One of the leading scientists in the debate over climate change has claimed that global leaders must take charge and suspend the construction of any new coal-fired power stations.

Speaking at the trial of six Greenpeace activists charged with causing £30,000 of damage to the Kingsnorth power plant in Kent last October, former White House advisor James Hansen said that one of the leaders of Germany, the UK or the US should "step up" and take leadership by imposing a moratorium on any new coal power plants.

"Humans are now in charge of atmospheric CO2 and the global climate... It is up to those of us alive today to take the bold steps needed," Hansen told a court in Maidstone, Kent.

Hansen, who also heads up the NASA Goddard Institute for Space Studies in New York City, was drafted in by Greenpeace as a defence witness to argue that the group's actions were justifiable in scaling the 200m tower at Kingsnorth and painting the word "Gordon" on it.

The environmental group argues that under the Criminal Damages Act of 1971, the campaigners had a lawful excuse to cause the damage as they were trying to prevent even greater damage being caused by the contribution the plant would have to climate change.

The climate scientist, who is also one of Al Gore's main environmental advisors, is seen by many environmentalists as one of the founding fathers of climate change science.

"This is the man who introduced much of the world to the idea of climate change 20 years ago, when he famously stood up in front of Congress and warned them about it. He has spent much of his time since warning a succession of US vice presidents, including Al Gore, about the same thing," said a Greenpeace spokesperson in a blog entry on the trial on the group's homepage.

Hansen originally became involved in the Kingsnorth campaign back in December when he wrote to Gordon Brown asking that the prime minister drop any plans for coal plants that do not include carbon capture.

The Kingsnorth power plant is owned by energy company E-ON UK, formerly known as Powergen. In October 2006, the company announced plans to develop the site by building what it calls two new "cleaner" coal plants. E-ON claims the 1,600MW facility would be 20 per cent more efficient than the current plant.

Last month E-ON conducted a survey of people living close to the Kingsnorth site which it claims showed that about 57 per cent of local people were in favour of plans to develop the site to make it more efficient.

"Clearly we as a company are also very concerned about climate change, which is one of the reasons we are closing the existing Kingsnorth plant by the end of 2015 and planning to build a much more efficient station," said Alan Sanderson, plant manager at Kingsnorth in statement.