What is the Greenhouse Effect?
Climate Science 101
Hello!
How many people, from street activists all the way up to highly credentialed scientists understand the basic, scientific theory that underpins climate change?
In my experience, fewer than you think.
In this letter I hope to equip you with the basics. The climate system is EXTREMELY complex. To the extent that nobody understands it completely. But at it’s core, The Greenhouse Effect is a relatively simple theory. Don’t let the seemingly impenetrable pursuit of ‘climate science’ put you off from understanding the basics. And keep on asking questions!
Summary
The sun provides a continuous source of energy to the earth, known as electromagnetic radiation.
Electromagnetic radiation is absorbed by the surface of the earth, which subsequently heats up.
The earth re-emits electromagnetic radiation in the form of ‘long-wave infrared radiation’ (LWIR), sending it back outwards towards space.
‘Greenhouse gasses’ in the atmosphere (such as carbon dioxide) absorb and re-emit LWIR in all directions.
The energy balance is therefore altered , such that the amount of LWIR that would otherwise be lost to space is reduced.
This energy imbalance is said to result in a net warming at the earth’s surface relative to that which would otherwise occur in the absence of a greenhouse gas-containing atmosphere.
The Earth and its atmosphere DO NOT behave like an actual greenhouse despite carbon dioxide being a key component of both!
Background
During my time working in the environmental consultancy sector I have witnessed climate change becoming increasingly prominent in much of the work being undertaken; developing carbon strategies, formulating greenhouse gas inventories, producing adaptation plans and so on are some of the projects I have personally been involved with.
But how many people, from striking school children all the way up to highly credentialed scientists actually understand the basic, scientific theory that underpins climate change?
Many understand at some basic level that something called the ‘greenhouse effect’ causes a warming of the earth’s surface. But it doesn’t often go much deeper than that. Given the magnitude to which the implications of this theory affects us all, I find this surprising!
I consider the ‘greenhouse effect’ to be a scientific theory. It a scientific proposition that leads to a set of predictions, which can be then be tested through experiment and observation, repeated, re-tested and so on. The prediction culminating from the theory, in this case, is a changing (warming) climate.
And for the record, my own personal curiosity and skepticism prevents me from ever accepting that any theory is ‘settled’. Keep on asking questions. The scientific method demands it!
A quick history lesson
The Swedish scientist, Arhenius, was an early proponent of the greenhouse effect theory in the late 1800’s. This was the same Arhenius who also thought that electrocuting young Swedish children would increase their intelligence, but that’s another story!
Arhenius himself was building on the discoveries of earlier scientists such as Fourier and Tyndall. Tyndall undertook a famous experiment, allegedly demonstrating experimentally for the first time the behavior of greenhouse gases using a form of spectroscopy in the lab.
If you hear people claim that the greenhouse effect has been understood since the 19th Century, this is basically what they mean.
How does the greenhouse effect work in the real world?
We need to start with the ultimate source of energy on our planet. The sun.
The sun emits energy in the form of electromagnetic radiation (light) across a wide spectrum of wavelengths.
Much of the light that doesn’t get reflected back out to space by clouds, the earth and other surfaces, is absorbed by the earth, causing its’ surface to warm.
The warmed surface of the earth re-emits radiation back out towards space. This radiation is in the form of long-wave, infrared radiation (LWIR).
Some of this LWIR is absorbed by certain gases, known as ‘greenhouse gases’. These gases have molecular properties that allow outgoing LWIR to be absorbed.
Greenhouse gas molecules cannot hold onto LWIR, but rather they become ‘excited’, by it, and then re-emit the energy back outwards again.
This phenomenon results in there being less LWIR at the outer reaches of the earths’ atmosphere, and more being present near its’ surface. Moreover, scientists can measure the amount of LWIR at these two locations - the earth’s surface and the top of the atmosphere - and the difference between them used as a means to quantify the greenhouse effect. This value is often expressed as a ‘power’ (given in watts per unit area).
Greenhouse gases don’t put heat into the system, it is more accurate to say that the presence of greenhouse gases inhibits the cooling of the earth.
Climate scientists calculate that, in the absence of an atmosphere, the earth would have an average surface temperature of 255 degrees Kelvin as opposed to the 288 degrees Kelvin that we have. The earth’s atmospheric blanket makes us 33 degrees warmer, preventing it from freezing over (255 degrees Kelvin is approximately -18 degrees Celsius).
What does it have to do with greenhouses?
The term ‘greenhouse effect’ can be misleading since the earth and its atmosphere doesn’t actually behave like a greenhouse!
Radiant energy from the sun penetrates greenhouses and warms up the surfaces inside. These surfaces, through a process of conduction, warms up the air within the greenhouse. A physical, usually glass, barrier prevents convection (mixing) of the warmed air inside the greenhouse with the colder air outside. Without this ability to moderate warming inside the greenhouse, the temperature inside will continue to rise as long as the sun keeps shining. Conversely, opening a ceiling panel will rapidly cool the greenhouse by allowing the warm air to mix with the colder air outside.
Unlike the closed greenhouse the earth’s atmosphere is effectively an openly convective system. Perhaps confusion arises from the fact that carbon dioxide – a greenhouse gas – plays a role in providing optimal greenhouse conditions. Gardeners increase carbon dioxide concentrations in their greenhouses to nourish their plants, but certainly not as a way to make the greenhouse warmer.
So there you have it - a simple introduction to the Greenhouse Effect !
The implications of this simple theory are HUGE! And now you can sound like a pro the next time you talk about climate change!
For what it’s worth, I believe there are some interesting question marks hanging over the greenhouse effect theory. I will be highlighting these points in subsequent letters to make sure that we can all keep on asking questions!
- Tristan