The existence or non-existence of the so-called atmospheric greenhouse effect continuously dominates the extremely emotional discussion about a human impact on global warming. Most scientists agree with the fundamental greenhouse theory, but like their opponents they are missing a reliable experimental verification of this effect. Measurements at the open atmosphere are too strongly affected by perturbations to quantify the relatively small contribution of greenhouse gases to local heating of the air or the Earth’s surface. Therefore, we have developed an advanced laboratory set-up, which allows to largely eliminate convection or heat conduction and to reproducibly study the direct influence of greenhouse gases under similar conditions as in the lower troposphere. We measure the additional warming of a pre-heated plate due to back-radiation of the greenhouse gases carbon dioxide, methane and nitrous oxide as a function of the gas concentration, and we derive from the observed warming the radiative forcing of these gases. The measurements are well confirmed by radiation transfer calculations and underline that there exists no climate emergency.
The very first explanation of the atmospheric greenhouse effect (GHE) goes back to Jean-Baptiste Joseph Fourier in 1824 , who was studying the Earth’s energy budget to explain the surface temperature. He was inspired by the hot box of Horace-Bénédict de Saussure consisting of an isolated compartment with several glass windows, which in the inside is strongly heated up by the transmitted solar radiation. Fourier assumed that the atmosphere is acting similar to the glass windows, transparent for the solar radiation but blocking the infrared (IR)-radiation emitted from the ground. Heat exchange with the environment by convection or heat conduction was largely neglected in this model.
First quantitative measurements with IR absorbing gases like water vapor or carbon dioxide (CO2) go back to Tyndall (1861) . He described the GHE by comparing the atmosphere with a dam built across a river and causing a local deepening of the stream; in a similar way should the atmosphere act as a barrier to radiation from the Earth, thus, producing a heightening of the temperature at the Earth’s surface. Although he could study the absorptive and emitting behavior of these gases, he had no direct evidence for a GHE.
Within the second half of the 19th century fundamental relations for the interaction of radiation with matter were formulated, e.g., Kirchhoff’s law of thermal radiation (1859) , the Stefan-Boltzmann law (1879) [4, 5] or Planck’s radiation law (1900) , which form the theoretical basis of the GHE. First calculations with a still strongly simplified climate model go back to S. Arrhenius (1898) , who already considered ice-albedo feedback in his model and found a CO2 climate sensitivity (temperature increase at doubled CO2 concentration) of 5 – 6°C. Since this time there existed continuous trials to confirm or to refute the GHE by more or less simple laboratory experiments. Direct measurements at the atmosphere are too strongly affected by convection, turbulence or scattering effects to quantify the relatively small contribution of greenhouse molecules to any local warming of the air or the Earth’s surface.
One of the frontier experimental investigations was performed by R. W. Wood (1909) , who used two boxes containing regular air. One box was covered with a glass window transparent for sun light, but blocking IR-radiation, the other covered with a NaCl window transparent also for IR. His measurements showed significant warming of the interior but no or only a negligible temperature difference between the boxes. From this Wood and other authors repeating his experiment (e.g., Nahle, 2011 ) concluded that infrared radiation, which can escape through the NaCl window, will not contribute to heating or only with an insignificant amount, while the observed temperature increase in both boxes – different to Fourier’s interpretation – is exclusively explained due to the blockage of convective heat transfer with the environment and not related to any kind of trapped radiation.
But experiments recording the temperature at the floor and ceiling of the interior, rather than looking only to a single temperature for each box, measure a 5°C larger floor to ceiling decline for the salt rock box than the glass box, while the bottom of the boxes have almost identical temperatures (V. R. Pratt, 2020 ). These results are principally confirmed with a slightly different set-up using an internal electric heating instead of external light sources (E. Loock, 2008 ). Such heating avoids differences in the incident radiation, which otherwise has to transmit windows of different materials and losses. A higher temperature of 2.5 – 3°C could be found for the glass box, and replacing the glass by a polished aluminum foil the temperature even rises by additional 3°C.
While the Wood-type experiments can answer the question, if and how far a reduced IR-transmissivity can contribute to warming of a compartment, respectively the troposphere, it gives no information about the interaction of greenhouse gases with IR-radiation. Thus, it still remained the question, to which extent such gases at least partially can withhold IR-radiation and how far simple absorption by GH-gases or the highly disputed back-radiation might contribute to additional warming of the floor. Such studies require to fill one compartment with the gas to be investigated and to compare this with a reference measurement using air or a noble gas.
Meanwhile different approaches have been carried out, partly with external irradiation or with internal heating (see e.g., Loock ), partly measuring the gas temperature or the IR radiation in forward and backward direction (Seim & Olsen, 2020 ). But either no warming was detected or, after closer inspection, the observed temperature increase could not be attributed to an IR-radiation effect.
Unfortunately some fake demonstrations with apparent temperature differences of more than 10°C are presented in the internet, which allegedly reveal the strong impact of the greenhouse gases (see, e.g., Ditfurth, 1978 [13). However, closer inspection shows that the higher temperature is mainly caused by a stratification effect combined with an increased isolation, when heavier CO2 is filled from the bottom into the compartment (M. Schnell, 2020 ). And really problematic is, when the co-recipient of the 2007 Nobel Peace Prize initiates a web-based campaign with multiple advertisements on television, focused on spreading awareness for a climate crisis and as “evidence” presents a completely unrealistic and unreproducible video experiment of the GHE (Al Gore’s Climate 101 video experiment, 2001 ), which meanwhile has been falsified by several revisions (A. Watts, 2011 ; J.-E. Solheim, 2016 ). It is a dirty propaganda using such a manipulated experiment to spread fear around the word and to indoctrinate our society with the message that we can only rescue our Earth by stopping all future emissions of greenhouse gases. This undermines any serious attempts to discuss and analyze the expected influence of GH-gases on our climate. Political imaginations, speculations or religious faith are no serious consultants to ensure a prosperous future, our knowledge and technical progress is based on scientific principles.
Therefore, it is time to stop the endless speculations about the disastrous implications of an atmospheric GHE and to concentrate on reliable investigations, which allow to quantify the size and limiting impact of GH-gases on global warming caused by anthropogenic emissions of fossil fuels.
In this contribution we present an advanced experimental set-up, which to a large extent allows to eliminate convection or heat conduction and to reproducibly study the direct influence of greenhouse gases under similar conditions as in the lower troposphere. In Section 2 we present the set-up, which different to other experiments uses a heated plate as radiation source and simultaneously as sensitive detector for the back-radiation from GH-gases. We measure the increasing temperature of this plate or, alternatively at stabilized temperature, the energy saving due to the back-radiation. Section 3 describes the theoretical concept for analyzing and evaluating the experiments. Section 4 contains some preliminary studies for the validation and calibration of the measuring system, and Section 5 presents our measurements together with an evaluation of the data, from which the radiative forcing of the gases CO2, CH4 and N2O can be derived. Our results are discussed in Section 6 and Section 7 gives a summary with future perspectives.