The answer is Climate Kids by NASA. Climate kids is aimed at, well, kids, but it serves as a fantastic primer of basic climate science. For example, under Big Questions and then How do we know the climate is changing? we find short explanations of the following questions (with links to further resources): So what if Earth gets a tiny bit warmer? Why is Earth getting warmer? (includes the CO2 graph copied here) How do we know what Earth was like long ago? How can so little warming cause so much melting? Doesn’t rising sea level just bring us closer to the beach? How does climate change affect other species?
The main menu of pages has Big Question. Weather and Climate. Atmosphere. Water. Energy. Plants & Animals. No matter how much you know about climate change, you’ll find something interesting on Climate Kids. You can also do a quick check of what you know with their Climate Trivia game.