AL GORE’S thought-provoking film – An Inconvenient Truth – on the environment and global warming was on TV last week. I hope you had the chance to see it. Apparently, ex-US President George W Bush said it was one movie he would never watch. Could there be a better endorsement?

The central theme of the former US Vice- President’s film is that human activity is the driving force behind climate change and that only a radical change in behaviour by governments, business and each of us as citizens can mitigate its effects.

As the title suggests, this change won’t be easy. It will involve extra effort and stopping doing things we enjoy or take for granted. It also means reminding ourselves it was our own reckless use of resources, our abuse of the planet that got us into this mess. Nobody likes being reminded they were wrong.

Very few of us like change, or having our beliefs and habits challenged. Those who do this tend to get a hostile reaction.

That is what leading environmentalist Jonathon Porritt, who chairs Forum for the Future, found recently when he linked the issue of climate change with population growth, suggesting it could be seen as irresponsible for people to have more than two children.

In support, he can rely on some impressive figures. By 2050, there will be nine billion of us on the planet, six billion more than in 1950, placing huge pressure on natural resources and adding to the causes of war and conflict.

The more people we have, the greater our collective “carbon footprint”. That means government targets on greenhouse gas emissions that already seem highly ambitious will become unattainable, unless we revert voluntarily to some form of pre-industrial existence.

This isn’t going to happen. So, we will go on multiplying and bringing our children into a world getting closer to the edge.

It is easy to see the logic behind Porritt’s arguments and easy, too, to see why they make so many people uncomfortable. They will be anathema to many people with religious convictions and disturbing to many civil libertarians. But I believe he was right to raise the issue and think it deserves to be debated maturely and rationally.

We need to be clear that no one is talking about forcing people to limit the size of their families. Intervention in family life, such as occurred in China with its “single child policy”, would be unworkable and plain wrong.

Another factor is that any population management must be coupled with an equally wide-ranging global assault on social injustice.

I am prepared to accept that having too many people may be a problem. But so is the fact that too many people lead lives untouched by the growing prosperity that the so-called advanced nations take for granted.

A system that allows us to pollute while they pay is unsustainable.

Ultimately, it is about persuasion, asking people to think about how their own personal behaviour and choices affect the planet.

The population issue is a personal and emotive one, of course, and that’s why Porritt’s comments have caused so much controversy.

Asking someone to separate food waste and cardboard from their weekly rubbish is one thing; suggesting they might limit the size of their family quite another.

But we have to be mature and debate this issue. Unlike old George W, we can’t stick our heads in the sand. Being serious about saving the environment means hard choices.

This could be one of them.

Inconvenient truths are often unpalatable ones as well, but they still have to be faced.