Options for reducing greenhouse gas emissions (GHGE), measured as global warming potential, in twelve crop and seven livestock systems were explored using a systems model-based life-cycle analysis of environmental burdens and resource use. Differences between crops in GHGE per kg product reflected differences in yield per hectare. Technological changes found to reduce GHGE per kg of crop were: (i) 20% decrease in total N (all crops except legumes); (ii) no-till (cereals and legumes only) and (iii) no straw incorporation (cereals and rape). Reductions in GHGE ranged from 2% (sugar beet) to 15% (cereals). GHGE per kg crop were also reduced by increasing crop yields by 20%. The maximum potential to reduce livestock GHGE was estimated by identifying for each livestock sector the system which gave the greatest reduction in GHGE per kg of product. Alternative systems were associated with reductions in GHGE of between 7% (beef from the dairy herd) and 21% (sheep meat). Nitrogen use efficiency (NUE) ranged from 48% for oilseed rape to 85% for sugar beet, and from 5.8% for sheep meat to 33% for poultry meat production. The results indicate that improvements in productivity and efficiency of resource use result in lower GHGE per unit of product and increased NUE.