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."
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."
"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 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."