This month, climate scientist Dr James Hansen retired as head of Nasa's Goddard Institute of Space Studies. Yet at age 72, instead of aiming for a leisurely life after work, Hansen plans to better focus his time on climate science, along with communicating the science and its policy implications.
It's an odd choice for a government scientist. But then, Hansen has long been a maverick, with a career that has taken him from investigating Venus' atmosphere, through studying and expressing concern about climate change, to being arrested for demonstrating against fossil fuel extraction and use.
In the late 1960s, Hansen received a Nasa graduate traineeship, leading to him spend more than 10 years studying Venus, which he proposed had an extremely hot surface due to its atmosphere trapping the sun's heat. He became involved in the Pioneer mission, but resigned to study the earth because, as he later wrote: "The composition of the atmosphere of our home planet was changing before our eyes… Surely that would affect the earth's climate."
By then, Hansen knew that carbon dioxide determined the climate on Mars and Venus. During another decade, he and a relatively small band of climate scientists showed that it was important here on Earth too, and rising emissions were likely causing rising global temperatures. In 1988, Hansen testified to a US senate committee, declaring that "It is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here".
This marked a shift from scientists being tentative regarding the issue. The next day, a New York Times headline announced "Global warming has begun".
Hansen's confidence was based on studying the earth's energy balance, and "forcings" that disturbed the balance. There were nine major forcings in recent years. Two were entirely natural: fluctuations in solar brightness, and volcanic activity. The rest were human-induced, and included land-cover changes, aerosols that scatter and absorb sunlight, carbon dioxide, and other greenhouse gases.
There was already evidence from the earth's past that even small changes in forcings can, over time, lead to remarkably large effects. Small tilts in the earth's axis and shifts in its orbit could determine whether ice ages begin or end, with the sustained changes in sunlight leading to feedback mechanisms such as carbon dioxide levels rising and causing more warming, in turn resulting in more carbon dioxide being released.
As Hansen headed to a meeting of the cabinet-level Climate Task Force in 2001, it was clear to him that the human-induced forcings added in the past several decades already dwarfed the natural forcings since 1750 - a period that included the latter part of the Little Ice Age. He showed a Christmas tree bulb during his presentation, and explained that the net effect was equivalent to having two of these burning day and night over every square metre of the earth. While the effect is small on a day-to-day basis, the forcing is huge over decades and centuries.
Even with the Christmas tree bulb, task-force members found the climate change science tough to follow. There was no agreement on an alternative scenario proposed by Hansen and colleagues, which would focus on reducing pollution. A second meeting was held, this time with a presentation by MIT professor Richard Lindzen, since described by Hansen as the "Dean of global warming contrarians". Lindzen said that there was uncertainty regarding global warming, and that some said some scientists made "alarmist" statements. Task-force members were confused.
But confusion about climate change is hardly confined to a few task-force members. In his book Storms of My Grandchildren, Hansen wrote "the undue influence of special interests and government greenwash pose formidable barriers to a well-informed general public". In 2004 and 2005, Hansen faced high-level censorship regarding climate change. A subsequent report found that Nasa's public affairs office reduced, marginalised or mischaracterised climate change science made available to the general public.
Hansen refused to be muzzled. Partly driven by thoughts of his grandchildren and the intergenerational inequity of human-made climate change, he has increasingly strived to make the implications clear for the public, especially young people.
According to Hansen, humanity is at a fork in the road. One potential path for the future has swift declines in fossil fuel emissions. The other is more or less business as usual, which is expected to lead to amplifying feedbacks, and climate change spinning out of control.
Earth's history indicates some of the potential consequences for the business as usual approach. Sea level has been unusually stable for the past 7,000 years, which probably contributed to the development of civilisation.
Yet the sea level can rise by several metres per century as ice sheets melt and collapse. Amplifying feedbacks can include release of methyl hydrates that are frozen on the seabed to discharge methane, a far more potent greenhouse gas than carbon dioxide.
Hansen believes that such changes will ultimately make parts of earth uninhabitable for humans. But even the short-term effects will have major impacts, such as the storms he believes will characterise this century's climate. Hansen writes of "climate dice" that are now increasingly loaded towards more severe storms, and more extreme heat waves.
Though Hansen is sometimes dubbed "the godfather of global warming", there are people who believe climate change is a threat yet are wary of his advocacy and outspokenness. Some argue against his belief that we need a rising fee on fossil fuels, with dividends returned to people rather than boosting government coffers.
But, to many people he is a hero. "James Hansen is a powerful voice of conscience," wrote Sierra Club executive director Michael Brune in a letter to the New York Times last month. "He deserves our respect for being one of the first to stand up in the early days of climate disruption awareness and demand that our leaders take action."
Martin Williams is a Hong Kong-based photographer and writer specialising in conservation and the environment. He holds a PhD in physical chemistry from Cambridge University.