CO2 stays 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 US contribution to global warming approaches 30 per cent for CO2 but a complete analysis (with methane and deforestation) shows an 18% share. Brazil’s contribution 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.
They do 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.
We have compiled historical greenhouse gas emissions and their uncertainties on country and sector level and assessed their contribution to cumulative emissions and to global average temperature increase in the past and for a the future emission scenario. We find that uncertainty in historical contribution estimates differs between countries due to different shares of greenhouse gases and time development of emissions. Although historical emissions in the distant past are very uncertain, their influence on countries’ or sectors’ contributions to temperature increase is relatively small in most cases, because these results are dominated by recent (high) emissions. For relative contributions to cumulative emissions and temperature rise, the uncertainty introduced by unknown historical emissions is larger than the uncertainty introduced by the use of different climate models. The choice of different parameters in the calculation of relative contributions is most relevant for countries that are different from the world average in greenhouse gas mix and timing of emissions. The choice of the indicator (cumulative GWP weighted emissions or temperature increase) is very important for a few countries (altering contributions up to a factor of 2) and could be considered small for most countries (in the order of 10%). The choice of the year, from which to start accounting for emissions (e.g. 1750 or 1990), is important for many countries, up to a factor of 2.2 and on average of around 1.3. Including or excluding land-use change and forestry or non-CO2 gases changes relative contributions dramatically for a third of the countries (by a factor of 5 to a factor of 90). Industrialised countries started to increase CO2 emissions from energy use much earlier. Developing countries’ emissions from land-use change and forestry as well as of CH4 and N2O were substantial before their emissions from energy use.