Save the trillionth tone

On 22 October 2009, a tonne of coal, designated the trillionth tonne of carbon that we will release into the atmosphere since pre-industrial days, was unveiled at the Science Museum in London. For Dr Myles Allen, lecturer in atmospheric physics, the trillionth tonne could be a very powerful symbol of our profligate use of energy and disregard for the environment. Or, depending on how long it survives before being consigned to the flames, it could embody our careful stewardship of resources and responsible use of them. A new website, trillionthtonne.org, hosted by the Oxford e-Research Centre, carries a ticking meter showing how fast carbon is being consumed. It currently predicts that the Science Museum’s tonne of coal will go up in smoke in 2045.

The concept of the trillionth tonne was born in an important paper published by Dr Allen in the journal Nature in April 2009, together with colleagues from other Oxford departments, the Met Office and the Potsdam Institute for Climate Impact Research. They argued that, given the evidence from climate modelling, it is difficult to support the current approach to framing targets for the control of carbon emissions. These targets are based either on reducing rates of emission – a 40 per cent reduction by 2020, for example – or on keeping the concentration of CO2 in the atmosphere below a predetermined level, such as 350 parts per million (ppm). ‘The difficulty is that [the emission rate] does not determine the risk of dangerous climate change’, says Dr Allen. ‘As for the 350 ppm target, we’ve already passed this.’

More or less everyone agrees that an increase of global temperatures of more than 2˚C would lead to dangerous climate change, and emissions quotas have been proposed nationally and internationally in the hope of avoiding such an increase. Dr Allen and his colleagues argue that the relationship between a particular level of atmospheric CO2 and the amount of warming – known as climate sensitivity – is still too uncertain to form the basis of such policy decisions.

Frustrated by this lack of scientific certainty, the team looked instead for a measure that would provide a more robust correlate of rising temperatures than either the concentration of CO2 or the rate of emissions. Comparing a variety of climate models, they demonstrated that the cumulative total of carbon released into the atmosphere since the beginning of the industrial age could be linked more consistently to the peak rise in temperature than either emissions rate or CO2 concentration. They predict that the most likely figure that will take us past the 2˚C barrier is a trillion (1,000,000,000,000) tonnes. ‘We’ve released just over half a trillion already’, says Dr Allen. ‘If we release the same again, we’ll be committed to 2˚C of warming, unless we extract it again before the system begins to warm up.’ In other words, we have a finite budget for carbon emissions, and the only way to avoid overspending – and triggering dangerous climate change – is to work towards eliminating emissions altogether. Hence the symbolic value of the trillionth tonne.

The preoccupation of policy-makers with percentage emission rate cuts stems from the wording of the UN Framework Convention on Climate Change, agreed at the Rio Conference in 1992 and made more binding at international conferences in Kyoto (1997). ‘That set us off in the wrong direction’, says Dr Allen. ‘It committed the parties to “stabilise” atmospheric greenhouse gas concentrations at a “level” that would avoid dangerous anthropogenic interference in the climate systems. The wording of the convention specified the problem that scientists had to solve, and it turned out to be very difficult: predicting the response to a stable concentration over an unspecified length of time turns out to be virtually impossible.’

In Dr Allen’s view, the focus on emission rates has already led to some bad policy decisions. ‘Europe’s favourite way of reducing emissions is to replace coal-fired power stations with natural gas’, he points out. ‘That leaves Europe feeling virtuous, but it has depleted gas reserves. Gas is a versatile fuel, and we should be conserving it to power transport in 50 years’ time, not burning it in power stations now.’ As for the drive towards increased energy efficiency, he concedes that it will buy us time, but argues that it does not go nearly far enough. This is depressing news for all those who have been virtuously replacing their boilers and double-glazing their homes. Instead, says Dr Allen, we should be badgering our energy suppliers to start capturing carbon and burying it, so that their net emissions are zero.

‘There are 4.5 trillion tonnes of carbon sitting underground that are exploitable, depending on how much you are prepared to pay for it’, he says. ‘Are we going to leave it underground? Or are we going to find ways of using it without dumping it into the atmosphere? If burning carbon more slowly does not help, the only thing that will help is starting the process of carbon burial in a major way.’

‘What I find worrying is the lack of debate. It’s not at all clear that the Kyoto process will work. The track record of internationally negotiated quotas is not good. We did not save the ozone layer by rationing CFCs: we saved it by banning them. This paper and others that appeared over the past 12 months all say that eventually you are going to need a ban on the practice of dumping CO2 in the atmosphere, and the sooner we recognise that the better.’

For more information, see Myles R. Allen et al., pp. 1163–6 in Nature 458, (30 April 2009).