How do greenhouse gases warm the planet?

Chris Smith answered this question...

Chris - One has to first of all think about why does the Earth have any kind of temperature in the first place? Well, energy comes to the Earth from the Sun. It does that in the form of heat given out by the sun which is invisible light - infrared and also, visible light because light is energy. That light reaches the Earth. It hits everything on the Earth and the energy in the light is absorbed by the things it hits and those things, because they now have more energy, have a higher temperature.

What normally happens is that the energy is then re-radiated as long-wavelength infrared light back into space. So, what comes in goes out and the Earth stays stable. But if something gets in the way of that re-radiation back into space then the amount of energy in the system goes up. In other words, the temperature goes up. It turns out that certain gases in the atmosphere are very good actually at interacting with long-wavelength infrared light. Things like carbon dioxide; things like water are very good at doing that. So, the rays of infrared come bouncing off the Earth's surface - and off of you - going skywards, but they see a molecule of carbon dioxide, and the bonds in that molecule soak up the energy and stop it going skywards.

As a result, you retain more of the heat close to the ground, which means that things on the ground are more likely to stay warmer. In other words, the overall global temperature has gone up by just a tiny amount but it's a significant amount in energy terms over a long period of time. Therefore, you will begin to shift the balance of energy distribution on the planet because the temperature is on average higher. So, just a tiny whiff of carbon dioxide in the atmosphere - and some more water - will make a difference in a global scale.

Georgia - That's really surprising. I would never expect water in the atmosphere to make this kind of a difference.

Chris - Water is a very, very powerful greenhouse gas. In fact, everyone maligns greenhouse gases but remember that greenhouse gases are good too, because were it not for our atmosphere - were it not for the ability of water and carbon dioxide to retain heat in the atmosphere - then we wouldn't be on a watery warm planet Earth - we would be on "snowball Earth".

It's down to the fact that we have this atmosphere that does retain some of the energy and keeps Earth cushioned and cocooned in a higher temperature than it will otherwise be that we do have this lovely balmy environment that's ideal for us to live in. So, we do need a greenhouse effect, but we don't want to exacerbate or exaggerate the greenhouse effect in an uncontrolled way, because that means the environment that has kept us nurtured us and nourished us, will begin to go off kilter and that could have unforeseen consequences...