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In this paper I discuss general conceptual issues in the estimation of the impacts of CO2 emissions from soils and biomass, over time, as a result of land-use change (LUC) due to increased demand for energy crops. The effect of LUC on climate depends generally on the magnitude and timing of changes in soil and plant carbon, and in particular on the timing and extent of the reversion of land to original ecosystems at the end of the bioenergy program. Depending on whether one counts the climate impacts of any reversion of land uses, and how one values future climate-change impacts relative to present impacts, one can estimate anywhere from zero to very large climate impacts due to land-use change (LUC). I argue that...
We evaluate the greenhouse gas footprint of natural gas obtained by highvolume hydraulic fracturing from shale formations, focusing on methane emissions. Natural gas is composed largely of methane, and 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well. These methane emissions are at least 30% more than and perhaps more than twice as great as those from conventional gas. The higher emissions from shale gas occur at the time wells are hydraulically fractured—as methane escapes from flow-back return fluids—and during drill out following the fracturing. Methane is a powerful greenhouse gas, with a global warming potential that is far greater...