A letter by Robert W. Howarth, Renee Santoro, and Anthony Ingraffea discusses the greenhouse gas (GHG) footprint of natural gas from shale formations, focusing on methane emissions. Methane is a potent greenhouse gas with a much higher global warming potential than carbon dioxide, especially over the first few decades after emission. The study finds that methane emissions from shale gas production are significantly higher than those from conventional gas, with 3.6% to 7.9% of methane escaping during well completion, particularly during flow-back and drill-out stages. These emissions are at least 30% higher and possibly more than twice as great as those from conventional gas. Methane emissions dominate the GHG footprint of shale gas on a 20-year time horizon, contributing 1.4 to 3 times more than direct CO₂ emissions. Over a 100-year time horizon, the GHG footprint of shale gas is 14% to 19% greater than that of conventional gas. Compared to coal, the GHG footprint of shale gas is at least 20% greater and possibly more than twice as great on a 20-year horizon. The study highlights the importance of considering methane emissions in assessing the climate impact of shale gas and calls for more research and better measurement techniques to accurately quantify these emissions. The authors argue that shale gas may not be a suitable bridge fuel for reducing global warming if its GHG footprint is not properly accounted for. They also note that current carbon-trading markets undervalue methane emissions by focusing on a 100-year time horizon and using outdated global warming potentials. The study emphasizes the need for improved regulations and technologies to reduce methane emissions from the gas industry.A letter by Robert W. Howarth, Renee Santoro, and Anthony Ingraffea discusses the greenhouse gas (GHG) footprint of natural gas from shale formations, focusing on methane emissions. Methane is a potent greenhouse gas with a much higher global warming potential than carbon dioxide, especially over the first few decades after emission. The study finds that methane emissions from shale gas production are significantly higher than those from conventional gas, with 3.6% to 7.9% of methane escaping during well completion, particularly during flow-back and drill-out stages. These emissions are at least 30% higher and possibly more than twice as great as those from conventional gas. Methane emissions dominate the GHG footprint of shale gas on a 20-year time horizon, contributing 1.4 to 3 times more than direct CO₂ emissions. Over a 100-year time horizon, the GHG footprint of shale gas is 14% to 19% greater than that of conventional gas. Compared to coal, the GHG footprint of shale gas is at least 20% greater and possibly more than twice as great on a 20-year horizon. The study highlights the importance of considering methane emissions in assessing the climate impact of shale gas and calls for more research and better measurement techniques to accurately quantify these emissions. The authors argue that shale gas may not be a suitable bridge fuel for reducing global warming if its GHG footprint is not properly accounted for. They also note that current carbon-trading markets undervalue methane emissions by focusing on a 100-year time horizon and using outdated global warming potentials. The study emphasizes the need for improved regulations and technologies to reduce methane emissions from the gas industry.