First, I think you're posting this in the wrong account. I specifically stopped visiting the "opinionated techie" blog because of these types of articles, but I still visited "Skinning the Frog" because it was about games and "My site for talking about the customization of Windows."
Second, the temperature data for europe in the middle ages disagrees with temperature data from other parts of the globe, suggesting that it was a local phenomena. This means the Greenland ice samples, alone, should be taken with a grain of salt.
Third, if you show longer periods of time (on the order of 500,000 years, instead of 4,000 years) you can see that temperatures do go up and down on the planet. For the last 10,000 years or so, we've been on a peak temperature - hotter than any time in the last 120,000 years. Global warming advocates don't claim that human-produced carbon dioxide is the sole factor involved in global warming, but rather, it's a major factor that is launching the earth to even hotter temperatures than would occur naturally, and to higher temperatures than anything the planet has seen in a very long time. It's a little bit like sitting inside on a hot summer day with the windows closed, and turning on the electric heater. Yeah, the space heater isn't the sole source of heat inside your home, and the external temperature does vary, but it doesn't mean you can deny the effects of your space heater on the inside temperature.
