"How did you become an activist?" I was surprised by the question. I never considered myself an activist. I am a slow-paced taciturn scientist from the Midwest US. Most of my relatives are pretty conservative. I can imagine attitudes at home toward "activists".

I was about to protest the characterisation – but I had been arrested, more than once. And I had testified in defence of others who had broken the law. Sure, we only meant to draw attention to problems of continued fossil fuel addiction. But weren't there other ways to do that in a democracy? How had I been sucked into being an "activist?"

My grandchildren had a lot to do with it. It happened step by step. First, in 2004, I broke a 15-year self-imposed effort to stay out of the media. I gave a public lecture, backed by scientific papers, showing the need to slow greenhouse gas emissions – and I criticised the Bush administration for its lack of appropriate policies. My grandchildren came into the talk only as props – holding 1-watt Christmas tree bulbs to help explain climate forcings.

Fourteen months later I gave another public talk – connecting the dots from global warming to policy implications to criticisms of the fossil fuel industry for promoting misinformation. This time my grandchildren provided rationalisation for a talk likely to draw ire from the administration. I explained that I did not want my children to look back and say: "Opa understood what was happening, but he never made it clear."

What had become clear was that our planet is close to climate tipping points. Ice is melting in the Arctic, Greenland and Antarctica, and on mountain glaciers worldwide. Many species are stressed by environmental destruction and climate change. Continuing fossil fuel emissions, if unabated, will cause sea levels to rise and species to become extinct beyond our control. Increasing atmospheric water vapour is already magnifying climate extremes, increasing overall precipitation, causing greater floods and stronger storms.

Stabilising climate requires restoring our planet's energy balance. The physics is straightforward. The effect of increasing carbon dioxide on Earth's energy imbalance is confirmed by precise measurements of ocean heat gain. The principal implication is defined by the geophysics, by the size of fossil fuel reservoirs. Simply put, there is a limit on how much carbon dioxide we can pour into the atmosphere. We cannot burn all fossil fuels. Specifically, we must (1) phase out coal use rapidly, (2) leave tar sands in the ground, and (3) not go after the last drops of oil.

Actions needed for the world to move on to clean energies of the future are feasible. The actions could restore clean air and water globally. But the actions are not happening.

At first I thought it was poor communication. Scientists must not have made the story clear enough to world leaders.

So I wrote letters to national leaders and visited more than half a dozen nations, as described in my book, Storms of My Grandchildren. What I found in each case was greenwash – a pretence of concern about climate but policies dictated by fossil fuel special interests.

The situation is epitomised by my recent trip to Norway. I hoped that Norway, because of its history of environmentalism, might be able to take real action to address climate change, drawing attention to the hypocrisy in the words and pseudo-actions of other nations.

So I wrote a letter to the prime minister suggesting that Norway, as majority owner of Statoil, should intervene in its plans to develop the tar sands of Canada. I received a polite response, by letter, from the deputy minister of petroleum and energy. The government position is that the tar sands investment is "a commercial decision", that the government should not interfere, and that a "vast majority in the Norwegian parliament" agree that this constitutes "good corporate governance". The deputy minister concluded his letter: "I can however assure you that we will continue our offensive stance on climate change issues both at home and abroad."

A Norwegian grandfather, upon reading the deputy minister's letter, quoted Saint Augustine: "Hypocrisy is the tribute that vice pays to virtue."

The Norwegian position is a staggering reaffirmation of the global situation: even the greenest governments find it too inconvenient to address the implication of scientific facts.

It becomes clear that concerted action will happen only if the public, somehow, becomes forcefully involved. One way citizens can help is by blocking coal plants, tar sands, and the mining of the last drops of fossil fuels.

However, fossil fuel addiction can be solved only when we recognise an economic law as certain as the law of gravity: as long as fossil fuels are the cheapest energy they will be used. Solution therefore requires a rising fee on oil, gas and coal – a carbon fee collected from fossil fuel companies at the domestic mine or port of entry. All funds collected should be distributed to the public on a per capita basis to allow lifestyle adjustments and spur clean energy innovations. As the fee rises, fossil fuels will be phased out, replaced by carbon-free energy and efficiency.

A carbon fee is the only realistic path to global action. China and India will not accept caps, but they need a carbon fee to spur clean energy and avoid fossil fuel addiction.

Governments today, instead, talk of "cap-and-trade with offsets", a system rigged by big banks and fossil fuel interests. Cap-and-trade invites corruption. Worse, it is ineffectual, assuring continued fossil fuel addiction to the last drop and environmental catastrophe.

Because the executive and legislative branches of our governments turn a deaf ear to the science, the judicial branch may provide the best opportunity to redress the situation. Our governments have a fiduciary responsibility to protect the rights of young people and future generations.

I look forward to standing with young people and their supporters, helping them develop their case, as they demand their proper due and fight for nature and their future. I guess that makes me an activist.

• The full version of this essay, entitled "Activist", will appear in the book The Day After Tomorrow; Images of Our Earth in Crisis by J Henry Fair, to be published in November by PowerHouse Books. Dr James Hansen's latest book is called Storms of my Grandchildren.