Global methane emissions from the human body: Past, present and future

Methane (CH4) is a potent greenhouse gas released to the atmosphere by various natural and anthropogenic sources. Numerous studies have been conducted to quantify the major and minor CH4 sources on spatial and temporal time scales. A minor source of the atmospheric global CH4 budget is the direct release of CH4 from the living human body. Based on available data from recent publications, for the first time, CH4 emissions from human breath and flatus are estimated on a global scale taking into account dominant factors influencing emission, such as age, ethnicity, and gender. Human CH4 emissions are compared between preindustrial times (1750), present age (2017) and future prediction (2100) using demographic data based on World Population Prospects 2017 of the United Nations (UN 2017). In preindustrial times the global CH4 release by humans is estimated at 34 +/- 28 Gg and then substantially increase by a factor of ten reaching 344 +/- 255 Gg by 2017. Emissions are estimated to further increase by almost fourfold to a value of 1221 +/- 672 Gg by 2100, even though the rise in population is predicted to only increase by 50%. This nonlinear relationship is related to the predicted change in population structure affecting the number of CH4 producers. In contrast, for the year 2100 the simplified non-weighted estimation which merely considers the expected increase in population would only account for 612 +/- 169 Gg. The discrepancy between the simplified non-weighted and weighted estimations of human CH4 emissions emphasises the importance of factor-based calculations in order to compile more accurate data.