Worried about your calorie count? You will be.

This article is about why the calories we consume make it so difficult for democratic societies to tackle climate change. If that sounds ridiculous then please bear with me.

We normally only worry about what the calories in our food will do to our waistlines, but calories can also be used to measure the energy in fuel.

What do you think your total calorie count would be if instead of calculating the energy you need to eat to stay healthy you added up all the energy you use to keep yourself warm, to travel around, to produce the food you eat and the entertainment you enjoy?

The difference between those two figures - how much it takes to keep us alive and how much energy we actually use - is a measure of the challenge that tackling global warming presents.

It also explains why democracies may never be able to get to grips with the problem.

I'll come to the actual figures soon, first let's explore the link between calorie counting and the big issues.

It's pretty simple really. In most modern societies the vast majority of the calories we use come from not from food but from fossil fuels. And fossil fuels - as everyone now knows - produce greenhouse gases which, in turn, cause climate change.

We all now also know the risks that climate change presents. We know that, if unchecked, scientists expect it to cause droughts, floods and storms which will disrupt agricultural production around the world and lead to famines, mass migration and conflict.

I know what you are thinking. You are thinking he's about to lay a dieting metaphor on me. And you are right - but only up to a point.

Of course a diet is the answer. Cut out all those fossil fuel calories we use and bingo! Problem solved. My question is whether our political systems will ever be able to put us on the diet the world needs.

Anyone who has been on a diet knows how unpleasant and difficult it can be. And make no mistake this fossil fuel diet is going to be uncomfortable too.

Fossil fuels are an extraordinarily concentrated energy source and moving to alternatives will have a cost. It will mean we have to change the way we do things, and many people will find that difficult and unsettling.

Regulating or taxing fossil fuel use will raise the price of energy and -at least in the short run - that's going to push up the price of pretty much everything.

Bizarre as it might sound, that may not actually mean we pay more. If you improve the insulation in your home or drive a more fuel efficient vehicle then energy prices can rise and you still end up paying less.

What's more higher prices for energy from fossil fuels should spur a move to renewable energy sources and make them much cheaper in the long run.

But the key point is this. Along the way it will mean some sacrifices and some big changes and many people are going to find that uncomfortable.

What is more, the systemic nature of the changes that are needed mean we can't expect individuals to do it on their own. This diet is going to have to be forced on us by government - which is where the tricky little problem of democracy comes in.

The challenge for democracy is this: will we - the electorate - ever vote for a government that will force us to use less fossil fuel?

President Obama said he'd try and do just that. You can see what happened to his promise and follow my madcap journey around America on BBC2 tonight at 7.00pm.

Of course, democratic societies have risen to some pretty big challenges in the past. They've overcome war, depression and social upheaval.

But climate change is a bigger challenge than even a world war.

Climate change requires people to recognise the need for a unique trade-off. We need to agree to make sacrifices now - the diet - to deal with a threat that lies way in the future.

And democracies tend to be poor at long term planning. Unpopular politicians don't get elected and telling people they have to make sacrifices is not going to make a politician popular unless there is a damn good reason.

Most voters would recognise that the German army threatening to invade is a good reason. The threat of global warming is a tougher proposition.

It requires that the electorate believe the science. They need to believe that the invisible gases produced by the very activities that make modern life so comfortable will slowly make the world less and less inhabitable.

And science is contestable. Just take a look what happened when I appeared on an American talk radio show.

Like dieters who find excuses to have that one cream cake or another glass of red wine: people are going to find excuses to avoid a fossil fuel diet.

And the tougher the diet, the more likely people are to try and avoid it. So what's our calorie count?

The average woman needs 2,000 calories a day to stay healthy. Men get a bit more. We need 2,500 calories a day.

In America the average person uses the equivalent of 216,728 calories every day, not including food - over 100 times more.

In Britain we use around half the amount of energy - a which works out at around 106,000 calories a day. It is more modest, but still a huge daily diet of calories.

So here's the big question, will democratic politicians be able to tackle climate change in societies as energy-hungry as ours?

The truth is that ultimately only you the voter can answer that question - at the ballot box.

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I am going to commit a green heresy.

What I am going to say will challenge everything you have heard about environmentally friendly lifestyles.

I am going to argue that cars can be greener than public transport.

But before I do, I should put my cards on the table. I'm a pretty ordinary bloke. I love driving and have been a car owner and driver ever since I passed my test. But I was forced to get rid of my car after I was press-ganged by the Newsnight editor into a year-long experiment in ethical living.

It was a sad moment as I stood with my family in 2006 and watched my precious Saab being winched away from outside my home.

You can see my humiliation for yourself here and judge for yourself whether you think my experiences have clouded my judgement on this issue.

Since that fateful day - and I've remained car-free all this time - I've been studying the carbon consequences of car ownership.

So let's look at the facts.

It is true that at first glance cars appear to compare very badly in terms of carbon dioxide emissions to other forms of transport.

The average car emits something like 180g per passenger kilometre.

You may be surprised to learn that, on that basis, taking the car is less environmentally friendly than taking a typical, well-filled short-haul plane, which emits 150g/km per passenger.

The figures show trains to be much greener, as you might expect. According to its publicity, one of Virgin's new Pendolino trains travelling half full up the West Coast main line clocks up just 27g per passenger kilometre.

But it is quite easy to tilt the statistics in favour of the automobile.

Cars appear very inefficient because, more often than not, just one person travels in a car. Pack in extra passengers and you quickly make the car a dramatically greener option.

The extra weight does increase fuel consumption - but only very marginally. Why? Because each additional passenger only weighs a tiny fraction of the weight of the car itself.

Think about it. The average car weighs in at over a tonne. Meanwhile the average passenger weighs around 70kg (the average British woman weighs 65kg and the average man something like 75kg).

That means each additional passenger adds just 7% of extra weight. Taking our average vehicle, that works out at just over 12g/km per extra passenger.

So, if I pack my family of five into a car, the average emissions per passenger falls to just 45g/km (and that's not taking into account the fact that most members of my family are very small).

But - I can hear the greens amongst you protesting - the train still wins out. Does it? Really?

Earlier this year, the Guardian and New Scientist writer Fred Pearce dug a bit deeper into the green claims made by train companies.

He found that most Virgin trains are nowhere near as clean as the Pendolinos. Virgin's most modern diesel train, the Voyager, emits 74g per passenger kilometre when travelling half-full. That's almost three times as much as the Pendolino, and half as much again as each member of my family travelling by car.

Travel on a Voyager when it is a quarter full and your emissions per kilometre travelled are about the same as sitting in a fullish plane. "More leg room", says Fred, "but no greener."

Catch a half-full sleeper up to Scotland and your carbon footprint is much worse. He calculates that with 12 people in a carriage you'd be lucky to emit less than 200g/km.

And the truth is, buses don't do much better. According to figures from the US Department of Energy, a bus with average occupancy (9 people) is more polluting than a car with average occupancy (1.57 people).

The figures are likely to be pretty similar in the UK. Some suburban bus routes in London are subsidised to the tune of £10 per passenger journey apparently (at least, that's what a mole inside the Mayor's office tells me). That suggests very low occupancy, and therefore a whopping carbon footprint.

But am I being fair comparing a car full of people with an empty bus or train?

It is true that full buses and trains are significantly less polluting per passenger than cars (even cars full of passengers). The problem is, for most of the day our buses and trains are far from full.

I couldn't find good occupancy figures for UK public transport systems - please help me if you can - but it seems most of our buses and trains are far from full most of the time.

That's because public transport systems are designed to meet peak demand.

Double-decker buses make a lot of sense in the rush hour but rumble around our cities almost empty in off-peak periods. Indeed, even at peak times every packed rush-hour bus is likely to make its return journey nearly empty.

And public transport has to offer a frequent service all day long. People won't use buses and trains unless they know they can get to where they want when they want.

We could run smaller buses in off-peak periods to cut emissions but think of the cost: bus companies would have to buy twice the number of vehicles. Read more about why public transport can be high-carbon here.

Should we conclude from this that the green choice is the car?

Here's where I get back on message. Even though travelling by car can be less polluting than public transport you should always take the public transport option.

That's because buses and trains are running anyway. They will be out there generating CO2 whether you ride them or not. So when you choose to take your car, the pollution you create will be on top of whatever the public transport option is producing.

The other key point is that my argument only really holds so long as you have other people in your car with you. Travel alone - the statistics show - and the car is, just as the greens claim, one of the most polluting forms of transport on earth.

So how have I got on without my Saab?

Well, despite my misgivings about getting rid of it, the truth is I found life without it much easier than I thought.

I recognise that giving up the car will not be so easy for most people. I live right in the heart of London, where public transport is almost always quicker and easier than taking the car.

But if you insist on continuing to use your car, you can get some tips on driving the low-carbon way in this video (made several months after I gave up the Saab). You can also see how a friend and I came up with a novel way of using my car to cut both our carbon footprints.

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The Secretary of State for Climate Change, Ed Miliband, said he would answer some of your questions, and he has.

Wolfgirl asked: Does Mr Miliband offset any of his personal emissions? If not, why not? Does he think that offsetting emissions can help us to tackle climate change?

"The first thing I do is look to reduce my emissions. So I take the train and have taken steps to reduce the energy I use in my home.
 
"Carbon offsetting is not a cure for climate change, but it can help raise awareness and reduce the impact of our actions.
 
"Offsetting can be useful when we need to compensate for unavoidable emissions. In this way, offsetting can play a useful part in helping to meet carbon reduction targets.
 
"Robust offsetting projects can, however, provide a route for investment in clean technology in the areas which lack it the most. This investment really does help lead to the spread of low-carbon development across entire regions.
 
"The government is becoming increasingly aware of the environmental impact of travel by our ministers and officials. That's why we have offset all our emissions from air travel since April 2006."

MrWaves asked: Given that so many technologies such as wave, tidal, carbon capture and large scale solar are still in their infancy, how will we grow at least some of these into worthwhile energy sources and how do we plan with so much uncertainty?

"Wave and tidal energy and clean coal have enormous potential, and I'm proud that the UK is seen as a world leader in the development of these cutting-edge technologies. All are going to be necessary, we can't pick and choose.
 
"We are working closely with industry to develop these technologies that aren't yet commercially viable. That's the government taking a strategic role in helping that technology along.
 
"Just last week we published our new clean coal policy - which is the most environmentally ambitious set of coal conditions of any country in the world. It says that there will be no new coal without carbon capture and storage.
 
"Our island nation also means we've got huge potential in the area of wave and tidal energy and there's loads of work already underway. We've recently made £22m available to developers to accelerate the commercial development of marine energy in the UK. Work began this month on a 'Wave Hub' in south-west England which will help developers test their designs, and we have a £50m Marine Deployment Fund for the projects once they are ready for commercial testing.
 
"So we are putting the measures in place to develop these technologies - which will all be vital as we decarbonise our energy supplies."

DocDim asked: Does Mr Miliband see such initiatives as Copenhagen as having more or less value than actions taken by individuals, such as assuming a suitable moral stance (intrinsically motivated) as opposed to one decreed by governments (extrinsically motivated)?

"The bottom line for me is that politicians and governments have got to take a lead in sorting the problem out, and that's where Copenhagen comes in. We can't afford for any country to be left out, or left behind. And the UK has been pushing really hard to get the most ambitious deal possible.
 
"But 'Doc Dim' is right to bring up the role of individuals. Individuals all over the world have and must continue to play a vital role in mobilising and putting pressure on their governments to do the right thing when it comes to the climate.
 
"There's also the simple things that we can do in our homes - we're all part of the solution. Some 40% of the UK's emissions are down to what we do in our personal lives - transport, heating, power use. It would be wrong - and impractical - for government to just leave it at that, and say 'it's over to you and it's your responsibility'.
 
"Governments should make sure that we put in place the measures and help so that people play their part in tackling climate change, find it easy, and that it doesn't cost too much. That's what we're doing through our pay as you save home insulation pilots, to the new clean energy cash-back schemes for people who start to generate their own energy.
 
"Personally, I think it's not a good idea for politicians to lecture the public. It's my role to communicate a message of optimism and hope - not a message of gloom and defeatism."

Brightyangthing asked: If a means of power generation for the future had to be 'in your back yard', and assuming that without it, you would have no domestic heating or lighting three days a week, which box would you tick: Wind farm, Coal-fired power station, Nuclear, Other?

"Firstly, no-one is going to have a nuclear power station or a coal power station in their back yard - there's pretty strict rules on where they get built, as you'd expect!
 
"But people's back yards could be great for small-scale technology such as a ground source heat pump or solar panels. And from next year, through the clean energy cash-back scheme, people are going to be able to produce their own clean energy in their back yard and get paid for doing it as well.
 
"In my local area of Doncaster, we already have a coal power plant which could be one of the first in the world to develop CCS, and there are plans to build wind farms too.
 
"But we need all kinds of energy technology across Britain - in the right places - to supply us with the secure low carbon energy we need in the future. And we shouldn't forget the jobs and economic benefit that this investment brings to each area."

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