERF. And Feldman also did not measure a global mean, but documented dLWR changes in two locations. Feldman was very clear about the distinction between dLWR and ERF. This is clearly stated in Feldman's paper:

Surface forcing represents a complementary, underutilized resource with which to quantify the effects of rising CO2 concentrations on downwelling longwave radiation. This quantity is distinct from stratosphere-adjusted radiative forcing at the tropopause, but both are fundamental measures of energy imbalance caused by well-mixed greenhouse gases. The former is less than, but proportional to, the latter owing to tropospheric adjustments of sensible and latent heat, and is a useful metric for localized aspects of climate response.

In other words, people raising this point either don't understand the distinction between ERF and dLWR or they have never read Feldman's paper, and likely both. If you're interested, a 2021 mdpi paper by Stefani made a big deal out of this misunderstanding of Feldman's paper, which I unpack here. It's actually quite funny.

2. Some also claim that Feldman said that CO2 caused only about 10% of the 0.2 W/m^2, but this is completely ridiculous. Feldman showed that the 0.2 W/m^2 observed change in dLWR was from increasing CO2 (they detected changes at wavelengths absorbed and emitted specifically by CO2). They also noted that this 0.2 W/m^2 is 10% of the overall trend in dLWR (about 2 W/m^2). This again is clearly stated in the paper:

This is approximately ten per cent of the trend in downwelling longwave radiation. These results confirm theoretical predictions of the atmospheric greenhouse effect due to anthropogenic emissions, and provide empirical evidence of how rising CO2 levels, mediated by temporal variations due to photosynthesis and respiration, are affecting the surface energy balance.

Feldman's paper provided observation evidence of the change in the amount of dLWR emitted by the atmosphere from the increase in CO2. It's one of several papers that provide direct observational evidence of CO2's role in the greenhouse effect (bibliography of other papers here), and even the magnitude of the change in dLWR confirms what climate scientists expected to find.

Reference:

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