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An observational study analyzing the effects of 300, 480, 3200, 7500, and 16,900 ppm CO2 on atmospheric (and soil) temperatures has determined “temperatures of atmospheric air in mesocosms [controlled outdoor experiments] with substantially higher CO2 concentration (ranging from 3200 ppm to 16900 ppm) were lower than that with the lower CO2 concentration (480 ppm)”.

Can we find a real-world experiment assessing CO2’s temperature impact?

Using real-world experimental evidence to directly measure the effects of incremental increases in CO2 concentrations on air temperatures is highly unusual in climate science.

Most studies utilize extrapolations from radiation modeling that rest on a series of assumptions to theoretically assess CO2’s effect on the temperature of atmospheric air (Kennedy and Hodzic, 2019, Gerlich and Tscheuschner, 2009).

An exception would be Lightfoot and Mamer, 2018, who directly assessed the air temperature changes associated with specific CO2 concentrations. They found that CO2 is sometimes a cooling gas, and sometimes it’s a warming gas.

Supporting the contention of a dual cooling and warming – or negligible – effect of CO2 variability on air temperature, it has been determined that natural cave air may vary between 500 ppm in summer to 5000 ppm in winter despite very minimal seasonal temperture gradients (Pla et al., 2017).

New experimental study challenges the CO2-rise-leads-to-warming paradigm

The seven co-authors of a new paper (Zhang et al., 2020) were intent on demonstrating the dangers of substantial CO2 emissions from soil and its contribution to the greenhouse effect.

Natural soil emissions are a legitimate concern for those who believe CO2 drives the Earth’s temperature changes.

After all, the air CO2 concentrations in soil can reach 20,000 ppm (Zhang et al., 2020), and natural yearly emissions from soil respiration/perturbation are 9 times greater than that from all human activity combined.

Image Source: ScienceDaily

Zhang and colleagues utilized mesocosms – controlled outdoor experiments – to assess the air and soil temperature effect of step changes in CO2 concentration.

They “unexpectedly” found the mesocosms with the 3 highest CO2 concentrations – 3200, 7500, and 16,900 ppm – actually had lower associated atmospheric temperatures than the mesocosms with 480 ppm.

The authors suggest the higher CO2 concentrations rise, the more they “may enhance net heat loss”.

Similarly, even soil temperatures were cooler with the higher CO2 concentrations (16,900 ppm), leading the authors to suggest that substantially higher CO2 “may cool the soil”.

These experimental results would appear to undermine the popular assumption that linear increases in CO2 cause linear increases in temperature due to an enhancement of the overall greenhouse effect.

“The increased atmospheric air temperatures with CO2 concentration (ranging from 300 ppm to 7500 ppm) at daytime with higher radiation were understandable. Unexpectedly, the magnitude of temperature increase of atmospheric air in mesocosms with 16900 ppm CO2 declined significantly compared to that with 7500 ppm CO2 at daytime with higher radiation. In addition, the temperatures of atmospheric air in mesocosms with substantially higher CO2 concentration (ranging from 3200 ppm to 16900 ppm) were lower than that with the lower CO2 concentration (480 ppm) at early morning and/or nighttime with lower heat radiation. These results emphasized that the molecules of CO2 not only absorb the infrared radiation but also re-emit it to the surrounding space (20). Thus an increase of CO2 concentration in atmospheric air may result in either an increase or decrease of the air temperature in the atmosphere, depending on the balance of heat gain and loss. In other words, CO2 with substantially higher concentration may enhance the net heat loss to colder surrounding interfaces when the heat absorption capacity of CO2 was saturated or heat input was much limited.”

“[T]he significant decrease of soil air temperature in mesocosms with CO2 concentration of 16900 ppm indicated that soil with substantially higher CO2 concentration may cool the soil probably by transferring more heat to surrounding space during colder periods when the temperature difference between soil and surface atmospheric air became larger. The realistic significance of these findings was greater than those in the atmosphere because CO2 concentration in soil air was often in the range of 1,000 ppm – 20,000 ppm [21-23]. Hence, the variation of soil CO2 concentration may regulate the balance of heat gain and loss in soil which determines the contribution of soil to surface warming of the earth.”