Actually, the research of Michael Mann and many other climate scientists has concentrated very specifically on understanding cyclical patterns, volcanic eruptions and other natural processes that impact climate. One of the key challenges has been to figure out how to provide accurate and reliable temperature reconstructions going back a significantly long period of time (way before there were thermometers to perform direct measurements) in order to ferrit out the relative impact of cyclical patterns, single event effects (like volcanic eruptions) and anthropogenic effects on temperature. The primary method to do this has been to use proxy data derived from things such as tree rings, ice cores and the like. Because this data exists overlapping periods when direct temperature measurements could be taken the proxy data could be calibrated to within a certain error factor. The resulting findings have shown that even allowing for the effects of cyclical patterns and single occurance events the anthropogenic impact has been the dominate contributor over the last half century.
Another thing to understand, and an issue even in this article is the incorrect use of the term "skeptic". These people are not skeptics (skeptics can be convinced in the presence of new data). These people are deniers who ignore data that disagrees with their world view. Scientists by their vary nature are skeptics and so to have this large of a consensus in the scientific community is even more of an indication of the validity of the conclusion that 1) the climate is warming by an unpresidented rate and 2) the dominant contributor is anthropogenic.