China is the new number one cause of global warming, yes? It just got ahead of the US, right? Maybe not. It all depends how you look at the numbers, says a new analysis that finds new heroes and villains in the story of global warming.
China's annual carbon dioxide emissions have indeed recently surpassed those of the US. Each country is responsible for about a fifth of global annual emissions of the most important greenhouse gas. But CO2 sticks around in the air for a century or more, so to measure the warming that a country can be blamed for, you need to measure not current but cumulative emissions – over centuries.
Industrialisation is new to China. As a result, when historical accumulations of CO2 emissions over the past century are taken into account, its contribution sinks to around 10 per cent. On the other hand, contributions from European industrial pioneers like Germany, France and especially the UK, with their long-accumulated legacies of emissions, increase significantly.
The findings come from the first comprehensive country-by-country analysis of who has contributed what to climate change to date, from an international team headed by Niklas Höhne from the Cologne, Germany, office of Ecofys, an international climate change consultancy.
Crucially, the team didn't take the usual short cut of looking at CO2 alone. Other greenhouse gases warm our world: the second most important is methane, produced by a range of human activities from draining marshes and deforestation to cattle farming. Methane does not accumulate as CO2 does, because its "half life" in the atmosphere is only around a decade, but while it is there it is a potent warmer.
When historical emissions and gases other than CO2 are all included, how do nations shape up?China looks good – its current CO2 emissions are high, but most are recent.
More surprisingly, perhaps, the US also comes out better than expected. It has been the world's largest emitter of CO2 from burning fossil fuels for most of the past century, so it's not surprising that if historical industrial CO2 emissions alone are taken into account its contribution to global warming approaches 30 per cent.
But once other gases and emissions from non-industrial causes like deforestation are included, that figure drops to around 18 per cent.
Spreading the guilt
Others do worse, says Höhne. Once methane and historical emissions were included, the team found that some poorer countries are contributing much more to current warming than would be expected from their CO2 emissions alone.
Brazil's contribution, for instance, rises from around 1 per cent to 5 per cent of the global total, and India's share doubles. The contribution of Indonesia, a hotspot for deforestation, rises by a factor of between 10 and 90, say the authors. The range depends on estimates of emissions due to deforestation in Indonesia. By some estimates, Indonesia is the third greatest contributor to global warming after the US and China – ahead of Russia.
These numbers matter, both for the atmosphere and for the politics of negotiating limits on greenhouse gas emissions. Some developing countries want historical contributions to be taken into account when setting those limits. It is ironic that one of the "losers" in Höhne's analysis is Brazil – which, back in the 1990s, was the first country to suggest the historical approach.
Journal reference: Climatic Change, DOI: 10.1007/s10584-010-9930-6
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