Michael Mann: The Medieval Warm Period was "a little" warmer than the Little Ice Age

Lateline - 15/03/2012: Climatologist slams intimidation of scientists

MICHAEL MANN: Yeah, so both in our estimates and many other estimates of how temperatures have changed in the past, we do see about 1,000 years, a period of relative warmth. Not warmth that actually rivals the most recent decades. The most recent warming takes us outside of that range of the past warmth. But there was an interval of time about 1,000 years ago that was a little warmer than the colder interval that we call the Little Ice Age which took place several centuries later. Now we can actually explain that period of moderate temperatures 1,000 years ago based on natural factors. A fairly high amount of solar activity, so the sun was a little bit brighter; there were relatively few volcanic eruptions, which are a cooling influence on the climate. So when we put those natural factors into the climate, we can actually explain that relatively warm medieval period. Now it turns out another element of that medieval period of climate is that certain regions like Europe appear to have been warm while other regions like parts of the tropical Pacific were cold. And it turns out a lot of that regional variation in temperature has to do with things like El Nino. And so it's pretty complex when you start to look at past climate changes and the different factors that can explain them. But the bottom line is that the recent warming is unusual in at least 1,000 years and other studies using climate models tell us we can only explain it from human activity.

...That "hide the decline" was referring to a specific tree ring study by a set of scientists in the UK. In fact they had originally published that study in 1998 in Nature and what their study was about was something that's known as the divergence problem. Those particular tree ring data that they were working with tracked temperatures very well up through about 1960, and for reasons that scientists are still investigating, and it may have to do with pollution and other factors, those trees stopped tracking temperatures after 1960.