What is the role of greenhouse gases in global warming?
They prevent heat from escaping.
Electromagnetic radiation travels in waves, and the distance from one wave crest to the next is known as the wave length.
Generally speaking when these wave encounter something they will either through it or be blocked by it. A general rule of thumb is the smaller the wavelength the more likely it will pass through the material.
In the illustration above you can see the smaller wavelengths are things like x rays and gamma rays. If you have had to have an x-ray you know that x-rays travel freely through a lot of material (such as your skin). The calcium of your bones will block some x-rays (which is why they appear on the xray film), but the reason that you wear lead everywhere that you do not need x-rayed is that it takes lead to fully stop x-rays. Gamma rays are even harder to block, and need a much thicker lead shield.
What does this have to do with greenhouse gases?
Well you see on the picture above the area that is a rainbow? That is where most of the radiation from the sun falls, and we call it visible light. Energy at this wavelength passes freely through the atmosphere, and can strike the Earth. When it strikes the Earth the energy is absorbed and the Earth is heated. Heat is actually another form of electromagnetic radiation called infrared radiation. What is crucial to note is that this type of radiation has a longer wavelength.
Greenhouse gases (carbon dioxide, methane, water vapor, etc.) are transparent to visible light, but opaque to infrared radiation. That means the sunlight travels through them but the heat is blocked by them.
All this is actually natural and is part of the energy budget of the Earth.
If you look at the diagram above you see that a certain amount of heat is supposed to be blocked by greenhouse gases. The problem is when greenhouse gases increase and block more heat than they are supposed to. This causes the temperature of the Earth to increase so that the amount of heat the Earth releases can increase and the budget can get back in balance.
They trap heat released by the Earth.
An increase in the amount of greenhouse gases causes less energy to escape from the atmosphere. Over time, this means that more and more energy is trapped within our planet and the overall, average temperature increases.
Greenhouse gases (GHGs) are the main cause of global warming according to scientists.