Up until a few years ago, Richard Muller was often quoted by sceptics as a credible, high-profile scientist who doubted the consensus on climate change.

Today, he starts his lectures by stating a few things he believes to be facts.

Professor Muller says people need to remain sceptical about how we tackle climate change. ( Getty: Dan Tuffs )

"Al Gore has grossly exaggerated global warming. And if you watch his movie you have more misinformation than information.

"However, global warming is real. It is caused by humans. It is caused by the human emission of greenhouse gases, and I personally feel we have to stop it somehow."

In 2010, Professor Muller from Berkeley University was funded to carry out a comprehensive study by a group of individuals who doubted the Intergovernmental Panel on Climate Change (IPCC) data.

They believed that urban heat islands, data-selection bias, and inaccurate climate models were being glossed over by scientists.

'Don't trust us' approach key to convincing sceptics

Professor Muller and his team — all of whom doubted climate change was happening or that carbon dioxide was its cause — were shocked to find a correlation between carbon dioxide emissions and warming.

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"That was the biggest surprise of all," he said.

"Volcanoes, sunspots, orbital changes, we could all rule out. What we could do is show [that warming] matched the carbon dioxide exceedingly well."

To address what he sees as a lack of transparency in some IPCC reporting, his team made all their data available online.

"The teams that did [the previous studies] said 'trust us'. We said 'don't trust us, here's what we did'. And for that reason I think we were able to win over the sceptics," he said.

However, he said there was still room for scepticism.

"Yes I am a converted sceptic. However, anybody today who is not a sceptic about the solutions being proposed is not thinking them through."

Dr Anthony Purcell, Research Fellow, ANU

"I doubted that the climate could be so dramatically sensitive to small perturbations, especially on a short timescale," Anthony Purcell said of his initial position on climate change.

Dr Purcell, a mathematician and geophysicist, said his view was largely shared by the members of his social circle.

Dr Purcell says his views of climate change have affected some personal relationships. ( Supplied: ANU )

"Pretty much everybody in my personal life was either agnostic or deeply hostile to the concept of anthropogenic climate change," he said.

"[My father is] a deeply entrenched climate denialist."

In 2010, he read an article in The Australian newspaper in which the distinguished scientist Professor Frank Fenner predicted that "Homo sapiens will become extinct, perhaps within 100 years" due to climate change.

Dr Purcell said that statement shook him.

"This was a responsible scientist making a public declaration and it seemed to me to raise alarm bells," he said.

"I hadn't been aware that this was even something that might be considered as a possible outcome."

Personal relationships 'severely eroded' by change of view

A deep-sea core sample, containing layers of sediment laid down over millions of years, demonstrated first-hand that Professor Fenner's prediction had a historic precedent.

One of the layers in the core sample was around 55-million-years old, from what is known as the Palaeo-Eocene thermal maximum (PETM).

Core samples taken from a diamond drill show sediment as it was laid down. ( Getty: Mo Morad. )

"All of a sudden that [carbon-rich material in the sediment] vanishes and it's just this dingy mud with no evidence of biological activity at all," he said.

"That was a response to a four-degree temperature increase that occurred over 20,000 years. We're looking at something that's a hundred-times faster than that."

Dr Purcell said his acceptance of the climate science had a significant personal toll.

"It's deeply impacted my relationship with my father," he said.

"It's severely eroded our relationship and our ability to communicate."

Professor David Karoly, University of Melbourne, Atmospheric Science

Professor Karoly initially set out to disprove climate change. ( Supplied: David Karoly )

David Karoly is one of the world's pre-eminent climate scientists, and advises the Australian Government through the Climate Change Authority (CCA).

But back in 1986, he believed he had a way to prove climate change had nothing to do with carbon emissions.

"I was aware from theoretical considerations that we would expect a specific pattern of temperature changes associated with increasing greenhouse gases," he said.

"Which is warming in the lower atmosphere and cooling in the stratosphere at heights above 15 kilometres.

"And I thought I would use that signature to show that in fact what was actually occurring in the data was due to natural variability like El Nino or increasing sunlight, and not due to human-caused climate change."

Sorry, this video has expired Climate Change Authority's Professor David Karoly speaks with ABC News Breakfast

Professor Karoly submitted an abstract of his forthcoming research to a climate-change conference in Melbourne in 1986. He expected his findings to show that there was no carbon-driven-warming signature.

"The more I looked, the more I found this signature of warming in the lower atmosphere and cooling in the upper atmosphere," he said.

By the time he came to present his paper, he had to switch his position in support of anthropogenic climate change.

But that was the '80s and climate science had yet to be deeply politicised.

He says changing his view on climate change was not a big deal at the time.

"I was coming into it as what I believe all scientists are — sceptical and wanting to test a hypothesis — and that's what I did."