Cows, Sheep and Science: A Scientific Perspective on Biological Emissions from Agriculture

Biological emissions from agriculture (methane and nitrous oxide) make up almost half New Zealand’s total greenhouse gas emissions, so their importance relative to carbon dioxide is of particular policy interest. Motu Economic and Public Policy Research brought together a group of New Zealand climate change and agriculture specialists to respond to questions posed by the Parliamentary Commissioner for the Environment on the science. The paper finds that the overriding need to reduce carbon dioxide emissions is scientifically uncontentious. For the climate to stabilise, net carbon dioxide emissions must ultimately be cut to zero. There is debate about whether, when and how much action to take on other gases. Some scientists advocate a comprehensive multi-gas approach, arguing that will be more cost-effective. It may already be too late to limit warming to two degrees without mitigating agricultural greenhouse gases. Others advocate a focus on carbon dioxide or on all long-lived gases (including nitrous oxide), with concerted mitigation of methane (a short-lived gas) only once carbon dioxide emissions are falling sustainably towards zero. There is support for ‘easy wins’ on all gases, but it is unclear how easy it is for New Zealand to reduce total nitrous oxide and methane emissions while maintaining production. The report summarises current and emerging options, and discusses methods to calculate methane and nitrous oxide emissions at the paddock, farm, regional and national scale. Finally, the report considers metrics used for comparison between gases, focusing on Global Warming Potential (GWP) and Global Temperture change Potential (GTP). The authors reached a consensus that the ‘right’ value depends on the policy goal and could change substantially over time; and if the main policy goal is to cost-effectively limit global average warming to two degrees above pre-industrial levels, then the value of methane should be less than the GWP100 value of 28 until global carbon dioxide emissions have begun to decline steadily towards zero. There is no agreement beyond this on the best value to use; the arguments reflect judgments about politics, economics, and the intersection of policy and science.