One July morning in 2012, climate scientist Michael Mann woke up to a terse email from a fellow scientist.
“Holy crap,” read the message, from Phil Plait, an astronomer and science communicator. “This is truly the most awful thing I’ve ever seen said about a climate scientist. If someone wrote this about me, I’d be calling a lawyer.”
A conservative media outlet and a right-leaning research organization had published commentaries comparing Dr. Mann, then a professor at the Pennsylvania State University, with Jerry Sandusky, the onetime Penn State football coach convicted of sexually assaulting multiple children. The writers claimed that Dr. Mann had created fraudulent graphs, and accused the university of mishandling investigations into both the coach’s crimes and the scientist’s research.
Dr. Mann did indeed call a lawyer. He sued the writers and their publishers for libel and slander. Now, 12 years later — after a pinball journey through the obstacle course of free speech and defamation law — the case is being tried in District of Columbia Superior Court. Only the two writers as individuals are on trial. A verdict is expected as soon as Wednesday.
“For me to be compared to Jerry Sandusky, as the father of a 6-year-old girl, was maybe the worst thing that I’ve ever experienced,” Dr. Mann testified in court on Jan. 24. “I felt like a pariah in my own community.”
The court case has played out over a time period when outright denial of climate science has decreased, but scientists’ integrity has become a bigger target.
“The nature of climate denial has changed,” said Callum Hood, head of research at the advocacy organization the Center for Countering Digital Hate. The group recently published a report analyzing YouTube videos, which found that personal attacks on scientists are now one of the most common types of online content dismissing climate change.
The lawsuit has caught the attention of climate scientists and legal scholars, among others. This trial marks one of very few instances in American courts that a climate scientist has taken the stand to defend their research, according to Michael Gerrard, the faculty director at Columbia University’s Sabin Center for Climate Change Law.
“It’s a rare case where a climate scientist is fighting back against climate deniers,” said Mr. Gerrard, who also is a member of the board of directors for the Climate Science Legal Defense Fund, which previously helped Dr. Mann with a different legal battle.
Because Dr. Mann is legally considered a public figure, he must clear a higher bar than most people would in order to win a defamation lawsuit. He faces the difficult task of proving the authors he sued knowingly lied in their writings. The authors have argued that their posts merely state opinions. Their publishers have also petitioned the Supreme Court, unsuccessfully, to review the case.
Katharine Hayhoe, the chief scientist at The Nature Conservancy and a professor at Texas Tech University, said that Dr. Mann’s case resonates among other climate scientists. “I cannot go one day without being attacked,” she said. “He’s fighting for all of us.”
In court, Dr. Mann is defending his most famous research, which was published in the late 1990s and showed average temperatures in the Northern Hemisphere rising so sharply in recent decades that the graphs resembled the shape of a hockey stick.
The research came under fire in 2009 in an incident known as “Climategate,” when hackers broke into a computer server at the Climatic Research Unit at the University of East Anglia and released thousands of emails between scientists, including Dr. Mann. Skeptics seized on the emails to claim he had manipulated data to exaggerate the hockey-stick graph.
Penn State investigated his research, as did the National Science Foundation, the Department of Commerce and others. All cleared Dr. Mann of misconduct. Both before and after the outcry, other scientists have replicated his findings using different data sources and statistical methods.
The matter seemed settled until 2012, when Mr. Sandusky was convicted and the former director of the Federal Bureau of Investigation published a report that said the administration at Penn State had failed to stop the coach’s criminal actions.
The day after that report’s release, Rand Simberg, at the time an adjunct scholar at Competitive Enterprise Institute, published a blog post on the think tank’s website comparing Dr. Mann to Mr. Sandusky. “Mann could be said to be the Jerry Sandusky of climate science, except that instead of molesting children, he has molested and tortured data in the service of politicized science that could have dire economic consequences,” Mr. Simberg wrote.
A few days later, Mark Steyn, an author and then guest host of conservative radio and television shows, republished part of Mr. Simberg’s post on National Review online. “Michael Mann was the man behind the fraudulent climate-change ‘hockey-stick’ graph, the very ringmaster of the tree-ring circus,” Mr. Steyn added in his own commentary.
In short order, Dr. Mann filed his lawsuit.
The scientific consensus on climate change has been clear for 20 years now. A 2004 paper that reviewed more than 900 scientific studies about climate change didn’t find any that rejected the idea that human activity is producing greenhouse gases that are warming the planet.
But public acceptance of that fact has fluctuated.
In 2008, 71 percent of Americans acknowledged that climate change was happening, according to a long-running biannual survey conducted by the Yale Program on Climate Change Communication and George Mason University. But between 2008 and 2010 — the years before and after Climategate — the portion of Americans accepting climate change fell to 57 percent.
It has since rebounded. A 2023 survey by Yale and George Mason found 72 percent of Americans accepted that climate change is happening.
In recent years, research on climate skepticism, denial and campaigns to delay climate action has also advanced. In 2021, an international group of researchers trained a machine-learning model to sort climate-related claims in 255,000 documents scoured from conservative think-tank websites and popular blogs published over the past 20 years. Included in this data set was Mr. Simberg’s post about Dr. Mann.
The study, published in the journal Scientific Reports, sorted the claims into five broad categories: global warming is not happening; human greenhouse gases are not causing global warming; climate impacts are not bad; climate solutions won’t work; and the climate movement/science is unreliable.
The model labeled the claims in Mr. Simberg’s blog post under the “climate movement/science is unreliable” category, according to an analysis provided by Travis Coan, a computational social scientist at the University of Exeter and an author of the study.
Within this category, scientists are even bigger targets than activists or politicians, said coauthor John Cook, a psychology researcher at the University of Melbourne. Attacks on scientists are “actually one of the most prevalent forms of climate misinformation,” he said.
Claims that “climate solutions don’t work” have also been gaining prominence and now make up more than half of the assertions coming from conservative research organizations, according to his group’s research.
No matter the form, all of these claims share the goal of delaying climate action, Dr. Cook said. “They try to get there through different pathways.”
Building on the 2021 study, the recent report from the Center for Countering Digital Hate used the same methods to analyze 12,000 YouTube videos posted over the past six years. The researchers found that what they call “old denial” — claims that global warming isn’t happening or isn’t caused by humans — now makes up only 30 percent of all dismissive claims, down from 65 percent in 2018. “New denial,” which includes attacks on scientists as well as misinformation about solutions, now makes up 70 percent of these claims, up from 35 percent in 2018.
A spokesman for Competitive Enterprise Institute declined to comment on the trial. Mr. Simberg’s attorney, Mark DeLaquil, said, “We don’t think that this case is really about climate science. We believe it’s about the right of individuals to express their opinions freely, even where they disagree with government reports of the type Dr. Mann claims exonerate him.” An attorney assisting Mr. Steyn, who is representing himself in court, also declined to comment for this article. When asked for a comment, National Review’s editor in chief Rich Lowry pointed to an editorial published at the start of the trial in January.
No matter the outcome, legal experts say this lawsuit is significant not just for climate science, but also for defamation and free-speech law.
“The case sits at the intersection of some of our hardest questions,” said RonNell Andersen Jones, a law professor at the University of Utah. The courts must balance people’s rights to express their opinions freely, while preventing lies that damage people’s reputations, she said.
If Dr. Mann wins, his case would show that “there really is some teeth to defamation law,” said Sonja West, a law professor at the University of Georgia. If he loses, the case could “feed into this greater debate on how very strong our First Amendment rights are.”