1909 Johns Hopkins physicist, R.W. Wood did an experiment that disproved “The Greenhouse Effect” for greenhouses. That is, greenhouses mainly heat by trapping convection (air exchange), not by trapping infrared (heat) re-radiated. In as sense, the effect is misnamed. This has ticked-off global warming enthusiasts for years. In 1980, physicist Vaughan Pratt of Stanford tried to duplicate the experiment and pronounced it a failure. Global warming enthusiasts rejoiced. The heck of it is, they only read the headlines. I laugh because I have done the experiment many times and Wood was basically right but a greenhouse is not an atmosphere. Pratt was correct too. Put on your thinking cap and realize that climate change is terrifically complex.
Wood's experiment tried to test the effect of a visible-light transmitting glazing that won’t transmit infrared (heat) and compare it to one that will. Glass won’t transmit heat, rock salt will. So he built two “greenhouses” of insultated cardboard, one with a glass cover and the other with a slab of rock salt. Set both in the sunshine and they both heat to about 130 F. Pratt built similar vessels and emplaced a number of thermocouples at various heights within the interior and found some differences in various vertical temp measurements of the two. More heat was trapped under glass.
Well, that’s what you’d expect. The glass greenhouse not only traps convection but also infrared radiation. But the difference is not large in the two final enclosures. What this shows is that convection is the larger method of transfer. And because of this, you can make a greenhouse out of acrylic or polycyanurate or a host of plastics (Ha! even visqueen plastic) that are often used today as a substitute for glass without regard to heat losses due to infrared radiation losses. It explains why, when you sit next to a large window on a cold night you get a draft. The room air convects over the surface of the window and descends as cold air. Convection counts for 90% of the transfer! And this also explains why you can spend a fortune on windows of low emissivity and it only changes the R-factor (heat transfer rate) from .61 to .68. Add cellular shades and the R-factor goes up by 2.0 or 3.0. Open the window or the greenhouse door and all the heat goes out (convection). And so the windows and greenhouse industry has happily shrugged and used the Wood result. And Pratt noted the bigger importance of convection deep within his paper.
Pratt’s right as well. There is an infrared component and a difference in the Wood greenhouse's temps. Moreover, the atmosphere is not a sheet of glazing. The only way for the atmosphere to transmit energy to space is by radiation. There is no convection. So does atmospheric chemical composition make all the difference? That’s where the naïve go wrong in supposition. There are other trapping mechanisms and they apparently work in differing conditions giving surprising results. Cloud cover is not constant but apparently increases with CO2 content predominantly in the tropical regions. This is Roy Spencer’s finding. And the tops of clouds reflect about 98% of incoming radiation. (When we add CO2, the earth corrects somewhat) There may be other self-correcting traps. Ocean currents affect only the upper 500 feet of the ocean. Below that depth, very little temperature change is observed even though the oceans are almost uniformly14,000-18,000 feet deep. Several historic climate changes due to melting ice ages have shown that current changes have altered temperature in large amounts, over 10 times as much as CO2 compositon. What’s going on? Is the lower temp of deep ocean water being tapped like a heat(cold) sink? This needs more study. And what of overall solar radiation? We now realize that the sun’s magnetic field must change but how and what does it do? More study. Mega-vulcanism? More study. Finally there’s the wobble of the earth, known as nutation that occurs every 100,000 years and correlates almost perfectly with the previous 26 ice ages. Until we can understand at least all but the last effect, we should admit our ignorance of man’s effect on global warming.
Oh, if you want to do the Wood experiment with your grandkids, you can use a flat plate of acrylic and a plate of clear glass to make your “greenhouse”. (Acrylics transmit part of the infrared spectrum) Make sure you put the thermometers in the exact same location to equalize measurements.