All that additional carbon we worry about in the atmosphere has to come from somewhere. As climate policy has inched forwards over the last couple of decades, working out who put it there, how and when, has become a political as well as a scientific requirement.
Scientifically, the answers are now in better shape than ever. The Global Carbon Project, established a dozen years ago, has created an impressive synthesis of data about carbon emissions and flows which is now used to produce an annual global carbon budget.
It assembles the best information on fossil fuel use, cement production, and land-use change, as well as absorption by land and ocean sinks. The results make vividly apparent the failure to curb emissions. Globally, we have to accept that global emissions of CO2 have gone up three times faster since 2000 than they did in the 1990s (roughly three per cent a year instead of one per cent). That steady increase means that total emissions were 58 per cent above 1990 levels in 2012. In 2013, annual global carbon dioxide emissions are set to reach record 36 billion tonnes.
Underlying the headline numbers is a laborious compilation of data, and a great deal of detail about the source of emissions. The data, and the way it is treated, are also openly available, so everyone can see how the numbers are worked out.
The whole thing is now established as an authoritative source, as Corinne Le Quéré, co-chair of the Global Carbon Project scientific steering committee explains: “We have a history of pulling together information for the public, policy-makers, advisers and NGOs who need information on the perturbation of the climate cycle, essentially”. That has been, she says ruefully, “an awful amount of work”.
Now they need the results to be easier to use for everyone else. The Global Carbon Atlas, launched today, makes that happen. Once you start putting information out there, people ask for more. The Atlas ought to meet the demand the project team have themselves created.
"Publishing the budget is a nightmare for us", says Le Quéré, director of the Tyndall Centre at the University of East Anglia. "We get many, many requests for comparisons. How does New Zealand compare with Australia, and so forth".
Now, everyone will be able to answer such questions themselves. The new site invites visitors to explore, display and download data and figures on carbon dioxide emissions from all sources, cut any way they choose. You can visualise changes over recent decades, and investigate global, regional and national comparisons.
As well as the emissions data displays, there is an outreach section which offers past and future views of carbon emissions and human development, and a researchers' section, which will include global and regional results from several different carbon cycle models.
The Atlas ought to increase the impact of the work invested in the Global Carbon Project, which Le Quéré describes as "trying to assist evidence based policy-making, where everyone’s emissions are out in the open. Having the numbers really kills exaggeration."
Both projects together provide a clearer view of responsibility for emissions. For example, the Global Carbon Project and the Atlas both focus attention on the difference between location of carbon release and the destination of embodied carbon when traded goods have been shipped round the world. This is an effect which, like carbon itself, is largely invisible to most people – millions of shipping containers cross the oceans unseen by the people who end up buying their contents. But it has a big influence on international comparisons.
On geographically based measures some countries, Britain is a prominent example, appears to be doing well on reducing emissions. But including embodied carbon in the national totals shows essentially no overall reduction.
Overall, carbon accounting based on consumption rather than territorial emissions gives a rather different picture of the current global distribution. Both will play a part in future discussions of how to limit the inexorable increase of CO2.
That complex, ongoing negotiation is now often framed in terms of how much additional greenhouse gas we can afford to release if there is consensus on aiming to keep the ultimate global average temperature increase to some agreed limit.
The landmark estimate here was published by Myles Allen and colleagues in Nature in 2009, and suggested that total cumulative carbon emissions should be limited to a trillion tonnes by 2050, and that we were about half way there.
This shifts attention from the target of parts per million of CO2 often discussed – most often 450ppm – to the quantity of emissions it takes to get there. The gap between that total and what is already released is then a kind of global carbon budget.
The basic notion was elaborated in the most recent IPCC report in September this year, which included a table depicting likely temperature responses to a range of projected future emission pathways, on the assumption that "a lower warming target, or a higher likelihood of remaining below a specific warming target, will require lower cumulative CO2 emissions".
Le Quéré has mixed feelings about the rise of this new sense of the term "carbon budget" – not, as the Atlas depicts, what has been spent, but what remains to be spent. “It is a double edged sword. This is the most accurate information scientifically that we have. However, there is the danger that there’s too much focus on sharing the cake at the expense of practical approaches to cutting emissions.” She is keenly aware of the equity issues in plotting past, present and future carbon pathways, as countries which historically began burning fossil fuels in quantity much earlier than others negotiate with those whose total present-day contribution is higher. But the folks who began their contribution earlier aren't going to stop any time soon. "Industrial countries cutting their emissions down to zero? It is hard to imagine".
As far as the future emissions ceiling goes, "I am trying to stress a little that we don't know exactly what the carbon budget is - there are uncertainties. And it depends on your target in terms of temperature rise".
And she speculates about the advantages of renaming the key quantity - calling anything a budget can come out in ways that encourage spending up to the limit. "Maybe we should talk of a carbon deficit! Or a carbon allocation."
Meanwhile, she has high hopes for the Carbon Atlas as a communication aid, because it gives people, including policy-makers, direct access to data. "It's at the base of scientific learning, that you learn by doing, essentially by repeating the experiments that others have done before you. People don't like to believe what they are told, they like to check and that's good". Now everyone can frame their own questions, and discover the answers for themselves. "People can go and check the data for their own country, and compare to any country they are interested in. We hope to let them draw their own conclusions".
The version now live could be expanded in several directions, she says – to include cities' emissions, disaggregate sectors such as transport, housing, industry and agriculture, or include more of a historical perspective. But it is a good start. “It gives us something to build on".
Further reading - there is a longer, Question and Answer interview with Corine Le Quéré in the June 2013 edition of Carbon Management, available on open access.
A presentation on the 2013 Global Carbon Budget is available from our Slideshare.