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Making Clear the Science of Climate Change

Judy Weiss
June 18, 2016

Michael Mann Takes It On

by Rabbi Judy Weiss

Mann_HockeyStickClimateWarsWHEN 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 I read several books dealing with the reasons why the public has trouble accepting climate change, how climate change affects agriculture, and carbon pricing policies. Still I had a hard time finding a climate-science book that was neither too dense and technical nor too elementary.

Eventually I found Michael Mann’s The Hockey Stick and The Climate Wars (Columbia University Press, 2012), which presents the material at a level that’s just right.

Mann is a climate scientist at Penn State University. His book gave me insight into what it takes to become a climate scientist, including the long educational path. 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. This is a field, in short, to which part-time dabblers (or even other physicists) are unlikely to contribute meaningful critiques of the work of full-time climate researchers.

Mann’s book interweaves 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.

I found it fascinating to hear about his choice of a dissertation topic. In the early 1990s, he notes, climate scientists could still "legitimately be skeptical about whether the human climate change signal had yet emerged.” His own opinion at the time was that “natural climate variability might be more important than some scientists thought.” As a result, he chose to study for his dissertation the planet’s natural long-term oscillations (fluctuations that recur every decade, or every few decades). He explains that climate scientists were then 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-1990s, 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.” The dissertation project he chose in the early 1990s 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 six hundred 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 weakened the argument that the planet’s current warming is just like that earlier medieval warming. Mann and his research partners also determined that while the degree of medieval warming was comparable to mid-20th century warming, it was exceeded by the warming we're experiencing in the 21st century. Mann's research thus led him away from his original thinking, and unexpectedly led to a “hockey stick” that destroyed a main myth used by the climate-change doubt-manufacturers.

ONE OBJECTION those doubt-manufacturers raise is that the Earth's climate system is incredibly complicated and that models representing it are inevitably faulty. Can complex climate models tell us anything if they aren’t perfect? Mann calls this a "red herring," reflecting "the common fallacy that ‘because we don’t know everything, we know nothing.’. . . [C]limate models had already passed critical tests by the late 1980s and early 1990s and since then they had become increasingly realistic in their ability to reproduce key features of the climate system."

What "critical tests" had they passed? Mann explains that by the mid-1990s, 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! Mann here gives 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 summarizes by saying 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.”

Mann also tells 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.

ONE CHAPTER in Mann's book 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 that scientists don’t overlook critical factors. As a non-scientist, I thought statistical techniques simply 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.

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 another chapter, he goes on to list a variety of negative feedbacks (cooling factors) that have been hypothesized but have not held up under scrutiny. In particular, he describes several important critics who object to conclusions about current warming, including, one "of the more formidable among them . . . Richard Lindzen,” an MIT scientist with impressive credentials who has raised several arguments regarding missing negative feedbacks (that is, cooling factors). Mann notes that teams of climate researchers have found flaws in some of Lindzen’s analyses, and in a couple of cases have suggested that Lindzen’s suggested negative feedbacks may actually have been positive feedbacks.

Other types of objections or questions about global climate change have also been raised, and climate scientists have dutifully investigated these concerns. 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. Mann describes how both issues were researched thoroughly and disproved. His 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 (with Tom Toles) to be released in September -- The Madhouse Effect: How Climate Change Denial is Threatening Our Planet, Destroying our Politics and Driving Us Crazy -- a satirical, comical, graphic look at the twisted logic of climate-change denial.

Rabbi Judy Weiss lives in Brookline, Massachusetts and is a volunteer climate change advocate with Citizens’ Climate Lobby, Boston Jewish Climate Action Network, and the Boston chapter of Elders Climate Action. This article is adapted, with her permission, from the website of Citizens’ Climate Lobby.