The carbon offset industry cannot agree on how much to charge for CO2 emissions, nor how much you produce when you fly, writes Fred Pearce

Flying somewhere this Christmas, or planning a ski trip? Arguably, given the carbon emissions involved, you just shouldn't.

But if you do, will you offset those flight emissions? Some people fuss that the offset companies are a green con. How do we know the trees we pay for won't die? Are we just subsidising renewable energy projects that were going to happen anyway?

Fair questions. But questions for another day. I have another problem. Why does the price of offsetting vary so much? Are we being ripped off?

Spend even a few minutes searching the internet offsetters and you will find two things. First, the prices charged for offsetting every tonne of CO2 you emit vary hugely. Second, the offsetters can't even agree on how great your emissions are for any particular flight.

Let's start with ClimateCare, based in Oxford. I have offset with them before, because I like the people and the projects. For a return economy flight from Heathrow to JFK in New York, they reckon my emissions are 1.53 tonnes. Earlier this week they wanted to charge me nearly £9 a tonne, making a total of £13.22. Type in your credit card details and it's done. Your money goes to fund some cooking stoves in Cambodia or wind turbines in Inner Mongolia.

But a more or less random sample of other offsetters this week provided me with some very different offers. The London-based CarbonNeutral company and Carbonpassport in Glasgow both say my New York return journey emits just over 1.3 tonnes. Terrapass in San Francisco puts it at just 0.84 tonnes. While Atmosfair in Berlin suggests I will be responsible for 3.48 tonnes. All are measuring the CO2 the same way; all are assuming a regular economy flight. The differences are baffling.

Then there is the price charged per tonne, which ranges from £17.50 at Carbonpassport to only half that with CarbonNeutral.

Put it all together, and Terrapass swears that I can offset my transatlantic hop with them for a measly $11.90 or £8.00. CarbonNeutral sound competitive at £11.90. But Atmosfairs wants €81, or £69.85.

And my spot survey didn't find the full range. A couple of weeks ago, Paul Hooper of Manchester Metropolitan University's centre for air transport and the environment published his own study, conducted last winter, of more than 42 online offsetters. He found a sixfold difference in the price charged per tonne of carbon emitted. And, taking in the higher charges that some offsetting companies make for a bigger, business- or first-class seat, discovered price tags for a return trip from London to Sydney that ranged from £9.48 to a staggering £643.39, almost a 70-fold difference.

Now, if I was buying a laptop or something similar and got offered such a range of prices, I'd probably just pay the least and send it back if it didn't work. But with offsets, there is nothing to take out of the box. At the end of the day, I have no real idea what exactly it is that I have bought. And maybe it is ethically better to pay more. The offsetters are all supposed to be good guys, doing their bit for the planet, after all. The more money they get, the more they can help. But maybe not.

So what's going on? I'm still not quite sure why some companies reckon they can absorb a tonne of carbon so much more cheaply than others. I'd welcome inside information on this from any companies not delivering.

But after a bit of pestering, I have established why they can't agree on the mileage. There are a few technical things like how full you assume the plane is. And maybe the odd discrepancy over flight routes and aircraft type. New planes generally emit less. But the big difference is a scientific disagreement.

It turns out that the companies with low emission estimates simply calculate how much carbon dioxide planes kick out of their engines per passenger-kilometre. But the rest try to factor in other emissions from the engines that also add to the global warming. Things like the contrails and the nitrous oxide emissions that do a bunch of different things to atmospheric chemistry that I won't go into here.

The problem is that factoring these in is complicated. There is no single answer. Some companies reckon these emissions double the global warming effect. Some triple it. Some go even higher.

This is because the answer depends on timescales. If you mostly care about the short-term effects over the next decade or so, then these other gases are big players. But if you have your ambitions set on protecting the climate for your grandchildren, then they will have long since gone, while the CO2 will still be hanging round in the atmosphere.

You would have thought the offsetting companies might have come up with some agreed rules about how to measure the overall global warming impact of greenhouse gases. But they haven't. Instead confusion reigns.

Once, we might have shaken our heads indulgently, thinking that at least they are encouraging us to cough up our cash for good projects that somewhere along the line will help clean up the atmosphere. Maybe the details don't matter too much.

After all, you wouldn't insist on personally checking the health of an Oxfam goat before giving that to your nearest and dearest for Christmas.

But in recent months, there has been a shake-down in the carbon offsetting business. The start-ups are being taken over. The enthusiasts in cardigans and riding bicycles are giving way to money men in sharp suits driving limos. A few months ago my own favourite, ClimateCare, got gobbled up by Wall Street investment bank JPMorgan. Call me prejudiced, but suddenly I don't want to give them the benefit of the doubt any more.