Below is a picture of a greenhouse. With one of those you can grew tomatoes in places were it normally is too cold for tomatoes.
The greenhouse is made of glass. Glass is transparent to sunlight (apart from the UV light which is why you don’t get tan / skin cancer in a greenhouse). The heat from the sun can enter the greenhouse but it can’t get out.
How is this related to the atmospheric greenhouse effect with the warming planet and so on?
This story begins in 1824 when Joseph Fourier (when he got bored working on his transforms) noticed that the earth is warmer than its distance from the sun justifies. He suggested that the atmosphere acts as an insulator. As in a glass greenhouse two conditions have to be met for this to work:
- The atmosphere has to be transparent to sunlight. If you look up on a clear day you can easily convince yourself that this is indeed the case.
- The atmosphere has to be absorbing emission from the earth. This emission is infrared and therefore invisible (the earth is twenty times colder than the sun so the radiation has twenty times longer wavelength or 10 micrometers instead of 500 nanometers). Luckily there are instruments that can measure this and indeed: Some of the elements in the atmosphere, notably CO2 and water vapour do absorb radiation from the earth.
So the atmosphere is a functioning greenhouse. The next step was realizing that humanity can actually make this greenhouse more efficient by emitting CO2. This step was taken in 1896 by Svante Arrhenius who also tried to quantify the effect: On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground. He concluded that doubling the amount of CO2 in the atmosphere would increase the temperature of the earth by 5 degrees C.
Following this there has been a long debate (saturation blah blah, aerosols blah blah, clouds blah blah, absorption band widening blah blah, atmospheric layers blah blah) regarding the size of the effect. The interesting thing is that 117 years of scientific debate has changed very little. The latest estimate from the IPCC puts it at 1.5-4.5 degrees C.
Fortunately (from a scientific point of view), we also have experimental data to compare with. Faced with the question: What will happen if we release a thousand billion tons of CO2 into the atmosphere? Mankind has actually gone ahead and done the experiment. A graph with CO2 levels versus temperatures looks like this: (CO2 here, temp here).
The results can vary a little depending on your starting temperature and how long you think the delay between emissions and temperature rise is (I picked 5 years, a number I got from the same place Irish bankers find bailout estimates) but it doesn’t matter all that much for the end result.
To sum up: We have a scientific theory that has been largely unchanged for 117 years and which is verified by experiment. There are still some important questions of course:
- Exactly how big is the effect? Lower or higher than the current 3 degree C estimate?
- What is the cost of doing nothing? A temperature increase could be nice for some people and bad for some people.
- What is the cost of doing something? Obviously there are some problems associated with changing the worlds energy system.
BUT THERE IS NO EXCUSE FOR LISTENING TO THE TIN FOIL HAT BRIGADE RAMBLE ON ABOUT THE WHOLE THING BEING A HOAX! THEY ARE FLAT EARTHERS AND SHOULD BE TREATED AS SUCH.