The evidence keeps piling up. Climate sensitivity to an increase in atmospheric CO2 is effectively zero.

Scientists Hermann Harde and Michael Schnell published a paper in 2021 entitled “Verification of the Greenhouse Effect in the Laboratory.” The study alleged to experimentally determine the atmospheric CO2 greenhouse effect not only exists, but functions in concert with physical laws.

“To our knowledge we present the first demonstration of the atmospheric greenhouse effect in a laboratory experiment, which also allows quantitative measurements under conditions as in the lower troposphere. We use an experimental set-up consisting of two plates in a closed housing, one plate in the upper position heated to 30°C, the other at the bottom and cooled to -11.4°C.”

A few years later Thorstein Seim and Borgar Olsen (2023) analyzed their experimental setup in further detail. They noted that when the CO2 is increased 500-fold, or from 0.04% (400 ppm) to 20% (200,000 ppm) in the Harde and Schnell greenhouse effect experiment the plate temperature increases by just 1.18°C, and by just 0.4°C more (1.6°C total) when the CO2 concentration reaches 100%.

Image Source: Seim and Olsen, 2023

But in their 2023 paper Seim and Olsen modified the Harde and Schnell (2021) experiment to better simulate “the earth/atmosphere situation.” They found that instead of modestly increasing the plate temperature by a little more than one degree, their tweaked experimental setup showed increasing CO2 from 0.04% to 100% actually cools the blackbody (plate) by about -0.2 to -0.3°C.

And now, in another new lab study, Harde and Schnell, 2025 provide experimental evidence that further supports this negative (or near-zero) greenhouse effect at high CO2 concentrations. Their experimental setup shows increasing CO2 from 20,000 (2%) to 80,000 ppm (8%) leads to a negative greenhouse effect, resulting in tenths-of-a-degree cooling.

This may be the second experiment to illustrate the negative greenhouse effect in a lab. Radiation studies published by other authors (Schmithüsen et al., 2015, Chen et al., 2024) also indicate the cooling influence from adding CO2 may occur in polar climates (Antarctica, Arctic). A CO2-induced high-latitude negative greenhouse effect fully undermines the anthropogenic global warming (AGW) narrative that says humans are driving “polar amplification,” or rapidly melting glaciers, ice sheets, and sea ice.

“CO2 radiation is measured for three standard concentrations of 2, 4, and 8%. Addition of CO2 leads to a significant cooling of the gas temperatures…”

The latest Harde and Schnell study also affirms water vapor (WV) dominates the greenhouse effect (GHE), and thus the assumption that CO2 is a driver of greenhouse effect forcing has “significant limitations.” Unlike the 2, 4, and 8% CO2 volumes utilized in this experiment, the real-world atmosphere, with 0.042% CO2, is overwhelmed by WV concentrations that are 30-40 times greater. Thus, the saturated absorption bands where CO2 could potentially have an effect are “overlaid” or “overlapped.”

“[D]ue to the long propagation paths in the atmosphere and the 30-40 times higher WV concentration, the weak overlap of the spectra leads to a significant limitation of the CO2 climate sensitivity…”

“As the H2O concentration increases, the gas radiation of the other GH-gases is overlaid, and their effectiveness is correspondingly reduced.”

Image Source: Harde and Schnell, 2025

It is assumed that the 100 ppm (0.01%) atmospheric CO2 increase since 1950 – from ~320 to 420 ppm – has been the primary cause of 1950-to-present global warming. However, these experiments show increasing CO2 2500-fold (0.04% to 100%) or even 100-fold (~40,000 ppm) leads to insignificant warming or even tenths-of-a-degree cooling. This tiny thermal impact is too insignificant to measurably affect Earth’s climate.