Global warming is an issue that has become controversial, not because of the science behind it, but because of politics. More than almost any other issue, it can be used as a litmus test of your political affiliations and beliefs. And yet, like many other scientific problems (take evolution as an example), it is fairly easy to settle, even if the science behind it is complex and difficult to understand. The data supporting the theory that our planet is getting warmer, and that we are largely responsible for it, hes been around for many years. The climate models agreed with observations, yet a small, but vocal minority kept distorting reality, making large portions of our society very confused in the process. This was an orchestrated and very deliberate process, not much different from the efforts of tobacco companies years ago, trying to squelch data showing how bad smoking was for your health (see "Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming").
But data does not lie, no matter what kind of ideological glasses you are sporting.
In a recent study, led by anthropogenic climate change "skeptic" Richard Muller, data collected from over 39,000 measuring stations was re-analyzed again to ensure that some of the "biases" alleged previously were correctly taken into consideration.
While there is always additional research that can add to our current knowledge, this study is just another nail in the coffin of the global warming deniers.
At this point, with all the data available and analyzed, and with 99% of climate scientist agreeing, you have to either be dumb, uneducated, or a hard-core republican (or still watching Hannity of Fox) not be believe.
This illustrated a larger principle: Our world is increasingly complex and the issues we face require more and more expertise to understand. Therefore, always trust the experts, not a single one, but a consensus of many experts, experts in a given field of knowledge. This ensures that you get real, unfiltered knowledge and not some ideological woo-woo, paid by a corporation or a political figure.