If the ten biggest food corporations were a country, it would rank 25th in total greenhouse gas emissions, according to Oxfam, the aid group confederation.

The "Big 10," which include household names like Coca-Cola, General Mills, Kellogg and Nestlé, emit an estimated 263.7 million tons of greenhouse gases - more than Finland, Sweden, Denmark and Norway combined.

Farming to blame for most emissions

Direct emissions from the companies themselves are responsible for only about 11 percent of this total, while the lion's share of pollution is caused by agricultural operations in the corporations' supply chains.

Now Oxfam has asked major food producers to work with their suppliers toward reduce emissions from farming, saying that supply-chain emissions are not covered by the carbon reduction targets the companies have set for themselves.

Oxfam said it expected major food producers could cut their combined emissions by 80 million tons by 2020. Such a reduction would have the same effect as taking every car in Los Angeles, Beijing, London, and New York off the road.

Agriculture: a net carbon sink?

Many food companies already regard climate change as a potential threat to their supply chains.

Extreme droughts and floods have reduced agricultural yields in some regions and the United Nations' Intergovernmental Panel on Climate Change recently confirmed in its Fifth Assessment Report that the problem is only expected to get worse.

Today, agriculture accounts for roughly 25 percent of total greenhouse gas emissions.

But some experts maintain that if better practices were to be adopted, agriculture could become a so-called net sink, rather than a net source, of carbon. In other words farming could soak up more carbon from the air than it releases.

Some key innovations that could help improve farming's carbon balance include no-till farming and soil enrichment.

nz/cc (Oxfam)