'A four-year-old child could understand this! Run out and find me a four-year-old child." For the past year or so, this Groucho Marx line from the 1933 classic Duck Soup might well have been tattooed on the inside of my eyelids. This is what happens when you undertake the challenge of writing a book for children about a subject as complex and, at times, depressing as climate change. With every turn of phrase, I've had to remind myself that we must never underestimate a child's intelligence, or their capacity and eagerness to learn something new.

But as I struggled to explain concepts such as, say, the albedo effect, chlorofluorocarbons and the Milankovitch cycles, I was forever troubled by a central question: what is the right age to tell a child about climate change? And, furthermore, how do you go about discussing a subject that will be an increasingly impactful and predominantly negative presence in their lives? Should we be shielding children from the bad news for as long as possible? Or do they deserve to know the truth as early as possible? After all, their generation will have to pick up the tab, as well as live with much of the fallout. If there had been enough room on the insides of my eyelids to write it, I would probably have squeezed in another pertinent quote: "War is never so ugly as when you explain it to children."

Climate change has joined a long and growing list of difficult subjects - a death in the family, terrorist attacks, poverty, drugs, bullying, natural disasters, racism - that can leave parents and teachers flailing in search of ways to explain them without leaving a child traumatised, perplexed or angry. (Could it even supersede that oh-so-awkward conversation about the birds and the bees as the on-the-knee chat that parents most dread?)

As a topic for discussion with children, climate change is challenging on two levels. First, in terms of the underlying science, it can be very complicated to explain. But it also throws up so many vexed issues, especially when you move on to who's to blame and what the solutions might be. Climate change oozes politics from every pore, which is what makes it such a controversial subject with so many adults.

Inevitably, my own children were my first point of reference. My six-year-old daughter, Esme, the eldest of three, is so far the only one with any concept of climate change. I don't think she knows the term itself, but she has brought home from school related talk of how "leaving lights on can cause the ice that polar bears live on to melt".

"That's right," I responded warmly, congratulating her on knowing such a thing. But then I wavered: do I really want her to be fretting about the fate of polar bears, a cuddly, miniature version of which currently sits on her bed? This is a child, after all, who will start crying when she sees roadkill lying limp on a verge. Would it be better to couch talk of conserving energy in terms of saving money for her parents?

I can well remember some of my own vertiginous epiphanies from childhood. For example, reading Raymond Briggs's When the Wind Blows left me cowering under the sheets at night, fearful that the Russians would nuke us all at any moment. Did this instruct me about the realities of the big, bad world? It helped to politicise me, but I could have done without the cold sweats. My conclusion from having written the book is that it must be all about tone, balance and timing. Esme's primary school has been excellent at instilling in her a sense of wonder about wildlife and the wider environment, as well as the need to recycle and not waste resources such as water and electricity. Naturally, I too try to nurture an interest in her, but joining the dots to reach a wider understanding that our climate is changing - and that we humans are to blame - seems quite a way off and I'm happy not to push it too hard.

In fact, the national curriculum dictates that climate change need not enter a child's formal education until the age of 11, the start of secondary school and key stage 3. Since September 2008, it has stated that the study of science should include an understanding that "human activity and natural processes can lead to changes in the environment", which includes the impact on the climate of burning fossil fuels. In geography, "environmental interaction and sustainable development" are now taught as "key concepts" to key stage 3 students aged between 11 and 14. The topic of climate change is creeping into citizenship classes, too.

However, the subject's introduction has not come without controversy. In 2007, a school governor in Kent took the government to the high court over its decision to distribute An Inconvenient Truth, Al Gore's Oscar-winning documentary, to secondary schools across England, saying he did not want "our young people indoctrinated with this political spin". He failed to get the film banned, but the judge did order that the DVD be accompanied by guidance giving the "other side of the argument".

But should we be leaving the job of explaining climate change to secondary school teachers? Can the subject be raised with younger children? Debi Gliori, the popular children's illustrator, believes so. Last year, Bloomsbury published The Trouble with Dragons, Gliori's book aimed at five- to seven-year-olds, which describes how when "dragons" chop down trees, build roads, eat lots of food and "blow out lots of hot air", it helps to destroy the world around them. "Children don't like being lied to," says Gliori. "This is their world, too. They will inherit it. It isn't a Disney movie. I like to use euphemism in my books because I don't want to frighten them. I want to tell them the truth, but I use dragons to talk about the big stuff. I have had children become upset at talks I've given, though. Once they realise that the dragons represent us, they have asked questions such as, 'Does that mean there will be no world left for us?' I do sometimes feel guilty about interrupting their world. It doesn't make you many friends writing about things like this, but I just don't like lying to children."

Professor Hugh Montgomery, an intensive-care consultant and the director of University College London's Institute for Human Health and Performance, spends his downtime writing children's books. The Genie in the Bottle is aimed at the 7-to-11 age group and tells the "simple story of climate change: how fossil fuels were laid down, how humans have changed the environment, how carbon is affecting the atmosphere and therefore how it might affect our future on this planet". The book forms the basis for lesson plans now being used by primary school teachers as part of the BT-sponsored Project Genie, an education-based programme focusing on climate change and sustainability, that has been trialled in 150 primary schools. Montgomery is adamant that there is no such thing as too young when it comes to telling children about climate change: he has even told his three-year-old about the "stinky gases" damaging the planet.

"You can tell them as young as you want," he says. "Children are no more readily frightened than adults. It's a question of context. We must avoid the conspiracy of silence. In my work in healthcare, I find that children dying of cancer usually know well before their parents think their child knows. We mustn't underestimate them. I've spoken to five-year-olds in schools about climate change. They understand the idea of carbon dioxide. I show them a piece of coal and describe it as "fossilised salad". But I will also talk about how people are at risk of dying if we don't tackle climate change. We shouldn't give them mixed messages."

He stresses that the way we tell them about climate change is important. "It's exactly the same with my patients. If someone is at risk of coronary disease - they're overweight, they smoke etc - they need to be told the truth. You have to tell them they are in a desperate situation. But rather than overwhelm them with a new health regime, I will try to encourage them step-by-step by getting them to do some exercise first, before I get them to, say, stop eating buns. Children can actually achieve massive change. Pester power is crucial. We know that from what we've learned in healthcare. Your own daughter telling you you're going to die from smoking is much more powerful than any government poster saying so.

"Children love being given responsibility and being told they can do things without their parents. In our Project Genie primary schools, children have achieved a 42%-72% reduction in electricity use, simply by policing energy use themselves."

Current thinking among educationalists and child psychologists seems to confirm that children can grasp a subject as cognitively and emotionally challenging as climate change.

"Children are much cleverer than previously thought," says Frances Gardner, professor of child and family psychology at the University of Oxford's Department of Social Policy and Social Work. "They think outside themselves. We generally believe now that we need to be more emotionally open about things with children. A generation ago, death and grieving were shielded from children. Children weren't taken to funerals, for example, but this is changing."

Christine Howe, professor of education at the University of Cambridge's Faculty of Education, believes that climate change should now be a recurring theme throughout the school curriculum. "Relevant work could even be done in pre-school facilities, familiarising children with sources of 'hot' and 'cold'," she says. "However, early teaching should focus on manageable constructs and simple relations - with gradual scaling up over a very long period of time."

But there are those who worry that by speaking to young children about climate change we could risk encouraging a sense of inertia, either by leaving them paralysed with fear, or cosseting them with reassuring talk of switching off phone chargers when we know that it won't make a blind bit of difference.

"I think telling five- to seven-year-olds is too young," says George Marshall, a veteran climate-change campaigner, author, and founder of the Oxford-based Climate Outreach and Information Network. "Maybe 10- to 11-year-olds is OK, but, in general terms, I try to teach my own children [aged five and seven] that we are living through an extraordinary period of change. They actually seem quite excited by that. I don't talk to them about guilt, western lifestyles and things like that. I don't want to scare them. It's very different from, say, how children were told about the threat of the Nazis or the nuclear bomb. They were an external enemy, whereas climate change is an enemy within. We should be careful what we tell children when we haven't yet put our own house in order as adults. Ultimately, I'm not really persuaded by the argument that they are the next generation, because we have to tackle climate change so fast. We have other priority audiences more important than children."

No matter how many times I toss that last sentence around in my mind, I just can't accept it, no matter how ice cold and pragmatic its logic might be. Surely, our children must be a priority when it comes to invoking a meaningful response to climate change? I believe so, and it's why I've spent the past year writing a book aimed at 9- to 12-years-olds that will hopefully spark in its readers a lifelong concern about climate change and, more importantly, a passion to be part of the solution. As my daughter Esme said to me the other day: "Why don't we just stop hurting the planet?"

I couldn't have put it better myself