Stewart has been at the forefront of Greenpeace's climate change comms, amid concerns that Copenhagen may not deliver a binding deal.
How is it going so far in Copenhagen?
It's extremely expensive and extremely cold. We have to remain optimistic because failure is not an option. The recent talking down of the potential of a good deal is a not-unusual precursor to these things. We saw it at Kyoto. Many people coming here are elected leaders who don't want to be blamed for failure. Passing the buck is a tactic - they are engaged in expectation management and they are very good at it. Over the next couple of weeks we are going to be surprised - we are expecting the unexpected.
What is the biggest comms challenge you are facing?
The leaders coming here won't do the right thing by virtue of their good hearts and their appreciation of the science. They'll do it because otherwise they will pay a political price back home. Our most difficult job is to explain extremely complex negotiations to a public that is often uninterested and sometimes sceptical, and ensure those leaders know they will pay a political price if they don't come here and do a deal. That is not like selling ice cream - it is an extremely difficult comms challenge.
What is the one thing the comms community could learn from the Greenpeace approach?
I wouldn't be so arrogant as to suggest people can learn much from us, but what we try to do is explain extremely complex negotiations and issues in a way that resonates with normal people and makes sense to them in their normal lives. Without passion from many people - a critical mass - it's difficult to imagine world leaders are going to come to an agreement that we need.
The other thing is to beat our opponents to the button. Perceptions are going to be formed very quickly, and we do have opponents here from the lobbying and fossil fuel industries. They are going to be trying to get their line out - and we have to beat them to it. We also have to be accurate and informative and not find we are contradicted later on.
When it comes to climate change, which communicator do you most admire?
I've always been a fan of Plane Stupid actually. I'm here with one of the founders Joss Garman, and I've always been very impressed with the passion they bring to the debate. They've been patronised but there is something immensely gratifying about a bunch of young people doing something they feel passionate about in their spare time and beating the big guys. They have radically changed the debate around aviation and climate change. I think NGOs can learn a lot from them.
What is the one media channel you are finding it impossible to live without?
I find it extremely difficult to live without the US political blogs, in particular the ones that translate the to and fro in Washington into an analysis I can understand and apply here. Talking Points Memo and Andrew Sullivan, but also Climate Progress and Real Climate. Fortunately I can get them here. I do miss (BBC show) Newsnight, which is my meat and potatoes when I'm at home in the UK.