Definition The greenhouse symbol
A greenhouse is a closed construction designed to suppress air convection while remaining transparent to natural sources of light so that the plants within can photosynthesize. Traditionally the cladding was glass (hence glasshouse) but transparent plastic is now often used. The suppression of air convection means heat from the sun and internal furnaces can be better retained, thus enabling plants to be grown out of season and in hostile colder regions.
exploit the fact that air has a great capacity for thermal
convection - the movement of masses of warmer gas molecules to
cooler regions. We experience the forces of this thermal convection as
wind (breezes, gusts, gales etc) and these movements of air moderate
temperatures on Earth.
comparison air has a small capacity for thermal
conduction – the direct movement of heat between atoms. Hence
stilled air is a relatively effective thermal insulator compared to most
There is a common misconception that greenhouses retain heat because the glass lets sunlight through, the sunlight hits the soil and other contents and is transformed into longer wave infrared radiation, which cannot pass so easily back out through the glass. However polyethylene is almost as transparent to thermal infrared radiation as it is to solar radiation and polyethylene greenhouses also retain heat.
work primarily because their design enables us to exploit the thermal
properties of air and thus control and suppress thermal convection. It
is valuable to understand this for a number of very important reasons:
Atmosphere = greenhouse
endemic symbolization of the atmosphere as a greenhouse reveals a
profound denial of stewardship/change that puts humanity at peril.
the general principles for building greenhouses had been known for
millennia, the physicist Horace-Benedict
de Saussure is credited with constructing the first solar oven
in 1767 using an insulated box with three layers of glass.
The physicist-mathematical genius, Joseph Fourier, knew of the work of de Saussure. His experiences of the thermal extremes of the Sahara Desert also provided him with insight of the powerful role of the atmosphere on thermal balances. In 1827 he contrasted the elevated temperature in the sealed insulated box with that of the turbulent convecting atmosphere and attempted a mathematical explanation. It is from this work that he is popularly attributed with discovering the “greenhouse effect” .
greenhouse effect symbol is the notion that air is largely
transparent to visible light while being relatively opaque to infrared
and that without its envelope of air, then Earth’s surface would be
about -18ºC i.e. 33ºC cooler on average. We now know that a group of
trace gases with powerful capacities to retain thermal energy are
responsible for the current sustaining 15ºC average.
it is probably incorrect to attribute Fourier with symbolizing the
atmosphere as a greenhouse the image resonated deeply in the culture of
the “Industrial Revolution”.
In Fourier’s time greenhouses were already a symbol of great social prestige and industrial power. European manors competed to grow rare flowers and foods out of season. This use of the greenhouse symbol expanded in profound ways to express national pride, global aspirations and religious visions.
For instance, the Great Exhibition of 1851, was designed to display Great Britain’s industrial might and reinforce the image of humans (Britons in particular) fulfilling ancient Biblical and other incantations that Man is ordained by God to live in dominion over all other creatures and the resources of Earth. (The dominion symbol was largely stripped of its association with conservation and associated with the right to control and exploit the resources of Earth.) This dominion over all was symbolized by housing the Great Exhibition in a giant greenhouse (The Crystal Palace).
greenhouse symbol soon became the ultimate expression of the
religions that enabled the Industrial Revolution with its fundamental
belief that humans can exploit engineer all elements of our planet at
will. Thus it is now endemic for climate experts, teachers and the media
to evoke images of Earth encased in a greenhouse in myriad ways.
These include preaching of greenhouse gases and greenhouse
effects in a greenhouse world.
symbolization of Earth’s atmosphere as a greenhouse is a profound
denial of change because our atmosphere is very dynamic and organic
structure, characterized by a powerful capacity to transfer thermal
energy by convection. Greenhouses are the converse. This denial enables
maladaptive behaviour on scale. As
a result our impact on the thermal balances that sustain humanity may
well induce changes to our climate characterized by enhanced convection
with more extreme weather events. We deny this change out our peril.
is also a profound denial of stewardship for it replaces the image of
humans as part of a dynamic, organic structure with an image of humans
transcending atmospheric processes and being able to manipulate them at
will. This disconnection enables a dangerous denial of stewardship
of our actions that also puts humanity at grave risk.
the potential of the greenhouse symbol by only associating it
with human constructions. Thus our children will better understand the
thermal properties of air and be more able to design buildings, clothing
and other products using all manner of material in ways that exploit
those properties while living in harmony with Earth’s climate.
Page last updated: July 2010