Less than a week before U.N. negotiators convene in South Africa for a new round of talks aimed at forging a global climate pact, a hacker has released an apparent second round of e-mails from the University of East Anglia in Britain, seeking to portray climate scientists in a negative light.

The e-mail exchanges, which appear to have been pulled from the same set of pirated electronic files taken from servers at the university’s Climatic Research Unit more than two years ago, do not contain any new revelations about research linking human activity to global warming. But the release highlights the ongoing conflict between some bloggers and climate-change skeptics who challenge this scientific consensus, and those who support it.

In 2009, an anonymous hacker posted more than 1,000 e-mails from the University of East Anglia on the Web, sparking a controversy dubbed “Climate-gate” by some media outlets and prompting many conservatives in the United States and elsewhere to question whether human activity induces global warming. British police have investigated the ­e-mail piracy but have yet to identify who was behind it.

Those e-mails painted the scientific climate establishment as combative and clubby, but a half-dozen investigations in the United States and Britain found no evidence that the scientists had manipulated data, as critics alleged.

This second batch deals less with climate science than with how some prominent scientists framed the issue and recruited colleagues to serve on panels such as the U.N. Intergovernmental Panel on Climate Change (IPCC).

In a statement Tuesday, the University of East Anglia said that “we have no evidence of a recent breach of our systems” and that it could not confirm yet whether the 5,000 new e-mails are genuine because of “the sheer volume of material.”

But it added that, if authentic, “these emails have the appearance of having been held back after the theft of data and emails in 2009 to be released at a time designed to cause maximum disruption to the imminent international climate talks. This appears to be a carefully-timed attempt to reignite controversy over the science behind climate change when that science has been vindicated by three separate independent inquiries and [a] number of studies — including, most recently, the Berkeley Earth Surface Temperature group.”

Jonathan Overpeck, who co-directs the Institute of the Environment at the University of Arizona and whose comments are quoted in the recent batch of e-mails, said this round “seems to be more of the same, and the conclusion will be the same, which is nothing was done wrong.” He added that when he and other researchers were writing e-mails several years ago, “no one was thinking, ‘What if these get stolen and get taken out of context — how will they sound?’ ”

In one set of e-mails from 2005, researchers discuss whether an early draft of a 2007 IPCC report accurately depicted the temperature rise in the lower atmosphere. An official from the British Met Office, a scientific organization that analyzes the climate, writes to the Climate Research Unit’s then-director, Phil Jones: “Observations do not show rising temperatures throughout the tropical troposphere unless you accept one single study and approach and discount a wealth of others. This is just downright dangerous. We need to communicate the uncertainty and be honest. Phil, hopefully we can find time to discuss these further if necessary.”

Later, the official adds, “I also think the science is being manipulated to put a political spin on it which for all our sakes might not be too clever in the long run.”

Another missive, written by Stephan Singer, who heads the European climate and energy policy team for the advocacy group WWF in Brussels, said that when it came to publicizing the state of climate science, “we as an NGO working on climate policy need such a document pretty soon for the public and for informed decision makers in order to get a) a debate started and b) in order to get into the media the context between climate extremes/desasters[sic]/costs and finally the link between weather extremes and energy.”

Singer could not be reached for comment. WWF’s U.S. managing director of climate change, Lou Leonard, wrote in an e-mail: “We would be best served by avoiding the rehashing of old personal attacks on scientists and instead focusing on tackling climate change at the United Nations climate talks next week. In Durban, we have an opportunity to address the real and growing impacts of climate change and ensure that people around the world all have access to ample food supplies, clean drinking water and sustainable sources of energy for years to come.

Some e-mails appear to show scientists’ push to close ranks. In one, Jones writes about enlisting reliable researchers to join the team writing the IPCC’s summary report.

“Getting people we know and trust is vital,” he writes, referring to a group charged with studying tornadoes. Jones was not immediately available for comment.

Michael E. Mann, who directs the Earth System Science Center at Penn State University and had some of his e-mails publicized in the release two years ago, called the new documents “a rehash” of stolen correspondence.In a phone interview, he noted that a probe by the National Science Foundation’s inspector general had cleared him and other researchers of charges of academic malpractice.

Mann has successfully fought the mass release of e-mails that he and more than three dozen climate researchers wrote to one another when he taught at the University of Virginia between 1999 and 2005. Both Virginia Attorney General Ken Cuccinelli II and the American Tradition Institute, a conservative group, have sought the e-mails in court.

Marc Morano, a prominent climate-change skeptic and publisher of the Web site Climate Depot, welcomed the e-mails’ release. “The new emails further expose the upper echelon of the U.N. IPCC as being more interested in crafting a careful narrative than following the evidence,” he wrote.

Rep. F. James Sensenbrenner Jr. (R-Wis.), vice chairman of the House science committee, said that the news “further highlights the need for an unbiased, comprehensive investigation of ‘Climategate.’ As I said after the first round of IPCC emails were released, we need to be sure that our policy decisions — decisions which would impact the American economy and cost hundreds of thousands of jobs — are based on sound evidence.”