As the covert deception and climate change denial programs gained traction in the late 1990s, they started to focus their attention on the scientists who were the most outspoken in the defence of mainstream climate science–where the evidence that anthropogenic emissions of carbon dioxide was causing global warming and climate change was becoming increasingly persuasive.
Michael Mann is Distinguished Professor of Atmospheric Science at Pennsylvania State University and director of the University’ Earth System Science Center. He is well known for his work on climate change and for his outreach work speaking about climate science.
In 1998 and 1999, Mann, along with colleagues Malcolm Hughes and Raymond Bradley, published studies that reconstructed the average temperature of the northern hemisphere over last 1000 years. The graph was reprinted in the IPCC Third Assessment Report and received widespread attention. It was dubbed the ‘hockey stick’ curve because of its distinctive shape.
The graph became an iconic image in the global warming and climate change debate because it showed that rapid global warming in the northern hemisphere had started sometime around 1900—and that this phenomenon was not only continuing, but that temperatures were now higher than at any time in the last 1000 years. The hockey stick graph generated a significant amount of controversy—with climate contrarians repeatedly attempting to refute the validity of the analysis.
Although the data and statistical analysis have been challenged, the findings have been reaffirmed by several high-level and reputable reviews including one by the National Academy of Sciences in 2006. 
Inevitably, it wasn’t long before Michael Mann became a personal target of the anti-climate science movement.
In 2005, Congressman Joe Barton of Texas demanded that Mann hand over detailed information about the sources of his research support, and the source and location of all his data—even though the scientific results and all the data had been published in peer-reviewed journals and was in the public domain.
At the time, Barton stated that the idea that we must take action to slow global warming is “absolute nonsense.” The Washington Post said Barton was “hunting witches.” Mann responded to Barton, saying the data he wanted had been available in the public domain for years, and could be found in a public archive. Many US politicians objected to Barton’s demand including Republicans Sherwood Boehlert and John McCain.
In the lead-up to the 2009 United Nations Copenhagen Climate Change Conference, conflict between the anti-science movement and climate scientists arose when email exchanges between researchers at the University of East Anglia Climatic Research Unit and other climate scientists (including Mann) were hacked. The scientists were accused of falsification, manipulation and suppressing key data. However, eight subsequent independent investigations cleared the scientists, finding no evidence of fraud or scientific misconduct.
As a result of the hacking, Mann was subjected to personal attacks: he was called a liar, a charlatan, and a scumbag. He and his family received thinly veiled death threats. In 2010, he was sent a letter laced with white powder and an email that said, “You and your colleagues who have promoted this scandal ought to be shot, quartered, and fed to the pigs along with your whole damn families.” The white powder turned out to be cornstarch.
“Those behind this campaign to discredit the case for climate action have long recognized that one very effective means of attacking the science is to vilify individual scientists, to make examples of us, so that other scientists are afraid to speak out,” said Mann. “I think it’s a very carefully crafted means of attack.”
In 2010, Ken Cuccinelli, Virginia’s attorney general, still searching for evidence of fraud, used a civil investigative demand under the state’s Fraud Against Taxpayers Act (generally used to prosecute Medicaid fraud) to demand every email, record, or document involving Mann from 1999 to 2005, when he was on the faculty at the University of Virginia. An editorial in the journal Nature strongly objected saying: “Given the lack of evidence of wrongdoing, it’s hard to see Cuccinelli’s subpoena… as anything more than an ideologically motivated inquisition that harasses and intimidates scientists.” The journal was right.
State courts ultimately considered Cuccinelli’s efforts an abuse of power, with the Virginia Supreme Court rejecting the demands. But when the court rejected Cuccinelli’s demands, the American Tradition Institute, now known as the Energy and Environmental Legal Institute (E&E Legal), a Koch Brothers-funded group, attempted to use state open records laws to demand the same emails.
The case went all the way to the Virginia Supreme Court, which ultimately rejected E&E Legal’s efforts as being in violation of the protections afforded academics in furtherance of academic freedom.
The attacks on Michael Mann were clearly intended to harass and vex him and his family to the point where he would perhaps eventually withdraw from working on climate science and switch to a less stressful area of research.  This has not happened.
For more background:
 See the IPCCC 3rd Assessment Report (TAR) at: //www.ipcc.ch/site/assets/uploads/2018/05/SYR_TAR_full_report.pdf . The graph is shown on page 174.
 Check out Michael Mann’s book: The hockey stick and the climate wars: Dispatches from the front lines . At //www.michaelmann.net/books/hockey-stick
 Environmental Defense Fund. Perspectives of scientists who become targets: Michael Mann. At //climatesciencedefensefund.org/2017/07/20/perspectives-of-scientists-who-become-targets-michael-mann/
 Union of Concerned Scientists : How the fossil fuel industry harassed climate scientist Michael Mann. Accessed at //www.ucsusa.org/how-fossil-fuel-industry-hrassed-climate-scientist-michael-mann#.W8Y782hKjIU