Climate scientist Michael Mann says he has received hundreds of them -- threatening e-mails and phone calls calling him a criminal, a communist or worse.
"6 feet under, with the roots, is were you should be," one e-mail reads. "How know 1 one has been the livin p*ss out of you yet, i was hopin i would see the news that you commited suicide, Do it."
"I've been called just about everything in the book," Mann, who runs of the Earth System Science Center at Penn State University, told ABC News. "It's an attempt to chill the discourse, and I think that's what's most disconcerting."
Mann is not the only one. The FBI says it's seeing an uptick in threatening communications to climate scientists. Recently, a white supremacist website posted Mann's picture alongside several of his colleagues with the word "Jew" next to each image.
One climate scientist, who did not wish to be identified, told ABC News he's had a dead animal left on his doorstep, and now sometimes travels with bodyguards.
"Human-caused climate change is a reality," Mann said. "There are clearly some who find that message inconvenient, and unfortunately they appear willing to turn to just about any tactics to try to suppress that message."
Scientists See Political Campaign by Senator
Many climate scientists, however, say the most disturbing recent example of what they call intimidation is not anonymous hate mail.
Rather, they point to a governmental report released in February by Sen. James Inhofe, R-Okla., one of the most vocal climate skeptics in office, which names 17 climate scientists and argues some of them may have engaged in "potentially criminal behavior."
Inhofe's report referred to an incident late last year known as "Climate Gate," in which e-mails hacked from computers at the University of East Anglia in Britain gave the impression some climate scientists may have been trying to hide flaws in their research. Several subsequent investigations have exonerated the scientists' work.
One section of Inhofe's report outlined the laws the scientists may have violated, including the Federal False Statements Act, which the report noted could be punishable with imprisonment of "not more than five years."
Climate Scientist: 'Modern-Day McCarthyism'
"It's reminiscent of other periods in American history," Gavin Schmidt, a NASA climatologist named in Inhofe's report, told ABC News. "People were smeared not on the basis of anything they did but just by powerful people seeming to ... insinuate that they've somehow done something wrong."
"Some of the attacks that are being made against climate scientists smack of modern-day McCarthyism," he said.
Inhofe: 'The Public Needs to Know'
Inhofe refused to grant ABC News an interview, citing the network's previous coverage of climate change. But in a statement, he said: "One of the most basic principles of good science is openness and transparency. ... As the climate e-mail controversy revealed, it appears some of the taxpayer-funded science used to advance cap-and-trade legislation is being hidden from public view. ... The public needs to know whether the research they fund is reliable, objective, and easily accessible."
Marc Morano, who used to be Inhofe's spokesman and is now one of the most vocal climate skeptics in the country, told ABC News that the skeptics have been getting threats for years.
"No one is advocating violence," Morano said, "but it is refreshing to see these scientists hear from the public. When you go to a used-car salesman and you get conned ... you don't go back to the used-car dealer all happy and pleasant. You have a lot of anger, and that's what these scientists are appropriately feeling."
Gavin Schmidt of NASA dismissed the idea that climate change is a hoax.
"It's nonsense," Schmidt said. "Scientists are hyper competitive. ... The idea that you could fool all of these people or get them to just buy into some global conspiracy, is laughable."
Many scientists say the harassment they endure stifles scientific discovery and progress.
"Good scientists just go where the evidence takes them," said Peter Gleick, a hydro-climatologist and president of the Pacific Institute. "The idea that they would be limited by politicians in what they can evaluate or study or say, is anathema to them."
Scientists Issue Public Letter
Gleick, along with 254 other members of the National Academy of Sciences, recently signed a public letter calling for an end to the intimidation of climate scientists.
"Many recent assaults on climate science, and, more disturbingly, on climate scientists by climate change deniers, are typically driven by special interests or dogma, not by an honest effort to provide an alternative theory," read the letter, which was published in Science magazine earlier this month.
"We ... call for an end to McCarthy-like threats of criminal prosecution against our colleagues based on innuendo and guilt by association, the harassment of scientists by politicians seeking distraction to avoid taking action, and the outright lies being spread about them," the letter continued.
Gavin Schmidt of NASA said attacks are meant to keep scientists from speaking out about climate change in public.
"They're trying to dissuade people from being involved in the public discussion about what this means and what we can do to prevent it," he said. "I think people who care about the integrity of science in this country should find that worrying."