For the past two decades, Pennsylvania State University climate scientist Michael E. Mann has been front-and-centre in the climate change debate—and has faced vicious attacks because of it. As lead author of the infamous “hockey stick graph,” a dramatic representation of estimated temperatures stretching back 1,000 years that shows a sharp uptick this last century, Mann became a target of those who sought to discredit his work. In is new book, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines, he describes doing battle with climate change deniers. Mann spoke to Maclean’s from Washington, D.C.

Q: What was the hockey stick graph, and why did it become such a flashpoint in the climate change debate?

A: It was this graphic my co-authors and I published back in the late 1990s. We only have about 100 years of widespread thermometer measurements, and know it’s warmed about one degree Celsius over that time frame. But to get an idea of how the climate has varied naturally in the more distant past, one needs to turn to indirect measures of climate. So we were working with so-called proxy data, like tree rings, corals, and ice cores, because it’s only with indirect measures of climate that we can get an idea of how climate changed in the distant past.

The picture that emerged was this descent from the relatively warm period of the Medieval era, into the cold of the little Ice Age of the 17th to 19th centuries, followed by this dramatic warming of the past century, which took temperatures outside of the range of anything that we had reconstructed in the past. We realized that the warming trend in modern time appeared to be unprecedented as far back as we could go, which was 1,000 years. In more recent years, researchers have extended these estimates further back in time—almost 2,000 years, in several cases—and the same picture emerges. The recent warming appears to be without precedent.

It was as we began to analyze the data that this so-called hockey stick emerged, the blade being the abrupt modern warming.

Q: What kind of reaction did you get?

A: The hockey stick graph became an icon in the climate change debate in substantial part when featured in the 2001 United Nations [Intergovernmental Panel on Climate Change] report. It’s visually compelling: for the public, you don’t need to understand the physics of how a climate model works to understand what that graphic is telling you. That has made it a particularly dangerous scientific result to those who seek to deny the problem, so they targeted it specifically. I became very much an accidental, and certainly reluctant, public figure in the debate over climate change, and I found myself in the centre of it. It was only later that I fully appreciated what was actually unfolding.

Q: What sorts of attacks did you face?

A: It’s hard to even keep track. I’ve been accused of having perpetrated a fraud by prominent politicians like James Inhofe and Joe Barton. I’ve had death threats against me. I received an envelope a couple years ago with a white powder in it: I had to have the FBI send it off to their lab to make sure I hadn’t been exposed to anthrax. I’ve been attacked in just about every way imaginable, whether it’s by climate change deniers, advocates for fossil fuel industry, powerful politicians, right-leaning media outlets in the U.S. and abroad. I’ve been called all sorts of nasty things; I’ve been accused of fraud; I’ve been called a criminal. It’s using the tools of character assassination—to discredit the science by going after the scientists.

Q. Good science is built on healthy debate. How can you tell the difference between that, and an attack that’s below the belt?

A: Skepticism is an essential thing in science. Scientists are expected to support their results with solid reasoning, and solid data, and their findings have to be subject to replication by other scientists. It’s all part of the give and take, and sometimes it’s rough-and-tumble. Scientists can be brutal, but as long as it’s an honest debate, that’s what moves science forward. Unfortunately, in the areas of science that have become politicized, there are those that exploit that culture of open debate by engaging in bad faith attacks. There are usually telltale signs, like going after the researcher rather than the results. Many of those looking to discredit mainstream scientific findings or individual studies like ours are not participating in the normal scientific process. They’re not going to meetings, giving lectures, or taking questions from an audience. They’re not publishing in peer-reviewed journals. They’re issuing attacks in op-eds in newspapers, or on websites with no degree of critical quality control.

Q: Other than discrediting your specific study, what, in your mind, is the larger purpose of attacks like this?

A: The motive is to create an air of controversy: to make it seem like this entire area of science is too controversial for the public to trust the findings of the science. And it’s intended, I think, to make [scientists’] lives more onerous, so that I and other colleagues attacked in this way find it more difficult to advance our science—to actually continue to do research in this area—because we’re saddled with vexatious legal assaults and attacks we need to respond to on a day-to-day basis. We worry that perhaps it’s intended to dissuade younger scientists from even going into this field.

Q: Do you think the attacks could actually discourage scientists from pursuing research like yours in the future?

A: It’s of course my concern. You can never know how many of my colleagues won’t talk to the media, or lecture about climate change, over the fear of being vilified. It’s hard to know how many have been influenced, but [the tactic has] backfired, to some extent. Scientists don’t take kindly to being misrepresented, having their work distorted unintentionally; we scientists can be a cantankerous and stubborn group. I think we’ve seen in recent years real signs of fight within the scientific community, and especially among the younger scientists. Younger scientists are far more involved, whether through social media or other means of participating in public discourse and defending science from attack.

Q: What’s motivated you to continue with this work?

A: Confronting the challenge of climate change is often framed as a scientific, or political, or economic issue. Not often enough is it framed as what it most truly is: an issue of ethics. I have a 6-year-old daughter. To me, it’s very much a question about the planet we leave our children and grandchildren. Decisions we make will impact earth for decades and centuries to come. We have to decide what sort of legacy we want to leave our children. It isn’t futile; it isn’t too late. There’s a lot of pessimism, and the problem is urgent, but there’s still time to confront it.

Michael Mann tweets from @michaelemann