James Hansen, director of the NASA Goddard Institute for Space Studies and an Adjunct Professor at Columbia University's Earth Institute, admits he'd prefer to spend his time out of the public eye, building on the decades of scientific research that has won him a reputation as the world's top climate scientist.
For many years, he refused media interviews and avoided public comment on climate change policy.
But he explained to a packed public meeting at Sydney's Seymour theatre on March 8 that the birth of his first grandchild in 2001 led him to change his mind. "I did not want my grandchildren, someday in the future, to look back and say, 'Opa understood what was happening, but he did not make it clear'."
He told the meeting: "I realised that if the scientists don't try to connect the dots from the climate science all the way to the end policy, then it's the special interests that connect the final dots instead."
Hansen's recent book, Storms of my Grandchildren, is one important result of his decision to speak out.
The book combines a thorough explanation of what is and isn't known about global warming with an impassioned plea for ordinary people to get active before it is too late.
And he says without hesitation that "too late" is very soon.
The ongoing use of coal, oil and gas for energy, and fossil-fuel use in agriculture and in other industrial processes is the most important cause of dangerous climate change, he told the meeting.
Hansen also said other "climate forcings" — such as changes in the sun's intensity and the climatic effects of volcanic eruptions — are measured very accurately. No evidence suggests the Earth's recent warming can be put down to factors other than human activity.
Two hundred years of treating the atmosphere like a carbon dump has caused the Earth's average temperature to rise by about 0.8°C. Yet even if all emissions were to miraculously cease tomorrow, a similar amount of extra warming is already in the pipeline.
In his book Hansen explained: "The effects of climate change have been limited in the near term because of climate system inertia, but inertia is not a true friend. As amplifying feedbacks begin to drive the climate towards tipping points, that inertia makes it harder to reverse direction."
His warning is that if business-as-usual pollution continues, it will soon trigger climate feedbacks that would lead to catastrophic, runaway climate change.
There is too much carbon in the atmosphere, he told the Sydney meeting. To preserve a safe climate, we must aim to reduce the level of atmospheric carbon dioxide to below 350 parts per million. Today, the level is about 390ppm and rising. He said the most important immediate step is to end the burning of coal for energy.
Years of advising the US government to change course has also led Hansen to draw new conclusions about politics. He said the mainstream politicians have not acted on climate change because they are too compromised by the powerful fossil fuel lobby.
At first, he presumed politics would shift once the scientific knowledge of climate change was put out. He now says his early confidence was "disastrously wrong" for two reasons.
"First, special interests were remarkably successful in preventing the public at large from understanding the situation", he wrote in his book. "The result was a growing gap between what was understood by the relevant scientific community about human-caused climate change and what was appreciated by the public.
"Second, it had become clear that greenwash was a near universal response of politicians to the climate change issue."
Hansen also strongly criticised carbon offsets, "clean coal" and emissions trading schemes as solutions to the crisis. At the Sydney meeting, he outlined his proposal for a carbon tax as a policy alternative.
A strong point in his plan is that the tax would be levelled at the point of pollution, not on individual consumers. Power stations, mines, steel and aluminium plants and other big emitters would pay the tax.
He said a tax of about $115 per tonne of carbon dioxide, levelled on a national basis, would be enough to prompt a rapid phase-out of fossil fuel use and make carbon-free energy alternatives far more attractive.
Hansen said the funds raised by the tax should be returned directly to the population — a monthly "green cheque" to compensate for price rises. He estimated this would leave most people better off financially.
A big plus with Hansen's carbon tax is its simplicity. Unlike highly complex trading schemes, such as PM Kevin Rudd's proposed Carbon Pollution Reduction Scheme, a direct tax on big polluters leaves no room for traders and speculators to manipulate a carbon market to their own advantage.
Also, given the climate crisis, a high price for carbon is far better than a low price — as long as steps are taken to ensure the tax is not simply passed on to consumers.
However, is it really a good idea to give all the proceeds of a carbon tax back to the population?
Couldn't a carbon tax be designed to also fund the transition to a low-carbon economy? After all, to phase out fossil fuels quickly, we need to have the renewable infrastructure ready to replace it. Shouldn't a carbon tax then make the big polluters pay for this?
From the audience, I put these questions to Hansen at the March 8 meeting. His response was unambiguous — no.
He gave two reasons. First, we can't trust governments to spend the money wisely. Second, we can't win public support for a carbon tax unless people are certain they won't be worse off.
However, this leaves the question: how can a rapid transition occur and who should pay for it? His tax plan assumes a high carbon price will make big business invest heavily in clean alternatives. However, big business will only do this if it creates short term profits.
The most realistic way to make the transition in the short time we have left is for government's to do in conjunction with a strong climate movement.
Pro-capitalist governments don't generally act in humanity's interest but serve the greenhouse mafia. The climate movement must rise to the challenge and become strong enough to force governments to act — or change the government.
To be effective, any carbon tax on big polluters should be seen as part of a wider plan for a public program to quickly decarbonise our economy — not as an alternative to it.