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On my last entry, TandF1 posted a comment about a subject I've been planning to write about for a while - so what better time than now to delve into it?

The issue is this: how much carbon dioxide should each person on Earth be "allowed" to emit?

Put another way: if emissions of CO2 and other greenhouse gases are to be limited, at some target date, to a figure that science suggests can stave off "dangerous" climate change, then how does that figure break down at the personal level, when shared out among the world's citizens?

(We are not talking here, pretty obviously, only about emissions directly produced by you or me or them over there, but also each person's share of emissions produced by others for things that benefit us, such as heating our house, manufacturing cement to build our house, or creating garden fertilisers to help our house look beautiful.)

As TandF1 noted, the figure that's being bandied about these days is two tonnes of CO2 (or its equivalent) per person per year.

It's derived from the ambition of halving global emissions by 2050 compared with 1990 levels, during last year's summit in Japan. In turn, this may be enough to constrain the global average temperature rise to within 2-3C at most, which (IPCC) would avoid many of the most damaging projected impacts.

Currently, the average Briton produces about 10 tonnes per year - the average US citizen more, the average Chinese or Indian considerably less.

So a great deal of convergence is implied, and I should be remiss if I did not point up here the important role of Aubrey Meyer and the in developing the concept of Contraction and Convergence.

To get a quick idea of how the two tonnes figure translates to everyday life, you would reach it simply by driving a Tata Nano for 20,000km a year, or by taking a return flight from London to Bangkok or from Moscow to San Francisco. It does not, then, sound very much.

(These aren't intended to be exact figures, by the way, merely indicative; just like economic prudence, it depends on how you calculate it.)

is among the leading figures on the political climate change stage that have promulgated the two tonne per person idea.

I had a chat with him a couple of months ago now and raised the question of what this actually means in practice. If the average Briton were to cut his or her emissions by 80%, as the idea implies, what would probably be producing those two tonnes, and what would have to change?

The big one, he said, was agriculture. It's relatively difficult to reduce or eliminate greenhouse gas production from ruminant animals or fertiliser use, so this is where much of our two tonnes per person would probably have to go.

Cutting the amount of meat eaten, as by IPCC chief Rajendra Pachauri, would reduce agricultural emissions, but that's a different story.

So, assuming this goal becomes adopted by a working majority of the world's governments and translated into policy - a huge assumption I know, but stick with it for the moment - every source of greenhouse gas emissions other than agriculture would have to shrink markedly or disappear completely.

So does that mean you wouldn't be able to drive from one end of the country to the other, or fly off to your favourite holiday destination?

The conventional answer to this is "no". According to the UK's , for example, which backs the 80% figure, many low-carbon or zero-carbon technologies would come in to do the job instead.

The sees almost all electricity generation switching to renewables, nuclear, or fossil fuels with carbon capture and we would produce more than we currently use in order to power a substantial slice of transport. A similar picture pertains in the idea developed by Robert Socolow (there are other conceptualisations too).

There are some areas where the idea that there's an easy technological replacement still looks a bit tricky, such as aviation, but the proposals carry no distinguishable whiff of hair-shirts or a bread-and-water diet.

There's also the question of carbon offsets, where rich countries (or potentially even rich people, if we had ) pay others to reduce emissions for them - which could allow the rich to continue above two tonnes, while financially rewarding poorer people for staying below the figure.

However, other factors might conspire to reduce the two tonnes figure. TandF1 points out that population growth is one: the more people on the planet, the lower each person's emissions would have to be under this kind of regime.

The , prepared by the Chinese Academy of Social Sciences and released in December at the , adds another factor to the mix: historical equity.

It starts with the standpoint that the warming by, say, 2050 is caused by all the emissions that have gone into the atmosphere by that point, starting from the dawn of the industrial age.

Each country, it argues, should be entitled to a share of that total amount that is proportional to its population over the period.

By this measure, countries that began emitting early, such as the UK, have by now already used up more than their share. The report proposes a financial mechanism whereby westerners would have to buy spare shares from developing nations, the annual price tag measured in hundreds of billions of euros.

I won't go into the financial aspects here; but the important bit from the carbon sharing point of view is that this approach to the issue effectively lowers the two tonnes per person figure even further if you live in a rich, developed nation.

(There is a fair amount of wrangling in learned discourse as to the merits of the "historical equity" idea, as you might guess. Some commentators argue it makes today's western citizens responsible for the actions of previous generations who knew nothing of radiative forcing; others say true, they didn't know, but developed countries still benefited from the economic growth associated with those emissions, and so should pay up.)

If things weren't already complicated enough, the Chinese report also points out that equal historical shares for all might not, in fact, be equitable.

If you live in a country where the weather's generally very hot or very cold, you might need to use more energy than in places where the seasons run more equably.

In a country with a sparsely-spread population, such as Canada or Australia, it might be reasonable to allow transport emissions a bit more leeway, and if your country is rich in coal but poor in natural gas, again, your allowance should also be bumped up a bit.

(The country that does best out of all this, by the way, is Russia, which nets about a 50% increase in its permitted pot.)

So let's go back to the original question; how much carbon dioxide should each person on Earth be "allowed" to emit?

Two tonnes by 2050 might be a starting point for discussions; but precisely how much, and by what means, are clearly questions where important nuances pertain.

Where there are grey areas, there is also much room for political wrangling; and of course any agreement on contraction and convergence towards some figure like two tonnes, with whatever caveats, in the end has to be negotiated between governments.

Now, my guess is that some of you are going to approve of the idea and think the world should move towards it as soon as possible, and others are going to hate it, and I'm sure you'll post comments as usual.

But for those turned on by two tonnes or thereabouts, I have a particular question: politically, how can it be turned into reality?

I ask not because I endorse the idea - that's not my job - but because I am struggling to see, through the realities of credit crunch and business pressures and electoral concerns, a political path that leads to its adoption, and I'd be interested in seeing whether anyone else has succeeded where I am failing.

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