Oh, I don't know. The compairing organic farming to modern methods is not as simple as it seems. All other thing being equal, organic farming uses more fuel for tilling, requires more acreage and releases more CO2 and methane. Organic farming trades the use of herbicides for increased tillage to control weeds.
Every time organicaly grown field are plowed, more fuel is consummed.
All other things being equal, more soil erodes when useing organic farming techniques. One of the biggest water polution problems in the eastern US is the silting of rivers and streams. Organic farming needs much more acreage than conventional farming. One of the main reasons so many US farmers are going out of business is that they grow about 20 times as much produce per acer as they did 100 years ago. Much of the surplus farm land is now growing trees that quietly soak up CO2 without the need for constant attention required by annual food crops.