James Hansen

US scientist James Hansen was one of the first people to warn the world about global warming and introduce the concept of climate change into public debate. Now in his seventies, he could be sitting in quiet retirement looking back on a successful career that saw him serve as Director of NASA's Goddard Institute for Space Studies. But he opted for something altogether different.

He decided to leave his prestigious position to concentrate his energies on climate activism. It is that ambition that pushes him around the world to share his ideas. And it is that same ambition that has seen him arrested for demonstrating.

Hansen has come to Paris to bang the drum for science, which he says is not sufficiently factored into the climate targets. "The goals were not chosen on the basis of scientific criteria, but because they are comfortable."

Many NGOs are now calling for a global warming cap of 1.5 rather than two degrees, and Hansen agrees that the goals on the table at COP21 fall short of the mark.

"We know two degrees is not a safe target. All we have to do is look at the earth's history. The last time the earth was two degrees warmer, sea levels were six to nine meters higher. That would mean all coastal cities would be dysfunctional, and the economic consequences are incalculable."

Avoiding that scenario, the scientist says, requires us to do away with fossil fuel entirely. Forever. Even if that means relying on highly contentious nuclear power. The main thing is to move fast."Science does not say we can aim for a 50 percent reduction by 2050 or some other fractional reduction. It actually says we need to phase down emissions rapidly and move to carbon-free electricity."

Pennsylvania-based Hansen, who is also a professor at Columbia University, is both a father and grandfather. A few years ago he thought it would be nice to introduce his grandchildren to nature by planting seeds that would later become food for butterflies.

"I didn't realize I was introducing them to a problem where the butterfly started to disappear," he said. "Six or seven years ago we could see more butterflies, now we hardly see them. So I stopped talking about that."

But he didn't stop thinking about it. And it was in part his grandchildren that helped him switch careers.

"I don't want my grandchildren to say 'Opa understood what was going on, but he didn't try to make it clear.' That was when I started communicating and I still thought it was going to be easy."

Turns out it's not. But that won't stop him from doing what he can to get across the message that even if every country makes good on the climate pledges they volunteered ahead of the Paris conference, the world will still be looking at global warming of 2.7 degrees.