I've been trying to put this into simple ways to discuss this with friends of mine, I've spent years learning about climate change and I'm probably more well read than the average person. I try to use this approach, first I try to get them to understand this:

There is cause and effect, we can agree on that. In this case, cause is rather simple but effect plays out in incredibly complex ways that we don't fully understand or can't fully predict, though we know that just about all of it is "not good". Deniers use the complexity of the effect to cast doubt on the cause, which is crazy if you think about it, the cause is rather simple.

Leading from there, with discussing cause, I ask a series of questions: 1. Do you believe Carbon is released when we burn a fossil fuel? 2. Do you believe scientists can measure the amount of carbon in the air? 3. Do you believe scientists can predict how much carbon will be released from a specific amount of fuel? 4. Do you believe certain gasses in the atmosphere trap heat energy from the sun?

At least I get a feel for what exactly their grounds for believing denial are (they usually aren't very strong) and then I can work from there.