His report was crucial in awakening awareness of the economic impact of global warming. Published in 2006, the Stern Report, commissioned by the British government, marked a turning point and opened the eyes of leaders around the world to the real and damaging effects of inaction — of standing still in the face of climate change. Lord Nicholas Stern is the economist who today heads the research institute on climate change and the environment at the London School of Economics. He has just published a new book, Why are we waiting? – The logic, urgency, and promise of tackling climate change, because, despite the years that have passed and the various warnings, little has changed.

Almost ten years on from publishing your landmark report on the climate, you have written a new essay. Why do those in government not understand that action must be taken immediately?
The first thing to understand is that we are in a hurry because the concentration of greenhouse gases in the atmosphere is already at a level that will make it difficult for us to keep the rise in average global temperature within two degrees (as promised by the Unfccc countries, ed.). Despite this, we must try. The longer we wait, the more CO2 builds up in the atmosphere, the more decisive action will be needed, and everything will become even more complicated. If we do not act now, a point will come when it will be impossible to keep that promise. The consequences are the melting of the polar ice caps and the rapid rise in sea and ocean levels. We are already close to two degrees.

How do population growth and urbanisation affect the rise in CO2 emissions?
Cities are being built at an ever faster pace as a result of migration towards urban centres. Over the next 30 or 40 years, cities will come to house 6.5 billion people compared to the 3.5 billion of today. But the kind of cities we build today has long-term consequences. This means that if we do not move beyond the current model based on dirty, inefficient, congested and polluted urban structures — if we remain stuck for the next 20 to 30 years — it will be extremely difficult to halt the emissions trend. Cities have been what they are for too long. That is why the time has come to change; waiting is dangerous. So when I say "why are we waiting?" it does not mean "we shouldn't wait" — it means "we must act now".

What can we as citizens do about global warming?
Insulate our homes, use public transport, walk more, ride a bicycle. All things that also make us happier!

Under what conditions would you consider the Paris conference (Cop 21) a success?
I have no doubt that an agreement will be reached and that it will be better than nothing. The right direction will be taken. My concern is that it may not be enough to honour the two-degree promise. Our emissions by 2030, the year by which a stocktake must be made, could still be too high. Reaching an agreement is positive, but the real challenge is putting it into practice. After Paris, an acceleration will be needed.

Do you think Pope Francis's commitment is helpful in the fight against global warming?
I think Pope Francis's encyclical Laudato si' is enormously helpful. I had the privilege of meeting him in September; it was a moment of great inspiration. I am not religious and I am not Catholic, but in him I found an extraordinary, visionary, ethical and at the same time analytical leadership. And the Pope has a truly remarkable way of expressing himself. Having the gift of language and strong leadership, together with good ideas, is what we need right now, because this is the moment to press on the accelerator. Francis says that God always forgives, people sometimes, nature never. It is a simple but very effective way of making us notice that we are going wrong and how difficult it will be to retrace our steps and repair the damage we are doing to the environment.