Michael Mann’s ‘Hockey Stick’ gives readers a handle on the science of climate change
By Judy Weiss
When I first learned that climate scientists thought climate change is potentially catastrophic and advancing faster than expected, I felt obligated to try to learn what I could. I took a MOOC (a Massive Open Online Course) that was an introduction to climate change for non-scientists, and read several books dealing with: reasons the public has trouble accepting climate change, effects on agriculture, and carbon pricing policies. But I had a hard time finding a climate science book that wasn’t too dense and technical, but also wasn’t too elementary.
Eventually I found Michael Mann’s The Hockey Stick and The Climate Wars (Columbia U. Press, 2012) which presents the material at a level that’s just right. I’ll try to convey why I found Mann’s book accessible and compelling.
Mann’s book gave me insight into what it takes to become a climate scientist. He began by describing in Chapter 1 the long educational path that leads to becoming a climate researcher. Mann majored in physics and studied more physics in graduate school. When it was time to choose a dissertation topic, he decided to do two more years of study in order to specialize in geophysics. By my count, it seems like six years of physics classes, math, a ton of statistics courses, computer science, probably a few chemistry classes, plus two years of geophysics studies.
Why does this matter? Some people think that anyone can offer useful opinions on climate science theories, models, or data analysis. I infer from the educational path of a climate scientist that this is a field where part-time dabblers (or even other physicists) are unlikely to contribute meaningful critiques of the work of full-time climate researchers.
Also, Mann’s book interweaves the telling of his personal story with explanations of climate terminology, summaries of work by other researchers, and questions researchers are still trying to answer. His discussion of technical details rarely became too dense to digest.
And I found it fascinating to hear about his choice of a dissertation topic. In Chapter 3, he reveals that in the early 1990’s climate scientists could still honestly disagree over whether human influence on the climate was detectable (“they could legitimately be skeptical about whether the human climate change signal had yet emerged” page 26). His opinion (page 27) at the time was that “natural climate variability might be more important than some scientists thought.” As a result, for his dissertation he chose to study the planet’s natural long-term oscillations (fluctuations that recur every decade, or every few decades). He explains (page 28) that climate scientists in the early 1990’s were still trying to discover “if there was specific evidence for natural long-term oscillations in the climate system that could be competing with–or even masquerading as–apparent anthropogenic (human-caused) climate change.” However, by the mid-1990’s, enough evidence had been accumulated that “there was no longer reason for real scientific debate over the proposition that humans had warmed the planet and changed the climate. . . What scientists were still debating with each other at scientific meetings and in the professional journals was the precise balance of human versus natural causes in the changes observed thus far, and just what further changes might loom in our future (page 17).” The dissertation project he chose in the early 1990’s was smack in the middle of this issue.
Mann extended his thesis work with research partners Ray Bradley and Malcolm Hughes, and together they compiled and analyzed temperature data and proxy data going back about 600 years. They found evidence for even longer term patterns of natural variability, and evidence for the era often referred to as a “Medieval Warm Period.” However, they also determined that during this medieval period, only parts of the planet warmed (parts of Europe, China, western North America and Australia), but other parts of the globe did not experience warming (such as southeastern North America, and the Mediterranean). This weakens the argument that the planet’s current warming is just like the earlier medieval warming.
This conclusion grabbed the attention of the climate change doubt machinery who argue that the warming we’re having now, also happened in the Middle Ages and therefore is not due to industrialization. In addition, Mann and his research partners determined that the medieval period warming was comparable to mid-20th century warming, but not as warm as early 21st century. I was impressed with a graduate student, working in the early and middle part of the 1990’s, who set out to investigate the possibility of natural oscillations being a significant contributor to our current warming, and his research not only led away from his original thinking, but unexpectedly led to a “hockey stick” that destroyed a main myth used by the doubt-manufacturers.
One objection that is sometimes raised regarding climate science projections is that the climate system is incredibly complicated and the models representing it are faulty. Can complex climate models tell us anything if they aren’t perfect? Here’s what Mann says in Chapter 12:
“Critics will always be able to point to something that climate models can’t resolve. But that’s a red herring, the common fallacy that ‘because we don’t know everything, we know nothing.’ As noted previously, climate models had already passed critical tests by the late 1980’s and early 1990’s and since then they had become increasingly realistic in their ability to reproduce key features of the climate system.”
What tests had they passed? Chapter 2 of Mann’s book explains that by the mid-1990’s the models could help investigate causal mechanisms, including humanity’s role in the planet’s current climatic changes:
“Only when human factors were included could the models reproduce the observed warming — both its overall magnitude and, equally important, its geographical pattern over Earth’s surface and its vertical pattern in the atmosphere.”
Holy spewing smokestacks! Our climate models don’t just predict a temperature spread, they anticipate varying amounts of warming for different geographical regions and at different elevations (see Chapter 2, footnote 3)! Mann gave me a window into the types of factors that matter. For example, sulfate aerosols have a cooling effect and their cooling pattern may reveal humanity’s hand: Since natural volcanic sulfate aerosols reach high into the atmosphere, they disburse around the globe cooling everywhere. However, industrial sulfate aerosols stay in the lower atmosphere, don’t spread as far, and thus provide only localized cooling.
Mann explained that some patterns in the predicted warming constitute “a unique fingerprint of what the human influence on climate should look like if the models were correct — and the fingerprint matched.”
In Chapter 2, Mann also told a story demonstrating how Dr. James Hansen took advantage of an unexpected opportunity, and in the process demonstrated, “with flying colors,” how models had become very successful at making predictions.
“James Hansen, in the late 1980’s, successfully predicted the continuing warming that would be observed by the mid-1990’s. Even something the model couldn’t have predicted in advance — the 1991 eruption of Mount Pinatubo in the Philippines — provided yet another key test. As soon as the eruption occurred, Hansen put what was known about the reflective qualities of volcanic sulfur particulates (known as “sulfate aerosols”) into the simulations. The aerosols cooled surface temperatures for several years in the model by shielding the surface from a fraction of incoming sunlight, leading Hansen to make what turned out to be a successful prediction of the temporary cooling that was seen over the ensuing few years.”
In essence, the planet set up an experiment for climate scientists. In the aftermath of the eruption, scientists could study how one unusual change to the climate system affected temperatures, and how well our models predicted the change. Mann told very engaging stories about individual climate scientists and about how good our modern climate models are.
Preventing bias in models
One chapter was a little hard for me to read, but it too told an important story: Chapter 4 basically offers details about the sophisticated statistical methods that climate scientists use to eliminate biases from their data. Statistical tools can also eliminate biases in analyses by identifying important patterns in the data so scientists don’t overlook critical factors. As a non-scientist, I thought statistical techniques just give ranges of expected outcomes, margins of error or probabilities. I was particularly impressed by Mann’s story about how scientists “validate” climate models by comparing model results to historical, measured temperature records. I’m told this is “standard operating procedure,” but as an outsider, it says to me that these models really are top-notch.
Feedback in Models and Accepting Feedback
Some people who accept that humans are causing climate change, nonetheless argue that scientists overstate the impact of positive feedback effects or ignore a variety of negative feedbacks. Chapter 2 of Mann’s book explains that models point to some feedbacks “that are almost certainly positive” (more water vapor, less ice, more ground/ocean surface), and most models also reflect negative feedback effects from low clouds. He asserts that clouds are still a “daunting challenge.”
In Chapter 5, he goes on to list a variety of negative feedbacks that have been hypothesized, but none of them held up under scrutiny. In particular, he describes several important critics who object to conclusions about current warming, he says: “One of the more formidable among them is Richard Lindzen.” Mann describes Lindzen’s impressive credentials and several arguments that Lindzen has raised regarding missing negative feedbacks. Mann also identifies teams of climate researchers who studied Lindzen’s arguments for additional negative feedbacks. They found flaws in some of Lindzen’s analyses, but in a couple of cases researchers suggested that Lindzen’s proposal for a negative feedback may actually have identified a positive effect. It was abundantly clear that feedback, criticisms and suggestions are not ignored.
Other types of objections or questions have also been raised, and climate scientists have dutifully investigated these concerns, too. For example, Mann mentions concerns that records of global temperatures could be overstated due to urban heat islands, and concerns that satellite-based temperature measurements seem to contradict claims of warming. In Chapter 12, Mann describes the researchers who worked on the question of urban heat islands, and the various teams who investigated satellite data contradictions. Both issues were researched thoroughly and disproved. Mann’s recounting of the details gave me an appreciation of how carefully climate scientists listen to their peers, and how seriously they follow through on questions.
I can’t wait for his next book to be released in September — it’s a satirical, comical, graphic look at the twisted logic of climate change denial–it should be an even more engrossing read than the Hockey Stick: The Madhouse Effect: How Climate Change Denial is Threatening Our Planet, Destroying our Politics and Driving Us Crazy by Michael Mann and Tom Toles.