The climate scientist Michael Mann, who has been under relentless attack from sceptics since the exposure of emails at East Anglia's Climatic Research Unit, was cleared of research misconduct by a university investigation yesterday.

The four-month internal investigation by Pennsylvania State University found no evidence for charges made by climate sceptics that Mann had violated university ethics. The committee cleared Mann of the much more serious charges of falsifying and manipulating data last February.

The climate scientist is the author of the iconic "hockey stick" graph, showing a recent and rapid rise in the earth's temperature. The graphic depiction of global warming made Mann a prime target of those who deny the existence of climate change.

But the campaign against Mann escalated to new heights after emails taken from the Climatic Research Unit's server showed discussion over his use of a statistical "trick" to "hide the decline". Climate sceptics accused Mann of science fraud.

In its announcement yesterday, the university noted Mann's "outstanding" work was widely recognised in science circles – discounting accusations of misconduct. However, it chided him for circulating the unpublished work of other researchers without their consent.

Mann told Climate Science Watch that the decision was a vindication for scientists, who have been under attack since the release of thousands of emails from East Anglia last November.

"It's become clear that all of the claims that they had made originally, about the stolen CRU emails, are incorrect and do not stand up to scrutiny," he said. "There is no evidence of any impropriety on the part of the scientists. There's no indication of the fudging of data. There's no indication of any of the things they claim that these emails showed. And every investigation that's been done thus far has concluded that."

Mann added: "They have fundamentally failed in their effort to prove that climate change is a grand hoax."

His personal battle will go on however. Virginia's Republican attorney general is trying to investigate Mann for defrauding government research grants while he was at the University of Virginia.