In an unusually strong attack on politically powerful deniers of global warming, 255 members of the National Academy of Sciences, including 32 from Northern California, have charged that opponents are using "McCarthy-like tactics" against legitimate climate scientists.

The letter condemning "political assaults" on climate researchers was published Friday in the journal Science, and was sent earlier to the White House Office of Science and Technology, where John Holdren, its director, is President Obama's science adviser.

Members of the Academy of Sciences, who are frequently called upon to advise the federal government and its agencies on scientific questions, normally debate controversial issues sedately. But with the global warming debate becoming increasingly politically charged, the scientists struck back.

"We are deeply disturbed by the recent escalation of political assaults on scientists in general and on climate scientists in particular," they said in their letter. "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 distractions to avoid taking action and the outright lies being spread about them."

The reference was directed at Sen. James M. Inhofe, R-Okla., who scoffs at most climate change data as a "hoax" and has threatened a criminal investigation of an international climate science team whose e-mail exchanges were hacked by opponents. Critics accused the team of manipulating and hiding data regarding climate change. The scientists were later cleared.

The Heartland Institute, a conservative public policy think tank, brands climate scientists as "global warming alarmists," and a former director of the National Hurricane Center, who is a strong skeptic, has declared "it is high time to question the true agenda of the (climate) scientists" - clearly suggesting that their "agenda" is political, not scientific, and far to the left.

The letter in Science insisted climate change is real.

"There is compelling, comprehensive and consistent objective evidence that humans are changing the climate in ways that threaten our societies and the ecosystem on which we depend," the scientists' letter declared.

The lead signer of the letter was Peter Gleick, director of the Pacific Institute for Studies in Development, Environment, and Security in Oakland. Others from Northern California are at Stanford, UCSF, UC Berkeley and UC Davis.

An editorial in the same issue of the journal also warned that the debate over global warming has become dangerously polarized and noted that several politically inspired lawsuits against federal regulation of greenhouse gas emissions have claimed that "climate change is a conspiracy."

"The debate has become polarized," warned the editorial, and as a result "the scientific enterprise and the whole of society are in danger of losing their crucial rational relationship."