"The burning of fossil fuels that came with industrialization released massive amounts of carbon dioxide emissions into the atmosphere," Julia Pongratz, formerly of Carnegie Institution for Science, US, and now at Max Planck Institute for Meteorology, Germany, told environmentalresearchweb. "But clearing forests and other wild areas for agricultural purposes also contributes to the atmospheric carbon dioxide increase, and that has been happening since long before the Industrial Revolution – a fact that has not previously been taken into account in studies attributing climate change to countries."

Now for the first time Pongratz and colleague Ken Caldeira from the Carnegie Institution have considered emissions from land-use change prior to 1850. The findings shift attribution of global temperature increase from the industrialized nations to less industrialized countries such as India and China by up to 2–3%. That is "a level that is relevant for political discussions" according to their paper in Environmental Research Letters (ERL).

"Our study found that considering emissions from pre-industrial land-use change increases the amount of total global warming that can be attributed to emissions from South Asia from 5.1% to 7% – an increase of 37% in the amount attributed to this region," said Pongratz. "However, it needs to be noted that these fractional increases are large partly because the amount of overall warming attributed to these regions is small. Emissions from North America, Europe and the Former Soviet Union have caused more than half of today's global warming, even though fewer people live in those regions combined than live in India alone."

According to Pongratz, up until the 1950s emissions from land-use change – primarily due to clearing of forest for agriculture – were similar in size or larger than those from fossil-fuel burning. "Unlike our industrial activity, they began thousands of years ago," she said. Around 20–40% of all land-use change emissions to date took place before the Industrial Revolution.

Between AD 800 and 1850, the world's population increased roughly five-fold to more than a billion. Around half of the population growth took place in India and China, leading to large-scale deforestation in the late pre-industrial era.

"Industrial-era emissions have been low in India and China compared with the industrialized countries, because India and China started only recently with substantial industrial activity," said Pongratz. "On the other hand, in these regions much forest was cleared for agriculture prior to the Industrial Revolution, which still affects today's atmospheric carbon dioxide concentration."

Pongratz and Caldeira used established methods of climate-change attribution alongside recently published datasets of land-use change emissions. These datasets cover the entire last millennium and had yet to be used in attribution studies.

"We show that considering greenhouse-gas emissions many centuries into the past gives the most scientifically defensible method for attribution of today's climate change," said Pongratz. "This is because substantial parts of these emissions are still in the atmosphere today."

According to Pongratz, just as relatively small amounts of carbon dioxide emitted many centuries ago continue to have impacts on atmospheric carbon dioxide concentrations and climate today, the relatively large amounts of carbon dioxide we are currently emitting will continue to have a relatively large impact on atmospheric carbon dioxide concentrations and climate many centuries into the future.

"Assessing current impacts of events that occurred deep in the past also comes with substantial uncertainties and raises issues beyond the realm of science concerning the responsibility of present generations for historical activity," said Pongratz.

The researchers say that key points under debate when it comes to sharing the burden of past emissions include whether countries should be held responsible for emissions at times when their effect on climate was not understood; should present generations be held responsible for historical activity, in particular as national identities were subject to change; and should emissions be attributed to countries based on where they occur or where the end products are consumed.