Climate Change: Michael E. Mann Explains His Hockey Stick Graph

Michael E. Mann is a renowned pioneer in the study of climate change and the understanding of how humans impact the Earth’s climate. In 1999, Mann and a team of researchers published a graph which came to be known as the “Hockey Stick Graph,” now an iconic symbol in the fight against climate change.

In a recent episode, he spoke about his infamous “Hockey Stick Graph,” what it means, and the controversy surrounding it.

Mann: “The hockey stick graph that my coauthors and I published a decade and a half ago is an estimate of how temperatures changed in the distant past. We only have about a century of widespread thermometer measurements, and we know the Globe’s warmed by about a degree Celsius (about a degree and a half Fahrenheit) over the last century. To place that warming in a longer term context and assess how unusual it is, we need to turn to other indirect measures of climate that we call proxy data, like tree rings and ice cores.

We used those sorts of data to reconstruct the pattern of how the climate changed over the past thousand years, and what we found was that indeed the modern warming spike is unprecedented as far back as we could go. If you look at the graph, it looks sort of like an upside down hockey stick, where the handle represents the long term slow decline, with a slight cooling as we enter into the little Ice Age of the 17th, 18th, and 19th centuries, and then of course the blade of the hockey stick being the abrupt warming spike of the last century which has no precedent as far back as we can go. In fact, many studies are now are finding that the modern warming spike is probably unprecedented in tens of thousands of years.

That graph became an icon in the climate change debate. I think it became an object of attack by industry funded climate change deniers and fossil fuel industry front groups because it represented a threat. You didn’t need to understand the complexities of the science to understand what this graph was telling us, that there is something unprecedented about the changes that are taking place today. And by implication, it probably has to do with human activity. As a result, as a post-doctoral researcher who you know wanted nothing other than to continue to work on interesting problems in science, I found myself at the center of what is arguably the most suicidally contentious issue that we face today: the issue of human caused climate change and what to do about it.”

"I found myself at the center of what is arguably the most suicidally contentious issue that we face today: the issue of human caused climate change and what to do about it."

Q: “That was 18 years ago. Has science validated your basic findings of the hockey stick curve? I know over those 18 years, lots of people have challenged it. How has it held up in the last 18 years?”

Mann: “Yes. Many of the challenges actually came from outside science, and that’s one of the important things to understand. Skepticism is a good thing and all good scientists are skeptics, which means that we hold a claim up to scrutiny. As Carl Sagan famously said, ‘The more extraordinary the claim, the more extraordinary the evidence must be.’ The way you get ahead in science is actually by disproving other people, by making a name for yourself, and so of course other scientists are going to be challenging a finding that receives as much prominence as the hockey stick did. But most of the attacks came from outside the world of science, not actually from scientists but from industry funded climate change deniers, conservative news sites that were attempting to discredit our work, in part by discrediting me personally. And that’s not the way science works.

If you actually look at what the science has found over the intervening 18 years, all of the studies that have sought to reconstruct past temperature changes published in the peer reviewed literature have actually reaffirmed our finding. That’s why it is now not just a finding that is attributed to an 18-year-old article by us, but it’s the consensus as represented in the most recent report of the Intergovernmental Panel on Climate Change.”