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Normally the birth of a child is a moment of pure celebration. Not for me.

When my lovely daughter Elsa was born (our third child) I had to justify her very existence.

Her birth came half-way through my year of carbon cutting as the Â鶹ԼÅÄ's Ethical Man so my family life was under intense scrutiny.

I discovered that lots of people (who I am sure are very pleasant in their everyday lives) believe very strongly that the world is already full, and new Elsas are simply not welcome.

So is the burgeoning human population really the cause of our climate crisis?

It is easy to see why so many people believe that it is. People are responsible for greenhouse gas pollution, so - the argument runs - the more of them there are, the more damage they will do.

One reader of my blog last week asserted that "the human population could do with a good 25% knocked out."

He goes on to suggest that we should: "restrict every woman to a single pregnancy, once she has had that then sterilize her, restrict every man to causing a single pregnancy, after that castrate him, stop ALL forms of artificial preganancy (test tube etc.) This way we will reduce the population - and quite quickly."

Strong stuff! But it is certainly true that for the last couple of centuries population growth has been inextricably linked with the use of fossil fuels.

Indeed, the big mistake made by the original prophet of population doom, Thomas Malthus, was his failure to appreciate how fossil fuels would transform the world economy.

To be fair, it would have been hard to foresee, back in 1798, how industrialisation - powered by fossil fuels - would create the vast agricultural surpluses that would sustain a huge increase in population.

But his mistake points to the real culprit here: the problem is pollution not people.

Why? If we were to successfully "decarbonise" our economies then - in terms of the climate at least - the vast human population would not be a problem.

But that is a big "if". The suggests that the world isn't going to reduce greenhouse gas emissions anywhere near as quickly as the science says is necessary.

And the population is rising. You'll find no shortage of frightening statistics at the website of the new Malthusians at the , which campaigns to reduce population. It details how world population is expected to swell from 6.8 billion this year to 9.1 billion in 2050.

But with the New Year looming let's not dwell just on the negatives.

The OPT's forecasts stop in 2050 and there may be a good reason for that. After 2050 it is expected is that world population will stabilise and then, very gradually, begin to reduce.

That's because there has been a dramatic fall in fertility worldwide.

The key figure in population statistics is 2.1. That's what is called the "replacement rate", the number of children per couple that would keep the population stable. Any higher than 2.1 and the population will rise, any lower and it will fall.

As recently as the 1970s only 24 countries - all of them rich - had fertility rates of 2.1 or below. Within the next few years and will include countries like Brazil, Indonesia, China and even South India.

Sometime between 2020 and 2050 the world's fertility rate will fall below 2.1, at which point the long term trend will be downwards.

So while we undoubtedly face a population explosion now, pressure will ease over time.

And away from the queues and chaos of Copenhagen there is cause for optimism about the world's move towards a low-carbon economy.

On my travels around America earlier this year, I saw all sorts of exciting developments - technological breakthroughs, vast investment in renewable technologies, and a growing army of climate activists with friends in the highest places.

But the most inspiring thing of all was the sense that many Americans realise that a low-carbon energy revolution could be the key to America's future prosperity, kick-starting the American economy.

And, despite its obstructive position in Denmark, - make no mistake about that. The country is also investing billions trying to steal a march on the rest of the world.

Indeed, the coming decade should see an international race to develop the world-beating sustainable technologies.

The question now is whether these two trends - the move to a low-carbon economy and falling fertility - will happen fast enough.

But there is a further factor to consider when assessing how ethical it was to have Elsa.

Another commenter on my blog last week, Krupt, hits the nail on the head when he writes "20% of the worlds population consumes 86% of the world products and food. Bon appetite."

It's .

"To suggest", he concludes, "as many of my correspondents do, that population growth is largely responsible for the ecological crisis is to blame the poor for the excesses of the rich."

Which, unfortunately, still doesn't get me off the hook.

Let's not beat around the bush. Christmas is a carbon catastrophe and the reason is our ludicrous culture of present giving.

I know I'll be called a mean old Scrooge but here's the Christmas manifesto of this (former) Ethical Man: if you must give, give money.

I wrote these words two years ago but I'm pleased to see that more people seem to be coming round to this way of seeing things.

Professor Joel Waldfogel, who's been writing on the subject for far longer than me has been and he gave a great interview to Â鶹ԼÅÄ Radio 4's More or Less programme.

The professor, author of , is an economist. He's concerned about the $25bn of "missing satisfaction" incurred every Christmas, because gifts are under-appreciated. I'm more concerned about the carbon - but we reach the same conclusions.

You don't have to want Tiny Tim to starve in the workhouse to recognise what a bloated consumer nightmare the festive season has become.

Take my family, for example. We try to meet up over the Christmas period but I have mixed feelings about it.

Not because I don't enjoy seeing everyone and eating and drinking far too much before falling asleep in front of the telly. No, the reason is that I hate giving presents.

There you go you see - you are thinking I'm an old Scrooge aren't you?

Well I say bah humbug to that. I hate receiving presents too and here's why.

I've got three children and they have 10 cousins. If each of my children buys everyone a present that's thirty-six presents. If all the cousins do the same we're talking 13x12 presents - a staggering 156 in all.

And that is just the start. We've got to get presents for my folks, my three sisters and their husbands, my mother-in-law and her partner as well as my father-in-law, my wife's grandmother and her brother. Then, of course, they've all got to do the same for us. So that's another 9x8 presents - 72 more gifts - even if we assume that couples get just one present.

So my direct family alone could buy each other as many as 228 presents every single year and that's before I've popped over to my cousin Xand's for a mulled wine or two (another present plus one for my Aunt Anthea) or even begun to think about presents for friends (not that I do, you understand).

So why are these 228 presents a carbon catastrophe? That's simple: because every single one has a carbon cost. It wouldn't be a problem if this glut of gifts were actually useful. But be honest, when was the last time you actually got something you wanted or more importantly, needed?

My children now get so many presents that we collect up the ones we don't like and give them to the local charity shop. I know it sounds cruel, but we live in a small house and we would be engulfed in a veritable tsunami of tat if we didn't.

The real problem is that giving presents is an inherently inefficient activity. It means guessing what someone else may want or need. Every now and then you'll buy the perfect shirt but more often than not the ornament or tie or garden thermometer will end up in the attic or more likely in a landfill site and all the carbon that went into making it is completely wasted.

A few decades ago you probably needed the socks that your mum gave you or the saucepan she was given by her Aunt. These days it is different. Consumer goods are so cheap and plentiful that even people on very low incomes have no shortage of stuff.

Indeed, if you need proof of how corrupt our present giving culture has become look no further than the "gift" shops that have colonised every high street. You know the ones; they sell things no-one wants like scented candles, little vases and foot massage kits. Nevertheless they seem to do reasonable business.

Some of you may be thinking that I'm missing the point. You're thinking that present-giving isn't about the inherent value or utility of the gift but is about the act of giving itself.

I'll concede there's something in that, but gifts don't have to be useless. I've got a rule of only buying consumables as presents: food and drink (and sometimes fireworks). At least you know someone is going to enjoy them.

But surely it would be more ethical if we all bought each other stuff that you knew we really needed. Stuff like washing up liquid, toilet paper and breakfast cereal. Or better still, cash.

I've never understood why giving money is considered bad form. Wasn't that £5 note folded into Granny's card the very best present of all? You could use it to buy something you actually wanted. Not only that, until you buy something, cash is completely carbon free.

Hence my Christmas manifesto.

Can I apologise right now if the content of this blog dampens your Christmas spirit? It is about something many of us believe we should do, but very few of us actually get round to doing.

It was certainly the hardest thing I did during my "year of living ethically" for the Â鶹ԼÅÄ.

But Adolf Hitler managed it and so did Linda McCartney. Indeed, the government's .

Are you there yet?

Yes, I am talking about giving up meat. Or, in my case, giving up all animal products.

But I should warn you we started our exploration of the ethics of what we eat with a lustrous Norfolk Black turkey chick we named Ned.

We watched him grow into a magnificent one-and-a-half stone stag... and then came Christmas.

Viewers with a sentimental nature should NOT watch this film.

I said at the time that I regretted not killing Ned.

"An ethical man should be able to stomach dispatching his own supper or should decline to dine upon it, shouldn't he?" I wrote.

And I am sure lots of us carnivores would be a lot less keen on our mixed grills if we had to look all the animals that go into them in the eye before they were served up on our plates.

But this blog isn't about sending you on a vegan guilt-trip - though if that's what you want, you can learn more about the mechanics of turkey slaughter here.

Neither is this blog about the bizarre animal ingredients I discovered might be lurking in even the most innocent-seeming foods - bread anyone?

It is also not about the incredible health benefits I experienced from my brief flirtation with ethical eating - I shed 2kg in 31 days and saw my cholesterol level plummet from 5.6 mmol/L (rather high) to just 3.4 mmol/L (very low for a man of my age).

Nor is it about how . Everyone knows that now - though, if you will allow me a little boast - we in the Ethical Man team pretty much got their first.

So what is this blog about?

It is about another aspect of the food we eat - the threat of an impending food crisis.

There was a hint of what could be to come back in 2007-8 .

Well, don't imagine that the worldwide depression has got us off the hook. Food prices have risen dramatically this year even as economic activity has fallen.

According to the UN Food and Agriculture Organisation (FAO) a billion people on earth will go hungry this year - one in six of the world's population. That's a thought that will haunt you as you sit down to enjoy your Christmas dinner isn't it?

But, lets be clear about this, there is no shortage of food in the world. Agricultural output is pretty near its historic high. So why are so many people going hungry?

The problem is that, increasingly, we don't actually eat the food we grow. Some is converted into bio-fuels - and rising oil prices makes that more profitable - but even more is used to fatten up the animals so many of us eat.

There has been a huge increase in meat consumption around the world in recent years. That trend should be a cause for celebration because it reflects that fact that people in developing countries are getting significantly richer. One of the first things people do when their income rises is to buy themselves some meat.

The problem is, these trends - coupled with population growth (which I will be discussing next week) - .

The FAO estimates that by 2050 the amount of food available in developing countries will need to double - which is the equivalent of a 70% increase in food production.

We would need a lot less if people stopped eating meat because it would require so much less land.

It is yet another powerful argument for changing our diet. So the question is: how can we get people to change what they eat?

We can try persuasion, working through some of the arguments, as I have here. But don't underestimate how difficult it is to change people's behaviour on this.

If you want a measure of just how tough a problem this is to crack, look no further than me.

I know the arguments pretty well (I hope you will agree) and I've experienced the health benefits first hand. But I will still be sitting down to a turkey dinner come Christmas.

So perhaps some gentle coercion might therefore be more effective. There is already a lobby for "fat taxes" - higher taxes on fattening foods. It is a short step from there to taxing foods that have an adverse impact on the environment.

But would any politicians have the courage to impose a tax on meat? They are reluctant enough to impose taxes on other, more directly polluting, behaviours.

There may be other ways - please use the comment box below to send in any ideas you have - but, in the meantime, I have two suggestions for determined meat eaters who want to reduce the environmental impact of their food.

First off, eat less meat - that's something my family is doing (though not this Friday).

The second is even more straightforward, actually eat the stuff you buy!

In developed countries a quarter of all the food that is produced goes uneaten, most of it no doubt growing mould at the bottom of all our fridges.

So here's a festive challenge: I want you to craft that limp carrot, half-eaten packet of cheese and the remains last night's pizza into a delicious Christmas spread. It has to be possible to rustle up something palatable... doesn't it?

I don't mean one of your stray locks that fell into the butter. What I want to know is whether amino acids produced from human hair were used to process the flour that went to make that piece of toast you wolfed down on the way to the bus stop.

It sounds unthinkable doesn't it? But when I became a temporarily became a vegan for Newsnight, I developed a keen interest in what goes into the food I eat and I discovered that a food additive which is sometimes produced from human hair can be used as an additive in some baked goods.

But first, the veganism. I did not do it out of high principle. The idea was to test the claim made by a number of people who e-mailed in to insist that becoming a vegan significantly reduces one's impact on the environment.

I was vegan for one month- January 2007. So this did not preclude me eating Ned the Newsnight turkey for Christmas 2006.

I am happy to report that Ned was as tasty as he was ethical. My family gnawed our way through his ample carcass over the course of a full week. We ate Ned roast on the big day, then sandwiched, curried, as a supreme and finally in a tasty soup. Then, as the last few slices of Ned grew an extravagant mould in the bottom of our fridge, the New Year turned and my diet became completely meat and dairy free.

It wasn't easy. I did not just cut meat and fish out of my diet. Vegans don't eat any animal products including milk, eggs and honey. So did cutting out all animal products reduce my carbon footprint?

I need a bit of persuading about the bees but cows certainly produce an impressive quantity of greenhouse gases - some 500 litres of methane a day per animal.

When my vegan experiment was just getting under way, the then environment minister . Agriculture is reckoned to account for 7% of all greenhouse gas emissions, about the same as aviation.

And methane isn't the only issue. It is claimed that one acre of arable crops can produce enough food for up to 20 people. Turn that field over to beef production and it will feed just one person.

Not only that, raising animals is a lot more carbon intensive than growing vegetables. David Pimentel, an ecologist from Cornell University, has calculated that animal protein production requires more than eight times as much fossil-fuel energy than plant protein yet yields proteins only 1.4 times as nutritious for humans.

That's the average. When you look at individual sectors the figures are even more startling. Take beef, for example. Using US Department of Agriculture figures he found that beef production requires an energy input to protein output of 54:1 (as well as 100,000 litres of water per kilogram of meat).

Vegetarians shouldn't feel too smug, though. Milk protein has a ratio of 17:1. In fact, rather depressingly the most efficient form of animal production - perhaps not surprisingly - is battery chickens. .
My problem has been eradicating all these inefficient animal proteins from my diet. Take my very first day of vegan living, New Year's Day.

I hadn't prepared very well and hadn't got any margarine in. The local corner shop, a Londis, was open and they stock a good range so I wasn't too worried. But as I worked my way through the eight or so different varieties of margarine I was amazed to find that every single one contained milk or dairy products in some form.

It makes you realise just how common the use of animal products in food is. Before I became a vegan I would eat animal products in every single meal. Indeed the points out that some vegans consider tap water unacceptable because it contains chemicals that have been tested on animals.

I am not going that far but I have certainly developed a mania for reading food labels and there are all sorts of unexpected animal additives.

Most people know that gelatine is produced from animal skin and bones and that the rennet used in some cheeses comes from calves' stomachs. But did you know that bone char (from cow bones) is still occasionally used to whiten some sugars or that some wines and many beers (particularly real ales) include isinglass - a substance obtained from the swim bladders of fish?

Which brings me back to the possibility that human hair may be used in bread. A vegetarian friend alerted me to the existence of an animal-based flour additive called L-Cysteine. It is an amino acid which is used as a flour improver. It is known as E920 and is permitted for use in all biscuits, breads and cakes except those that claim to be wholemeal.

The problem for a would-be vegan is that traditionally L-Cysteine is produced from feathers, pig bristles and sometimes even human hair. These days L-Cysteine can also be produced synthetically but apparently human hair remains one of the richest sources of this amino acid - it makes up about 14% of your hair - and there is a small industry in China making the additive from hair clippings.

There's even a paper on the web written by a Rabbi about whether L-Cysteine from human hair is kosher.

So how commonly is L-Cysteine used? My vegetarian friend claims that the problem with E920 is that - even when it is used - it doesn't have to be listed in the ingredients. She says that's because it is broken down in the baking process so the manufacturers argue that doesn't constitute an ingredient.

That is something the Food Standards Agency flatly denies. It says that L-Cysteine must always be labelled. Indeed, the industry says the reason you so rarely see E920 on labels is that these days it is very rarely used (apparently it was much more common fifteen years ago). The industry also says that the only L-Cysteine their members would use is the synthetic variety.

That is a little odd because according to the Food Standards Agency the European regulation specifies that only L-Cysteine produced from duck and chicken feathers or from pig bristles can be used. That means that, so long as your daily bread was baked in Europe, it almost certainly does not include human hair.

But it's a little confusing. If British bakers are using synthetic L-Cysteine are they breaking EU guidelines? It is hard to get a straight answer. Biscuit makers told me it would be added when the flour is milled - and the millers say it's something the bakers would add.

So if anyone can put this hairy issue to bed once and for all I'd be very grateful. And while I am on the subject, if anyone knows of any other animal-based (or human-based) food ingredients a vegan needs to steer clear of, please do tell me.

NB - Look familiar? This blog is made from 100% recycled material from Justin's 2007 Ethical Man series - keeping it ethical.

The word "sceptic" is in danger of becoming a term of abuse. A "climate sceptic" is used to mean someone who rejects the evidence of global warming. But scepticism is actually a healthy instinct and should be celebrated.

We are lucky here in Britain to be home to the most august scientific institution in the world, . It celebrates its 350th anniversary next year. Its motto is "nullius in verba" which means "take nobody's word for it" - which is pretty much a charter for scepticism.

It is a fitting motto, because healthy scepticism is the foundation of good science.

The urge to question accepted truths, to doubt received wisdom, to investigate things for yourself, is the basis of scientific enquiry.

So let's not damn people for being sceptical of the climate science... unless, that is, they don't make the effort to make a reasonable examination of the evidence.

The opinion polls suggest that .

So Newsnight decided to do a little (and very unscientific) experiment of our own.

We challenged two leading British scientists to try to prove the science of global warming to a group of people whose views very loosely reflect national opinions.

And, as if that wasn't tough enough we asked them to do it in my kitchen.

Can they do it? Well, you can see for yourself.

Obviously, we had to radically cut down the scientists' presentation to squeeze it into the tight TV time constraints but if you want to dig a bit deeper into the science of global warming the best place to go is the .

The is more accessible to non-scientists, or you might try which has a section on climate change science. There's also loads of stuff .

Get stuck in!

I'm happy to celebrate the instinct to question authority because scepticism is also the basis of journalistic enquiry.

Right from the get-go, the idea of the Ethical Man series was that it should be a sceptical inquiry into .

That's why I have no problem reporting that micro wind turbines don't work or that cars can sometimes be more carbon efficient than public transport - if that is what the evidence suggests.

But not everyone is comfortable with scepticism.

A couple of days after my blog on cars was published, I was shocked to find an e-mail from an environmentalist who said it should never have been posted. He made no effort whatsoever to refute my claims, his argument was simply that it undermined the debate to publish such heresy.

That instinct to suppress evidence that challenges preconceptions is very dangerous. Any hint that the climate change science is anything other than transparent will - understandably - encourage people to be even more sceptical.

If it looks like that's what people are trying to do, the scientific community should be very clear in its condemnation - as Sir David King, a former chief scientific adviser to the government, was in my kitchen.

Because, if our "experiment" says anything it says that people are hungry to understand the science of global warming and that when the evidence is explained clearly, lots of people find it very persuasive.

So let's celebrate scepticism and follow the Royal Society's injunction to interrogate accepted wisdom. But let's do it with an open mind and on the basis of a reasoned examination of the evidence.

That's how scientific hypotheses get proved and - yes - disproved. And wouldn't it be great if someone proved the science of global warming was wrong?

The most dispiriting thing about trying to live a more environmentally friendly lifestyle is that it is all about not doing things.

We are told we have to stop flying, stop driving, stop eating meat, stop heating our houses... the list goes on and on.

So it is a nice change to be told that there is something you can do which will reduce your impact on the environment AND requires that you buy yourself a nice bit of kit to boot.

Bring on the domestic wind turbine!

What could be more environmentally friendly than harvesting electricity from the wind and what better ethical badge of honour than a turbine spinning on my roof?

That's certainly what I thought three years ago, when my family and I were challenged by the Newsnight editor to try to make our lifestyle greener.

Unfortunately, I wasn't the only wannabe ethical man to want to reap the wind. As I began exploring the possibility of erecting a turbine on my terraced London home the .

The question was, who would get theirs up first?

Three years on and neither I nor the Tory party leader have a turbine on our roof.
Why?

The answer is very simple. In most urban locations in Britain wind turbines simply do not work.

Yes, they spin, but they do not generate significant amounts of power. Why not?

Here's the science bit... (don't worry, you will be able to follow it).

A simple equation gives the power of the wind. Power = 0.5 x collection area x the wind speed cubed.

What it tells us is that the power of a turbine is related to two factors: the size of the turbine and the strength of the wind.

Let's look at size first.

Cast your mind back to your GCSE maths (I'm old enough to have done O-levels). No doubt you dimly remember that the area of the circle is equal to the constant pi (3.14) times the radius of the circle squared.

What that means is that as you increase the length of a turbine blade, the collection area increases disproportionately.

Take the micro turbine I was planning. Its blades were 1.75m long, giving a collection area of just under 10sq m. Tiny.

Compare that to the wind turbines I visited in Texas earlier this year. Some had turbine blades 45m long, giving a collection area of 6,358sq m. Huge.

The message is clear from the maths - small turbines have disproportionately smaller collection areas and therefore generate dramatically less power.

And what about wind speed?

The key here is that cube function on the wind speed. The power of the wind is related to the cube of the wind speed. So, at low wind speeds you get virtually nothing. When it really blows it you get a lot of power.

Here's why. Double the wind speed and you get eight times the power. Quadruple it and you get 64 times as much. Eight times the speed and we're talking more than 500 times the power.

The figures given by Windsave, the company that was going to install my wind turbine, confirmed that.

It boasted that its 1.75m turbine would generate 1kW of power at speeds of 12.5m per second.

Pretty good, but 12.5m per second is a force 6 wind, a decent breeze.

Halve the wind speed to six meters per second (a moderate breeze) and - thanks to that cube law - you now get just 120 Watts - that's two standard incandescent lightblubs (10 energy friendly compact fluorescents).

Hum, not bad.

My house is on the flanks of the highest hill in London and is relatively exposed but I'm told that average wind speeds are likely to be between 4m and 5m per second. (You can find out the wind speed in your area .)

At those speeds I'd be lucky to get 25 Watts. That is barely enough for two energy saving light bulbs. Nowhere near enough to live up to the company's promise of reducing my electricity bills by "up to 30% a year".

The message is clear. In most UK locations micro wind turbines will never generate significant amounts of electricity.

It makes a nonsense of the claim made by , when I was planning my turbine, that domestic wind turbines could supply 4% of all the UK's electricity needs and cut carbon dioxide emissions by 6%.

It also suggests the government should think again about offering a generous feed-in tariff for power generated from micro wind turbines.

And, if any more proof of my point was needed, in September this year .

Of course, not all wind energy is a dead end. What our calculations tell us is that power increases dramatically as you increase the size of the turbine and the wind speed. So, a 10m turbine in a 10 knot breeze generates 100,000 times the power of a 1m turbine in a 1 knot breeze.

Indeed, if Camden, my local council, gave me planning permission for one of those Texan whoppers it would generate significant power - something like 200kW - even at 4m per second.

But even these impressive figures can't disguise the inconvenient truth about wind power: except in storm conditions it is - compared to fossil fuels - a very dilute energy source.

Professor David MacKay, the new chief scientist at the Department for Energy and Climate Change, has done the maths on this. Instead of kW, he calculates power in kWh, and he estimates that if we put wind turbines across the windiest 10% of the country, .

According to MacKay, it takes 40 kWh to drive the average car 50km.

Add in offshore turbines covering a third of the available shallow water locations (44,000 turbines) and installing deep water turbines in a 9km-wide strip all round the entire British coast and you get an additional 48kWh day per person.

That's a lot of power, but even on quite conservative estimates .

It leads to a dispiriting conclusion. Wind is, at best, only a very partial solution to the problem of how to generate low-carbon energy.

Here's the challenge: how do you illustrate, in a television report, the impact of flying on the environment?

It isn't as easy as you might think.

Three years ago, I pondered the problem with the Ethical Man producer, Sara, for a couple of days before we had a moment of genius.

Our solution infuriated my wife, enraged my colleagues and alienated a large section of our audience but I still stand by it.

So what was this brilliant idea?

We decided that I should illustrate the impact of flying by jetting off for a weekend in Jamaica.

I know. That's what my wife thought too.

And to make matters worse I filmed the moment I revealed our plans to her. You can watch my marriage disintegrate here.

Please tell me if you think our approach was misguided.

For the record, here's my defence. We reasoned that if you watched our film and thought the idea of a man calling himself "ethical" flying off to Jamaica for the weekend smacked of hypocrisy, it might make you reflect on your own behaviour and consider flying less.

And, because we were keeping a tally of my carbon footprint, we reckoned the record of my flight would serve as a reminder of just how carbon-intensive flying is.

I did say this at the time but, judging by the avalanche of outraged emails we received, it fell on deaf ears.

The truth is, flying is the single most intractable climate change issue.

There is a solution to most of the other stuff - we can cut our energy use, change how we generate power, drive electric cars, eat less meat etc... but there is no alternative to flying.

And how we love to fly. A least one foreign holiday a year now seems to be regarded as pretty much a right of citizenship. Which is why politicians are so worried about stopping us doing it.

Why is flying such a problem?

Let's start with the good news: a couple of years ago I looked into the numbers and found that modern jet aeroplanes are actually a very efficient form of transport .

The jet engine is actually one of the most effective ways to convert the energy from fuel into thrust. The best jets are 37% efficient. By contrast modern petrol engines are around 25% efficient while a finely tuned diesel will achieve, at best, 32% efficiency.

How does that translate into actual fuel consumption?

Take a look at some figures: my old car - a two litre petrol Saab 9-5 estate - uses 8.6 litres per 100km.The most efficient cars do better than that. The Toyota Prius, for example, is much more frugal. It uses 4.3l/100km.

So what about aircraft? The average jet plane now uses around 4.8l/100km per passenger - just a little worse than a Prius with no passengers. But the manufacturers say modern jets are much more efficient.

Airbus claims it makes the most efficient aeroplane currently flying, the A380. It says this behemoth uses just 2.9l/100km per passenger. (Here's the dull bit: that's the fuel consumption when you assume a three class configuration operating at capacity with 525 passengers.)

So far as I can tell, the latest jumbos are similarly efficient - it is hard to be certain because the manufacturers do not publish comparable figures - but Boeing's 747-8 uses 3.7l/100km per passenger when operating at 70% capacity. (Assuming it is configured to hold 470 passengers in three classes.)

So, if jet engines are more efficient than car engines why do they get such a bad rap?

The anwer is pretty obvious - we use planes to travel extremely long distances. That weekend in Jamaica racked up just over 15,000km. That's pretty much what the average British driver would do in an entire year.

The other problem is that planes release their pollutants high up in the atmosphere where they have an even stronger greenhouse effect. The process is known as radiative forcing and means aircraft emissions are reckoned to be almost twice as damaging as emissions at ground level.

So, combine the distance you fly with the effect of radiative forcing and you can see why our appetite for air travel is so worrying.

You can do as I did - get rid of your car , switch to energy efficient bulbs , eat locally grown food - but take one holiday flight and you will erase all your careful carbon cuts.

So what are the alternatives?

Here's the rub. As my figures show, even if you did take the car instead of the plane you would still emit huge amounts of carbon, because of the vast distances covered in most journeys by air.

I've already done the maths on trains and buses. If you pack them full of passengers they will offer some carbon savings but, like cars, they leave the tricky little challenge of crossing oceans. Boats will do that job but there's a hefty carbon price to pay there too.

And these alternatives ignoring perhaps the most important feature of flying: it is extremely fast.

Lots of environmentalists regard speed as some kind of crime but much (not all) time spent travelling is time not spent doing something else - often a productive activity. So there is an economic cost to slow travel.

So what's the answer?

It's a tough one isn't it? Do write in if you've got any good ideas.

In the meantime how about this: establish a price system that accurately reflects the impact that carbon emissions have on the environment? That way the price of a plane ticket would include all the costs of our holiday.

It would mean we would all fly less, of course. But, given the problems my weekend in Jamaica caused me, maybe that would not be such a bad thing.