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PSU Climatologist Michael Mann answering questions often raised by climate skeptics, warning against intimidation of scientists

"The lead climate scientist in much of that work was Michael Mann. Mann says he's the central object of attack in what some have characterised as the best funded, most carefully orchestrated assault on science the world has known."

From the ABC Australia's Lateline program (transcript available):

From the transcript:

MICHAEL MANN: Well in fact it's the fact that thermometer measurements don't go further back than about 100 to 150 years that leads scientists to use what we call proxy data, indirect measures of how the climate change in the more distant past, like tree rings and corals and ice cores, to attempt to reconstruct how climate change farther back in time. And so back in the late 1990s, my co-authors and I published a graph that came to be known as "the hockey stick" which attempted to estimate temperatures, the average temperature of the Northern Hemisphere back a thousand years using these sorts of data and what it showed was that the recent warming is indeed unprecedented as far back as those estimates go, about 1,000 years. In the more than decade period since we first published our work, many other researchers using independent approaches, different types of data have actually come to the same conclusion and even extended that conclusion a little bit farther back in time. So we do now know that not only has the globe warmed by a little less than a degree Celsius over the past century, that warming does appear to be unusual in a longer-term context.

EMMA ALBERICI: Back in 1998 a television interviewer asked you if your research proved that humans were responsible for global warming. Your answer was that it was highly suggestive of that conclusion, but you wouldn't go further than that back then. At what point were you finally convinced that that link did exist?

MICHAEL MANN: Right, so, you know, all we could conclude with our work was that the recent warming was unusual in a long-term context. That alone doesn't mean it's due to us. It's due to fossil fuel emissions and rising greenhouse gas concentrations. That conclusion has actually been established by taking models, theoretical models of the climate and subjecting them both to natural factors like volcanic eruptions and changes and the behaviour of the sun and the human factors of increasing greenhouse gas concentrations and what those more recent studies show is that you can't explain that anomalous recent warming from natural factors. We can only explain it when we include the effect of humans on the climate.

EMMA ALBERICI: Your critics refer to what's known as the medieval warming period of around 1,000 years ago when there were no coal-fired power stations, no motor vehicles and other modern phenomena that could explain the temperature rises as you suggest, and yet the planet was going through an extended heat spell between that period of 11th and 14th centuries. How do you explain that?

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 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.

EMMA ALBERICI: Now you've just published a book called The Hockey Stick and the Climate Wars and I have to say it's a book that reads much more like a thriller than a scientific textbook. You've had death threats and charges that you misappropriat