The Man Behind the Hockey Stick

Climatologist Michael Mann discusses the science of global warming, and takes on critics of his work.


Michael Mann enjoys taking on big challenges, which explains why he feels right at home trying to untangle the mysteries of global warming. The University of Virginia climatologist is best known for his “hockey stick” graph—the famous chart showing that the earth’s temperature has sharply increased over the past 100 years or so, after centuries of gradual change. His work has gained wide acceptance among scientists, who point to the chart as the most elegant symbol of human-induced climate change, and was recently showcased in the 2001 IPCC Assessment on Climate Change, the most wide-ranging compilation of climate change science to date.

Nevertheless, Mann’s work—like that of many climate change scientists—has come under heavy attack from skeptics of global warming, including various politicians and well-funded industry opponents. Not only that, but he faces the daunting challenge of convincing a doubtful and often disinterested public that climate change is not only real, but has been going on for the past century. As Mann notes, he’s hardly the first climate scientist to face this sort of criticism, and he certainly won’t be the last. And while he believes that the climate-change science ought to speak for itself, he and his colleagues have decided to go a step further and take an innovative step to get their message out there—they’ve started blogging.

Mann and eight other climatologists have created a new website,, devoted to educating the public on the intricacies of the evolving debate over global warming, as well as to debunking the work of industry-funded pseudo-scientists. Since first appearing online last December, the site has received several thousand visitors and substantial media coverage. As a result, Mann’s critics are finally on the retreat—at least for the time being.

Mann, who in 2002 was named one of 50 leading visionaries in science and technology by Scientific American, recently spoke by phone with Mother Jones.

MJ: How did you get into climate science?

MM: When I went to graduate school at Yale, my focus was originally on theoretical physics. But I didn’t find the problems of the time as interesting as some of the great problems of physics that intrigued me in the first place. So I looked around to see if there were other fields where I could use my training and expertise to work on some large, yet still unsolved problems. I talked to a scientist named Barry Saltzman who worked in the climate sciences, which I found to be fascinating, and I ended up doing my PhD with him.

When I entered the field back in the early 1990s, the issue of whether or not we could detect the human influence on climate was still an open question. But we were on the verge of big change. The field was advancing quite rapidly and by the mid-late 90s scientists were convinced that humans were altering the climate. Despite that conviction, many problems remain to be solved.

MJ: Was there ever any component of the issues and implications of climate change that lured you in as well?

MM: I think there was an attraction in that scientists in the field asked questions like “Are humans influencing the climate?” which had many societal implications. But I never felt like I came into it with an agenda in that regard. I was really more interested in natural climate variability and my PhD thesis in fact had almost nothing to do with the issue of humans altering climate.

What I was really into was reconstructing trends of past climates and trying to understand how climate varied within its natural state. In the late 1990s some colleagues and I produced this reconstruction of climate in prior centuries and we began to realize that there were some deep implications for our work—now we could compare what had happened in the twentieth century with what had happened in the past. So through these studies, I sort of stumbled into working on discerning human influence on climate.

MJ: What was it that caused so many scientists to start believing that humans were altering the climate?

MM: At the time that I got into the field there were a large number of what I would call legitimate, “honest” skeptics—that is those who demanded to be convinced that human-induced climate change was a real phenomenon. They were open to being presented with evidence that might change their minds.

But advances in climate modeling, an influx of new scientists working on the problem, and in some cases advances in other scientific communities like the physics community and the statistics community played important roles in the developments within our field. The evidence became more and more compelling—it wasn’t a coincidence that the scientific community reached a consensus on the issue in the mid-1990s. My advisor, Barry Saltzman, was originally a skeptic—yet I saw him come totally around on the matter. By the late 90s, what skepticism remained generally amounted to misleading arguments that may have made for good sound bites on Crossfire but which didn’t stand up to any bit of scientific scrutiny.

MJ: Are there any legitimate scientific arguments left that humans aren’t altering the climate? Or is that just off the table completely?

MM: As a scientist you never want to say that anything is completely off the table. But I would say it’s become extremely difficult to make a plausible argument that the changes in the climate are not in large part due to human influence. Where the legitimate debate now lies is on the sensitivity of the climate—how it responds to an increase in, for example, greenhouse gas concentration. We know that there has been a certain amount of warming, we know that carbon dioxide levels have increased from pre-industrial levels of about 280 parts per million to about 370 parts per million today, and that those levels are unprecedented in the last several hundred thousand years. We know this increase is due to human activity—there is no legitimate scientific argument to indicate otherwise.

Where things get a little less clear is that there is still some uncertainty in how the climate responds to a variety of natural factors such as changes in the sun’s intensity and volcanic eruptions. As well, some human influences, such as certain industrial aerosol emissions, can actually have a cooling effect. It can be very difficult to detangle all those factors and to figure out exactly how much warming is due to human influence.

MJ: Can you talk about the degree to which we know how much humans are altering the climate?

MM: Sure. The aforementioned uncertainties not withstanding, it’s very difficult to argue that the Earth would warm any less than 1.5 degrees Celsius and any greater than 4.5 degrees Celsius for a doubling of atmospheric carbon dioxide—which is a standard benchmark in the science of climate change. At current rates of emissions this doubling would happen midway through this century—but of course exactly when it will occur is impossible to predict, because humans themselves are unpredictable.

MJ: That sounds like a lot. But what does this mean in tangible terms?

MM: Even at the low end, there are significant implications for our climate. There is reason to expect a fair amount of sea-level rise, perhaps on the order of a meter, with increased flooding in coastal regions as well as increases in certain types of extreme weather events such as droughts and extended heat-waves. We expect atmospheric circulation to change, with some areas becoming wetter and some becoming drier.

The more rapidly this happens, the more difficult is it for humans, animals and natural ecosystems to adapt. We’re already seeing significant disturbances in bird migration patterns and increased mortality in certain animal species which appear to be directly related to climate change.

In the higher end of that range there would be more profound changes in the climate, like major melting of ice sheets and glaciers, significant sea-level rise, and probably a fairly dramatic increase in the incidence of certain types of extreme events such as severe hurricanes. The potential disturbances to humans and ecosystems could be quite severe. We’re talking about profound changes on a timescale that we really haven’t seen in the past. Some of us argue that this is more than enough evidence suggesting we ought to proceed with caution.

In fact, a paper published in Nature earlier this year predicted the earth could potentially warm by more than twice as much most of us believe, somewhere close to 20 degrees Fahrenheit. Now one could argue that this range is too large because it was based on just one set of model simulations. But there are other constraints. By looking back at various periods in the past where we have reasonably good ideas of what the greenhouse gas concentrations and the average temperature of the earth were, you can come up with a sensitivity of anywhere from 2-4 degree C for a doubling of carbon dioxide. The authors of the study know this, and I suspect that they were probably taken a bit out of context in some of the dramatic news reports that came out following their study.

MJ: Many people have the presumption that all of our predictions are based on models—which of course, they often don’t trust. But you’re telling me that past evidence provides credibility to predictions of warming as well?

MM: Absolutely.

MJ: You can’t argue with the past.

MM: No you can’t.

MJ: But people have trouble buying the argument nonetheless. For example, a lot of times you hear, “Hey—if we can’t predict 10 days in advance, how can we predict a hundred years?”

MM: (Laughs) One of the classic blunders in talking about climate is to make an inappropriate analogy with weather—but it doesn’t stop people from doing so. It’s a disingenuous argument, and in fact it’s an argument that was parroted by Michael Crichton in his recent book, State of Fear. There’s a big difference between weather and climate. Let’s say it’s December and I’m heading out to San Francisco for a conference. I know enough about climate that I better bring my umbrella because it’s very likely going to rain while I’m there. If I’m going out there in July, I needn’t bother.

MJ: Here’s another common mistake: Next year might be colder and the skeptics, or rather the “contrarians” will say, “How can this be so, if global warming is real?” Yet if it’s a little warmer, the advocates on the other side come out and say “Hottest year ever!”, as proof that climate change is real.

MM: There’s often this tendency on both sides of the debate—at least with advocates—to take evidence out of context. Describing climate change as a warming of some small number of degrees C of the global mean temperature makes it difficult for people to understand what the big deal is.


However, you can talk about the frequency of events and how that might change as climate changes. In fact there are solid scientific reasons for why there could be dramatic changes in some of these events—more dramatic than you might think.

MJ: Hurricanes in Florida are a commonly used example.

MM: There’s been a lot of confusion on that topic. Some of what the scientific community has had to say has been distorted by the “contrarian” filter, but it’s a more subtle issue than it’s often made out to be. There are two different things we care about with hurricanes—their frequency and their intensity—and those are two separate matters.

First let’s address the matter of frequency. If you look at the Atlantic basin, it’s still somewhat unclear what is happening, and it depends on how El Niño would change in a human-altered climate. It turns out that El Niño has a very important influence on the formation of hurricanes in the Atlantic Basin. During an El Niño event there is more vertical shear in the atmosphere which is very disruptive for hurricane formation. So if you don’t know how El Niño will change, it’s very difficult to know how the frequency of hurricanes will change in the future.

On the other hand, as sea surface temperature warms, the hurricanes that do end up forming can become far more powerful. There’s a great deal of sensitivity to this—it isn’t that you just change the sea surface temperature by 1 percent and hurricanes get 1 percent stronger. As you increase sea surface temperatures in the tropics, even by a little bit, you can greatly enhance the maximum potential intensity of a hurricane. It’s what we call a “nonlinear” relationship.

MJ: So to say that the Earth is warming and thus there will be more hurricanes hitting Florida is not necessarily true.

MM: Right. It could be true, but we don’t know the answer to that question yet. However, the models are quite clear about the storms getting stronger, and the observations on that point thus far seem to be consistent with predictions.

MJ: One of the problems with getting people to buy into this notion of climate change is the fact that regional predictions—as opposed to the global mean predictions—are quite a bit more uncertain.

MM: Well, that brings us back to the fact that an El Niño influences climate patterns all over the globe. To have truly reliable regional climate predictions, we would need to be certain how anthropogenic forcing will affect El Nino’s. The answer isn’t precisely known.

However, that doesn’t mean we can’t say something about regional impacts. Most models predict increased severity of summer droughts over continental regions of the extra-tropics, such as the United States. Moreover, we can still be confident about predictions for increased global surface temperatures and a more vigorous hydrological cycle over much the earth despite the uncertainty of regional effects.

MJ: Human behavior is also unpredictable.

MM: Right. There are really two types of uncertainties that are difficult to compare. One of them of course is an uncertainty in the nature of the physical system. And that’s the sort of uncertainty—one would hope—that will continue to be reduced over time. But what will we do when faced with a given set of considerations about what humans will do? This is where I generally say that this is outside my domain of expertise. It’s hard enough modeling the climate, let alone predicting human nature.

One can be hopeful. But that’s not a prediction.

MJ: Lets move on to the topic of the hockey stick. What is the hockey stick? It’s been all over the news.

MM: The hockey stick is a term that many have attached to a reconstruction of the earth’s past temperature that my colleagues and I performed back in the late 1990s. We estimated—with considerable uncertainty—the earth’s mean temperature over the last 1000 years using tree rings, ice cores, corals, and other indicators of past climate. We used statistical methods to relate those sorts of data to modern instrumental temperature measurements and using those relationships were able to describe temperature patterns back many hundreds of years.

This reconstruction showed the past few decades to be the warmest period of the past 1000 years even taking into account the fairly large uncertainties. The “hockey stick” term, originally coined by a scientist named Jerry Mahlman, refers to the fact that the late 20th century warming is much larger than the variations in previous centuries, making the reconstruction somewhat resemble a hockey stick.

Since the time of our reconstruction, more than a dozen other groups have produced similar reconstructions using different methods and data, supporting the conclusion that late 20th century global warmth is unprecedented in modern history.

MJ: Correct me if I’m wrong, but I’ve heard that the last century is anomalous not for its average temperature but for the rate of change during that century.

MM: Well, the 20th century in almost all reconstructions is the warmest century—but owing to the uncertainties involved, its hard to conclude this with total certainty. On the other hand, the latter half of the twentieth century is so dramatically warmer than the rest of the reconstruction that you can easily conclude that it’s the warmest period during the past 1000 years. So both the rate of change and the overall temperature during that time are historically high.

MJ: Your hockey stick has come under heavy attack in the last few months.

MM: Yes. The contrarians have tried to make it seem that there’s just one reconstruction and have attempted to narrowly define the debate on the premise that if they can debunk this dataset, the whole warming theory would come into question. What they will do is take our particular reconstruction and make disingenuous claims about it’s various attributes and hold it up as a straw man for trying to argue that these are not the consensus conclusions.

But that’s ridiculous. These days, scientists in the field prefer not to talk about the “hockey stick” anymore because of the sheer number of corroborating reconstructions; we now talk in terms of the “hockey team”. They might be able to take one member of the team out of the game for a while with a cheap hit, but there are others that can easily fill in.

MJ: Is this just a big PR campaign?

MM: I’ll leave it to you as a journalist to investigate some of the links, some of the funding sources, and come to your own conclusions. Ross Gelbspan—he’s a former editor of Boston Globe—has written two books on the connections between industry funding, in particular funding by ExxonMobil, and these climate contrarians. The vast majority of them appear to receive funding from industry sources.

MJ: Earlier this year, the hockey stick was a hot topic, but it seems the debate has died down a little of late. Are they backing off this particular argument?

MM: The contrarians are always jumping from one argument to another. It’s sort of like a Hydra—when one of its heads is cut off it merely sprouts another. These contrarians—and interestingly it is often the same group of individuals—first tried to dispute the instrumental record of surface temperature data that demonstrates the dramatic warming of the earth’s surface. That was back in the in the early 1990s. They argued that urban heat island issues affected the temperature readings—but of course, scientists had already accounted for that.

Then they moved on to the argument that recent satellite data—which appeared to show less warming than surface data—argued against surface warming. The flaws of this argument were exposed as well, and if you analyze the satellite data properly they actually reaffirm the other evidence of surface warming. So then the contrarians began to go after the “hockey stick” because it was perceived as an icon of the global warming debate. It just goes on and on.

There are quite a few papers undergoing peer review now and studies in press which detail the critical flaws in the arguments that these contrarians have been putting forward about the hockey stick in the past few months. As it plays out in the peer-reviewed literature, it will soon be evident that many of claims made by the contrarians were fraudulent.

MJ: Is it fair to say that this head of the hydra has been cut off?

MM: As far as the legitimate scientific community is concerned, yes. But what’s often the case is that contrarians will pretend that they have an argument long after the scientific community has thoroughly discredited it. As long as something sounds good in a sound bite, they’ll keep at it.

MJ: This has to be difficult for scientists to combat in the public arena. How do you approach this?

MM: I’ve been involved with a team of other climate scientists in this project called which is a website with a blog format. It’s a commentary site on climate science created by working climate scientists for both the interested public and for journalists.

The idea is to provide a rapid response to developing stories and provide the scientific context that’s often missing in the media coverage. We’re not circumventing the peer review process, as some have claimed, we’re simply trying to provide the context of what existing peer reviewed science has to say about certain issues. And we’ve had a fair amount of success with this. We’ve received good media coverage, and have had over 200,000 visitors since we went online in mid-December. So it seems to be serving the purpose that we intended, which is to provide a resource for people looking for an honest broker for stories like the hockey stick attacks and State of Fear, both of which are riddled with fallacies.

MJ: Any other venues or concepts that scientists are throwing around?

MM: Well, this is our baby, and we hope that it might set an example for others who are interested in helping educate the public about these issues. I’m sure there are other creative approaches to this problem. This is the one we came up with.



Truth #1: The chaos is the point.

Truth #2: Team Reality is bigger than it seems.

Truth #3: Facebook owns this.


Truth #5: It's about minority rule.




payment methods



Truth #1: The chaos is the point.

Truth #2: Team Reality is bigger than it seems.

Truth #3: Facebook owns this.


Truth #5: It's about minority rule.




payment methods

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