Nasa's top climate scientist Jim Hansen told a jury at the trial of 20 environmental activists that he had begun speaking out about climate change again in the past five years because of his grandchildren.
"I did not want them to say: 'Pa you understood what was happening but you never made it clear'," the 69-year-old told the trial last Monday.
The activists were found guilty of conspiracy to commit aggravated trespass by the court today, and could now face suspended prison sentences.
Hansen had flown overnight from his home in the US to give evidence about the science of climate change and in particular the threat posed to humanity by the burning of coal at plants such as E.ON's at Ratcliffe-on-Soar.
During his two-hour testimony and cross-examination, Ed Rees QC, for the defence, stopped him repeatedly to ask him to explain the technical terms he was using. But the message he delivered was the clearest – and starkest – crash course in climate change the jury, the judge and members of the public in the gallery are ever likely to get.
Hansen, whose speech to Congress in 1988 is seen as pivotal in first bringing climate change to the world's attention, is well-versed in speaking out against the coal industry. Nottingham is not the first British court where he has given evidence. He testified last year in the case of the "Kingsnorth Six", who had climbed up E.ON's coal plant. They also used the climate change defence – that their actions were designed to prevent immediate harm to human life and property from climate change – and were acquitted.
The day after last Monday's trial, he flew to Switzerland from East Midlands airport to try to dissuade the government there from building a new coal plant. In the past 18 months, he has been arrested twice in the US in protests against coal mining.
In his witness statement submitted for last week's case, he argues that if the burning of coal continues to grow at its current rate, the level of carbon dioxide in the atmosphere will rise to between 500 and 600 parts per million. This is way above the current level – of about 389ppm – and the maximum level considered safe by an increasing number of scientists of 350ppm.
The Guardian interviewed Hansen at his hotel in Nottingham after the case. He was tired after his overnight flight and two-hour testimony and mindful of his 4.45am wake-up call the next morning. But he spoke passionately about his recent trip to Beijing for an energy conference. It is China that is doing the most to avert catastrophic climate change, he says, and has made him a lot more optimistic about the future. He no longer believes that "the solution to the problem requires an agreement between China and the US as the two big emitters". He believes that support is growing in China to introduce its own carbon tax to penalise the use of fossil fuels and make renewables and nuclear more economic.
"What I realised is that the most important thing is China," he says, cradling a beer and still wearing his trademark cowboy-style wide-rimmed hat. "They have shot past the US as the biggest emitter by far and they are going to go higher still. It's to their advantage to put a price on carbon. They stand to suffer more from climate change than most places. They have air and water pollution which is horrendous. They don't want to follow the path of the US where the country becomes addicted to fossil fuel and has to protect the supply lines around the world. They have taken the right first step. They are now the number one producer of solar cells, wind and nuclear power."
He is hopeful that Europe could follow suit and introduce its own carbon price. "I tend to be naive and gullible I guess but I try to believe that governments believe what they say. And European governments say the right things." He also acknowledges the recent progress made in the UK on coal, after coal plants which do not capture some of their emissions were banned last year.
But he is scathing about discussions at the UN climate talks in Copenhagen last year and Cancún, which reaches its climax today, over agreeing emissions targets, based on a cap-and-trade system. "Cap-and-trade with offsets will not work. Because by definition they are trying to trick you. By saying you can put a cap on but it's not going to make energy much more expensive – if it doesn't make energy much more expensive it's not going to be effective."
He is less positive about his own country. He points to the legal battle in the US in 2007 over tough new car emission restrictions which demonstrated the susceptibility of federal government to lobbying by big business he says. "The government stood in court alongside the automakers resisting improved vehicle energy efficiency. It's not democracy the way it was intended to be. It's money democracy where $1 is what counts rather than one vote." If the US does not start weaning itself off fossil fuels soon, he believes the country is "headed for second- or third-grade status in decades to come".
Hansen is also critical of how the media often gives climate change sceptics prominence to ensure a "balanced" debate even though they are unrepresentative of the mainstream scientific community. Such media distortion of the argument – and powerful lobbying – means that in democracies "the public is misinformed". Asked if it was more a case of the public not wanting to believe the frightening realities of climate change, he adds: "There is some of that. But the majority would go along."