Activists accused of conspiracy to trespass were arrested before they carried out plan to force E.ON's coal plant at Ratcliffe-on-Soar to shut down

The anger of 114 activists who planned to break into a coal plant near Nottingham was understandable because of the "lies" told by governments about climate change, Nasa's top scientist told the trial of 20 climate campaigners.

Professor James Hansen, the NASA scientist credited with doing the most to raise awareness of climate change, had flown from the US to be the star witness.

Twenty activists are accused of conspiracy to trespass on private property. They were arrested last Easter before the group were able to carry out their plan to force E.ON's coal plant at Ratcliffe-on-Soar to shut down for a week.

The activists claim that had they succeeded, 150,000 tonnes of CO2 would not have been emitted.

Hansen, a vocal critic of coal power, told Nottingham crown court: "The fact that we continue to burn more coal and build more coal plants shows governments are not telling the truth "If they are saying they understand climate problem but will continue to burn coal its easy for me to understand that young people get upset, because they know governments are lying or kidding themselves."

Hansen's evidence was crucial in the case of the "Kingsnorth six" who broke into the Kent coal-fired power station and scaled a chimney. Their "lawful excuse" defence, that the harm they did was committed to prevent the greater harm of climate change, was successful and they were acquitted in September 2008.

Sarah Shoraka, biodiversity campaigner with Greenpeace, from North London, was the first defendant in the witness box. Felicity Gerry for the prosecution asked her why, rather than spend £15,000 on the planned plant occupation, the group did not enlist a celebrity like Cheryl Cole to front a "green" campaign to encourage people to wear second hand clothes for example.

Telling the court that she had a compost toilet at home, she also said that the "Meat Free Monday" campaign to cut down on meat consumption and related emissions promoted by Coldplay's Chris Martin and Sir Paul McCartney was, "quite an effective campaign because some celebrities are involved."

She suggested the activists should have tried something similar. "Why did you not just do 'turn off Tuesday or switch off Saturday'".

Shoraka, who did not deny her involvement in the protest, said that it was not a publicity stunt. She also expressed her frustration at the time with politicians' failure to ban coal plants.

"I was very pessimistic at the time. I did not see there was any political will to go anywhere near tackling this problem. I felt it was hopeless. These people who were supposed to be there to protect us failed in their duty to do that."

Gerry also asked professor Hansen "it may be suggested… that if we all use biodegradable compost toilets and switch off the lights then that will achieve significant results in reducing the level of CO2. What do you say to that?"

Hansen responded "Unfortunately that will do little good… burning it a little more slowly, being a little more conservative in our use. We are going to have to leave fossil fuels in the ground. The biggest one to leave in the ground is coal."

Hansen told the court the UK was the world's biggest carbon polluter on a per person basis since industrialisation so the onus was on the UK to lead the way in phasing out coal plants.Gerry put it to him: "its grandma"s fault." Hansen responded: "Yes but grandma didn't know about it. We do now."

He also explained that he had begun speaking out more in last five years because he now had grandchildren. "I did not want them to say pa you understood what was happening but you never made it clear. You can see the changes now. The things which will happen during the lifetime of our young people if we continue as business as usual will dwarf these [current] inconveniences."