(Repeats column published July 31 with no changes to text. The opinions expressed here are those of the author, a columnist for Reuters.)
By John Kemp
LONDON, July 31 (Reuters) - Supporters of natural gas promote it as a clean "bridge fuel" between the coal-and-oil dominated energy system of the present and the zero-emissions system of the future based on renewables and nuclear power.
Burning gas rather than coal in a power plant produces roughly half the carbon dioxide emissions and sharply reduces airborne pollution from mercury and other heavy metals.
Switching to gas from coal could therefore slow the rise in global temperatures as well as produce significant health benefits.
Shifting to gas is at the heart of the U.S. Environmental Protection Agency's (EPA) proposed new rules for power plant emissions.
But gas is only more environmentally friendly if it is produced, transported and burned carefully, without too much leaking into the atmosphere.
Processed natural gas is almost entirely methane, which has a global warming effect more than 20 times more potent than carbon dioxide, according to the Intergovernmental Panel on Climate Change, so leaks are a big problem.
Methane accounted for almost 9 percent of U.S. greenhouse emissions in 2012, according to the EPA, making it the second-largest contributor to climate change after carbon dioxide. ("Inventory of U.S. greenhouse gas emissions and sinks 1990-2012" April 2014).
The Obama administration's Climate Action Plan, therefore, contains a separate "Strategy to Reduce Methane Emissions", published in March 2014, designed to tackle the problem.
The administration's methane strategy is a mixture of practical and political. On a practical level, the global warming effect from methane emissions is too large to be ignored and could get worse as gas becomes an increasingly important fuel in the next few decades.
Methane leaks from natural gas wells and pipelines are also associated with leaks of other heavier volatile organic compounds, which are a significant cause of air pollution.
But the White House also has a political motivation in that it is backing gas as a cleaner alternative to coal.
Some green groups question whether gas really is cleaner, given the leaks into the atmosphere and its greater potency as a greenhouse gas.
Precisely how much methane is leaking into the atmosphere is fiercely disputed. Some green researchers claim enormous quantities are being released, especially from shale wells. ("Bridge to nowhere: methane emissions and the greenhouse gas footprint of natural gas", April 2014)
Others are more sceptical, suggesting that if so much gas were really being lost, it would have created combustible clouds over oil and gas fields and led to frequent explosions.
In fact there is not really enough evidence to reach a definitive conclusion, because comprehensive survey data have not been collected.
But there is a political imperative for the White House to be seen to be taking the issue seriously to keep the support of prominent environmental groups for its renewables-plus-gas strategy for combating climate change.
Agriculture and forestry accounted for almost 40 percent of anthropogenic methane emissions in the United States in 2012, according to the EPA.
Methane emitted from enteric fermentation (the digestive systems of cattle) is estimated to account for 25 percent of all U.S. methane emissions.
To put that in context, it is the equivalent of 141 million metric tonnes of carbon dioxide (2 percent of all greenhouse emissions from all sources).
Other major sources of methane emissions come from manure management, forestry and rice cultivation.
But farmers are a politically powerful group the White House cannot afford to offend. Besides, there are currently no easy ways to tackle enteric and manure-related emissions.
"This strategy addresses emissions from agriculture exclusively through voluntary actions, not through regulations," the White House emphasised when it was launched.
With farming off limits, the methane strategy focuses on three other sources: gas and oil systems (28 percent of all methane emissions), landfills (18 percent) and coal mining (10 percent).
The administration has promised to introduce new standards or rules for petroleum, landfill and coal mining systems, using its powers to regulate air pollution (via the EPA) and activities on public lands (via the Bureau of Land Management).
Cutting fugitive emissions from the oil and gas systems has emerged as the priority for the White House as well as for environmental groups.
Of petroleum-linked methane emissions, three-quarters of emissions come from natural gas production, transportation and distribution, with the rest from the oil industry, according to the EPA.
"Within the natural gas industry, approximately 31 percent of this methane came from production sources, 15 percent from processing, 34 percent from transmission and storage, and 20 percent from distribution," the White House claimed in its strategy document.
"As our use of natural gas in manufacturing, transportation and power generation increases - creating jobs, reducing costs, cutting carbon pollution, and reducing dependence on foreign oil in our nation - we must continue to ... reduce methane emissions from this vital sector of our economy."
Almost half of fugitive emissions come from the production and initial processing stage, so this has emerged as an early focal point for the administration's efforts.
A large proportion of the fugitive emissions from gas wells are thought to occur during the initial "flowback" period, lasting perhaps ten days after a well has been hydraulically fractured, when natural gas mixed with fracking water comes to the surface.
If the methane is not captured during this period, before the well is hooked up to the gas-gathering system, large quantities of natural gas are lost to the atmosphere.
Environmentalists are pushing for companies to use "green completion" techniques, which capture substantially all this gas.
Gas producers have a strong commercial incentive to use green completions, since they capture valuable output, and many are doing it voluntarily. The federal government also is moving to require green completions for wells drilled on public lands and perhaps eventually for all wells.
For its part, the U.S. Department of Energy has agreed to help overhaul the U.S. transmission, storage and distribution systems with new efficiency standards and research. ("Department of Energy announces steps to help modernize natural gas infrastructure" July 29)
The Energy Department has also recommended to the Federal Energy Regulatory Commission (FERC), an independent regulatory agency the administration does not directly control, that it should find ways to help gas transmission companies invest in leak detection and reduction by giving them more certainty that they will be able to recover the costs from pipeline users.
Cutting fugitive emissions from trunk pipelines, storage facilities and local utility networks is set to be a major feature of the first Quadrennial Energy Review promised for January 2015.
Being seen to control leaks from the production and transportation of natural gas is essential to maintaining the political consensus that gas is a "clean" fuel that will play a big positive role in countering global warming. (editing by Jane Baird)