Image caption The Green Revolution brought major gains in rice yields, feeding millions

The Green Revolution of the 1960s raised crop yields and cut hunger - and also saved decades worth of greenhouse gas emissions, a study concludes.

US researchers found cumulative global emissions since 1850 would have been one third as much again without the Green Revolution's higher yields.

Although modern farming uses more energy and chemicals, much less land needs to be cleared.

The study is published in Proceedings of the National Academy of Sciences.

"Converting a forest or some scrubland to an agricultural area causes a lot of natural carbon in that ecosystem to be oxidized and lost to the atmosphere," said Steven Davis, from the Carnegie Institution's Department of Global Ecology at Stanford University in California.

"What our study shows is that these indirect impacts from converting land to agriculture outweigh the direct emissions that come from the modern, intensive style of agriculture."

Hunger tackled

The researchers constructed alternative scenarios for how global society might have developed since the 1960s had the new, high-yielding Green Revolution varieties of rice, maize and other crops that raised crop yields in Asia and South America never existed.

Image caption Non-intensive agriculture would mean much greater land clearance

These new varieties turned countries such as India, which imported food in the best of times and needed emergency aid in the worst, into major exporters.

Without the new crops - but with the growth in the human population and all the other socio-economic trends seen since the 1960s - feeding the world at current levels would mean the use of more than twice as much land as is currently used for agriculture, the researchers found.

Farming this way would have required less energy and use of chemicals such as fertilisers, whose production involves emissions of CO2 and whose use generates nitrous oxide, another greenhouse gas.

However, additional emissions from the extra land clearance, releasing carbon stored in trees and soil, would have been the more important factor by far.

Meeting extra food demand this way would have released about 160 Gigatonnes (billion tonnes) of carbon (GtC) over the decades - which, the researchers note, "corresponds to 34% of the total 478 GtC emitted by humans between 1850 and 2005".

"That's about 20 years of fossil fuel burning at present rates," observed Dr Davis.

Modern gains

Modern intensive agriculture is often criticised over its relatively heavy use of chemicals, which can impact insects, larger animals and plant life in the vicinity of the farm.

In addition, the run-off of excess fertiliser into rivers and lakes can generate blooms of algae and "dead zones" of water where nothing can survive.

However, strictly from the point of view of greenhouse gas emissions, intensive farming appears to be significantly the better option.

"Our results dispel the notion that industrial agricultural with its petrochemicals is inherently worse for the climate than a more 'old-fashioned' way of doing things," said Dr Davis.

He and his team suggest that policymakers keen to reduce greenhouse gas emissions should look towards further increases in crop yields, which they say might be more economical than other innovations.

Existing research shows that curbing production of meat - which is an inefficient user of land and water - would by itself have some impact on emissions, though by precisely how much is debated.