WALNUT CREEK, Calif. — A few years ago, Ben Santer, a climate scientist with Lawrence Livermore Laboratory in Washington, answered a 10 p.m. doorbell ring at his home. After opening the door, he found a dead rat on the doorstep and a man in a yellow Hummer speeding away and shouting curses.

Santer shared this story recently before a congressional committee examining the increasing harassment of climate scientists, and the state of climate science.

After the online posting in November of 1,073 stolen e-mails from climate scientists, including some from Santer, the threats took a more ominous turn, Santer told members of the Select Committee for Energy Independence and Global Warming, led by Rep. Edward Markey, D-Mass. Skeptics of climate change have dubbed the e-mail incident “Climategate.”

“The nature of these e-mail threats has been of more concern,” Santer said. “I’ve worried about the security and safety of my family.”

In the heated debate over global warming’s cause and to a diminishing extent its existence, the stolen e-mails fueled the rhetoric. And though skeptics of human-induced climate change have tried to use the e-mails to discredit established science and to derail policies such as the regulation of carbon dioxide emissions or cap-and-trade initiatives, scientists are fighting back.

They penned a consensus letter in May, testified before congressional committees to explain why they’re certain human activity is warming the world, and they’re trumpeting what they say is the growing harassment of climate change researchers.

In the written version of his testimony, Santer mentioned concerns “about my own physical safety when I give public lectures.”

Santer is accompanied by bodyguards at some conferences, Stephen Schneider, a prominent climate scientist with Stanford University, said in May. Santer and the lab declined to discuss details about security for him, saying it would be inappropriate to do so. Schneider, who testified at the congressional hearing, told the committee he has a history of fielding abusive e-mails. A typical one, he said, accuses him of being a “Communist dupe for the United Nations,” and states, “You’re a traitor and should be hung.'”

The threats escalated after publication of the hacked e-mails from the University of East Anglia in England. On blogs, talk shows and other forums, people heatedly discussed the content of certain e-mails, and Sen. Jim Inhofe, R-Okla., has requested a criminal investigation of 17 climate scientists, including Santer and Schneider, whose e-mails were among those stolen. Inhofe believes human-induced global warming is a hoax and that there is no scientific consensus.

Rep. James Sensenbrenner, R-Wis., also wrote the head of the Intergovernmental Panel on Climate Change, which produces reports widely regarded as the most authoritative assessments on climate change, requesting the 17 scientists be banned from contributing to the panel’s next report.

Those hacked e-mails revealed some climate scientists involved in a pattern of stonewalling, discussing ways to conceal data that didn’t agree with their findings, and deriding skeptics of global warming. In one e-mail, Santer wrote when he next encountered a certain climate skeptic at a scientific meeting, “I’ll be tempted to beat the crap out of him. Very tempted.”

In an interview with the Associated Press about the e-mail, Santer said, “I’m not surprised that things are said in the heat of the moment between professional colleagues. These things are taken out of context.”

Two independent investigations by British academic panel found no evidence of fraud or scientific misconduct in the e-mails’ contents, nor did an Associated Press analysis of all the e-mails.

The hacked e-mail incident was followed by the discovery of several embarrassing errors in the 2007 Intergovernmental Panel on Climate Change report. In light of the errors, a review of the intergovernmental panel report began in May in Amsterdamby a 12-person panel selected by the Inter-Academy Council. The council is independent of the United Nations, which publishes the intergovernmental panel report.

Schneider and other climate scientists note only a handful of errors were found, and the report’s conclusion is solid that human activity is very likely the reason for the rise in average global temperatures since the mid-20th century.

While four scientists at the Washington, D.C., hearing detailed why most climate scientists fear the ecological and economic consequences of a buildup of greenhouse gases, a fifth scientist offered a counterpoint.

William Happer, a physics professor at Princeton University, expressed far less concern about the heat-trapping threat of carbon dioxide from human activity and said scientists on the other side of the debate also face intimidation.

Happer called for creation of a “B-team” of scientists given funding to investigate other possibilities besides human-caused warming. The Department of Defense, the CIA and “many others routinely establish robust team B’s; that is, groups of experts who work full time, sometimes for several years, to challenge the establishment position,” Happer said. “This has given us much better weapons systems and intelligence.”

Happer said he believes increased carbon dioxide levels may cause only an inconsequential rise in temperatures and that plant life will flourish with more atmospheric carbon dioxide. He concurred when one congressman asked if he was in a “minority position” among scientists..

Ralph Cicerone, president of the National Academy of Sciences, detailed the science behind most climate scientists’ views. Cicerone described a 1 degree Fahrenheit rise since 1979 documented by NASA and other agencies. Declassified U.S. Navy and satellite data show Arctic ice sheet thickness has declined 50 percent in 50 years, he said, and sea levels are now rising 3.2 millimeters per year. He said the average ocean surface temperature “has increased significantly since 1980,” which scientists say lead to more extreme weather events.

“The year 2009 was the warmest on record for the entire world south of the equator,” he added.