Hi, this is a very interesting and important question and entire books have been written about this subject. For that reason I cannot cover this question in all its complexity but I will try my best to explain some of the most important concept.

You already know where nearly all of the energy that heats the surface of the Earth comes from. Yes exactly, the sun. Sunlight caries a lot of energy and when the sunlight reaches the Earth, approximately 70% of the light is absorbed by the atmosphere and the surface of the earth (the rest of the energy is reflected back into space – sort of like a mirror). This is the reason why the earth surface and atmosphere gets warm. Now anything that gets warm will send back heat to the surroundings (we say it “radiates” heat). This is why you feel warmth when you are close to a fire or close to an oven. So the Earth gets warm and then radiates some of the heat back into space. If you calculated how much sunlight reaches the Earth surface and how much heat gets radiated back into space, you would find that the temperature of the Earth should be about 60 degrees Fahrenheit (or 30 degrees Celsius) colder than it actually is. How can that be? Why is the Earth warmer than it should be? The answer to this question lies in the so called ‘greenhouse effect’ and that effect is a major cause for changes in the temperature of the Earth (and therefore for global warming). Some of the heat that gets radiated by the surface of the Earth is absorbed by certain gases (called greenhouse gases) in the atmosphere and send back to the earth. Thus some of the heat cannot get passed the atmosphere and essentially gets “trapped” on Earth. This greenhouse effect is quite similar to what? Yes, similar to a greenhouse where the sunlight shines through a big window into a building, warms up the air in the building but the heat cannot get past the windows and gets trapped in the building. The most important greenhouse gases are water vapor, and two gases called carbon dioxide and methane.

If you understood the above text then you have all the tools to explain one of the main effects that causes changes in the temperature of the Earth; namely changing the amount of greenhouse gases in the atmosphere. If there is a lot of carbon dioxide and water in the atmosphere, the Earth will trap more heat and get warmer. If there are fewer gases, the Earth will get cooler. It is important to realize that without greenhouse gases we would live on a very cold planet with average temperatures close to 0 degrees Fahrenheit. So greenhouse gases are a good thing!

However, if these gases become too abundant in the atmosphere, the temperature will get warmer and warmer and potentially cause many problems for humans. About 150 – 200 years ago there was a time called the industrial revolution when people on Earth started burning large amounts of coal and then oil to build and drive their machines and industries. Burning those fuels produced a lot of greenhouse gases in a very short time and all of the sudden the carbon dioxide levels rose very very quickly. In fact Earth has not had that much carbon dioxide in the atmosphere for many millions of years. Today carbon dioxide levels are growing faster and faster as more and more people use electricity, drive, fly or build factories. This increase in greenhouse gases is one of the main causes for the global warming that we see today and will see in the future. It is therefore very important that we find alternative sources of energy (like wind, the sun etc.) that do not produce greenhouse gases and can avoid too much warming of the Earth.

One last thing to mention here is that over millions of years the climate of the Earth has been changing a lot and the Earth has been much hotter at some times in the past than it is today. However, we should still try to avoid the Earth getting to hot. The earth might not “care” because it was at times much hotter and at other times colder than today but we humans will have a hard time living with the consequences if the Earth gets too warm.