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. 

Greenhouses 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.

By 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 common materials.  

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.  

Greenhouses 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:
We can improve our horticulture.
We can improve our design and use of insulated dwellings.
We can better understand how our atmosphere works.
We can enjoy a greater state of science and harmony with all. 

Atmosphere = greenhouse

The endemic symbolization of the atmosphere as a greenhouse reveals a profound denial of stewardship/change that puts humanity at peril.

Though 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” .

The 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 common to symbolise them as greenhouse gases - this evocation of greenhouses is doubly unhelpful because it obscures their trace nature and the powers of leverage..

While 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.

    
The Crystal Palace Britain built 1851

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).

King Leapold's greenhouse Laeken Belgium built 1874-

The 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.  

     
Samples of greenhouse earth  icon from Internet
 - posters of this symbol commonly adorn our school walls.

This 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.  

The greenhouse earth icon is particularly convenient for the egos of those who live in temperate zones and those in tropical areas dependent on air conditioning - it has comfortable subliminal associations with control, calm and personal comfort levels maintained at our will.

It 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.

Summary 

Page last  updated: Aug 2010

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