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"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."