DCS requested that GSI extend the work effort described in our initial Report on a Preliminary Investigation of Ground-Level Ambient Methane Levels in Manhattan, New York City, New York (16 December 2012) to assess the practicality of developing an estimate of methane emissions in Manhattan. Specifically the effort was to focus on providing an estimate of methane emissions that could be used in evaluating the role of natural gas leakage in Manhattan with respect to fossil fuel dependence, climate impacts and other environmental and economic concerns.
Currently the greenhouse gas equivalence of methane is widely accepted as at least 20 times the effect of carbon dioxide over a 100-year time frame. In other words, leakage of 1/20th, or 5%, of the methane moving through a natural gas production-transport-distribution system will effectively double the greenhouse gas impact of the use of that natural gas. That is, leakage of only 5% of natural gas from point of production to point of use would eliminate any greenhouse gas advantage of natural gas compared to other fossil fuels. More complex efforts by others have looked into the greenhouse gas emissions advantages of using natural gas instead of other fossil fuels. It appears that those more elaborate efforts are settling in at ≤3.2% gas loss to leakage as the maximum leakage rate at which use of natural gas retains an advantage. Hence, the loss of even a few percent of gas during production, transport, distribution and utilization is critically important to management and planning of present and future national and international energy supply and utilization systems. Therefore, it was concluded the extended GSI work effort should be focused on the need to assess total methane emissions. The available data was from Manhattan. Among the production, transport, local distribution and utilization systems, this work addressed the collective effect of only local gas distribution and utilization systems, along with any other methane sources that might be present in Manhattan.