Does certified organic farming reduce greenhouse gas emissions from agricultural production? Comment on the McGee study

A recent study by McGee (Agriculture and Human Values, 32, 255-263, 2015) from the University of Oregon has led to discussions in international media and on the web. This study addresses an interesting question and applies advanced statistics for its analysis. However, we identify several methodological flaws that invalidate the results. First, McGee tests a hypothesis that does not correspond to his main question and which does not allow McGee to derive the conclusions that are drawn in his paper and reported in the media coverage. Second, the data used are not adequate for the analysis because: i) the dependent variable does not reflect the greenhouse gas emissions characteristics of organic agriculture (e.g. different emission factors in organic and conventional agriculture or avoidance of emissions from fertilizer production), ii) the explanatory variables neglect the livestock sector, and iii) trade aspects are missing. Third, McGee fails to discuss his findings in the light of quite a substantial body of experimental, bio-physical research from the US and elsewhere.