By Matt McGrath Environment correspondent, BBC News One of the leading voices on the science of global warming is to retire from Nasa this week to be more active in the fight against fossil fuels.
Dr James E Hansen has been the head of the Goddard Institute for Space Research since 1981.
He is sometimes called the "father of global warming" for his early warnings about the impacts of rising levels of greenhouse gases.
But some critics say he has hampered the cause by overstating the risk.
In the 1970s, Dr Hansen focused on studies and computer simulations of the Earth's climate for the purpose of understanding the impact of humans.
He quickly became convinced that there was a clear link between increases in concentrations of carbon dioxide and rising temperatures. His work helped identify the ways in which the planet might respond to greater warming.
Sounding the alarmIn 1981, he published the first Goddard Institute Surface Temperature Analysis. In contrast to other models, this showed a rise on global temperatures between the 1880s and the 1970s.
In 1988, Dr Hansen testified before the US Congress, arguing that human induced climate change had begun.
"It is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here," he told reporters after that appearance.
Over the past 20 years, Dr Hansen's research output has continued to reinforce this theme, and he has become the pre-eminent global voice arguing in favour of restrictions on emissions of carbon dioxide. He was listed as one of Time Magazine's most influential people in 2006 and has won a series of awards for his work.
His role as an employee of the federal government proved contentious as his activism increased. In 2006, he told the New York Times that he was being censored by the Bush administration. Nasa officials eventually issued a statement underlining the right of scientists to speak out without restriction.
Dr Hansen's pronouncements were sometimes seen as extreme. In his 2009 book, Storms of my Grandchildren, he argues that the Earth could one day become like the planet Venus, where temperatures are warm enough to melt lead.
"I've come to conclude that if we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty," he wrote.
Dr Hansen has also argued that the executives of oil companies should be tried for crimes against humanity. Critics suggested that these type of remarks did little to convince sceptics. Even environmentalists sometimes objected to the nature and detail of his comments.
Temperature standstillIn January this year he acknowledged that the temperature rises predicted by global warming were at a standstill. He suggested that rather than being proof that global warming had stopped, as some sceptics had claimed, it was due to a combination of El Nino conditions and increased amounts of what are called aerosols.
"We conclude that background global warming is continuing," he said.
Dr Hansen says he wants to use his retirement to speak and give evidence in ways that he couldn't do as a government employee. He plans to take a more active role in lawsuits challenging both federal and state governments on issues such as the use of oil extracted from tar sands in Alberta, Canada. He wants to lobby European leaders to impose higher taxes on this type of oil as well.
"These plans should be vigorously resisted. We are already in a deep hole - it is time to stop digging."
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