I've talked to my friends about climate change, but I don't think I've ever really explained what convinced me that we need to take action.
Most people are coming around to that view by a kind of mass culture osmosis, but it kicked into high gear for me a couple of years ago, when Al Gore came to Portland.

Like everyone, I'd been aware that climate change was expected to become a problem, but I hadn't really internalized the fact that it's a problem right now.

That night, I was working the Oregon Bus Project table at the line to get in to Mr. Gore's presentation, and once it was about to start, we went in to see it. It was funny and terrifying at the same time. The one thing that I really took away from there was the graph showing the correlation between temperature and CO2 concentration. They march in lockstep with each other for eons, and then CO2 goes off the scale in the last few decades. It's hard to look at that graph and not be afraid that temperatures may rise quite a bit.

There are people who will argue that none of this means the Earth is warming. They make various assumptions to get from here to there, but their basic argument seems to be, "You aren't 100% certain that the rise in CO2 will cause warming, therefore it won't."
Looking at that graph gives me an idea which way to bet, though.
The best argument I've seen that this may not occur is in Skeptic magazine, where there's an article stating that the uncertainties about the future direction of temperature are greater than the certainties, leading to the conclusion that we can't reliably predict what's going to happen. In the same issue, though, there's a very convincing point-by-point breakdown of why we should expect warming to happen.

The main reason I expect it to occur, sooner and to a greater degree than is predicted, doesn't really have anything to do with those arguments. It has to do with the news.
It certainly seems that warming is happening right now. Every so often, there's an article about another ice shelf breaking off, or another species abandoning its habitat because of the change, or how the permafrost is melting.
Think about that: a region that people saw fit to name "permafrost", presumably because no one could remember a time when it wasn't frozen, is melting. I take that as a piece of evidence.

There's something else about those news stories, though. The scientists who are interviewed are always shocked by how quickly the event happened. There's always a statement about how this wasn't expected for another 20 years, or something similar.
This leads me to believe that one of two things is true. Either the scientists aren't adjusting their predictions by the increased rate of warming, in which case they're pretty incompetent, and warming is happening quicker that they're predicting. Or, they are adjusting their predictions for the acceleration in warming, and it's still happening even faster.
Neither of those possibilities is pleasant.

So, even though it isn't 100% certain that our spewing of CO2 is causing all this, I think there's enough reason to bet on it. And since we have the ability to cut our carbon footprint, we should do it. Now.