Understanding the greenhouse effect, and greenhouse gases

January 06, 2009
Written by Albert Grimm
These days, greenhouse gases seem to be on everybody’s agenda. But what are greenhouse gases? What does the greenhouse effect have to do with melting glaciers? And what gases do greenhouses produce to change global climate?

The idea of commercial production in greenhouses is relatively new. Originally, glasshouses were used to over-winter sensitive plants.
There is some confusion about these questions, not just among climatologists, but also among growers who may be confronted with questions about gases from their greenhouse. I am not really qualified to comment on global warming, but I have put together a few lines about the greenhouse side of the subject.

I have to start with some greenhouse history. The earliest greenhouses were simple structures, similar to our modern cold frames. The purpose of these early glasshouses had nothing to do with plant production as we know it today. During the 16th and 17th centuries, the explorers and travellers of the Age of Discovery often returned home with exotic plants for their European patrons. This was also the time when black pepper and other exotic spices were traded at prices that demoted fresh truffles to poor man’s food. So, these early exotic imports were not just exciting novelties to the Old World, but there were hopes to make some serious money by cultivating them in Europe.

Many of these plants, however, were poorly adapted to the cold European climate. The peasant farmers of that time had no capital, and too many other problems to see any need for production of exotic plant material. Exotic plants were primarily a hobby for the wealthy nobility and royalty. However, this hobby soon turned into prestigious international competitions, and groundskeepers throughout Europe were hard-pressed to protect their master’s precious specimen from common frost. That was when modern ornamental greenhouse horticulture was born.

In those days, ordinary window glass was still a luxury reserved for the super rich, and selling the glass from an acre of modern greenhouses would have easily financed a very plush retirement. Initially, large trenches were dug several feet into the ground in preparation for winter, and the plants were placed in these dugouts, and covered with small, glazed window frames.

The idea originated in the knowledge that soil stays frost-free at certain depths. The gardeners wanted to bring tender plants as close to the frost-free zone as possible. At night, a thick “thermal curtain” of layered straw bales was placed on top of the windows. During the day, the expensive window glass let some light in and kept the cold air out.

It soon became clear that the glass cover worked so effectively in trapping heat that the idea was applied to a variety of recreational royal buildings. The concept of the greenhouse was born. The first “modern” greenhouse, however, was not built until 1848, when London’s Kew Gardens constructed a house for its palm collection.

A few generations later, with the advent of modern science, scholars started to investigate why these early greenhouses were able to catch heat in such an effective manner. Around the year 1800, Sir Frederick William Herschel discovered thermal (infrared) radiation, just outside the spectrum of visible light. It was found that ordinary window glass allows visible light to pass, but blocks thermal radiation. When this brand new, high-tech knowledge was applied to the greenhouse question, it was concluded the energy entering the greenhouse as visible light was converted to thermal radiation inside the structure. This heat radiation was trapped below the glass. This, so it was thought, was responsible for the dramatic increase in temperature inside a closed glass greenhouse, even under moderately sunny skies.

Today, we know that there is indeed a small imbalance between the total radiation entering a glasshouse and the total radiation leaving the same structure. But we also know that this imbalance is only responsible for a very small portion of the heat energy that is trapped inside the greenhouse.

More important is the fact that the soil inside the greenhouse is heated by radiation from the sun. The soil in turn heats the air close to the ground. This warm air, when it rises, is prevented from leaving the greenhouse by the cover material. The trapping of warm air by the structure increases the air temperature in the greenhouse far more effectively than the comparatively small portion of thermal radiation that is trapped by glass. In fact, the ventilation system of an ordinary Venlo greenhouse is designed to allow for very accurate temperature control, by modulating the amount of air allowed to escape. If the trapping of radiation were effective, we would not be able to cool a conventional glasshouse in summer. Likewise, we would not be able to afford the heat for polymer-covered houses, because these cover materials are comparatively transmissive for infrared radiation.

In the early 1800s, however, this was not understood, and the dynamics of heat energy in the greenhouse had not yet been thoroughly investigated. In those years, French scientist Jean Baptiste Fourier, a man whose name most engineering students have learned to dread, was investigating the atmosphere of the earth. He discovered that water vapour and clouds high up in the atmosphere were able to radiate thermal energy back onto the surface of the earth and contribute to its warming. Since this process is so very similar to what people at the time thought to be happening in a greenhouse, Mr. Fourier coined the term “greenhouse effect” for his observations.

Largely obscure for a long time to anybody but growers and climatologists, the greenhouse effect has now entered everybody’s vocabulary because it is thought to contribute to global climate changes. Yet, this is one of the more significant misnomers in history, because it describes indeed the effect that Fourier discovered, and not the effect that keeps your greenhouse warm on a sunny winter day.

CO2, methane and – most importantly – water vapour are the gases that most effectively trap infrared radiation in the earth’s atmosphere. Visible light passes through these gases, but infrared radiation gets absorbed, and a portion of this thermal radiation is re-emitted towards the earth. This property of greenhouse gases is similar to the properties of ordinary window glass. Since these gases are largely responsible for the “greenhouse effect” described by Fourier, we call them “greenhouse gases.”

Climate systems are incredibly complex, whether they are observed in the atmosphere or in a production greenhouse. The cause-effect chains for temperatures and humidity are extremely difficult to trace. When an experienced greenhouse grower makes statements about the climate in his crops, this is generally based on experience and years of observation, but rarely ever on hard science. In the same manner, I think that climatologists have to use their experience to warn us of impending, grave and accelerating climate changes, even if they cannot predict exactly how to measure this change, or what impact it will have on each of us. In that sense, I am prepared to do my bit to keep our planet healthy, even if I have to trust the hunch of our climate experts.

Albert Grimm is the head grower at Jeffery’s Greenhouses in St. Catharines.

  p22_gas1 p22_gas2 p22_gas3
  Simple cold frames capture solar radiation energy and allow for earlier harvest without any additional source of heat.
Simplified energy dynamics in a vented greenhouse; the details of greenhouse climates are far more complex. Greenhouse gases such as water vapour, CO2 from fossil fuels, and methane from farms, capture infrared radiation just as glass does – but that’s where the similarity with a greenhouse ends.
Rice production and cattle farming generate huge quantities of methane – a very effective greenhouse gas. Global warming issues go far beyond the burning of fossil fuels.  

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