Working out who is responsible for climate change is important if we wish to share the cost of mitigation and adaptation strategies fairly. But calculating each country's contribution over time is no easy task. Previous calculations have included the effect of greenhouse gases and sulphate aerosols, but until now no-one had taken into account the full spectrum of short-lived gases and aerosols, ozone precursors and land-use change.

Now Daniel Ward and Natalie Mahowald, both from Cornell University, US, have created the most comprehensive rankings of climate change responsibility to date. To do this, they computed the radiative forcing from long- and short-lived greenhouse gases as well as the full spectrum of aerosols and albedo variations from land-use change. Using a simple climate model the pair worked out the proportion of temperature change that each country was responsible for, from 1850 through to the present day. They also employed two different emissions scenarios to estimate the temperature change each country would be responsible for up to the year 2100.

As expected, the US and EU are the top global warming sinners to date. But the newly included radiative forcing agents make some significant differences. Ward and Mahowald’s calculations reveal that developed countries are responsible for 58% of the temperature change up to 2010, rather than the 52% they were previously attributed.

“Short-lived species and aerosols account for about one third of the magnitude of radiative forcing in the current climate, both positive and negative,” said Ward. “In our calculations we found that the contributions of most developed countries are increased when the impacts of aerosols are considered, while the contributions of developing nations, particularly China and the African nations, were reduced. That is because the cooling influence of aerosols masks some of the warming from concurrent emissions of greenhouse gases and this is especially true for developing countries that are currently the dominant aerosol emitters.”

So even though current greenhouse gas emissions from developing countries have overtaken those of developed countries, Ward and Mahowald show that the cooling influence of aerosols mean that it will be another 15 years before developing countries become the majority contributor to climate change.

“We project 2030 to be the 'crossover year' when developing countries, led by China and India, become the majority contributors,” said Ward. The findings are published in Environmental Research Letters (ERL).

Whichever way you slice it, the study shows that the US, EU, China and India are between them responsible for around half of global anthropogenic climate change, up to the year 2100. “The idea that only a few countries contribute to the majority of climate change, which has been suggested previously, could allow a small subset of countries to meet together and come to an agreement, and perhaps sidestep the current stalemate,” said Ward.

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