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The way things are named in science can be confusing. In physics and physical chemistry, we have penguin diagrams and Proton-Enhanced Nuclear Induction Spectroscopy (PENIS)names that were intended to be funny but are not particularly descriptive. We also have naming conventions that reflected the best thinking at the time they were developed, but now remain as vestiges even after the science is revised. One such convention is Benjamin Franklin’s theory regarding electrical current. He concluded that when a glass rod and silk are rubbed together there is a flow of positive charge from glass to silk. He was very close, but electrons (negatively charged) move instead from the glass to the silk1. However, the convention of positive charge flow remains. This may make physics class more difficult to understand, but another somewhat confusing naming convention in science can affect much more than your grade—it may have consequences for the entire world.

That would be the greenhouse effect. While the greenhouse effect is real and results in warming of the atmosphere (specifically the troposphere), glass greenhouses operate in a different manner than our dear Earth does. There has been so much confusion over this apparent conflict that people have wrongly used it to argue against climate change.

In a nutshell, the greenhouse effect occurs because solar radiation in the visible spectrum hits Earth, gets absorbed by the ground, and is re-emitted in the form of infrared (IR) light. This infrared light is absorbed by carbon dioxide (CO2) and other greenhouse gases and can’t escape into space efficiently (much of it is re-radiated back to Earth), so the atmosphere warms up. Because infrared light is absorbed by greenhouse gas molecules, it increases the energy of those molecules, heating them and therefore the atmosphere. This definition is correct and describes the process that heats our planet’s atmosphere, but it does not fully describe the process that warms and cools greenhouses that botanists and gardeners use. So how did this strange naming convention develop?

The idea that gases in the atmosphere contribute to global warming has been around since 1824, but it was not until 1901 that Nils Ekholm coined the term “greenhouse” to describe the effect:

Firstly, the atmosphere may act like the glass of a green-house, letting through the light rays of the sun relatively easily, and absorbing a great part of the dark rays emitted from the ground, and it thereby may raise the mean temperature of the earth’s surface. Secondly, the atmosphere acts as a heat store placed between the relatively warm ground and the cold space, and thereby lessens in a high degree the annual, diurnal, and local variations of the temperature.

The important takeaway is that the greenhouse effect is a comparison of the atmosphere’s temperature with greenhouse gasses like CO2 compared to what the atmosphere’s temperature would be if there were no greenhouse gases. Unfortunately, using the term “greenhouse” is not the most apt analogy for atmospheric warming—and this choice of wording led to unintended confusion within a decade of Ekholm’s claim that persists to the current day.

In 1909, just eight years after Ekholm’s paper was published, Robert W. Wood, an optics professor at Johns Hopkins University, decided to test the idea that radiation trapping was the main contributor to warming of physical greenhouses. He argued, “If we open the doors of a greenhouse on a cold and windy day, the trapping of radiation appears to lose much of its efficacy.” This is not a far-fetched thought experiment: if botanists could keep plants warm in the winter by simply placing a glass table over them, they would, so there must be a reason greenhouses have walls and are fully enclosed. His hypothesis was that greenhouses stay heated because warmed air inside cannot mix with the cooler air outside; that is, there is no heat transfer due to convection currents. (Convection currents are basically wind; hot air rises and cool air sinks, so air circulates based on a temperature differential.) By contrast, and ignored by Wood, there is no analogous window or door that one can open into the atmosphere, as the Earth is surrounded by empty space.

Dr. Wood tested his hypothesis in a rather fun way. He created two black boxes, one with a glass lid and another with a salt lid (see illustration below). The box that had a glass lid was a greenhouse analog; glass is transparent to visible light but not infrared (IR) light, so it would allow visible light in, but trap re-emitted infrared light from the black sides and bottom of the box and heat up. The box with a rock salt lid was transparent to both visible and infrared light, so it would not trap any emitted infrared light like a greenhouse would. Wood reasoned that if trapped heat caused the greenhouse to warm up, then the one with the glass lid should be hotter than the one with the salt lid. If, however, the lack of convection currents was the cause of the heating, the boxes should have similar temperatures.

Schematic of Robert W. Wood's Greenhouse Experiment.

Schematic of Robert W. Wood’s Greenhouse Experiment.

With the experimental tools he had available in 1909, he observed that the boxes had the same temperature, suggesting that it is not the blockage of infrared light that keeps glass greenhouses warm. Instead, it’s the fact that the warmed air inside can’t mix with the cool air outside. I should note that he (mistakenly) was skeptical about the role that greenhouse gases play in warming our atmosphere. He ended his paper with this perspective: “I do not pretend to have gone very deeply into the matter, and publish this note merely to draw attention to the fact that trapped radiation appears to play but a very small part in the actual cases with which we are familiar.”

At this point in history, Ekholm’s definition of the greenhouse effect began to cause a lot of confusion. Wood had demonstrated that the greenhouse effect does not apply to greenhouses that are located on Earth and are spatially separated from an external atmosphere. However, the greenhouse effect has been shown (and is still being shown) to apply to our atmosphere. So what exactly did Wood test, and what are the differences between a greenhouse on Earth and our atmosphere?

First, let’s take a bird’s eye view on how greenhouses and the atmosphere can heat up to see if Wood’s test was well-designed to test that or not. There are three ways heat can transfer: conduction, convection, and radiation. Conduction plays a negligible role in both cases (it is not a dominant force with gases), and can be ignored. Convection, as defined earlier, is like wind: it moves gases around based on temperature differentials and doesn’t change the overall temperature of a greenhouse or the atmosphere. It will even out the temperature within the closed system, but not increase it. Radiative heating, on the other hand, occurs when radiated light will be trapped by the glass in the greenhouse or by the CO2 in the atmosphere and heat both systems. In the atmosphere, this trapped IR radiation can be radiation either emitted directly from the sun or re-emitted from the ground. The atmosphere is 400 parts per million CO2, and since the atmosphere extends 62 miles above the surface of the Earth, there is a lot of this gas.

So far, the atmosphere and a greenhouse are very similar. But Wood specifically wondered about what would happen if we opened the door of a greenhouse on a cold and windy day, so let’s examine the ways in which a greenhouse can cool as opposed to the ways our atmosphere can cool. There are two ways that Wood’s greenhouse can cool. First, energy radiated out from the greenhouse to the atmosphere would cool off the greenhouse. Second, taking off the lid would allow for convective exchange of air between the greenhouse and the cooler atmosphere outside, cooling the greenhouse. However, there is only one way to cool off the Earth and the atmosphere, which is for energy from the Earth to be radiated into space. This is the only path, as there is no way for the gaseous atmosphere on Earth to exchange with empty space; the Earth’s gravity keeps the atmosphere intact.

The key here is that there are two ways for a greenhouse on Earth to cool, while there is only one way for our atmosphere to cool. This is quite nuanced, and probably where Ekholm confused Wood in his comparison between greenhouses and our atmosphere. Wood acknowledges that there is an inside and outside of a greenhouse that can be separated by a wall, and that the warm air inside can mix with the cooler air outside when a door is opened. This spatial segregation is fundamental to his hypothesis. However, in the atmosphere there is no door to the outside that one can open. Put another way, an object is either in the atmosphere or it is in space. There is no exchange of the atmosphere with empty space (loss of gas to space, such as hydrogen and helium, is negligible). Again, this is a subtle point; greenhouses and the atmosphere share many of the same heating and cooling processes. However, a greenhouse on Earth has one more available path for cooling (convection with the external air) than our atmosphere does, and that makes all the difference. If a hermetically sealed greenhouse was put in space, it would behave exactly like the Earth and heat solely by the greenhouse effect because there is no external atmosphere.

Because the only way for the Earth’s atmosphere to cool is to radiate energy into space, the amount of CO2 directly controls the loss of heat. The more CO2 in the atmosphere, the more slowly the Earth cools, resulting in a net heating of the atmosphere.

Because of the confusion surrounding the naming convention, scientists felt that they had to work overtime to clarify that the now-ingrained term “greenhouse effect” was due to gases in the atmosphere like carbon dioxide and didn’t really have anything to do with physical greenhouses. In 1909, the same year Wood published his results, Charles Greeley Abbot, Director of the Astrophysical Observatory and later the Secretary of the Smithsonian, rebutted Wood’s claims mathematically (especially as they pertained to warming on a global scale). Importantly, Abbot calculated that without greenhouse gases, the average temperature on Earth would be a frigid -17 °C (1.4 °F), but the average temperature in 1909 was instead a balmy 14 °C (57.2 °F). He correctly attributed the 31 °C (55.8 °F) degree warming to gases in the atmosphere, which he called the blanket effect (he had not adopted Ekholm’s greenhouse terminology at the time). Basically, if greenhouse gases did not cause warming, Earth would be a frozen wasteland, and life as we know it would not be possible.

As far as I can tell, Wood’s conclusions were almost universally accepted in the years that followed and the new common wisdom was that atmospheric warming and physical greenhouses operate under different mechanisms. Unfortunately, Ekholm’s terminology, calling the mechanism of atmospheric warming the greenhouse effect, stuck. This led some who deny climate change to misinterpret Wood’s results to argue against global warming and climate change. They ask, if the so-called greenhouse effect doesn’t cause greenhouses to get hot, how can it cause the atmosphere to get hot? The answer is that the term “greenhouse effect” is a historical holdover from 1901 and is a misnomer, just like Benjamin Franklin’s convention for charge flow, but it accurately describes the mechanism by which the atmosphere is warmed.

To the best of my knowledge, Robert W. Wood’s experiment was unreplicated until 2009, when Stanford Professor Vaughan R. Pratt put it to the test using very thorough internal controls and modern technology. Dr. Pratt failed to reproduce Wood’s work: he found that the box with the glass cover (greenhouse analog that trapped infrared light) was several degrees warmer than the one with the salt lid (the one that did not block any infrared light). Specifically, Pratt observed that the glass box, or greenhouse simulator, was between 1 to 6 °C (1.8 to 10.8°F) hotter than the box with the salt cover, depending on the placement of the thermometers within the box. Thus, in contrast to Wood, Professor Pratt did demonstrate that there is a small contribution from trapped infrared light even in physical greenhouses.

I must emphasize that Robert Wood’s experiment only simulates a physical greenhouse and does not call into question atmospheric warming. A gardener can adjust the temperature of their greenhouse simply by opening a door to the cool atmosphere outside. On the other hand, our Earth can only release energy via radiation into space, so greenhouse gases like CH4 (methane) and CO2 trap radiation in our atmosphere and directly control how much heat our atmosphere retains. These gases cause rising temperatures on Earth, and the evidence for this is truly overwhelming, solid, and undeniable. So while the greenhouse effect is very real, the effect’s origins are rooted in greenhouse gases in the atmosphere, not glass greenhouses.

Print References:

  1. Asimov, Isaac. Asimov’s New Guide to Science. 1984. Basic Books, Inc. New York, NY. pp. 418-420.

Title image credit: louisredon via pixabay.

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  1. Pingback: El calentamiento de un invernadero y las dificultades de la física básica – La ciencia de Svante Arrhenius

  2. Anonymous

    Well written paper. It is really unfortunate that the name “greenhouse” effect has been applied to effect of retardation of IR radiation into space by several of the trace gases in the atmosphere.