There are multiple lines of evidence that scientists use to generalize that the planet is warming and mankind’s activities are the primary cause. But many of the explanations focus on evidence that the rank and file human being does not understand: proxy data (ice cores, tree rings, etc.), complex climate models, and chemical and mechanical process that combine to produce the effects we are beginning to see worldwide (heat and cold extremes, rising sea levels, erratic, severe weather events of all kinds.) In this article, Cause and Effect, Scott Denning explains the inevitability of global warming in a way that is far easier to comprehend. According to Denning, if we put more heat into the atmosphere, and at the same time put more gases that we know absorb heat into the atmosphere, voila, the atmosphere warms. There is some math here, only part of which I understand, but the basic notion that if we can show the previous two conditions to be true, then we have to accept with certainty that the planet will warm. If you have found it difficult to understand some of the more complex explanations of global warming, though there is a bit of scientific terminology here, I think you will find this a very useful article.