Myles Allen, BBC Green Room 2 Nov 09;
This week sees the final round of preliminary talks on a new UN climate treaty before December's Copenhagen summit, where delegates seem to be focusing on emissions in 2020. Myles Allen argues that they must not lose sight of the much greater challenges that lie beyond 2020 or they risk wasting another decade in the battle against dangerous climate change.
On Thursday, 22 October 2009, a single tonne of anthracite coal was unveiled in the Science Museum in London as part of a new exhibition on climate change.
Not, you might think, anything particularly remarkable about that, except that this is not any old tonne of coal: it will be, as close as we can estimate it, the trillionth tonne of carbon to be released into the atmosphere as carbon dioxide since industrialisation began in the 18th Century.
The Science Museum, London, and University of Oxford are committed to looking after it for as long as it takes, and solemnly escorting it down to a power station or wherever it can be used most efficiently when total carbon emissions from human activity reach one trillion tonnes.
If, that is, that time ever comes.
The trillionth tonne matters because carbon dioxide accumulates in the atmosphere. Once released, it continues to influence the climate more or less indefinitely unless active measures are taken to scrub it out again, which is not something anyone knows how to do on any scale.
Emissions since 1750 comprise of just over half a trillion tonnes of carbon (you can keep track of the number, and the countdown to the release of the trillionth tonne, on the trillionthtonne.org website).
This is estimated to have caused just under 1C (1.8F) of global warming (other things affect global temperature as well but, as it happens, their effects more-or-less cancel out over this period).
So if we release another 500 million tonnes, we commit the Earth to a most likely warming of about 2C, which is widely regarded as the threshold for dangerous climate change, and a rubicon that governments of G8 countries and other major economies pledged this year not to cross.
Over the past couple of decades, carbon dioxide emissions from fossil fuels have risen by an average of 1.6% per year, even allowing for the occasional blip like the collapse of the Soviet Union and this year's recession.
Emissions from deforestation have continued steadily.
Trend setting
If these trends continue, which is a relatively conservative "business-as-usual" scenario, we will release the trillionth tonne sometime in the 2040s - a date that is steadily advancing, as the underlying trend is for faster growth in recent years.
So, will the Science Museum only have to look after this tonne of coal for less than 40 years?
That rather depends on what happens in the next 40 days, at the end of which the climate negotiations will be nearing their climax in Copenhagen.
This week's negotiations in Barcelona will go a long way to determining whether agreement is possible in Copenhagen - and if it is, what sort of agreement.
If governments are seriously committed to limiting global warming to less than 2C above pre-industrial temperatures, then must preserve the "trillionth tonne", not just until 2020 or 2050, but until the whole issue of climate change is ancient history.
Other emissions resulting from human activity are expected, on balance, to add to the warming effect of carbon dioxide in the future, so if we are to keep the overall warming to less than 2C (or, for that matter, retain any hope of carbon dioxide levels eventually recovering back down to 350 parts per million, or avoid dangerous levels of ocean acidity), we cannot afford to release the trillionth tonne, ever.
The longer we put off reducing emissions, the faster they will have to fall to stay within the same total - just as dithering before you leave the house pushes up the speed you need to pedal to make your appointment.
And eventually, if you dither long enough, you simply won't make it.
This is where the trillionth tonne could be useful, even in the next 40 days.
If your government's negotiators in Barcelona and Copenhagen claim they want to keep global warming below 2C, and the chances are that they do, then they should have a plan to prevent cumulative carbon dioxide emissions from exceeding a trillion tonnes of carbon.
If they don't, then they haven't thought it through, no matter what they claim emissions will be in 2020.
This matters, because many of the measures they might take (or, more likely, ask you to take) to reduce the rate of emissions in 2020 would have no effect on, or even increase, cumulative carbon dioxide emissions overall.
Act now or pay later
What can you do?
Clearly, reducing your carbon footprint helps. Emitting carbon more slowly buys time, which we will certainly need.
But to solve the problem in the long term, we need to reduce net emissions, in effect, to zero.
You can't do this on your own, no matter how heroic a consumer you are.
You could reduce your lifetime carbon footprint to zero - by making your home zero-carbon, never use a car and grow your own food - and save the world from dangerous climate change for just a mere two seconds.
So the most important thing you can do is make sure your government recognises the importance of cumulative carbon dioxide emissions in climate policy.
At a previous round of negotiations, in Bonn in June, a group of us presented an open letter to the negotiators urging them to acknowledge the need to limit cumulative emissions of carbon dioxide.
We did not call for a specific cap: just an acknowledgement that the principle would fundamentally alter the focus of future negotiations. The aim would no longer be to ration out emissions; the aim would be to ban them, just as we banned CFCs. We didn't save the ozone layer by rationing deodorant.
As far as we can tell, that request fell on deaf ears: "This was not the focus of the negotiations at present."
Odd, when cumulative emissions of carbon dioxide are the principal determinant of the risk of dangerous long-term human-induced climate change.
And next time you are in London, drop in to the Science Museum to pay your respects to the trillionth tonne.
Take your children. Explain to them that if it is still there for them to show their grandchildren, they will have achieved a lot in the fight against climate change - but not enough.
Only when their grandchildren are showing their grandchildren the trillionth tonne, still safely on display in the Science Museum in the mid-22nd-century, will this whole saga be passing into history.
And wish them luck.
Myles Allen heads the Climate Dynamics Group in the Department of Physics, Oxford University, and is the principal investigator of climateprediction.net
The Green Room is a series of opinion articles on environmental topics running weekly on the BBC News website
