Global warming is a complicated subject. Most of us would be oblivious to it and ignore the fact that our planet Earth is dying. So we must do our part and help take care of our only home. The attitude should be taught to kids at an early age so they will grow into responsible adults that will take care of the environment. Teaching global warming for kids however will be difficult.
They see the world in their own eyes and it is filled with beautiful things. They do not understand that what some of their actions are hurting the planet. Giving them a lecture about saving the environment will just bore them. So you need to teach them by giving a lot of examples.
You can find example of global warming effect to the earth if you look around the house. For example the ice cap melting can be explained with a cup and block of ice. First get a block of ice and put it inside a cup. Make sure the ice is bigger than the cup and it stands out. Then fill the cup with water. You can ask the child to look at the cup and ice first and tell them that is the North Pole. Then put the cup under the sun and let it melt. Tell your child to see how the heat is melting the ice, turning it into water and overfilling the cup until it start tricking down the side.
Now you can explain to him how the excess water from the ice caps melting can cause disasters such as floods. You need to explain to them that it is happening right now and unfortunate people around the world are suffering from global warming. You do not need to change him into a Captain Planet but instead encourage him to do his part.
You yourself need to be a role model in matter. Kids always learn things from their surrounding and you, the parents are always together with him. Teach him how to separate the trash, save electricity and do not be wasteful of any resources. This will encourage him to learn to love and take care of his belongings.
Again you do not have to be the perfect nature lover so you can be an example to your child. You need to find that a balance of what you teach and how you act so they do not get the wrong idea from you.