...Agriculture inevitably makes a contribution to greenhouse gases, as does just about every human activity. You and I breathe out carbon dioxide all the time. But the effect of modern industrial agriculture is very much greater. Currently it is responsible for 25% of the world's carbon dioxide emissions, 60% of methane gas emissions and 80% of nitrous oxide. It's going to be hard to get fossil fuel consumption down, and agriculture has an important contribution to make which need not compromise our food supply.
The most energy-intensive components of modern industrial agriculture are the production of nitrogen fertiliser, farm machinery and pumped irrigation. They account for more than 90% of the total direct and indirect energy used in agriculture and are essential to it. It has been estimated that to produce a tonne of cereals or vegetables by means of modern agriculture requires 6 to 10 times more energy than by using sustainable agricultural methods...
More: http://www.indsp.org/IAGW.php
gelin