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Understanding the greenhouse effect, and greenhouse gases

January 6, 2009  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?

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 p22_gas4  
  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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