Spelunking in the archives here, I ran across this excellent article from 2009. I have linked to snips of it before, but the entire article is well worth the 15 minutes to read and digest. Yes, it's a number of years old but what is fascinating is that the objections raised in the article have yet to be addressed. Here's an excerpt:
I have looked at the raw temperature record for the USA (USHCN data) and the Bureau of Meteorology data for Victoria, Australia. Both show fluctuations of temperature with time but zero underlying trend for the last century. By contrast, the official IPCC endorsed data shows a strong underlying upwards trend. When I investigate why the difference, I find that the raw data has been adjusted for several supposed factors and every one of these adjustments created a warming trend. This implies that the claimed warming trend is due to the adjustments, not the raw data. In any less controversial scientific issue, such a result would be viewed with the greatest possible skepticism and would be extremely unlikely to be accepted.
The whole article is pack full of this sort of stuff. Highly recommended.