I was sceptical about climate change. I was cautious about crying wolf. I am always cautious about crying wolf. I think conservationists have to be careful in saying things are catastrophic when, in fact, they are less than catastrophic.
I have seen my job at the BBC as a presenter to produce programmes about natural history, just as the Natural History Museum would be interested in showing a range of birds of paradise - that's the sort of thing I've been doing. And in almost every big series I've made, the most recent one being Planet Earth, I've ended up by talking about the future, and possible dangers. But, with climate change, I was sceptical. That is true.
Also, I'm not a chemist or a climatologist or a meteorologist; it isn't for me to suddenly stand up and say I have decided the climate is changing. That's not my expertise. The television gives you an unfair and unjustified prominence but just because your face is on the telly doesn't mean you're an expert on meteorology.
But I'm no longer sceptical. Now I do not have any doubt at all. I think climate change is the major challenge facing the world. I have waited until the proof was conclusive that it was humanity changing the climate. The thing that really convinced me was the graphs connecting the increase of carbon dioxide in the environment and the rise in temperature, with the growth of human population and industrialisation. The coincidence of the curves made it perfectly clear we have left the period of natural climatic oscillation behind and have begun on a steep curve, in terms of temperature rise, beyond anything in terms of increases that we have seen over many thousands of years.
People say, everything will be all right in the end. But it's not the case. We may be facing major disasters on a global scale.
I have seen the ice melting. I have been to parts of Patagonia and heard people say: "That's where the glacier was 10 years ago - and that's where it is today." The most dramatic evidence I have seen was New Orleans, after Hurricane Katrina. Was that climate-change induced, out of the ordinary? Certainly so. Everyone who does any cooking knows that if you want to increase a chemical reaction, you put it on the stove and heat it up. If you increase the temperature of the oceans, above which there are swirling currents of air, you will increase the energy in the air currents. It's not a mystery.
So it's true to say these programmes about climate change are different, in that previously I have made programmes about natural history, and now you could say I have an engaged stance. The first is about the fact that there is climate change and that it is human-induced. I'm well aware that people say it's all a fuss about nothing, and even if it is getting warmer, it's nothing to do with us. So I'm glad that the BBC wanted some clear statement of the evidence as to why these two things are the case.
The second programme says, these are some of the changes that are now almost inevitable, these are the sorts of things that the nations of the world have to do, to forestall the worst. Will they do it? Who knows? And many people feel helpless.
Yet the fact of the matter is, I was brought up as boy during the war and, during the war, we actually regarded it as immoral, wrong, to leave food on your plate, you needed to eat what was on your plate because we didn't have enough. I feel in the same way that it is wrong to waste energy now, and if that sort of sea change in moral attitude were to spread amongst the world's population, it would make a difference.
During the past 50 years, I have been lucky enough to spend my time travelling around the world looking at its wonders and its splendours. I have seen many changes, some good many bad.
But it's only in the past decade that I have come to think about the question of whether or not what I, or anybody else, has been doing, could have contributed to the change in the climate of the planet that is undoubtedly taking place. When I was a boy in the 1930s, the carbon dioxide level was still below 300 parts per million. This year, it reached 382, the highest figure for hundreds of thousands of years.
I'm 80 now. It's not that I think, like any old man, that change is wrong. I recognise that the world has always changed. I know that. But the point is, it's changing more extremely and swiftly than at any time in the past several million years. And one of the things I don't want to do is to look at my grandchildren and hear them say: "Grandfather, you knew it was happening - and you did nothing."
As told to Michael McCarthy