Stuart Strand of the University of Washington has proposed making bales of crop residue, such as stalks, and sinking them into the deep ocean in order to sequester carbon.

Strand calculates that the the process of harvesting, transporting and sinking bales of crop waste, weighed down by stones, in ocean waters below 1,500 metres, would be 92 percent efficient at sequestering carbon for thousands of years. In comparison, leaving the residues on the ground is 14 percent efficient and using them to make ethanol is 32 percent efficient.

If 30 percent of the world’s crop residue was treated in this way – with the remainder being left to condition the soil – it would reduce carbon dioxide emissions by about 15 percent.

"The ocean waters below 1,500 meters do not mix significantly with the upper waters," Strand says. "In the deep ocean it is cold, oxygen is limited and there are few marine organisms that can break down crop residue. That means what is put there will stay there for thousands of years."

Strand also suggests that any environmental effects might be minimised by concentrating the residue in a compact area. For example, spreading 30 percent of U.S. crop residue in an annual layer 4 meters deep, would only cover 260 square kilometers. That’s about 0.02 percent of the area of the Gulf of Mexico